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Ecological recovery dynamics of the benthic and pelagic fauna in response to extreme temperature events and low oxygen environments developed during the early Triassic
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Ecological recovery dynamics of the benthic and pelagic fauna in response to extreme temperature events and low oxygen environments developed during the early Triassic
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Content
Ecological recovery dynamics of the benthic and pelagic fauna in response to extreme
temperature events and low oxygen environments developed during the Early Triassic
by
Carlie Pietsch
A Dissertation Presented to the
Faculty of the USC Graduate School
University of Southern California
In Partial Fulfillment of the
Requirements for the Degree
Doctor of Philosophy
(GEOLOGICAL SCIENCES)
August, 2015
1
Acknowledgements
Thank you so much Dave for your unwavering patience and positivity throughout these six years. I so
appreciate the encouragement you gave me to reach for each and every learning, funding, and job
opportunity. I have discovered so much about paleoecology and mass extinction under your guidance and
have had a lot of adventures along the way! Thank you also for all the tips on birds and travel!
I am so pleased to be returning to my roots in upstate New York for a postdoc. Thank you to Mom and
Dad and Stephen who have supported this endeavor since taking me on my first trips to the American
Museum of Natural History. Thank you to the rest of my family for your curiosity and interest in my
research. None of this would be possible without Ben. You inspired me to pursue higher education and
have been an excellent source of all things technical and grammatical along the way. Our adventures over
the years have rejuvenated me and our relationship is my core. Thank you also to Adrienne and Cooper,
for everlasting friendship and taking the time to visit California.
There have been so many brilliant faculty at USC who have helped to improve my research. Thank you to
Frank for treating me as if I was one of your own students. Your critical approach to research and your
writing and teaching style have left an indelible mark. Will Berelson has been an excellent sounding
board for all things geochemical. Thank you for letting me change things up as graduate student rep!
Lowell Stott and Miguel Rincón have been so generous with their time and equipment, especially with
last minute requests. My qualifying exam committee; Doug Hammond, John Long, and David Caron,
provided a creative set of questions, problems, and feedback that so improved the progression of this
dissertation. Greg Davis and John Platt got my structural and mapping skills up to speed.
My peers at USC have provided a rigorous academic environment and a lot of fun as well! Thank you so
much to Liz Petsios who has been an astoundingly patient and adept field assistant, conference wing-
woman, and friend. It has been so great to travel and work with you around the world. Thank you to
Lydia, Rowan, Sarah, and Kathleen for showing me the ropes. Kathleen shared a love of ammonoids and
was the driving influence behind Chapter 5. Scott Mata provided the sedimentogical insight that shaped
2
Acknowledgements
Chapter 2 and an extremely generous donation of teaching material. Jeff, Dylan, Joyce, Olivia, and Nate
have broadened my horizons through conversations on statistics, microbialites, geochemistry, ooids,
pterosaurs, and what it means to be an educator. Thanks Caty for climbing and commiserating!
Thank you to my many field and lab assistants over the years; Emily Hron-Wiegle, my cousins Linda and
Peter Ruff, Yadi Ibarra, Selene Gonzalez, Jessica Barnett, and Li Tian. Richard Twitchett provided key
information for accessing sites in the Italian Dolomites and Kevin Byland led us through the aptly named
Confusion Range. Sarah Bottjer taught me the finer points of driving with a manual transmission. Megan
Nathan donated so much of her time to almost every sample and became a true friend.
Thank you to the administrative staff, especially Cindy Waite, John Yu, and John McRaney for making
all university, technical, and financial problems disappear. Barbara, Karen, and Deb have always been so
generous with their time and expertise. Vardui made miracles happen every Wednesday at coffee hour!
I have been so lucky to call a group of super-human athletes, the USC Triathlon Team, my second family.
Thanks for letting me be a tri-momma and to learn and grow with you. Thank you for every swim, bike,
run, race, and GH. Rad, you have been more than an athletic instructor but a model of the patient mentor I
strive to be. Thank you for pushing me to the highest levels of sport. Lee, thank you for the Ironman and
many other cycling adventures. I would have never made it without you as training partner and friend.
Jeremy, Ena, Lily, Lacey, Corey, Jazmyn, Grace, Chin, and Steph fill my heart with laughter and love.
I’ve had generous funding sources that supported this research including: IGCP 572 Travel Grants, the
USC Provost Travel Grant, the USC Earth Sciences Department, the Geological Society of America, the
Paleo Society, AAPG, SEPM, Sigma Xi, the Theodore Roosevelt grant from the AMNH, the Women in
Science and Engineering (WISE) program at USC, and the Sonosky Marine Fellowship.
3
Table of Contents
1 Title Page
2 Acknowledgements
4 Table of Contents
5 Introduction
10 Figures
13 Chapter 1: The importance of oxygen for the disparate recovery patterns of the
benthic macrofauna in the Early Triassic and the development of a quantitative
recovery rubric for benthic ecosystems
54 References
60 Tables and Figures
81 Chapter 2: High temperature and low oxygen perturbations drive contrasting
benthic recovery dynamics following the end-Permian mass extinction
108 References
114 Tables and Figures
135 Appendix
138 Chapter 3: Shallow water and deep water depositional settings in the Southwest
United States and differences in the taxonomic and ecological recovery
167 References
172 Tables and Figures
198 Appendix
201 Chapter 4: The Early Triassic paleoecology in the Tethys Ocean. Rapid
resurgence and delayed diversity tied to local depositional environments and
global climate change
229 References
233 Tables and Figures
276 Appendix
283 Chapter 5: Rapid and resilient pelagic ecological development interpreted from
the shell shapes of Early Triassic Boreal Ocean ammonoid species
310 References
316 Tables and Figures
336 Chapter 6: Concluding l thoughts on ecological recovery dynamics of the benthic
and pelagic fauna in response to extreme temperature events and low oxygen
environments developed during the Early Triassic
4
Introduction
The end-Permian mass extinction was the most ecologically and taxonomically
devastating mass extinction in the Earth's history decimating both marine and terrestrial groups
(Alroy et al. 2010, McGhee et al. 2012). This event from the geologic past contains lessons on
abrupt climate change and extinction recovery dynamics that are relevant to today's
environmental changes. Anthropogenic climate change is already beginning to alter ocean
chemistry; acidifying the oceans and reducing the availability of dissolved oxygen (Feeley et al.
2008, Stramma et al. 2008). By understanding patterns in the biological and ecological response
to drastic climate change in the past, modern biodiversity might be preserved more precisely.
During the recovery from the end-Permian mass extinction different animal groups responded to
low oxygen and high temperature conditions using a variety of coping mechanisms which may
inform how communities today will rebound from climate change.
The end-Permian mass extinction occurred between 251.94 and 251.88 Mya based on the
most recent dates of the GSSP at Meishan (Burgess et al. 2014). The Early Triassic recovery
interval has been loosely constrained by additional ash beds in South China (Figure 1) although
major stage boundaries have yet to be themselves directly dated. The Early Triassic lasted 5
million years until the Middle Triassic Anisian 247.2 Mya (Ovtcharova et al. 2006, Galfetti et al.
2007, Brayard et al. 2009). Ammonoids and conodonts are used for biostratigraphic control in
other sections as well as variations in strontium (Korte et al. 2011) which is well constrained for
the Early Triassic. Variations in carbon isotopes were extremely volatile in the Early Triassic and
these excursions can also be used to correlate sections.
The extinction was a global event affecting both terrestrial and marine fauna. The marine
groups will be the focus of this dissertation and include communities in the three major ocean
5
basins of the Early Triassic; the Panthalassic Ocean, Tethys Ocean, and Boreal Ocean (Figure 2).
The Panthalassic Ocean represented 85-90% of the worlds ocean basin. The Tethys Ocean
represented 10-15% of ancient ocean volume and has been disproportionately well studied
compared to Panthalassa. The Boreal Ocean represents a part of Panthalassa that was an
embayment in the high latitudes of the northern hemisphere. Four of the studied environments
are from the eastern margin of Panthalassa deposited in what is now the Southwest United States.
One benthic environment was analyzed from the Tethys Ocean and the pelagic fauna was studied
in all three ocean basins.
The end-Permian mass extinction was likely triggered by the eruption of the Siberian
Traps in what is now present day Siberian Russia. The timing of these eruptions is constantly
being refined as new dates for the extinction boundary are developed (Sobolev et al. 2011, Kerr
et al. 2013, Ivanov et al 2013). The composition of the basalt as well as the country rock that it
passed through are under intense scrutiny as the magnitude and the rate of release of greenhouse
and other toxic gasses into the atmosphere will determine whether or not these eruptions were
the proximal cause of the extinction event. Carbonate, evaporate, and coal deposits add to the
volume of carbon and toxic gasses (HCl, HFl, SO
2
) released during the eruptive events and
evidence for explosive magmatism helps corrdinate the eruption rate to that of the rapid
extinction event (Svensen et al. 2009, Sobolev et al. 2011, Black et al. 2014). The erupted basalt
has been subject to erosion and sedimentary burial but surveys and cores estimate the volume to
be on the order of 3 million km
3
+/- 1 million km
3
which would be enough to bury the area of
China under 300 m of rock (Reichow et al. 2007, Saunders and Reichow 2009). The Deccan
traps at the end of the Creatceous were only 1.3 million km
3
.
6
The end-Permian eruptive event is tied to two major climate perturbations. Oxygen
isotopes from conodont apatite suggest that sea surface temperatures especially around the
equator experienced an extreme spike reaching up to 38°C (Sun et al. 2012). This excursion in
oxygen isotopes is associated with a negative excursion in carbon isotopes, thought to be the
result of the addition of isotopically light carbon from extensive volcanic eruptions during this
interval (Payne et al. 2004, Payne and Kump 2007, Cui and Kump 2014). Warming of the oceans
and the dissolution of CO
2
would have produced other environmental perturbations. Evidence for
ocean acidification at the extinction boundary comes from calcium isotopes as well as the
selective extinction of poorly buffered marine groups with a heavily calcified shell (Knoll et al.
2007, Clapham and Payne 2011, Hinojosa et al. 2012). Acidification would have been a
temporary phenomena focused at the extinction boundary and first carbon isotope excursion
while subsequent changes to the carbon system would have been buffered by increased alkalinity
from the initial event (Grenne et al. 2012). Evidence for low oxygen environments is widespread
across the Early Triassic globe but shows diachronous development in different ocean basins and
regions. Direct evidence for low oxygen environments includes changes in geochemistry
including total organic carbon, the presence of pyrite framboids, and changes in redox sensitive
elements. Indirect evidence includes modeling efforts based on projections of the volume of
carbon added to the atmosphere and the availability of nutrients to drive the biological pump
(Winguth and Maier Remier 2005, Winguth and Winguth 2012).
In the Tethys Ocean, evidence for basin wide stratification comes from positive carbon
isotope excursions which indicate the burial of isotopically light organic carbon (Horacek et al.
2007). Extensive black shale deposits corroborate the burial of organic carbon and in addition to
increased organic carbon, pyrite, and redox sensitive elements indicate a low oxygen
7
environment. Modeling efforts suggest that the Tethys Ocean acted as a nutrient trap which
would have driven the biological pump, burying more organic carbon and driving the basin
anoxic at times (Winguth and Winguth 2012). In Panthalassa, evidence exists for the
development of oxygen minimum zones which did not extend to the bottom of the ocean as
initially believed (Algeo et al. 2011 but see Isozaki 1997). Facies and geochemical evidence for
an organic rich section in Japan suggests that overlying water experienced low oxygen
conditions but the bottom of the ocean basin was not driven to anoxia during the extinction or
recovery interval (Algeo et al. 2010). Additional negative and positive carbon excursions in the
Early Triassic have been interpreted to represent additional inputs of isotopically light carbon
from furthur eruptive events. One of these excursions has been tied to a second drop in oxygen
isotopes and an inferred temperature increase to 40°C (Payne et al. 2004, Payne and Kump 2007,
Sun et al. 2012).
Dissertation Purpose: This dissertation strives to highlight the complexity of the
recovery of the marine system from the end-Permian mass extinction. Additional eruptive events
following the initial extinction mechanism created variable environmental conditions in the Early
Triassic. I will explore how the benthic and pelagic fauna responded to climate perturbations
especially extreme temperatures and low oxygen environments, because they were widespread,
repeated perturbations.
The focus of the thesis will be on the benthic recovery examining variations in
community development in shallow and deep marine environments in an effort to understand
how shoreface gradients influenced the recovery. The affects of extreme temperature and low
oxygen environments on the benthic fauna will be determined by comparison of communities
influenced by each of these perturbations. Differences in community structure will be tied to
8
depositional environment and the overarching climate regime of various stages of the Early
Triassic. Community structure will be determined not only by variations in taxonomic diversity
and abundance but also body size, tiering, and community interactions. Diversity and size data
will be quantitatively compared between communities throughout the Early Triassic using a
newly developed recovery rubric. The pelagic response to extreme temperature and low oxygen
conditions will serve as a comparison between the effects of environmental change on the
seafloor with the upper water column.
9
Figures
Figure 1. The time scale of the Early Triassic depicting the four stages; Griesbachian, Dienerian,
Smithian, and Spathian, and their relative lengths compared to the duration of the Early Triassic.
Time scale is based on dates from Shen et al. 2011, Lehrmann et al. 2006, and Mundil et al.
2004. These references can be found in Chapter 2.
Figure 2. A view of the continental configuration of the Early Triassic globe. Four major ocean
basins are labeled. Paleogeographic map modified from Scotese (2010).
10
Griesbachian
Dienerian
Smithian
Spathian
252.2
247.2
Early Triassic
Induan Olenekian
Figure 1
11
Panthalassic
Ocean
Paleo-Tethys
Neo-Tethys
Boreal Sea
Figure 2
12
Chapter 1: The importance of oxygen and the development of a quantitative recovery
rubric for the benthic macroinvertebrate
Introduction
The end-Permian mass extinction 252 million years ago was the most devastating crisis
effecting life on Earth, resulting in the loss of approximately 95% of species on the planet (Erwin
2006). The recovery from this event is commonly thought to have lasted up to five million years,
including the entire Early Triassic, and is often considered to be a step like process (Hallam
1991, Schubert and Bottjer 1995, Galfetti et al. 2007, Tong et al. 2007, Pietsch and Bottjer 2010,
Chen et al. 2010, Chen and Benton 2012). Study of the Early Triassic recovery interval provides
an extensive geohistorical data set on the relationship between biotic and environmental recovery
with application to the modern anthropogenic biological crises. To that purpose, the research
community focuses on the chemical, geological, environmental, and biological conditions that
existed during the recovery event. These studies by various interest groups span the globe and
have produced a plethora of contrasting results about the rates and processes that dictated the
benthic marine invertebrate recovery. This review will use the habitable zone (Beatty et al.
2008), an established relationship between oxygen and depositional energy gradients, to assess
the benthic marine invertebrate recovery. The purpose of this analysis is to provide an
overarching perspective for the Early Triassic literature and to develop evaluative tools for
continued study of disparate localities representing the recovery interval.
The hypothesized extinction mechanisms are the environmental changes resulting from
the eruption of the Siberian Traps flood basalts. Before the main body of the eruption, the
extinction may have been initiated by gaseous CO
2
and HCl released from the plume head and
13
pipes (Svensen et al. 2008, Sobolev et al. 2011). The basalts intruded through evaporite beds,
coal, and peat deposits leading to greater carbon and volatile release (Svensen et al 2004,
Svensen et al. 2008). This was followed by the eruption which covered 7 million square
kilometers (Courtillot and Renne 2003). The release of volatile gasses is thought to be
responsible for global warming, ocean stratification, and subsequent changes in ocean circulation
that led to the development of oxygen minimum zones and widespread anoxia (Wignall and
Twitchett 1996, Hotinski 2001, Winguth and Maier-Remier 2005, Joachimski et al. 2012, Chen
and Benton 2012). Increased atmospheric carbon is also associated with the development of
hypercapnic stress and ocean acidification at the extinction boundary and into the Early Triassic
(Caldiera and Wickett 2003, Payne et al. 2007, Kiessling and Simpson 2011, Chen and Benton
2012, Hinojosa et al. 2012).
Compounding stressful marine conditions including increased temperatures, anoxia,
euxinia, and hypercapnia were likely sustained beyond the extinction boundary throughout the
Early Triassic by repeated eruptions of the Siberian flood basalts (Payne et al. 2004, Knoll et al.
2007, Joachimski et al. 2012). Evidence for continued eruptions comes from carbon isotope
excursions documented at each of the major stage boundaries (Payne et al. 2004, Payne and
Kump 2007). Support for continued deleterious conditions comes from the geochemistry of the
Early Triassic rock record. High temperatures at low latitudes are calculated from oxygen
isotope data from South China sections (Joachimski et al. 2012). The oxygen isotope excursion
and inferred warming event is correlated with a carbon isotope excursion suggesting that
additional carbon excursions throughout the Early Triassic would have been matched by
temperature swings.
14
Anoxia also persisted in both the Tethyan and Panthalassic Oceans for much of the
Induan and was sustained in Panthalassa through the Olenekian (Wignall and Twitchett 2002,
Woods 2009). Evidence for extensive anoxia spanning the Early Triassic can be found in Table
1. Microbially dominated anachronistic facies suggest that the Early Triassic seafloor more
closely resembled the low oxygen Precambrian before metazoan grazers were ecologically
important (Pruss et al. 2006, Baud et al. 2007). Euxinia is represented by biomarkers such as
isorenieratane from sulfur reducing bacteria as well as changes in stable isotope ratios
representing burial of isotopically depleted sulfur as pyrite (Grice 2005). The effects of
hypercapnia (CO
2
poisoning) are evidenced by the well-buffered, high-metabolic physiology of
the fauna that were selected to survive the extinction event (Knoll et al. 2007). Recent reviews
have co-opted the effects of hypercapnia into the role of ocean acidification in extinction and
recovery intervals (Kiessling and Simpson 2011, Clapham and Payne 2012). A mechanism to
explain the Early Triassic recovery should be able to incorporate most, if not all, of the
aforementioned synergistic environmental perturbations.
This paper will present evidence in support of the relationship between depositional
energy gradients, increased oxygenation, and therefore improved environmental conditions for
the benthic invertebrate recovery. The habitable zone is a region of the Early Triassic shoreface
that showed the greatest bioturbation intensity and trace fossil generic diversity (Figure 1)
(Beatty et al. 2008). The habitable zone is located above storm wave base where moderate wave
activity persists. These higher energy environments are thought to be able to ameliorate dysoxic
and anoxic conditions by mixing in atmospheric oxygen at a faster rate and to greater depths than
diffusion processes (Wallace and Wirick 1993). The habitable zone is bounded on the shoreward
15
side by the high wave stress regions of the swash and upper shoreface where biodiversity and
trace preservation are reduced (Beatty et al. 2008, Zonneveld et al. 2010).
The habitable zone is an Early Triassic phenomenon resulting from unique environmental
conditions. The Middle Triassic (Ladinian), the Cenozoic, and the Modern are not limited by
deep water anoxia. Therefore, the greatest bioturbation diversity and intensity can be found in the
proximal offshore, offshore transition, and lower shoreface environments (Figure 1) (Zonneveld
et al. 2010). In Early Triassic sections from British Columbia and Alberta, Canada the highest
diversity and intensity of bioturbation is found in the lower shoreface and offshore transition
while the proximal offshore is depauperate. The addition of oxygen in the habitable zone
environment supported benthic re-diversification (Beatty et al. 2008). Anoxic conditions in the
proximal offshore and basinal environments, below storm wave base, cannot be reached by wave
mixed oxygen. These regions experienced reduced diversity and colonization in the Early
Triassic relative to well oxygenated and highly colonized offshore environments in the modern
(Beatty et al. 2008, Zonneveld et al. 2010). During the Early Triassic, anoxic offshore water
masses encroached on shelf settings in upwelling regions or during times of transgression
making broad shelves the best for forming and sustaining a wide habitable zone (Figure 2)
(Woods 2009, Zonneveld et al. 2010). In addition, water temperature is an important control on
oxygen saturation so that cooler, high-latitude settings were also preferential for lasting
oxygenation in the habitable zone (Beatty et al. 2008, Zonneveld et al. 2010).
The habitable zone was used as a lens by which to interpret recovery patterns for many
other Early Triassic shelf ecosystems. I explored the development of similar energy gradients in
other siliciclastic and carbonate shelf systems across the globe using sedimentological
descriptions from the literature. Benthic recovery was first documented and interpreted within
16
individual stratigraphic sections and then various shoreface localities with habitable zone
development were compared. Carbonate platform environments do not have the shelf capacity to
develop a habitable zone. However, the platform margin may have provided a moderate energy,
oxygenated setting for benthic recovery. The efficacy of various environments including isolated
platforms, shelves, and epicontinental seas in buffering the benthic invertebrate recovery is one
focus of this paper. I suggest that depositional settings that provided the largest range of
oxygenated environments best supported the benthic recovery. These results may help to explain
the observed patterns of differential recovery rates across the Early Triassic globe.
In addition to atmospheric oxygenation general ocean basin characteristics might have
also influenced the rate and pattern of recovery. In the Early Triassic there were three major
ocean basins, the Panthalassic Ocean (a paleo-Pacific), the Tethys Ocean, and the Boreal Sea
(Figure 3). Models of Early Triassic ocean circulation suggest that certain regions were more
susceptible to dysoxic or anoxic conditions. A model by Winguth and Maier-Reimer (2005)
found that the Panthalassic Ocean may have contained an oxygen minimum zone centered at
1500m depth and enveloping the equator. This zone of dysoxia would have negatively influenced
benthic recovery in upwelling zones along the west coast of Pangaea. The structure of the Tethys
Ocean as two separated basins, Paleo and Neo-Tethys, resulted in differential environmental
conditions in each basin. A hypothesis invoking basinal sills segregating Paleo-Tethys from the
open Panthalassic Ocean found that in Paleo-Tethys stratification, temperature, and salinity were
increased and oxygen content reached anoxic stages below 1800m (Osen et al. 2012). Based on
the delayed timing of extinction in some localities in Neo-Tethys it has been suggested that this
basin remained oxygenated beyond the Permian-Triassic boundary into the Late Griesbachian
17
(Wignall et al. 1996, Wignall and Twitchett 2002). I incorporated these hypotheses and models
into my assessment of oxygenated environments in the Early Triassic.
The following review will report on nine localities spanning all three Early Triassic ocean
basins Using descriptions from the literature on lithology, chemistry, and biology I will evaluate
the importance of depositional environment and oxygenation in the recovery from the end-
Permian mass extinction. My goal is to realize a potentially unifying recovery mechanism for the
Early Triassic.
Methods
Selection Criteria
In order to ascertain which depositional environments fostered benthic recovery I
undertook a review of the current literature on the Early Triassic benthic macrofauna. This
review spans all three major Early Triassic ocean basins; the Tethyan, Panthalassic, and Boreal
Oceans covering a range of paleo-latitudinal variability. The most important criterion was a
broad representation of the depositional and lithological regime including; mixed siliciclastic and
carbonate shelves and ramps, isolated carbonate platforms, and an epicontinental sea. Some of
these environments may be more susceptible to upwelling or transgression of anoxic deep water
than others, e.g. shelf vs. carbonate platform. More specifically, different depositional systems
produce unique local environments; for example, ramps can develop isolated, quiet water lagoon
systems behind shoals while carbonate platforms contain exposed, storm influenced platform
margins and slopes. The differences in energy stage for each setting could be the defining line
between a well-oxygenated, viable environment or a dysoxic, low diversity environment. The
second criterion is the amount and quality of depositional and biological information available in
18
the literature for each locality. To compare depositional energy with biological recovery this
review required well-described stratigraphic sections with detailed lithology, sedimentological
indicators of energy stage, as well as comprehensive occurrence and ecological data on the
benthic macrofauna.
Localities
Nine places met these criteria and include two Panthalassic, six Tethyan, and one Boreal
locality (Figure 3). The Panthalassic localities include an epicontinental sea represented by the
siliciclastic Moenkopi Formation of the Southwestern United States and a siliciclastic shoreface
represented by the Montney Formation in British Columbia and Alberta, Canada. The Boreal
Ocean is represented by the mixed carbonate siliciclastic shelf of the Vardebutka Formation in
Spitsbergen. The Tethyan localities are represented by mixed carbonate/siliciclastic shelves in
the Werfen Formation of Italy, the Mianwali Formation of Pakistan, and the Huangzhishan
Formation of South China. A mixed ramp is represented by the Ablaksokővölgy and Grennevár
Formations of Hungary. Carbonate platforms of the Wadi Wasit block in Oman and the Great
Bank of Guizhou in South China also occur in Tethys.
For each locality, I systematically reviewed the stratigraphy, following the authors
conclusions or making new suggestions about the depositional energy represented by each
lithology. Changes in energy stage throughout each section were then compared to available data
on benthic diversity and ecology. I looked for evidence of anoxia in the stratigraphic record and
noted the temporal extent of dysoxic and anoxic events and corresponding faunal changes (Table
1).
19
Ecological Recovery Rubric
When comparing the descriptions of benthic diversity from a variety of authors it is
important to have a rubric by which to evaluate the ecological recovery within members of a
formation and between different localities. I modified the recovery stages outlined by Twitchett
(et al. 2004, 2006) which created a qualitative model from 1 (low) through 4 (complete
ecological recovery) (Figure 4). The structure of this rubric suits the purpose of this review
because it not only considers overall taxonomic diversity but ecological complexity indicated by
dominance ratios, body size, tiering, and bioturbation depth and ecology. Including ecological
criteria helps reduce some of the subjectivity of taxonomic diversity arising due to the specific
interests of different research groups and the strength of research interest at various localities and
paleolatitudes. I made one important modification to the rubric, a definition of how many
characteristics were required to merit inclusion in a given recovery stage. For this review,
infaunal and/or epifaunal tiering characteristics defined by Twitchett et al. (2004) must be met,
and at least 50% of the remaining characteristics of general diversity and size must also be
present for each stage assignment. Other rubrics addressing the recovery from the extinction
have been put forth including a recent addition by Chen and Benton (2012). This rubric was not
selected for use because much of the requisite data was not readily available from the literature
and some recovery stages developed in the recovery scheme do not apply directly to the benthic
invertebrate recovery which is the focus of this review.
20
Results
Isolated Platform
South China
One of the best examples of an Early Triassic carbonate platform is the Tethyan Great
Bank of Ghuizhou in the South China block representing deposition in all four Early Triassic
stages (Figure 5) (Lehrmann et al. 2005, Payne et al. 2006). Sedimentology of the platform
interior begins with a calcimicrobial framestone spanning the Permian-Triassic boundary
followed by mudstones indicative of restricted conditions in the earliest Triassic followed by
dolo-ooid grainstones representing a higher energy shoal which formed in the platform interior
(Lehrmann et al. 2005). At the end of the Permian the corals and stenohaline echinoderms that
made up the reef community were replaced by a predominantly molluscan fauna composed of
50% bivalves and 50% microgastropods as well as some brachiopod taxa from the Permian
(Payne et al. 2006). In general, diversity and skeletal abundance were much reduced which is
indicated by a decrease in the abundance of metazoans in platform sediments from 14% in the
Permian reef to about 2% in the Griesbachian interior. In the Dienerian, the interior continues to
be dominated by mudstone and dolomite and a small percentage of ooids which suggest a
combination of restricted and shoal environments. The Smithian interior continues to be
dominated by mudstone and dolomite. For the rest of the Early Triassic the platform interior
fauna is mostly molluscs and ostracodes. Some indications of anoxia in the restricted interior
include non-bioturbated, laminated marls and the deposition of trace metal enriched framboidal
pyrite (Payne et al. 2006).
21
Compared to the restricted platform interior the platform margin and basinal
environments show unique recovery patterns. Early Triassic platform margins are known from
breccias and show that an echinoderm and bivalve fauna dominated the Early Triassic benthos,
however a comparative Permian deposit is lacking (Payne et al. 2006). The platform margin is a
high energy environment indicated by ooid-grainstone deposition (Lehrmann et al. 2005, Payne
et al. 2006). In the Griesbachian the platform margin is composed of 40% ooids and lacks
metazoan biota. Throughout the rest of the Early Triassic, ooid concentration decreases and
biodiversity increases. Within the oolites and breccias of the Dienerian platform margin,
intermittent packstone deposits were dominated by a bivalve and gastropod fauna. Upsection, the
fauna transitioned into a crinoid and sponge dominated fauna in the Smithian while the Spathian
shows a more balanced ecosystem including crinoids, bivalves, cephalopods, brachiopods and
ostracodes. The basin margin is predominantly composed of micrite and debris flows from the
platform margins (Lehrmann et al. 2005, Payne et al. 2006). The Permian-Triassic transition was
devastating on the basin margin, leaving Griesbachian deposits totally depauperate of metazoan
life. For the rest of the Early Triassic, the benthic fauna is composed of small amounts of
bivalve, echinoderm, and brachiopod fragments. By the Spathian a surprising proportion (4%) of
the sediment is composed of crinoid grains (Lehrmann et al. 2005, Payne et al. 2006).
Oman
Late Permian to Early Triassic aged strata of Oman represent a transition from reef to
platform deposition (Figure 6). The record of this event is best represented by large blocks
(200m
3
) of Permian to Griesbachian-aged strata found in Dienerian aged debris flow breccias
(Krysten et al. 2003, Twitchett et al. 2004). These blocks represent deposition of Permian reef
22
strata and a short hiatus which is followed by Griesbachian seamount limestone deposits. The
Permian strata were deposited on an isolated carbonate platform that contained a diverse fauna of
rugose corals, stromatoporids, and sponges. At the boundary the hiatus is represented by a thin
clay layer, which is topped by Griesbachian platform deposits consisting of a limestone coquina
followed by bioclastic wackestones and packstones. The fauna of the earliest Griesbachian-aged
limestone coquina contains a low diversity bivalve fauna dominated by Promyalina, and
including Eumorphotis, Claraia and a few unidentified gastropods. These deposits represent a
succession of high energy storms that left well-winnowed, erosive-based deposits with concave
down shells.
Up section, energy decreased producing packstones and wackestones with increased
diversity (Figure 6) (Krysten et al. 2003). Micritic interbeds with deep marine fauna such as
ammonoids indicate intervals of quieter deposition interbedded with disarticulated, abraded, and
angular fossil mollusc fragments suggesting regular storm reworking. The parautocthonous beds
have a diverse benthic fauna throughout the deposits consisting of bivalves, dominated by
Eumorphotis and containing Claraia, crinoids and echinoids, half a dozen microgastropod
species, and rhynchonellid brachiopods with the addition of pelagic taxa such as ammonoids and
conodonts (Twitchett et al. 2004). In addition to the resurgence of diversity, some gastropod taxa
exhibit increased average (6.9mm Bellerophon vs. 4.6mm) and maximum (25mm vs. 10mm)
body sizes compared to other Early Triassic gastropod faunas. The geologic record of the
recovery interval in Oman was fleeting. The Dienerian brecciation of the Permian-Griesbachian
reef and seamount was followed by the deposition of platy limestones of Dienerian and Smithian
age which represent a drowning event and transition to deeper water deposition for the remainder
23
of the Early Triassic in Oman, terminating the progression of recovery on the platform (Krysten
et al. 2003).
Siliciclastic Shelves
British Columbia and Alberta
The composite section from Western Canada represents Griesbachian and Dienerian aged
siliciclastic shelves that were deposited on the margins of large platform basins (Figure 7)
(Beatty et al. 2008, Zonneveld et al. 2010). The Montney Formation represents a low angle
clastic ramp with sands, silts, muds, and some bioclastic packstones (Zonneveld et al. 2010).
Parasequences deposited on these shelves signify a range of depositional environments from the
distal offshore to the proximal lower shoreface. Hummocky cross-bedded sandstone packages in
proximal offshore environments correspond to storm events that transported sediment, organisms
and oxygen into offshore environments that were usually characterized by laminated shales and
framboidal pyrite suggesting anoxic conditions. Further evidence of anoxia in these offshore
locations includes low bioturbation intensity (ii1-2) of small diameter (<5mm) Planolites (Beatty
et al. 2008)
The biota found in the studied sections are known primarily from trace fossils observed in
well cores (Zonneveld et al. 2010). The Griesbachian is represented by a transgressive systems
tract followed by a high stand systems tract within the offshore transition. The lowest
Griesbachian contains low bioturbation intensity (ii1), monotypic beds including Rhizocorallium.
Up-section diversity increases to an average of 4 taxa per bedding plane with predominantly
horizontal bioturbation (ii1-3) and vertical bioturbation depth ranging from 2-8cm. In the
Griesbachian section there are many bioturbated storm beds that are found within otherwise
24
laminated sediments suggesting that environments further onshore were a source of colonizing
biodiversity in deeper waters (Beatty et al. 2008, Zonneveld et al. 2010). The lower shoreface
environment in the Upper Griesbachian contains a consistently high diversity, high bioturbation
intensity (ii2-4) trace fauna with burrow penetration ranging from 8-12cm deep. Complex tiering
patterns and overprinting characterize this section which contains Diplocraterion,
Rhizocorallium and Thalassinoides. The top of the Montney Formation contains the Dienerian
and represents deposition during a flooding event and successive transgressive systems tract.
The bioturbators here show a contraction in diversity, decreased trace density, shallow burrow
penetration from 2-8cm, and simple tiering patterns. This change is interpreted by Zonneveld et
al. (2010) to represent a return of anoxic conditions in association with the flooding event.
Overall the Montney Formation shows the highest diversity, ecological complexity, and tiering
in the proximal lower shoreface environments and low diversity, low complexity, and shallow
tiering communities in the offshore transition.
Spitsbergen
The Boreal Realm is represented by a composite section from western Spitsbergen that
was deposited during the Griesbachian and Dienerian on a dominantly siliciclastic shelf on the
rim of a major basin now found in the Svalbard Archipelago (Figure 8) (Wignall et al. 1998).
The Early Triassic depositional environments are best explained when juxtaposed with the
preceding Permian conditions. The Kapp Starostin Formation of the Permian was a deep water,
dark chert with abundant sponge material and occasional (every 10m) carbonate bioclastic beds
containing a diverse fauna dominated by brachiopods and bryozoans along with crinoids, coral,
and molluscs. Major bioturbators in the Permian include Planolites, Diplocraterion, Chondrites,
and Zoophycos. The transition to the Early Triassic Vardebukta Formation is marked by
25
laminated, pyrite rich layers suggesting the onset of anoxic conditions. These sedimentary
indicators of anoxia were confirmed through tests of the carbon to sulfur ratios and the presence
of pyrite framboids. The Griesbachian is represented by the Selmanset Member and is comprised
of repeating parasequences containing shallowing upward sequences of anoxic, pervasively
laminated, and pyrite rich shales, siltstones, and rippled or HCS sandstones. The fauna of the
Selmanset Member is limited and the only identified bioturbator is Planolites. Additional
evidence for anoxia includes pyritized burrows, grain size sorting of pyrite framboids which
suggests persistent euxinia, and lasting anoxic conditions in deeper water sections compared to
more shallow study sites (Wignall et al. 1998).
The boundary between the Griesbachian Selmanset Member and the Dienerian Siksaken
Member is characterized by an abrupt change in lithology, oxygenation, and therefore diversity
(Figure 8). The Siksaken Member represents depositional environments ranging from the upper
offshore to lower shoreface as well as some intertidal deposition (Wignall et al. 1998). At the
lithological boundary a suite of trace fossils are found including Planolites, Skolithos, and
Diplocraterion. Further upsection additional trace fossils including Thalassinoides and
Rhizocorallium are combined with shelly fauna including bivalves, brachiopods, and
microgastropods.
Epicontinental Sea
Southwestern US
Across the Southwestern United States the pattern of transgression and regression allows
the preservation of individual Early Triassic stages at disparate localities. One of the best studied
sections is the Smithian stage Sinbad Limestone of the Moenkopi Formation which represents
26
onshore deposition along the shoreline of an epicontinental sea (Figure 9). In the Southwestern
US the Smithian stage is represented by a major transgressive-regressive event resulting in the
deposition of marine limestone in a predominantly terrestrial regime (Dean 1981, Goodspeed and
Lucas 2007).
In most sections the Sinbad begins with offshore facies deposits, a skeletal calcarenite
represented by skeletal and ooid wackestones and grainstones with bioturbated bedding surfaces
(Figure 9) (Blakey 1974, Nützel and Schulbert 2005, Goodspeed and Lucas 2007). The presence
of muds and coarser-grained beds suggests deposition below fair weather wave base which is
further supported by graded beds and Skolithos burrows which represent vertical bioturbation in
shifting sands (Figure 9) (Goodspeed and Lucas 2007). These deposits contain limited evidence
for anoxia, no chemical signals or dysoxic biota and only occasional laminations at the base of
the section (Nützel and Schulbert 2005. Goodspeed and Lucas 2007). Rather, the offshore
deposits contain abundant bivalves, gastropods, scaphopods, ammonoids, and occasional
echinoid spines suggesting normal salinity and open marine conditions. The end of the
transgressive event is represented by a transition into shallow foreshoal-shoal deposits which
contain coarsening upward deposits of peloid and ooid grainstones suggesting decreased sea
level or increased energy. The shift into a shallow marine environment is echoed by a loss of
biodiversity implying a change to a restricted, potentially high salinity backshoal/lagoon
represented by high energy storm events mixed with lagoonal muds and a restricted fauna of
bivalves, gastropods, and ostracodes (Nützel and Schulbert 2005, Goodspeed and Lucas 2007).
Additional work on the Sinbad Limestone has focused on the gastropod lagerstätte
represented by these deposits (Batten and Stokes 1986, Frasier and Bottjer 2004, Nützel and
Schulbert 2005). Within the offshore facies, tempestite beds contain extremely diverse gastropod
27
assemblages. One bed studied by Nützel and Schulbert (2005) contained 26 species of gastropod,
representing about one-third of all gastropod diversity present in the Early Triassic. In addition, a
unique phenomenon is represented by these snail deposits called the Lilliput Effect, decreased
body size thought to be related to stressful environmental conditions (Urbanek 1993, Twitchett
2007). Frasier and Bottjer (2004) found that 89% of the snails in the Sinbad Limestone could be
considered microgastropods, not reaching more than 1cm in length. The small size of these
gastropods is a subject of debate (Brayard et al. 2010) and the cause behind the small size
remains unknown. Potential causes are taphonomic sorting, restricted environmental conditions,
or decreased productivity (Fraiser and Bottjer 2004, Nützel and Schulbert 2005).
The trace fauna of the Sinbad Limestone have been well studied and include a range of
traces such as Planolites, Diplocraterion, Rhizocorallium, and Thalassinoides (Fraiser and
Bottjer 2009). Both horizontal and vertical bioturbators are represented with ichnofabric index
varying from ii1 up to ii5 but concentrated at low bioturbation intensity. The body size of the
trace makers however remains small, almost all trace fossils are less than 1cm in diameter. These
traces are found in low and high energy beds, including storm deposits, and represent an
opportunistic infauna (Frasier and Bottjer 2009).
Mixed Carbonate/Siliciclastic
Italy
The well studied Werfen Formation in Italy is composed of nine members and provides
us with a view of a mixed siliciclastic carbonate shelf system throughout the entire Early Triassic
(Figure 10). The Werfen Formation records two 3rd order parasequences with a sequence
boundary occurring at the Induan/Olenekian Boundary (Posenato 2008b). The Permian/Triassic
28
boundary is thought to be recorded in the first member of the Werfen, the Tesero Oolite Member
(Farabegoli et al. 2007).
The first depositional cycle begins in the Griesbachian with a sea level high stand in the
Tesero Oolite Member which represents a subtidal shoal environment below fair weather wave
base but with enough energy for the formation of ooid deposits and storm beds (Figure 10)
(Farabegoli et al. 2007). This member also represents the acme of extinction for fauna of
Permian affinity. Stromatolites also appear in the Tesero Oolite. Their presence indicates
anachronistic environmental conditions existed which allowed them to inhabit a wider range of
shoreface environments (Figure 10) (Schubert and Bottjer 1992, 1995, Mata and Bottjer 2011).
The Griesbachian Mazzin Member was deposited in the inner offshore, and represents a
deepening event (Farabegoli et al. 2007). The Mazzin Member contains biological and chemical
indicators of low oxygen conditions including small body size (<3mm), dominance of only a few
bivalves including the paper-pecten Claraia, Planolites traces (ii<2), abundant pyrite, and
depleted Thorium/Uranium ratios (Th/U less than 2 suggests anoxic conditions) (Wignall and
Twitchett 1996, Twitchett 1999). These authors note that from the mid-Mazzin upwards, Th/U
ratios are higher (Th/U greater than 2) in storm deposits suggesting oxygenation during these
events (Wignall and Twitchett 1996). A sudden shallowing upward event led to the deposition of
the supratidal Andraz Member (Wignall and Twitchett 1996). In this shallow environment
dysoxic/anoxic conditions are not present but the peritidal environment likely presented
challenging hypersaline conditions that did not support the recovering benthic fauna.
The second depositional cycle begins in the late Griesbachian and continues into the
Dienerian with a sudden deepening event leading to the deposition of the Suisi Member in an
offshore-transitional environment followed by a subtidal shoal (Posenato 2008b). The Suisi
29
Member is anoxic at its base, as evidenced by depleted Th/U ratios and dominant Claraia
(Wignall and Twitchett 1996, Posenato 2008b). The benthic fauna experienced extremely low
diversity in the Mazzin and Suisi Members, with only five genera of bivalves representing just
two major ecological life modes; shallow infaunal burrowers and epibyssate forms (Farabegoli et
al. 2007, Posenato 2008a). In contrast with low diversity benthic fauna, Hofmann et al. (2011)
find evidence for a diverse and complex trace fauna with bedding plane bioturbation ranging
between bpbi2-4 and ichnofabric index ranging from 2-4 in the Suisi Member at
Rosengarten/Catinaccio and at the Aferer Geisler. Nine to ten ichnogenera were recognized and
include Thalassinoides, Palaeophycus, and Planolites (Hofmann et al. 2011).
The Gastropod Oolite Member represents an inner shoreface storm environment where
wave energy led to the formation and deposition of oolitic shell fragments and intraclasts
(Wignall and Twitchett 1999, Nützel and Schulbert 2005). The oolite contains bivalves and a low
diversity (5 genera), high abundance assemblage of microgastropods dominated by
Pseudomurchsonia kokeni, in comparison to the 26 species represented in the Sinbad Limestone
microgastropod lagerstätte.
The Smithian Campil Member marks a highstand in the Werfen Formation (Hips and
Pelikan 2002). The depositional environment is similar to the underlying Gastropod Oolite
Member but contains a greater amount of terrigenous material in the form of red sandstones and
siltstones (Posenato 2008b). Siliciclastic input could be the result of increased humidity,
weathering, and runoff (Posenato 2008a) or a sea level lowstand indicated by the deposition of
high energy sedimentary structures (Broglio-Lorgia et al. 1990). The Campil Member also marks
an increase in bivalve taxonomic diversity and ecotypes including epibyssate forms such as
Avichlamys, Leptochondria, and Scythentolium, free swimming Entolium and shallow infaunal
30
Costatoria subrotunda (Figure 10, Farabegoli et al. 2007). Trace fossil diversity, bioturbation
intensity, and trace size decrease with only a few genera represented in the siliciclastic
environment with ichnofabric index ranging from ii1-2 and trace diameter no greater than 5mm
(Figure 10, Twitchett 1999).
The final three members of the Werfen represent a shallowing succession that began in
the Campil Member and continues into the open shelf environment of the Val Badia Member, the
supratidal Cencenighe Member and finally the siliciclastic dominated peritidal San Lucano
Member (Hips and Pelikan 2002). Moving upsection, additional gastropod species are found in
the Val Badia Member as well as the first appearance of the ammonoid Tirolites (Broglio-Lorgia
et al. 1990). In the Cencenighe Member new semi-infaunal bivalves, a new ammonoid genus,
and crinoids appear (Posenato 2008a). The addition of crinoids represents increased epifaunal
tiering and return to stenohaline conditions. The re-appearance of Rhizocorallium suggests stable
oxygen concentrations for increased infaunal tiering (Twitchett 1999). The San Lucano Member
was likely too shallow to support a diverse benthic fauna.
Hungary
The Early Triassic of Hungary can in many ways be compared to the progression of
lithologic changes and biologic recovery observed in the Italian section. While the Permian-
Triassic boundary has been the focus of many studies (Posenato et al. 2004, Haas et al. 2006,
Hass et al. 2007), the entire Early Triassic record is available (Figure 11) (Hips and Pelikan
2002). The Bükk Mountain section is well studied and represents deposition in an outer ramp
from offshore facies to high energy shoreface zones (Haas et al. 2006). An additional section in
the Transdanubian range represents deposition on the inner ramp.
31
In the Bükk Mountain section, the Permian-Triassic boundary shale is succeeded by the
Griesbachian Gerennevár Limestone which begins with meters of mudstone as well as thick
"crinkly" stromatolites which are many meters thick (Haas et al. 2007). Upsection bioclastic
grainstone interlayers appear and increase in thickness into an ooidal limestone which represents
a shallow subtidal environment (Hips and Pelikan 2002, Haas et al. 2007). A few brachiopods
and a "dubious" specimen of Claraia clarai are known from the lower Gerennevár Limestone.
Upsection Claraia aurita suggests a Dienerian age for the top of the Formation.
In the Lillafüred section the Ablaksokővölgy Formation represents the rest of the
Hungarian Early Triassic from the Dienerian through the Spathian (Figure 11) (Hips and Pelikan
2002). The Ablaksokővölgy Sandstone Member ranges from the Dienerian through the Smithian
and represents deposition in the shoreface and transitional zones of the shelf system based on
oolitic grains and coarsening upward siliciclastic dominated beds. The member contains a low
diversity bivalve fauna of Claraia, Eumorphotis, Costatoria, and 'Pseudomonotis'. The Lillifüred
Limestone member represents a transition to carbonate dominated deposition in a deep subtidal
environment. Limestones and laminated, fine grain size marls shallowed upward into the
offshore transition and lower shoreface environments where storm deposits of crinoid-ooid
wackestones and packstones are found. The Lillafüred Limestone Member contains, in addition
to crinoids, Eumorphotis kittli and Natiria costata. The appearance of Tirolites suggests a
Spathian age for the top of this deposit. The Savósvölgy Marl represents carbonate deposition
below storm wave base represented by laminated marls and shales with rich ammonite deposits
giving it a middle Spathian age. Gastropods have been reported to include Natiria costata,
Naticella subtilistriata, and 'Turbo rectecostatus' in addition to the bivalve Costatoria costata
(Hips and Pelikan 2002). Finally, the Újmassa Limestone represents storm tempestites deposited
32
below storm wave base. Bioturbated limestones are interbedded with marl and shale
intercalations. Many of the beds contain strong bioturbation approaching a nodular structure
suggestive of Thalassinoides. In addition to increased bioturbation, the shelly fauna includes
Costatoria, Unionites canalensis, Unionites fassaensis, Bakevellia, "Pecten", and Neoschizodus
laevigatus.
South China
The Griesbachian Stage Huangzhishan section in South China represents a transition
from a carbonate dominated lithology in the Permian to a mixed carbonate siliciclastic shoreface
during the Griesbachian (Figure 12). The Permian Changhsing Formation consists of a reef
ecosystem composed of bioclastic packstones and wackestones that contain a diverse benthic
macrofauna including corals, brachiopods, crinoids, sponges, gastropods, bryozoans, ostracodes,
foraminifers and algae (Chen et al. 2009). At the end of the Changsinghian, at the extinction
event, there is a change in lithology to shoreface deposition of muddy limestones with
Chondrites, fine laminations, and pyrite framboids, all indicators of extremely low oxygen
conditions. These muddy horizons contain epifaunal bivalves and chonetid brachiopods, both
flattened, surficial dwellers which suggest that infaunalization was limited by decreased oxygen
concentration.
Low oxygen conditions persist in the mixed carbonate siliciclastic mudstones of the Early
Triassic Huangzhishan Formation (Chen et al. 2009). Indications of low oxygen include
laminated mudstones, surficial bioturbation represented by Planolites, pyrite framboids
indicating euxinic conditions, and the paper-pecten Claraia. These Claraia beds also include the
ammonoid Ophiceras and the articulate brachiopod Lingula, another disaster taxon of the Early
33
Triassic. In addition, Chen et al. (2004) have described the lithology of the Permian-Triassic
boundary beds and reported additional brachiopod and ophiuroid faunas from the Huangzhishan
Formation.
Upsection, the Huangzhishan Formation transitions from low energy mud deposits into a
storm dominated, oxygenated, carbonate system (Chen et al. 2009). These lithological changes
are accompanied by a conversion to a benthic fauna composed of Permian holdovers including
crinoids and brachiopods and an Early Triassic disaster fauna. The Early Triassic fauna includes
bivalves, bryozoans, gastropods, ostracodes, ophiuroids and a blastoid. However, despite the
brief increase in diversity in the Early Triassic Huangzhishan Formation, the recovery is not
complete. In the overlying Yinkeng Formation deleterious, low oxygen conditions returned as
indicated by black shales and green mudstones. The benthic fauna is not diverse and consists of
brachiopods and Claraia in this part of the Early Triassic section.
Pakistan
The Mianwali Formation of Pakistan was deposited from the Griesbachian through the
Spathian on a mixed carbonate siliciclastic shelf in Neo-Tethys (Figure 13) (PJGR 1985). The
earliest Griesbachian begins with micrites in the Lower Kathwai indicating quiet water shelf
deposition (Wignall and Hallam 1993). Brachiopods, echinoids, and bryozoans occurred
sporadically within limestone coquinas (PJRG 1985). The depositional environment begins to
shallow upsection resulting in the deposition of massive echinodermal grainstones in the Middle
Kathwai (Wignall and Hallam 1993). Within these deposits, proximal localities contain micritic
horizons often as intraclasts. This higher energy environment supported a fauna of echinoderms
including crinoids, echinoids, and ophiuroids. Brachiopods including Crurithyris are also found
34
as well as rare forams. All of these taxa are abraded and are likely the result of winnowing and
storm deposition (PJRG 1985, Wignall and Hallam 1993). Transitioning into the Upper Kathwai,
the section begins to deepen, and lower energy bivalve packstones and wackestones replace the
echinoderm grainstones of the Middle Kathwai. Bivalve packstones contain Leptochondria,
Eumorphotis, and Entolium in the Upper Kathwai (PJRG 1985). Deep water deposition is also
represented by a condensed section between the Middle and Upper Kathwai indicated by
glauconite deposition and concentrated grainstones (Wignall and Hallam 1993).
The Mittwali Marl continues the deepening trend with dominant shales and marls
preserving bivalves and ammonoids (Wignall and Hallam 1993). The base of the Mittwali is
Dienerian in age and was likely anoxic as indicated by palynology, laminated sediments, high
organic content, abundant Claraia, and pyrite (Wignall and Hallam 1993, Hermann et al. 2011).
Deepening and decreasing oxygen content continued upsection in the Mittwali Marl with
decreased bivalve abundance as the lithology transitioned from fossiliferous packstones to shales
(Wignall and Hallam 1993). Black shales were organic rich and dominated by Claraia and green
shales contained other opportunistic, high dominance bivalves including Permophorus and
Unionites. The Dienerian/Smithian boundary is recognized by a carbon isotope shift correlated
with a change in sea level as well as extinction of pelagic taxa such as ammonoids (Hermann et
al. 2011). The majority of the Smithian represents a shallowing upward event composed of
siltstone and sandstones and representing oxygenated conditions. This shallowing event kept
dysoxic or anoxic conditions from developing and potentially served to protect the benthic fauna
(Hermann et al. 2011). The Smithian/Spathian transition occurs within dense bivalve beds found
towards the top of the Mittwali Marl which include Pinna, Bakevellia, Leptochondria, Eobuchia,
35
and Scythentolium (Wasmer et al. in Press). The carbon isotope excursion across the
Smithian/Spathian boundary has not been correlated with eustatic change (Hermann et al. 2011).
Discussion
Based on the modified Twitchett (et al. 2004, 2006) recovery stage scheme and body and
trace fossil data from the literature, a series of recovery stages were designated for each locality.
Analysis of sedimentological, biological, and chemical indicators were used to establish the
relationship between ecological recovery patterns and depositional environment. Finally,
depositional environment and recovery patterns were compared between localities in order to test
the importance of local environmental conditions compared to whole ocean basin settings.
South China
The extremely low diversity and low abundance of benthic fauna found in the
Griesbachian and Dienerian of each of the three environments; platform interior, platform
margin, and basin margin, represented on the Great Bank of Guizhou suggests a recovery stage
of 1 in the earliest Griesbachian and Dienerian stage deposition (Figure 5). The platform interior
was sometimes physically restricted as indicated by laminated deposits (Payne et al. 2006).
These alternated with a high energy shoal that would have physically disturbed the benthic
recovery resulting in a depressed recovery stage of 1 and 2. The platform margin shows an
improvement in the Dienerian with increased diversity of molluscs and brachiopods, leading to a
recovery stage 2. In the Smithian, the reappearance of sponges and increased epifaunal tiering
represented by crinoid debris characterized a recovery stage 3. The basin margin also achieves a
recovery stage 3 by the Spathian with the introduction of crinoid debris flow deposits which
suggest that crinoids were encroaching deeper into the basin.
36
The Smithian recovery of the platform margin of the Great Bank of Guizhou was likely
driven by the relatively high energy environment which allowed for increased oxygenation
compared to the deeper, lower energy basinal section with less disturbance compared to the
turbulent platform interior. The final step of recovery for the Great Bank of Guizhou came in the
Middle Triassic when diverse reef faunas that are comparable to other Middle Triassic Tethyan
reefs reappeared (Lehrmann et al. 2005). The return to these reef ecosystems indicates a recovery
stage 4, a fully recovered marine community five million years after the end-Permian mass
extinction. This full recovery is likely the result of a return to normal marine conditions at the
conclusion of Siberian Traps volcanism and environmental perturbations. The progression of
recovery on the Great Bank of Guizhou is best compared to other isolated platforms from the
Early Triassic, such as the Wadi Wasit block from Oman, as well as other additional Tethyan
localities.
Oman
The fauna of the late Griesbachian "Bioclastic Limestone" of Oman represents a diverse
assemblage including bivalves, microgastropods, ammonoids, brachiopods, echinoids, and
crinoids (Twitchett et al. 2004). The high diversity and abundance within this unit in addition to
a high stage of epifaunal tiering indicates a recovery stage of 3 (Figure 6). This "Bioclastic
Limestone" is the most diverse assemblage to date for the Griesbachian across the Early Triassic
globe and in many cases, a return to diversity of this stage does not occur until the Spathian
(Twitchett et al. 2004, Twitchett 2006). These authors suggested that extended oxygenated
conditions allowed the benthic fauna to re-diversify more rapidly in Oman (Twitchett et al.
2004). Two hypotheses for increased oxygenation have been put forth. The first that Neo-Tethys
experienced a later onset of the anoxic conditions, which otherwise existed globally in the
37
Griesbachian, allowing for the recovery process to attain a head start at this and other Neo-
Tethyan localities (Twitchett et al. 2004). Pakistan (see section 4.9) also shows an early recovery
suggesting environmental conditions in Neo-Tethys might have temporarily supported a benthic
resurgence. The second hypothesis, as discussed for the Great Bank of Guizhou, suggests that
local conditions were more important in dictating recovery; namely, the high wave energy
depositional environment on the seamount provided an oxygenated refugium for the recovering
benthos from low oxygen conditions present in deeper environments (Twitchett et al. 2004). The
benthic fauna of Oman likely owe their swift resurgence to some combination of atmospheric
oxygenation from the well documented storm deposits preserved in the Wadi Wasit block as well
as lingering high oxygen in Neo-Tethys.
British Columbia
The study sites in British Columbia and Alberta, Canada represent the classic habitable
zone depositional environment (Figure 1). Through time, recovery stage increases from a stage 1
in the basal Griesbachian up to a stage 3 by the Upper Griesbachian (Figure 7). The late
Griesbachian experiences a climax of tracemaker diversity, tiering depth, and the presence of
ecological complex traces including Diplocraterion, Rhizocorallium, and Thalassinoides. The
progression of the recovery was strongly tied to local depositional environment along the shelf.
Tracemaker diversity and bioturbation intensity were greatest in wave aerated environments
which are indicated in the stratigraphic section by wave and current influenced sedimentary
structures. The development of a recovery stage 3 infauna in the late Griesbachian provides
strong evidence that a lack of oxygen was a global limiting factor in the recovery from the end-
Permian extinction. In shoreface environments where shelf structure and wave energy combined
to create a strong habitable zone, benthic recovery was able to progress extremely rapidly even in
38
the presence of offshore anoxic conditions. The lack of diversity and bioturbation in deeper
water sections with sedimentological and chemical evidence for anoxic conditions provides
additional evidence for the importance of oxygenated environments in the recovery of the
benthic fauna. In the Dienerian a flooding surface results in the transgression of low oxygen
conditions and recovery stage temporarily returns to a low diversity, low tiering, stage 1
community.
The role of oxygen, depositional environment, and biotic recovery is echoed in trace
fossil deposits from the Boreal Sea at Spitsbergen as well as other Panthalassic and Tethyan shelf
environments discussed below.
Spitsbergen
Without information on complex tiering relationships, body or trace size, or bioturbation
intensity it is difficult to perform a comprehensive review of the ecological recovery in
Spitsbergen. However, the inclusion of a Boreal Sea locality is essential for comparison to
Panthalassa and Tethys in an effort to understand how ocean basin setting may have influenced
the benthic recovery. Overall, the Spitsbergen section shows a rapid recovery relative to other
Early Triassic sections indicated by increased and seemingly sustained shallow water
oxygenation and body and trace fossil diversification (Figure 8). Of note is the coincidence of the
shifts in lithology and diversity between the stage 1 recovery in the Griesbachian Selmanset
Member and the stage 2 recovery of the Dienerian lower Siksaken Member. The recovery stage 1
fauna is associated with deeper water anoxic conditions, which lacked the depositional energy
for sufficient wave oxygenation. Increased depositional energy during the parasequence
deposition of silts and sands occurs simultaneously with the transition to a recovery stage 2
39
fauna. Finally, a further progression into upper offshore and lower shoreface environments was
combined with a resurgence in trace maker diversity and ecological complexity leading to a stage
3 recovery. The return of high trace and shelly fossil diversity, bioturbation intensity, and trace
fossil complexity in environments with greater depositional energy provides more evidence for
the importance of oxygenation by local conditions in the benthic recovery. Deeper water sections
did not show this same pattern but instead anoxic conditions lasted into the Middle Triassic
(Wignall et al. 1998). It is evident for the Spitsbergen Vardebukta Formation that depositional
energy stage and position along the shelf of a major basin led to the development of a strong
habitable zone which fostered a rapid recovery of the benthic marine fauna. The recovery
documented here is very similar to the rapid return of infaunalization found in the habitable zone
in British Columbia and Alberta, Canada (Beatty et al. 2008, Zonneveld et al. 2010). Both
localities represent cooler water sections deposited on broad shelves with the potential for
widespread lasting habitable zone development. Using the modified Twitchett (et al. 2004, 2006)
(Figure 4) recovery rubric the recovery pattern at these localities can be easily compared to body
fossil data from other shelf environments.
Southwestern USA
The high diversity fauna of the Smithian stage Sinbad Limestone contains up to 26
gastropod species as well as many bivalves, echinoderms, and scaphopods indicating a recovery
stage 3 (Figure 9). While many of the gastropod species are small, less than 1cm, some grow to
larger sizes and bivalve species range in size from mm to many cm in diameter also supporting a
recovery stage 3. The presence of tracemakers Rhizocorallium and Thalassinoides and increased
depth of tiering also point toward a recovery stage 3 (Fraiser and Bottjer 2009). The strong
recovery represented in the Smithian Stage Sinbad Limestone may be the result of normal marine
40
conditions that were maintained by the establishment of a habitable zone in the broad
epicontinental sea (Figure 2). Proximal offshore depositional environments, which are dominated
by storm waves, preserve the highest diversity faunas including specialized shelly infauna
suggesting that oxygen was penetrating into the sediment allowing for infaunalization. The
broad shelf structure of the epicontinental sea and the onshore position of the Sinbad Limestone
likely combined to protect the recovering fauna from deep water circulation and upwelling. More
proximal environments such as the backshoal and lagoon environments showed reduced
diversity and potentially restricted faunas stemming from a lack of circulation and/or increased
salinity suggested by the absence of stenohaline crinoids. Overall, the Sinbad Limestone is most
similar to other broad shelf environments, such as the Italian Werfen Formation, that mimic the
conditions found in this epicontinental sea.
Italy
The recovery of the benthic fauna in the Werfen Formation occurs in two waves. The first
recovery is in the late Griesbachian and early Dienerian of the Suisi Member followed by
taxonomic and ecological setback until the Spathian members of the Werfen. Oxygenation of
depositional environments likely played a large role in buffering the benthic fauna from
widespread anoxia in the Tethys Ocean basin. The Tesero Oolite and Mazzin Members of the
Griesbachian represent a recovery stage of 1 with low diversity fauna and Planolites bioturbation
(Figure 10) (Wignall and Twitchett 1996, Twitchett 1999, Farabegoli et al. 2007). The Dienerian
Suisi Member represents a sudden jump to recovery stage 3 due to increased diversity
(Farabegoli et al. 2007, Posenato 2008a) but more importantly the presence of deep, more
complex ichnogenera documented by Hofmann et al (2011) including traces of crustaceans
which are classically sensitive to low oxygen conditions (Song et al. 2014). The Mazzin Member
41
was deposited in the Griesbachian and therefore its low diversity can be explained by proximity
to the extinction horizon as well as extensive evidence for low oxygen conditions in the Tethys
Ocean basin during this interval (Wignall and Twitchett 1996, Horacek et al. 2007, Winguth and
Winguth 2012). One potential explanation for the resurgence of diversity and complex
infaunalization during the Dienerian Suisi Member is the development of a shallow water
habitable zone (Beatty et al. 2008). Geochemical and modeling evidence suggest that the Early
Triassic Tethys Ocean experienced stratification and potentially basin wide overturn which
would have limited the benthic recovery. The deeper water Mazzin Member shows signs of low
oxygen concentrations including low Th/U ratios and therefore a low diversity and complexity
benthic fauna. In contrast, the Suisi Member represents a slightly more shallow water
environment indicated by cross bedded siliciclastics and storm wave influenced carbonates
(Broglio-Loriga 1990). The Suisi Member may have been deposited above the low oxygen or
anoxic stratified horizon of the Tethys Ocean and/or wave activity in this more shallow section
may have buffered the benthic fauna allowing for the rapid resurgence of complex infauna within
500, 000 years of the extinction boundary. This environment closely mirrors that seen in
Spitsbergen and British Columbia (Wignall et al. 1998, Beatty et al. 2008, Zonneveld et al.
2011). The recovery does not progress linearly from the Dienerian into the Spathian in Italy but
instead suffers a taxonomic and ecological setback.
The increase in wave energy represented by the Gastropod Oolite member might be the
beginning of increased oxygenation in the Werfen Formation and the start of accelerated benthic
recovery. However, this member remains a recovery stage 2 because while new taxa are present,
diversity does not increase into the stage 3 range nor do epifaunal or infaunal tiering show a
significant increase. The slight increase in diversity in the Campil Member also falls short of a
42
recovery stage 3. The high energy depositional environment represented by the siliciclastic
dominated Campil Member could have fostered benthic diversity by providing an extensive
interval of oxygenated conditions within a habitable zone but the high energy environment seems
to have simultaneously limited diversification or colonization of a diverse and complex fauna.
The environments of the Spathian stage Val Badia and Cencenighe members fostered
increased diversity, tiering, and body size placing these members at a recovery stage 3. Both
members are well within the habitable zone and far from the extinction boundary and therefore
represent a recovered fauna for the Early Triassic with epifaunal tiering indicated by crinoid
remains and increased ecological diversity of infaunalization represented by Rhizocorallium
(Posenato 2008a, Twitchett 1999). The San Lucano Member, while poorly studied, is likely too
shallow to have supported a very diverse fauna although it represents the top of the Spathian
Stage of the Early Triassic (Broglio-Lorgia et al. 1990).
Local depositional environment was important in dictating the benthic recovery in the
Italian Werfen Formation. A rapid resurgence of benthic diversity and complex infaunalization
in the Dienerian aged Suisi Member was likely driven by the formation of a shallow water
habitable zone above low oxygen stratified environments in the Tethys Ocean. In the second half
of the Werfen Formation, a return of high energy and therefore well oxygenated environments
showed the greatest improvements in benthic ecological recovery. The step-like increases in
ecological recovery stages are mirrored in sections with similar depositional environments
especially the Hungarian ramp, another Paleo-Tethys section.
43
Hungary
Similar to the Werfen Formation of the Italian section, the Bükk Mountains also show an
incremental increase in recovery stages throughout the Early Triassic (Figure 11). The
Gerennevár Limestone Formation and Ablaksokővölgy Sandstone Member each contain fewer
than 5 taxa and no indication of epifaunal tiering or bioturbation suggesting a recovery stage of
1. The Lillafüred Limestone Member also contains a low diversity fauna and no indication of
increased infaunalization. However, crinoid storm beds suggest that nearby, recovery had
progressed to include epifaunal tiering. The Lillafüred Member could be considered a recovery
stage 2 with the additional benthic fauna found there. While anoxia indicators have not been
studied in the published literature the dearth of biodiversity and tiering in the deep water,
Spathian Lillafüred Limestone suggests low oxygen conditions persisted in the Hungarian basin,
limiting the benthic recovery. While diversity increased in the Savósvölgy Marl there is no
indication of increased tiering and recovery stage remains a 2. For the Újmassa beds diversity
had increased to include many species of bivalves and the presence of Thalassinoides designates
a recovery stage 3.
Again, the storm influenced, shallow water sections of the habitable zone show the
greatest improvements in the benthic recovery suggesting the importance of local environmental
changes. The sections in Hungary and Italy are both influenced by low oxygen conditions in the
Tethys Ocean. While shallow water depositional systems in Italy fueled a rapid return to diverse
and complex benthic community in the Dienerian, deeper water deposition in Hungary did not
experience the same benthic renewal. Instead, the section at Hungary experienced a slow, step-
like increase from a recovery stage 1 to a 3 by the Spathian as the Early Triassic climate
44
stabilized and the Tethys Ocean experienced fewer ocean overturns or anoxic events (Horacek et
al. 2007, Winguth and Winguth 2012).
South China
The base of the Huangzhishan Formation represents a recovery stage 1 with extremely
low diversity and horizontal bioturbation (Figure 12). An increase in depositional energy and
likely increased oxygenation led to a rapid increase to a recovery 3 stage indicated by crinoids,
molluscs, ostracodes, and bryozoans. The advance to higher recovery stages in the Griesbachian
is cut short by a return to anoxic conditions and a recovery stage 1 in the Yinkeng Formation.
The loss of storm energy during the deepening event at the boundary between the Huangzhishan
and Yinkeng Formations resulted in the encroachment of anoxic conditions and the loss of the
benthic habitable zone that had been developing in the Upper Huangzhishan. The early formation
of a habitable zone and rapid recovery to Stage 3 by the benthos mirrors the rapid recovery found
in the habitable zones of British Columbia and Alberta, Canada and Spitsbergen. These localities
all showed high tiering and biodiversity within oxygenated depositional environments. Taken
together they provide evidence for the importance of local depositional environments in all three
Early Triassic ocean basins.
Pakistan
The Griesbachian of Pakistan is unique because it is the only shelf environment that
contains a recovery stage 3 fauna directly following the end-Permian extinction event (Figure
13). The sections in British Columbia and Alberta, Canada, Spitsbergen, Italy, and even Oman
showed Stage 3 recovery only by the late Griesbachian or Early Dienerian. In Pakistan, crinoid
grainstones in addition to a diverse mollusc fauna signifies a highly recovered benthos (Figure
45
13). The formation of grainstones interbedded over quiet water mudstones implies increasing
energy level and oxygenation in storm dominated earliest Triassic environments. The trend from
the Dienerian through Spathian is one of decreased diversity and ecological recovery (Figure 13).
A deepening event in the Dienerian was associated with the encroachment of low oxygen
conditions and the development of a low diversity, high dominance bivalve fauna implying a
recovery stage 1 (Figure 13). Increased bivalve diversity throughout the Smithian and Spathian
points toward a recovery stage of 2 (Figure 13). The bivalve species that constituted the recovery
are thought to be endemic to Pakistan (Wasmer et al. 2012). This could be caused by the unique
environmental conditions present there; a well oxygenated Griesbachian that allowed for initial
survival and a shallow and also seemingly well oxygenated Smithian, that supported further
species diversification. However, this pattern may also result from poor scrutiny and lack of
comparison of Early Triassic bivalve faunas across the globe (Wasmer et al. 2012). It should also
be noted that many of the colonizers of the Early Triassic in Pakistan are composed of bivalve
genera that survived the extinction and produced new species in the Early Triassic which lends
support for Griesbachian oxygenation as a primary driver of endemic diversity (Wasmer et al.
2012).
The Pakistan section represents the synergistic effects of ocean basin oxygenation
combined with the formation of a habitable zone depositional environment. Similar to the
carbonate platform deposits of Oman, the Griesbachian stage deposits in Pakistan appear well
oxygenated and contain holdover Permian faunas suggesting a delayed arrival of deleterious,
anoxic environmental conditions. Once anoxic conditions reached the Pakistan section in later
stages, depositional environment became more important. The shallow conditions in the
46
Smithian and Spathian supported continued re-diversification of the benthos in a reconstructed
habitable zone environment.
Summary
The results presented clearly indicate that the presence of oxygenated environments was
an important aspect controlling the benthic marine invertebrate recovery from the end-Permian
mass extinction. In addition, acidification likely played a role in the initial extinction event and
high temperatures and hypercapnia helped produce lasting deleterious environmental conditions
(Clapham and Payne 2011, Chen and Benton 2012, Joachimski et al. 2012). Following the
extinction event, the Siberian Traps continued to erupt for 5 million years, evidenced by a global
record of continued carbon isotope perturbations (Payne and Kump 2007). Atmospheric carbon
input led to the formation of acidified, anoxic environments at the extinction boundary. Ocean
warming and increased temperatures stressed marine faunas and produced stratified ocean basins
resulting in a shut-down of ocean conveyor belt circulation (Hotinski et al. 2001). Our review of
nine Early Triassic localities representing different ocean basins and depositional environments
shows that one uniting factor that directly influenced the recovery from the extinction event was
lasting ocean basin oxygenation and/or local oxygenation via wave aeration (Figure 14).
Each locality showed improved benthic ecology under oxygenated conditions, in some
localities even directly following the end-Permian extinction boundary (Table 2, Figure 14).
Permian holdover faunas were sustained in both Oman and Pakistan suggesting oxygenated
conditions at these two disparate localities (carbonate platform vs. shelf) did not wane
throughout the Griesbachian (Twitchett et al. 2004). Instead it supports the hypothesis that Neo-
Tethys remained oxygenated through the Griesbachian, well beyond the rest of the Early Triassic
47
globe and may have acted as a refuge for further Triassic diversification (Figure 14). Further
ocean modeling that takes into account the phenomenon of lasting oxygenated conditions in
Neo-Tethys has yet to be performed and is necessary for improved understanding of the
environmental factors controlling the re-population of the benthic fauna in the Early Triassic.
The impacts of lasting oxygen concentrations could include endemic recovery stemming from
the remaining hold-over fauna and the spread of taxa across the globe from these refugia. The
taxa that populated the Pakistan section were an endemic fauna that diversified from the Permian
holdover fauna of the region (Wasmer et al. 2012). Whether or not these taxa invaded the globe
to establish new populations is poorly understood. The final onset of anoxia in the Pakistan
section and the drowning of the Oman platform reset the recovery trajectories for both localities
(Krysten et al. 2003, Twitchett et al. 2004). Oman shows anoxic deep water deposition
throughout the Dienerian and Smithian while the deepening event at Pakistan led to the
development of a local, step-like recovery of the benthic fauna (Table 2, Figure 14).
The Spitsbergen locality representing the Boreal Sea also shows evidence for lasting
oxygenated conditions in the earliest Triassic. The Griesbachian age deposits show evidence for
oxygenated conditions and a correspondingly diverse and ecologically complex trace fossil fauna
(Table 2, Figure 14). The diverse fauna has in the past been attributed to the high latitudinal
position of Boreal deposits, cooler water temperatures, and therefore increased oxygen saturation
(Wignall et al. 1998). However, the role of depositionally mediated oxygenation presented in this
review provides an addition recovery mechanism. The environments that supported recovered
tracemaker assemblages occurred in high energy environments that would have supported the
formation of a habitable zone though wave aeration of the water column. The exceptionally early
48
development of the habitable zone in the late Griesbachian Spitsbergen section could be partially
attributed to increased oxygen saturation in cold water or remaining oxygen concentrations.
The majority of the Early Triassic sections studied showed an important connection
between conditions that supported habitable zone development and a strong benthic ecological
recovery. In the Modern, storm and fair weather wave aeration can bring atmospheric oxygen 20-
30m into the water column (Wallace and Wirick 1993). In ancient shelf environments wave
aeration was crucial for the amelioration of offshore, deep water anoxic conditions. The studied
shelf environments each provided evidence for the importance of this oxygenation mechanism
for the improved recovery of the benthic fauna. In carbonate platforms, the depositional
environment that most closely represented the conditions that characterize the habitable zone, the
platform margin, was also the focus of benthic recovery. The platform margin is an energetic
environment that would have allowed for oxygenation of the water column without being too
physically destructive. Thus, the ecological recovery of the benthic fauna on the Great Bank of
Guizhou was most improved on the platform margin.
Some localities, including British Columbia and Alberta, Canada and the Italian Werfen
Formation showed the potential for habitable zone development early in the extinction recovery
(Table 2, Figure 14). Other localities such as the Hungarian Bükk Mountain sections experienced
a delayed onset of habitable zone conditions. The Smithian Sinbad Limestone represents the
development of a strong habitable zone on a broad epicontinental shelf far from anoxic deep
water. However, the Sinbad fauna lacked crinoids and larger taxa and was therefore not a fully
recovered stage 4 fauna, suggesting another limiting environmental force inhibited re-
diversification and growth. The inhibition or differential timing of habitable zone development
across the globe can be attributed to some combination of a lack of requisite energy gradients,
49
exceptionally low oxygen concentrations, or very likely, the influence of an additional
environmental perturbation such as high temperature or acidification. Oxygen isotopes from
sections in South China indicate Permian-Triassic boundary temperatures were between 29°C
and 35°C, well over the heat stress limit of 25°C for many taxa. High temperatures or
acidification could have played a ubiquitous role at the Permian-Triassic boundary and also the
set-backs or delayed recovery in the late Smithian Stage of many sections. The deposition of
stromatolites in the Griesbachian in both the Italian and Hungarian sections could be attributed to
increased temperatures at the Permian-Triassic boundary allowing microbialites to flourish
(Joachimski et al. 2012). The Sinbad fauna may have been subjected to increased temperatures in
this shallow epicontinental sea (Galfetti et al. 2007, Joachimski et al. 2012). More work on the
sedimentology and chemistry of other localities will be required before the relative roles of
oxygen, temperature, and acidification at the Permian-Triassic boundary and throughout the
Early Triassic can be fully understood. In addition, further work on the direct relationship
between oxygenated environments and benthic recovery needs to be made. Insight into whether
anoxic environments in the Early Triassic also contained ecologically rich benthic fauna is
lacking. The detailed requirements that dictating habitable zone development throughout Early
Triassic space and time also need to be better understood.
I would suggest that the evaluative tool by which to discuss future studies be the
modified recovery rubric from Twitchett et al. (2004, 2006) (Figure 4) used here. While other
models have been more recently proposed the scheme presented in this review allows for a
comprehensive and focused understanding of the benthic marine invertebrate ecological recovery
(Chen and Benton 2012). The modified Twitchett et al. (2004, 2006) rubric defines the
composition of a recovered fauna using quantitative species data and qualitative ecological data
50
and therefore better captures the complexity of the benthic community. In the Chen and Benton
(2012) trophic pyramid, the infauna and epifauna considered in this study fall into only three of
the seven components defined by their model resulting in a limited representation of the benthos.
However, their findings on recovery patterns for the complete trophic web echo many of the
observations made here for the benthic fauna. They concluded that the recovery of the complete
trophic web was a step-like process spanning the Early Triassic and beyond much as the return to
infaunalization and epifaunal tiering was often observed to be a stepped process for the benthic
taxa (Chen and Benton 2012). In addition, the delayed recovery of the benthos was echoed in
studies of all trophic stages suggesting that the deleterious environmental conditions that limited
benthic recovery had a similar effect on other marine ecological regimes including apex
predators (Chen and Benton 2012). For future studies that focus on the benthic recovery, the
modified Twitchett et al. (2004, 2006) model will allow for more direct comparisons between
localities and therefore more transparent research results.
Conclusions
The end-Permian mass extinction devastated life on Earth 252 million years ago followed
by an extensive recovery interval. The Early Triassic spans a 5 million year period in which the
benthic marine invertebrate fauna slowly recovered to pre-extinction stages of diversity and
ecological complexity, the climax of which is marked by the development of reef ecosystems
(Schubert and Bottjer 1995, Galfetti et al. 2007, Chen and Benton 2012). In this review I
investigated the relative role of oxygenated ocean basins and local depositional environments as
an overarching mechanism to describe the progression of the benthic marine invertebrate
recovery at disparate localities. Other contributors to the extinction event and the delayed
51
recovery of the benthos likely included ocean acidification and high temperatures (Clapham and
Payne 2011, Joachimski et al. 2012).
Nine sections representing all three Early Triassic ocean basins and a variety of
depositional environments including carbonate platforms, shelves, and an epicontinental sea
were compared (Table 2, Figure 14). Neo-Tethyan localities experienced lasting post-extinction
oxygenation allowing for the maintenance and evolution of diverse and ecologically complex
benthic communities in the early Griesbachian. Wave energy that led to increased atmospheric
aeration of the water column and the development of a habitable zone fostered benthic recovery
on most Early Triassic shelf environments by buffering the fauna against oxygen minimum
zones and anoxic deep water upwelling events (Figure 1). Ocean acidification may have played
an important role at the initial Permian-Triassic extinction boundary (Knoll et al. 2007, Kiessling
and Simpson 2011). However, while continued carbon perturbations would have maintained
increased ocean temperatures and low oxygen conditions, further acidification events were likely
buffered by weathering inputs (Greene et al. 2012). Like low oxygen, increased temperatures
were a likely stressor throughout the Early Triassic (Hotinski et al. 2001, Joachimski et al. 2012,
Sun et al. 2012). At the Permian-Triassic boundary and in the Early Triassic, isotopic evidence
suggests extreme temperatures were present at the equator (Joachimski et al. 2012). In addition
to environmental energy gradients and oxygenation, increased temperatures from the equator to
the poles would have had a synergistic, limiting effect on the re-diversification and recovery of
the benthic fauna. By the Spathian, a pole to equator temperature gradient was re-established as
evidenced by the specialization of ammonoid faunas (Galfetti et al. 2007). Many of the sections
reviewed here also show a return to more thoroughly oxygenated conditions in the Smithian and
the Spathian suggesting continuously improving environmental conditions.
52
The modified rubric developed and used in this review is an important tool for future
interpretations of the benthic marine invertebrate recovery from the end-Permian mass extinction
(Figure 4) (Twitchett et al. 2004, Twitchett 2006). Continued work on the recovery of the benthic
fauna should focus on more in-depth studies of the biological and lithological structure of the
habitable zone in conjunction with chemical studies and models focusing on the oxygen content
of the surrounding environment.
53
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59
Table 1. The sedimentological, biological, and chemical indicators of ancient oxygen levels used
in this review. Summarized from Wignall and Twitchett (1996, 2002) and references therein.
Table 2. A summary of the ocean basin, depositional system, and time span represented at each
of the eight localities. The major benthic marine invertebrate faunal constituents and the
progression of recovery stages are also provided.
Figure 1. A diagram of the habitable zone and the distribution of Ladinian and Early Triassic
trace fossils. The habitable zone occurs in the lower shoreface and offshore transition where
wave aeration mixed in atmospheric oxygen. Breaking waves further up the shoreface created
too much physical stress while environments below storm wave base did not receive enough
oxygen from mixing. Early Triassic trace fossil distributions are compared to the Middle Triassic
(Ladinian) indicating that the Early Triassic was an anomaly. Modified from Beatty et al. (2008)
and Zonneveld et al. (2010).
Figure 2. A comparison of the habitable zone in two different environments. The development,
size, and maintenance of the habitable zone varies between A., a narrow shelf and B., a broad
shelf. Modified from Zonneveld et al. (2010).
Figure 3. A view of the continental configuration of the Early Triassic globe. Four major ocean
basins are labeled as are the nine localities used in this study. 1. South China Platform, 2. Oman,
3. British Columbia and Alberta, Canada, 4. Spitsbergen, 5. Southwestern United States, 6. Italy,
7. Hungary 8. South China Shelf, 9. Pakistan. Paleogeographic map modified from Scotese
(2010).
Figure 4. A representation of the four stages that comprise the ecological recovery model. The
four stages are determined by epifaunal diversity, size, dominance ratios, tiering height, and
Tables and Figures
fossil composition as well as infaunal diversity, burrow size, burrow depth, and trace fossil
composition. For a fossil deposit to be considered for a given recovery stage it must meet 50% or
more of the diversity, size, dominance, or depth characteristics and must contain evidence for
epifaunal and/or infaunal tiering depending on available data. Rubric is modified from Twitchett
et al. (2004) and Twitchett (2006). Body fossil dominance is based on Clapham et al. (2006) and
Clapham and Bottjer (2007). Trace fossil diameter is based on Zonneveld et al. (2010). Future
work could further develop quantitative scales for body size. In addition trace fossil dominance
metrics could be expanded to include ichnofabric index in addition to the abundance of certain
trace genera. The key for this rubric can be found in Figure 5.
Figure 5: The Great Bank of Ghuizhou includes the Early Triassic platform interior, platform
margin, and basin settings. A. The section at Guandao on the Great Bank of Guizhou contains
Griesbachian through Spathian deposits representing the basin margin setting with occasional
breccias containing platform margin deposits. B. The section at Dajiang represents Griesbachian
through Smithian deposits from the platform interior of the Great Bank of Guizhou. The master
key provided here applies to the recovery rubric in Figure 4 and the stratigraphic sections
presented in Figures 5-12. Modified from Lehrmann et al. (2005a), Lehrmann et al. (2005b), and
Payne et al. (2006).
Figure 6. A section from the Wadi Wasit Block from Oman. This locality includes Permian reef
deposits and Griesbachian platform deposition. Modified from Krysten et al. (2003) and
Twitchett et al. (2004).
Figure 7. A composite section from the Pedigree-Ring and Kahntah regions representing the
Montney Formation. This section from British Columbia and Alberta, Canada represents the
61
lower shoreface and offshore transition of a siliciclastic shelf. Modified from Zonneveld et al.
(2010).
Figure 8. The composite section of the Vardebutka Formation. The Festningen Section from
Spitsbergen contains a siliciclastic shelf representing the Griesbachian and Dienerian. Modified
from Wignall et al. (1998).
Figure 9. The Batten and Stokes (Batten and Stokes 1986) section represents the Sinbad Member
of the Virgin Limestone in Utah (USA). This section shows Smithian Stage deposition in the
shallow marine environment of an epicontinental sea. More details on the trace fossils can be
found in Fraiser and Bottjer (2009). Modified from Nützel and Schulbert (2005) and Goodspeed
and Lucas (2007).
Figure 10. A composite section of the Werfen Formation. In Italy, the Werfen represents
Griesbachian through Spathian deposition on a mixed carbonate-siliciclastic shelf. Average
member thicknesses are as follows Tesero Oolite, 1-10m; Mazzin Member, 35-50m; Andraz
Horizon, 20-30m; Suisi Member, 80-90m; Gastropod Oolite Member, 15-40m; Campil Member,
100m; Val Badia Member, 80m-90m; Cencenighe Member, 70-80m; and San Lucano Member,
50-60m. Modified from Broglio Lorgia et al. (1983), Broglio Lorgia et al. (1990), Wignall and
Twitchett (1996), Twitchett (1999), Farabegoli et al. (2007), Posenato (2008a), and Posenato
(2008b).
Figure 11. A composite section from the Bálvány North and Bálvány East sections in the Bükk
Mountains as well the Gerennevár and Lillafüred sections showing the Gerennevár Limestone
Formation and Abla ksok ővölg y Formation in Hungary. These sections represent Griesbachian
62
through Spathian deposition on a mixed carbonate-siliciclastic ramp. Modified from Hips and
Pelikan (2002), Posenato et al. (2004), Haas et al. (2006), and Hass et al. (2007).
Figure 12. A composite section of the Mianwali Formation. These localities represent a mixed
carbonate-siliciclastic shelf in Pakistan spanning the Griesbachian through Spathian. The
Griesbachian and Dienerian sections range from 5-10m in thickness. The thickness of the
Smithian and Spathian deposits range from 115-125m. Modified from PJRG (1985), Wignall and
Hallam (1993), and Hermann et al. (2011).
Figure 13. A summary of the data used in the modified recovery rubric for the most advanced
recovery stage observed at each locality. A filled box represents a characteristic that was
observed and used to identify the given recovery stage. A line through the box represents that did
not have data available from the literature. For the Great Bank of Guizhou in South China the
recovery stages of the platform interior are shown in black and the platform margin is shown in
grey.
Figure 14. A summary of recovery progression through time and space in the Early Triassic. The
first map shows the Griesbachian and Dienerian, the first two stages of the Early Triassic. The
second map shows the Smithian and Spathian, the second two stages of the Early Triassic. Lower
level recovery stage 1 and stage 2 faunas are combined and represented by red circles. Stage 3
recovery faunas are indicated by yellow circles. Localities that do not have an available record
for a given time period are marked by black circles. For each localities the highest recovery stage
that occurred in a given time period is indicated on the map. From this figure it is apparent that
recovered faunas could be found in the Griesbachian and Dienerian in Neo-Tethys and Northern
63
Panthalassa/Boreal Sea. The second phase of recovery in the later part of the Early Triassic was
concentrated in Paleo-Tethys. Paleogeographic map modified from Scotese (2010).
64
Sedimentological
Fine laminations Anoxic/low oxygen conditions reduce bioturbation, can also result
from high sedimentation rate
High organic carbon content/black shales Anoxic/low oxygen conditions reduce organic remineralization. High
carbon content can also result from changes in productivity and
sedimentation
Biological
Low diversity, horizontal bioturbation or shallow bioturbation depth,
small body size (Savrda and Bottjer 1986)
Anoxic/low oxygen conditions reduce bioturbation depth and result in
smaller trace maker body size.
"Paper pectens" such as Claraia Thin shelled bivalve taxa resistant to dysoxic conditions (Wignall
1994)
Chemical
Depleted Thorium/Uranium Th/U<3 suggests enrichment of authigenic uranium (Langmuir 1978,
Myers and Wignall 1987)
Pyrite (framboids) Pyrite deposition and Framboids are diagnostic of euxinic conditions
(Wilkin et al. 1996)
65
Locality Ocean Basin Depositional System Time Periods Common Biota/Turnover Recovery Stage
Changes
1 S. China Paleo-Tethys Platform Interior,
Margin and Basin
Griesbachian-Spathian Molluscs to Echinoderms 1 to 3
2 Oman Neo-Tethys Platform Griesbachian Disaster Fauna to diverse
invertebrates
1 to 3 to 1
3 B.C. and
Alberta
Panthalassa Siliciclastic Shelf Griesbachian and
Dienerian
Planolites to
Thalassinoides and
Rhizocorallium
1 to 3
4 Spitsbergen Boreal Mixed Shelf Griesbachian Planolites to
Thalassinoides
1 to 3
5 SW USA Panthalassa Epicontinental Sea Smithian Diverse epifauna and
infauna, no crinoids
3
6 Italy Paleo-Tethys Mixed Shelf Griesbachian to
Spathian
Disaster fauna to diverse
invertebrates
1 to 3
7 Hungary Paleo-Tethys Mixed Ramp Griesbachian to
Spathian
Stromatolites and disaster
fauna to diverse
invertebrates
1 to 2
8 Pakistan Neo-Tethys Mixed Shelf Griesbachian Early Crinoids to Disaster
fauna
2 to 1 through 2
66
FWWB
SWB
Wave Aeration Diffusion
Swash
Upper
Shoreface
Lower
Shoreface
Offshore
Transition
Proximal
Offshore
Distal
Offshore/
Slope
Basin
Autochthonous
Assemblages
Allochthonous
Assemblages
Ladinian Diversity
Griesbachian Diversity
Habitable Zone
Figure 1
67
Habitable Zone
High Wave
Stress
Storm Wave Base
Storm Wave Base
anoxic upwelling
A. Narrow Shelf
B. Broad Shelf
Figure 2
68
5
6
7
3
4
2
1
8
Panthalassic
Ocean
Paleo-Tethys
Neo-Tethys
Sea Boreal
Early Triassic Continental Shelf
Modern Continents
Early Triassic Land Mass
Figure 3
69
Body Fossils Trace Fossils
5-10 sp. 10+ sp.
high
pre-extinction
45-75% 25-45%
2-5 sp. 5+ sp.
<20 mm 5-20 mm
reefs
Diversity
Size
Dominance
Diversity
Diameter
Fossil
composition
Fossil
composition
Stage 2 Stage 3 Stage 4 Stage 1
<5 sp.
small
>75%
1-2 sp.
<5 mm
Tiering lowest infaunal
bivalves
high,epifauna
appear
highest, coral
reefs return
Dominance
Tiering shallowest
Planolites
simple
burrows
disaster
taxa
increasing
vertical traces
appear
45-75%
increasing
25-45% <25%
increased
Thalassinoides
pre-extinction
high
<25%
deep,
complex
Figure 4
70
Stage 3
Spathian
Sedimentary
Structures Biology
Recovery Level
(This Study)
Lithology
Time
0
m
50
m
100
m
150
m
200
m
250
m
Smithian Dienerian Griesbachian Permian
Dalong
Formation
Guandao Section
Stage 1 Stage 2
Ripples
Cross Bedding
Planar Laminations
Ooids
Herringbone
Mud Cracks
Sedimentary Structures
Th/U
Thorium to
Uranium Ratio
Pyrite Framboid
Chemistry
C Org
High Organic
Carbon Content
Bioturbation
Claraia
Gastropod
Stromatolite
Diplocraterion
Rhizocorallium
Biology
Thalassinoides
Skolithos
Planolites
Bivalve
Brachiopod
Crinoid
Echinoid
ii # Ichnofabric Index
PM
PM
PM
Master Key
Congomerate
Mixed Carbonate/
Siliciclastic
Lithology
Sandstone
Limestone
Dolomite
Marl
Siltstone
Shale
Ooid Limestone
Bioclastic Limestone/
Packstone
Chert
Muddy carbonate
Fossiliferous Rock/
Wackestone
Breccia
Spathian
0
m
100
m
200
m
300
m
400
m
Smithian Dienerian Griesbachian Permian
Dajiang Section
Stage 1
Sedimentary
Structures Biology
Recovery Level
(This Study)
Lithology
Time
500
m
Th/U
Stage 2
A. B.
Figure 5
71
Wadi Wasit Block
Bioclastic Limestone
Griesbachian
Stage 3
Biology
Recovery Level
(This Study)
Lithology
Time
Promyalina Beds
Permian
Reef
Limestone
0
m
1
m
2
m
3
m
4
m
Figure 6
72
Stage 3
Dienerian
Montney Formation
Griesbachian
Sedimentary
Structures
Biology
Recovery Level
(This Study)
Lithology
Time
Debolt
Formation
Carbon-
iferous
0
m
10
m
20
m
30
m
40
m
50
m
60
m
70
m
80
m
90
m
100
m
ii 1
ii 4
ii 1
ii 2
ii 1
ii 2
ii 3
Stage 1 Stage 1
Figure 7
73
Siksaken Member
Vardebutka Formation
Selmaneset Member
90
m
210
m
150
m
30
m
0
m
Kapp Starostin
Formation
60
m
120
m
180
m
Griesbachian Dienerian
Sedimentary
Structures
Biology
Chemistry
Recovery Level
(This Study)
Lithology
Permian
Stage 3 Stage 2 Stage 1
Time
Uppper Hovtinden
Member
Figure 8
74
Stage 3
Smithian
Sinbad Member
Sedimentary
Structures
Biology
Recovery Level
(This Study)
Lithology
Time
2
m
4
m
6
m
8
m
10
m
0
m
Figure 9
75
Tesero
Oolite
Campil Member Val Badia Member Cencenighe Member
San Lucano
Member
Gastropod
Oolite
Member
Suisi Member
Andraz
Horizon
Mazzin
Member
Bellerophon
Formation
Stage 3
Werfen Formation
Richthofhen
Conglomerate
Sedimentary
Structures
Biology
Chemistry
Th/U
<2
Griesbachian
Th/U
<2
Dienerian Smithian Spathian
Th/U
>2
Th/U
>2
Stage 2 Stage 1
Recovery Level
(This Study)
Serla Dolomite
Anisian Permian
Lithology
Stage 4
Time
ii2
ii2
Figure 10
76
Nagyvisnyó
Limestone
Permian Griesbachian
Gerennavár Limestone
Ablakoskővölgy Sandstone
Member
Nammalian
Spathian
Lillafüred Limestone
Savósvölgy
Marl
Újmassa
Limestone
Hámor
Dolomite
Anisian
Sedimentary
Structures
Biology
Lithology
140m
120m
100m
80m
60m
40m
20m
0m
160m
180m
200m
220m
240m
260m
280m
300m
320m
340m
360m
380m
Recovery Level
(This Study)
Stage 3 Stage 2 Stage 1
Time
Figure 11
77
Stage 2
Spathian Permian Anisian
Kathwai
Member
Mittwali Marl
Mianwali Formation
Chhidru
Formation
Tredian
Formation
Narmia
Member
Landa
Member
Griesbachian Dienerian Smithian
Sedimentary
Structures
Biology
Chemistry
Recovery Level
(This Study)
Lithology
Time
Stage 1 Stage 2
C Org
C Org
Figure 12
78
1. South China
Great Bank of Guizhou
8. Pakistan, Mianwali Frm.
Middle Kathwai
2. Oman Wadi Wasit Block
3. Western Canada
Montney Frm.
4. Spitsbergen Vardebutka Frm.
Siksaken Mbr.
1 3 2
Diversity
Size
Domin.
Content
Tiering
Content
Tiering
Domin.
Diversity
Diameter
Body Fossils Trace Fossils
5. SW U.S.
Sinbad Limestone
1 3 2
Diversity
Size
Domin.
Content
Tiering
Content
Tiering
Domin.
Diversity
Diameter
Body Fossils Trace Fossils
6.Italy Werfen Frm.
Cencenighe Mbr.
1 3 2
Diversity
Size
Domin.
Content
Tiering
Content
Tiering
Domin.
Diversity
Diameter
Body Fossils Trace Fossils
7. Hungary Lillafüred Mbr.
Újmassa beds
1 3 2
Diversity
Size
Domin.
Content
Tiering
Content
Tiering
Domin.
Diversity
Diameter
Body Fossils Trace Fossils
1 3 2
Diversity
Size
Domin.
Content
Tiering
Content
Tiering
Domin.
Diversity
Diameter
Body Fossils Trace Fossils
1 3 2
Diversity
Size
Domin.
Content
Tiering
Content
Tiering
Domin.
Diversity
Diameter
Body Fossils Trace Fossils
1 3 2
Diversity
Size
Domin.
Content
Tiering
Content
Tiering
Domin.
Diversity
Diameter
Body Fossils Trace Fossils
1 3 2
Diversity
Size
Domin.
Content
Tiering
Content
Tiering
Domin.
Diversity
Diameter
Body Fossils Trace Fossils
Figure 13
79
Griesbachian and Dienerian
Smithian and Spathian
1
2
8
6
7
4
3
5
1
2
8
6
7
4
3
5
Panthalassic
Ocean
Paleo-Tethys
Neo-Tethys
Boreal Sea
Panthalassic
Ocean
Paleo-Tethys
Neo-Tethys
Boreal Sea
No Data for the given time period Recovery Stage 1 or 2 Recovery Stage 3
No Data for the given time period Recovery Stage 1 or 2 Recovery Stage 3
Figure 14
80
Chapter 2: High temperature and low oxygen perturbations drive contrasting benthic
recovery dynamics following the end-Permian mass extinction
Introduction
The end-Permian mass extinction 252 million years ago resulted in the most devastating
loss of biodiversity on the planet (Payne and Clapham, 2011; McGhee et al., 2012). Following
the initial volcanic events that contributed to the extinction, additional environmental
perturbations during the recovery interval continued to inhibit the rapid return of ecological
complexity and biodiversity. Using new collections made from the previously studied Smithian
Sinbad Limestone and the Spathian Virgin Limestone of the Southwestern United States (Fig. 1)
(Blakey, 1974; Dean, 1981; Woods et al., 1999; Schubert and Bottjer, 1995; Pruss and Bottjer,
2004; Frasier and Bottjer, 2004; Nützel and Schulbert, 2005; Goodspeed and Lucas, 2007;
Brayard et al., 2010; Marenco et al., 2012; Hofmann et al., 2012) the paleoenvironmental context
of each sample is used to improve interpretations of Early Triassic benthic paleoecology and
recovery patterns during successive climatic events (Fig. 2).
The end-Permian mass extinction event coincides with emission of volcanic gases that
preceded flood basalt volcanism of the Siberian Traps (Svensen et al., 2009; Sobolev et al.,
2011). Associated with this volcanic activity was the release of carbon dioxide and other toxic
gases such as sulfur, chlorine, and fluorine due to the heating of evaporite, carbonate, and coal
deposits rich in petroleum and organic matter (Svensen et al., 2009; Black et al. 2012). The
emissions from the Siberian Traps elevated pCO
2
levels to multiple times pre-industrial values
(Saunders and Reichow, 2009, Sobolev et al., 2011). The immediate effects of these eruptions
would have included the climatic consequences of coal ash entering the stratosphere and
81
encircling parts of the globe (Grasby et al., 2011; Ogden and Sleep, 2012). Oxygen isotopes
preserved in conodont apatite suggest that equatorial sea surface temperatures were over 35°C
(Fig. 2) (Joachimski et al., 2012; Romano et al. 2012; Sun et al., 2012). The quantity of carbon
dioxide released over such a short interval would have been capable of generating an ocean
acidification event. This is supported by excursions in calcium isotopes and rhenium/osmium
isotopes synchronous with carbon isotope swings at the Permian-Triassic boundary (Georgiev et
al., 2011; Hinojosa et al., 2012) and the loss of calcium carbonate skeleton building reef fauna
and other invertebrates (Clapham and Payne, 2011; Kiessling and Simpson, 2011).
Environmental conditions did not improve following the extinction event. Rather,
repeated perturbations within the carbon and oxygen isotope records indicate recurrent volcanic
eruptions extended throughout the Early Triassic, which lasted for five million years following
the extinction event (Fig. 2) (Payne et al., 2004; Sun et al. 2012). The largest negative carbon
isotope excursion, occurring across the Smithian to Spathian boundary within the Olenekian, is
correlated with the most negative oxygen isotope values suggesting that a large eruptive pulse
drove extensive warming, producing equatorial sea surface temperatures over 35°C (Sun et al.,
2012). Extreme warming from increased atmospheric CO
2
resulted in gentle thermal gradients
between the equator and poles that likely promoted sluggish ocean circulation and subsequent
anoxic conditions (Wignall and Twitchett, 2002; Algeo et al., 2011; Grasby et al., 2013).
Extensive equatorial oxygen minimum zones in the Panthalassic Ocean are indicated by
dysaerobic and anaerobic facies in sections in Panthalassa as well as models of the Early Triassic
ocean system (Wignall and Twitchett, 2002; Algeo et al., 2010; Winguth and Winguth, 2012). In
addition to enduring expansive oxygen minimum zones, transient regional low oxygen events
have been documented within each of the three major Early Triassic ocean basins, including the
82
Panthalassic Ocean, Tethys Ocean, and Boreal Ocean (Twitchett, 1999; Woods et al., 1999; Mata
and Bottjer, 2011; Beatty et al., 2012; Grasby et al., 2013). These low oxygen events were
devastating for the benthic fauna and resulted in low diversity and low abundance of recovering
taxa (Beatty et al., 2008; Grasby et al., 2013).
The effects of prolonged environmental stress on the recovery of the benthic fauna were
ecologically profound. The Early Triassic benthos is often dominated taxonomically by a low
diversity bivalve "disaster fauna" assemblage and limited additional taxa (Hallam and Wignall,
1997). Five particular genera; Claraia, Eumorphotis, Leptochondria, Promyalina, and Unionites
are considered the most numerically abundant bivalves in the Early Triassic and all but Claraia
remain important throughout the Early Triassic (Hallam and Wignall, 1997). The inarticulate
brachiopod Lingula and a few additional bivalve species were also opportunistic taxa becoming
abundant in the wake of the extinction (Rodland and Bottjer, 2001; Benton, 2003; Zonneveld et
al., 2007; Chen and Benton 2012). In addition to the low diversity and low evenness that typify
the Early Triassic benthos, the prevalence of extremely small body sizes, termed the "Lilliput
Effect" (sensu Urbanek 1993) is another ubiquitous phenomenon. Many classes of molluscs,
foraminifera, and echinoderms show extremely small body size following the extinction event
(Frasier and Bottjer, 2004; Twitchett, 2007; Payne et al. 2011) although this is not ubiquitously
the case (Chen and McNamara 2006, McGowan et al. 2009, Brayard et al. 2010). This
phenomenon is especially pervasive in the Smithian Sinbad Limestone of the Southwestern
United States and the Lower Triassic Werfen Formation of the Italian Dolomites where
"microgastropods," under 10 mm in height, dominate entire assemblages (Twitchett, 1999;
Fraiser and Bottjer, 2004; Nützel and Schulbert, 2005; Twitchett, 2007). Their small size reflects
the suite of environmental conditions that limited their growth, including high temperature and
83
resultant metabolic changes, decreased oxygen solubility, and potentially limited nutrient
availability (Irie and Fischer, 2009; Melatunan et al., 2013).
To determine the roles that high temperature and low oxygen conditions played in the
recovery from the end-Permian extinction, this study examined two Lower Triassic units from
the Southwestern United States, the Sinbad Limestone of Utah and the Virgin Limestone of
southern Nevada (Fig. 1). The Smithian Sinbad Limestone of Utah was deposited during the lead
up to the hottest sea surface temperatures in the Early Triassic and potentially the entire
Phanerozoic (Fig. 1) (Blakey, 1974; Dean, 1981; Goodspeed and Lucas, 2007; Sun et al., 2012).
The Spathian Virgin Limestone of Nevada represents a benthic environment influenced by
recurrent low oxygen events (Mata and Bottjer, 2011) during a cooler period of the Early
Triassic (Sun et al., 2012). For each location: i) depositional environments were identified using
grain size, sedimentary structures, and petrography; ii) samples were collected in stratigraphic
succession; and iii) paleoecological analyses were conducted on recovered fossil communities.
Our new approach contextualizes the recovery through depositional environments and
stratigraphy, yielding new insights into Early Triassic paleoecology and elevating the importance
of regional dynamics and discrete climate events in the study of extinction events in the past and
present.
Methods
Bulk rock samples were collected from fossiliferous beds 15 cm or thinner and each
sample was taken at a vertical distribution of 0.25 to 7 m between samples. Samples were
collected in a stratigraphic context in order to capture fossil communities from the range of
depositional environments represented in each section. Sample size was standardized at 9 litres
in order to control for fossil abundance. In the lab, samples were disaggregated into 2 cm
3
84
fragments to expose all conspicuous fossils. For each bulk sample, specimens that were more
than 50% complete and over 2 mm in length were identified to the genus level. The poor
preservation of most faunal constituents as molds and casts, especially in the Virgin Limestone at
the Lost Cabin Spring locality, precluded more specific identification. Each genus was then
consistently measured across its widest or longest dimension. Most bivalves were measured
along their length, unless the width was the greater measurement for the particular genus as was
the case for some pectenids. Gastropods were measured from the apex to the base of the aperture.
In addition to bulk samples, bedding plane observations of diversity and body size were also
used when bulk fossil disaggregation was not possible or was not congruent with outcrop and
petrographic observations of approximate fossil content. A recent study of the Virgin Limestone
in Utah discusses the benefits and biases resulting from each of these methods (McGowan et al.,
2009). Bulk samples are considered to be biased towards capturing small, common faunal
elements. In contrast, bedding plane analysis covers a greater area and captures rare and large
faunal elements but underestimates small taxa (McGowan et al., 2009). Bulk samples were most
useful for statistical, quantitative assessment of recovery in the Virgin Limestone. Bedding plane
samples were used for qualitative evaluation of the dominance of disaster bivalves and body size
changes.
One or more thin sections were made for each sample in order to refine the
environmental interpretation and to gather echinoderm abundance data. Each petrographic slide
was point counted (1 mm grid, 126 points) for the abundance of mollusc and echinoderm
fragments as well as ooids, peloids, stylolites, micrite, and spar matrix. This data was used to
interpret the abundance of echinoderms as they are commonly abraded and their small size limits
their visibility in hand sample. Beds with 20% or more echinoderm grains were treated as
85
echinoderm storm beds and did not include molluscs, if any, that were found in these samples in
the analysis. Micrite or spar matrix distinguished packstones and grainstones and the presence of
micritized and abraded grains as well as ooids throughout the studied sections supplemented our
field observations.
Using the collected data, summary metrics were calculated including generic richness,
abundance, Shannon-Wiener diversity index, and average size. Richness is the count of species
collected from a given sample or environment. Abundance and Shannon-Wiener diversity index
were calculated for each individual sample and then averaged for samples contributing to each
depositional environment. Fossil size data was averaged across each individual genus and then
averaged for each sample. Both individual genus trends and average sample trends were plotted
through stratigraphic sections. Histograms of size data with 1 mm size classes were used to
interpret the importance of transport in the preservation of these samples (Westrop, 1986).
The generic richness and abundance of each depositional environment was generated by
summing the contents of constituent samples. These data were arcsine transformed and absence
data was coded as .001. In order to test for significant changes in diversity between depositional
environments both within and between different sections, a Mann-Whitney U test, which
compares the differences in medians between two samples, was run on raw and transformed data
(Bakus, 2007). P values less than .05 were accepted as statistically significant and are reported,
along with U values in data tables.
Two way cluster analyses were generated using the statistical software program PC-
ORD. The flexible method which approximates multiple clustering strategies including nearest
and farthest neighbor as well as group average was applied with β = -0.25 (Bakus, 2007;
McCune et al., 2002). Sorensen/Bray-Curtis distance measures were used as they measure the
86
frequency of co-occurrence which informs community structure more than Jaccard presence-
absence measurements. Data was arcsine transformed prior to cluster analysis and absence data
was coded as .001 (McCune et al., 2002). For the Sinbad Limestone the transformed abundance
of molluscan genera in each sample was used to generate clusters. For the Virgin Limestone both
molluscan and echinoderm abundance were used in cluster analysis. Non-metric
multidimensional scaling (NMDS) methods were also used to interpret relationships between
individual samples. This procedure is superior to PCA for this analysis as it is well suited for
non-normal data (Bakus, 2007). NMDS arranges objects in multidimensional space using the
original distance between samples as well as rank order and dissimilarity values. The data was
arcsine transformed and absence data was coded as .001. Six dimensional trials were run each
time with 10 real and 50 random runs, .0005 stability criteria, 250 iterations, and a step length of
0.2 (McCune et al., 2002). Final solutions were chosen based on final stress levels and using a
Monte Carlo Test. Stress below 20 indicates that the resultant analysis can be used for ecological
interpretations (McCune et al., 2002). Monte Carlo compares real and random data runs. When
stress in real data runs is significantly less than stress in randomized data runs (p=.05) then the
preferred ordination has been found (McCune et al., 2002).
Geologic Background
Sinbad Limestone
The Smithian Stage Sinbad Limestone was deposited in an epicontinental sea on a
pericratonic ramp 160 to 500 km wide (Fig. 1) (Goodspeed and Lucas 2007). The ammonoids
Anasibirites kingianus and Wasatchites indicate that the Sinbad Limestone was deposited during
the late Smithian Anawasatchites tardus zone (Lucas et al. 2007). The Sinbad Limestone
represents the farthest transgressive event of the Smithian and occurs as a discrete marine
87
interval of silty limestones deposited between exclusively terrestrial units. The unit represents
one single transgressive-regressive couplet (Blakey 1974, Goodspeed and Lucas 2007). Blakey
(1974) identified three major facies within the Sinbad Limestone: 1) skeletal calcarenite; 2) silty
peloidal calcilutite; and 3) dolomitized calcarenite. These facies have been interpreted by
Goodspeed and Lucas (2007) as representing five depositional environments: 1) peritidal; 2)
offshore; 3) foreshoal/shoal; 4) lagoonal/backshoal; and 5) tidal channel. This study follows the
environmental interpretations of Goodspeed and Lucas (2007) with reference to the facies
descriptions of Blakey (1974) and additional observations made herein. Three of the five main
environments were identified as potential fossil-bearing horizons for this study: 1) peritidal; 2)
offshore; and 3) shoal (Fig. 3). The advantage of sampling multiple depositional environments is
that it provides an environmental gradient that serves as a context for faunal analysis.
Microfacies analysis was the primary tool for facies identification, as many of the differences in
depositional environment are represented by the ratio of fossil, peloid, and terrigenous grains
(Fig. 4). At the outcrop scale, beds of the Sinbad Limestone are poorly differentiated and
commonly structureless but do show low angle cross-stratification in parts (Blakey, 1974;
Goodspeed and Lucas, 2007).
Virgin Limestone
The Virgin Limestone Member of the Moenkopi Formation is a mixed carbonate-
siliciclastic succession deposited on a distally-steepened carbonate ramp during the Spathian
Substage of the Lower Triassic (Marenco et al., 2008; Mata and Bottjer 2011). Tirolites in the
Utah equivalent of the Virgin Limestone member indicates placement in the Columbites zone
(Larson 1966). The Lost Cabin Spring locality in the Spring Mountains of Nevada represents a
distal ramp environment (Fig. 1) (Larson 1966). Parasequences, representing repeated cycles of
88
upward-shallowing deposits, can be traced through the stratigraphic section using sedimentary
structures and ichnofabric indices (Fig. 5) (Mata and Bottjer, 2011). Each parasequence typically
initiates with a generally laminated micritic mudstone facies commonly bearing low ichnofabric
indices representing low-energy offshore deposition (Mata and Bottjer, 2011). The laminated
mudstones that reoccur at the base of each parasequence suggest the periodic recurrence of an
oxygen-limited environment that suppressed infaunalization. Rapid deepening events at the base
of each parasequence likely resulted in the encroachment of low oxygen water into these deeper
water environments. Overlying the laminated mudstone within these parasequences are massive
beds of mollusc packstones with convex up shells that result from storm-generated currents in
the proximal offshore environment (Mata and Bottjer, 2011). Low-angle cross-stratified crinoid
packstones interbedded with shales suggest storm reworking in the offshore transition (Mata and
Bottjer, 2011). At the top of some parasequences, hummocky cross stratified sandstone beds
represent deposition above fair-weather wave base in the lower shoreface (Mata and Bottjer,
2011). Microfacies analysis supplemented our interpretation of the depositional energy level of
each sample.
Depositional Environments and Sampling Strategy
Sinbad Limestone
Two sections of the Sinbad Limestone were analyzed in this study: 1) the Batten and
Stokes section which has been widely studied for its gastropod lagerstätte beds (Batten and
Stokes, 1986; Fraiser and Bottjer, 2004; Nützel and Schulbert, 2005); and 2) the Rod's Valley
section which was recently established as a lectostratotype section for the Sinbad Limestone,
which has no type section (Goodspeed and Lucas, 2007) (Fig. 3).
89
At the base of each section, the dominance of ooid packstone and grainstone with
secondary mollusc shell hash, mud, peloids and rip-up clasts suggests deposition in a peritidal
setting (Goodspeed and Lucas, 2007). Rhombs and spar represent secondary dolomitization of
this facies (Fig. 4B).
Upsection, the main transgressive event is recorded by the deposition of cross-bedded
skeletal packstone with bivalves and gastropods, echinoid ossicles, and scaphopods (Nützel and
Schulbert, 2005; Goodspeed and Lucas, 2007). This interval also includes the previously studied
microgastropod lagerstätten (Batten and Stokes, 1986; Fraiser and Bottjer 2004; Fraiser et al.,
2005; Nützel and Schulbert, 2005). These deposits represent an offshore depositional
environment within the skeletal calcarenite facies (Blakey, 1974; Dean 1981; Goodspeed and
Lucas, 2007). In thin section, two energy regimes can be identified. One is more turbulent,
represented by abraded skeletal grains with abundant micritic coatings, ooids, and intraclasts in a
drusy spar and micrite matrix, and the second is a calmer depositional setting with increased mud
accumulation and complete skeletal grains (Fig. 4C,D) (Blakey, 1974).
Increased ooid, peloid, and terrestrial grain content upsection represents the higher
energy shoal environment (Fig. 4E) (Blakey, 1974; Goodspeed and Lucas, 2007). These deposits
are also secondarily dolomitized (Blakey, 1974) as indicated by replaced micrite matrix and
secondary rhombs.
Samples were taken from each facies in both studied sections in order to test for
differences in faunal composition between varying amounts of environmental energy. The
peritidal is represented by samples 1 to 4 at the Batten and Stokes section and sample 1 at the
Rod's Valley section, the offshore includes samples 5 to 9 at the Batten and Stokes section and
90
samples 2 to 4 at the Rod's Valley section, and the shoal includes samples 10 to 15 at the Batten
and Stokes section and sample 5 at the Rod's Valley section.
Virgin Limestone
Three parasequences were sampled for this study containing a range of depositional
environments from the proximal offshore to offshore transition and the lower shoreface (Fig. 5).
Based on the micritic matrix and low abrasion of fossil fragments, molluscan packstone deposits
are interpreted as autochthonous or parautochthonous assemblages of fossils deposited below
storm wave base. Crinoid grains have high porosity that makes them more easily transported
than siliciclastic or carbonate grains of similar size (Savarese et al., 1996). Ooid grainstones and
echinoderm packstones and grainstones contain abraded crinoid, echinoid, and occasionally
mollusc material, sometimes with micrite envelopes, indicating reworking in a high energy
environment. These packstones and grainstones are interspersed throughout the parasequences
suggesting high energy storm events interrupted molluscan packstone deposition. Progressing
upsection through each parasequence is an overarching shallowing trend punctuated by higher
energy storm events. To establish whether faunal composition varied along an environmental
gradient, molluscan packstone beds from the low energy proximal offshore and high energy
offshore transition environments were compared. In the lower parasequence, samples 1 and 2
represent low energy deposition and were compared to sample 3 representing high energy
deposition. In the middle parasequence sample 4 was compared to samples 5 through 7. In the
upper parasequence, samples 8 through 10 were deposited under low energy conditions and 11
through 14 formed within high energy environments.
91
Results
Taphonomy
To rule out the effects of taphonomy, specifically size-sorting associated with
transportation, histograms of size data were created for every individual sample with n>10.
Storms vary through time in terms of intensity and spatially in terms of depth (Westrop, 1986).
Therefore, a succession of storm beds generated by a variety of storm events in a given section
are expected to produce beds with different body size means and ranges (Westrop, 1986).
Additionally, winnowing and transportation of allochthonous grains have the potential to
generate samples with bimodal size distributions (Westrop, 1986). Size histograms for samples
in this study show a normal, Gaussian, distribution sometimes with a tail of slightly larger size
classes (Fig. 6). This suggests that deposition was mainly autochthonous or parautochthonous.
Paleoecology of the Sinbad Limestone
The Smithian Sinbad Limestone of Utah is one of the more diverse Early Triassic marine
assemblages in Panthalassa and includes an abundance of gastropod genera in a series of
lagerstätte deposits in addition to the remains of bivalves, scaphopods, and echinoids (Fig. 7)
(Fraiser and Bottjer, 2004; Nützel and Schulbert, 2005). The total generic richness, average
abundance, and Shannon-Weiner diversity indices and average body size are shown for two
stratigraphic sections highlighting the shallow peritidal, the offshore, and the shoal (Table 1,
Figure 8). At the Batten and Stokes section, Mann Whitney U tests show that generic diversity
significantly increased from the peritidal to offshore environment and experienced no significant
change in diversity between the offshore and shoal (Fig. 8, Table 3). At the Rod's Valley section,
diversity also significantly increased from the peritidal to the offshore and then significantly
92
decreased into the shoal (Figure 8, Table 3). Between the two localities, there is no significant
difference in generic diversity for the peritidal or offshore environments, but there is a difference
in diversity for the shoal (Table 3). Gastropod abundance is highest in the offshore environment
at both localities and gastropod generic diversity is highest in the offshore at the Batten and
Stokes section and in the peritidal at the Rod's Valley section. A Mann Whitney U test shows
that there is no significant difference in the proportion of gastropods in each of these
environments. Maximum size measurements for molluscs do not show a directional trend at the
Batten and Stokes locality. The average size of molluscs collected from shoal deposits is greater
than those from the offshore, but there is no directional trend in individual samples in the
stratigraphic section. There is a slight increase in maximum size at Rod's Valley from the
peritidal to the shoal but these values do not deviate from the size range of any individual
sample.
Two way cluster analysis of generic diversity at the Batten and Stokes locality shows
similarities exist between the offshore and shoal environments (Fig. 9). Disaster bivalves
including the genera Eumorphotis and Promyalina dominate the community structure in many
depositional regimes. Other high abundance bivalves such as Leptochondria, Pleuronectites and
Myalinella form the basis of additional clusters. Samples in the shoal environment (10 through
15) show some of the highest concentrations of high abundance bivalves. Neither bivalves nor
gastropods are the sole contributor to Sinbad Limestone samples. Observed microgastropod rich
beds (samples 6 and 7) still contain a relatively diverse and abundant bivalve fauna. Low
Shannon-Wiener diversity indices which range between .962 and 2.5, indicate the Sinbad
Limestone contains a high dominance, low evenness community (Table 1).
93
A three dimensional NMDS solution generated the lowest stress of rank ordering and
dissimilarity values for samples from the Batten and Stokes section. Stress ranged between 8 and
12.3 for the 3-D solution which is well within the 10 to 20 range for interpretation of ecological
communities (McCune et al., 2002). Scatterplots comparing each set of two axes show that axis 2
is determined by sample. Axis 1 and 3 are determined by the constituent fauna. NMDS results
reflect the sample distribution previously determined by two way cluster analysis but provide
additional insight into the arrangement of samples in the third dimension.
Two way cluster analysis at Rod's Valley was limited by low sample size but did show
groups formed based on abundance and gastropod diversity. Samples four and five, from the
offshore and shoal respectively, share a relatively high diversity, high abundance fauna. Sample
one from the peritidal also contains a high abundance fauna with the gastropods Naticopsis,
Worthenia, and Chartonella. NMDS did not find a significant solution to ordination of these five
samples which is not surprising given the limited data available for this test.
Like previous studies, there is evidence for an abundant and diverse microgastropod
fauna in the Sinbad Limestone. Both studied sections contained a wide diversity of
microgastropod genera, numerically dominated by a few taxa, all of which show extremely small
body size, on average 3 mm in length (Fig. 10). Size frequency histograms correspond closely to
previous work on the microgastropods of the Sinbad Limestone (Fig. 10) (Fraiser and Bottjer,
2004; Fraiser et al., 2005). From the 136 gastropods sampled in this study, only one remarkable
specimen measured over 10 mm in length; a specimen of Coelostylina measuring 17 mm,
deposited in an offshore environment at the Rod's Valley section (Fig. 7A).
Scaphopods are also abundant in hand sample and thin section and occur in fossil-rich
horizons deposited in offshore environments of the Sinbad Limestone (Fig. 4, 7C). One
94
scaphopod genus has been recognized in the Early Triassic, Plagioglypta of the order Dentaliida,
and has been reported previously in the Sinbad Limestone (Nützel and Schulbert, 2005). In some
point counted samples Plagioglypta was as abundant as the remains of other mollusc classes
including bivalves and gastropods, comprising 5% of sample 8 and 9.5% of sample 9 at the
Batten and Stokes Section.
Echinoderm diversity in the Early Triassic is low. One genus of crinoid, Holocrinus, and
one genus of echinoid, Lenticidaris, are found in samples from Panthalassa (Twitchett and Oji
2005, McGowen et al. 2009). The Sinbad Limestone appears to be devoid of crinoid remains,
although echinoid spines are sporadically distributed in thin sections from each of the studied
depositional environments (Fig. 4, 7D). They are not the primary skeletal constituent of any
facies or sample.
Based on mobility, feeding strategy, and tiering, nine trophic guilds are represented in the
Sinbad Limestone. Molluscs are predominately epifaunal suspension feeders and grazers with
some semi-infauna and infauna also present. Scaphopods constitute a unique niche as infaunal
microcarnivores and detritivores (Reynolds, 2002; Bush et al., 2007).
Paleoecology of the Virgin Limestone
The Virgin Limestone Member of the Moenkopi Formation has been relatively well
studied in shallow water settings in the Muddy Mountains of Nevada and the Beaver Dam
Mountains outside of Hurricane, Utah and might be considered the representative deposit of
Spathian Substage diversity in Panthalassa (Schubert and Bottjer, 1995; McGowan et al., 2009;
Mata and Bottjer, 2011; Hofmann et al., 2012). This study focuses on the generic diversity of,
likely synchronous, deeper water deposits in the Spring Mountains (Fig. 5) (Schubert and
Bottjer, 1995; Mata and Bottjer, 2011). Generic richness, abundance, and Shannon-Wiener
95
diversity indices for low energy and high energy depositional environments are shown in Table
2. For each parasequence there is no significant difference in generic diversity between low and
high energy depositional environments (Fig. 11, Table 5). However, there is a significant
difference in generic diversity between parasequences in that diversity significantly increases
upsection (Table 6). In the low energy environments there is a significant increase between the
middle and upper parasequences and in the high energy environment, a significant increase
between the lower and upper parasequences (Table 6). In the studied bulk samples the size of
individual genera did not show directional trends up stratigraphic sections. Bivalves from each
bulk sample range, on average, between 5 and 10 mm. These contrast with bivalve beds from the
top of the upper parasequence which were not possible to analyze via the outlined bulk sampling
method. Promyalina and Eumorphotis dominate these bedding plane samples and are typically
longer than 5 cm, an order of magnitude larger than bivalves from all other sampled beds (Fig.
12 A,B)
Two way cluster analysis of each sample from the Virgin Limestone show similarities
between samples from different parasequences and energy regimes (Fig. 13). The first major
difference in samples is determined by the presence or absence of echinoderm clasts. Samples
5,7,8,9, and 10 are from high energy environments in the middle and upper parasequence and
each lacks echinoderm fragments. They are clustered based on molluscan constituents; samples
five and seven contain abundant Eumorphotis and Leptochondria while samples 9 and 10 are
dominated by the gastropod Coelostylina and infaunal bivalve Unionites. Echinoderm-rich
samples were secondarily clustered by generic diversity and strongly represent the influence of
numerically abundant taxa. Samples 1,6, and 12 form a cluster of high abundance, high diversity
bivalves. In samples 2,4,13, and 14 Bakevellia was more abundant and disaster bivalves had a
96
lower abundance compared to the primary cluster. Samples cluster almost regardless of
parasequence placement or depositional energy level. NMDS failed to find a low stress
ordination for these samples. Real data runs never resulted in significantly less stress than
randomized data.
In addition to cluster diagrams, the importance of high dominance disaster bivalves in
Virgin Limestone community structure is again illustrated in Shannon-Wiener diversity indices
calculated for these samples, ranging from .55 to 1.7 (Table 2). Microgastropods are present in
these samples but unlike the Sinbad Limestone, they are not a dominant component of the
assemblage. Scaphopods were also present in each parasequence in both low and high energy
depositional environments. In comparison to the Sinbad Limestone, their abundance was much
lower, representing only 1% of fossil clasts in samples where they were found. Echinoderms, in
contrast, are extremely abundant in the Virgin Limestone at Lost Cabin Springs. Crinoid and
echinoid clasts are found in both low and high energy facies. In addition, allochthonous abraded
crinoid storm beds and echinoderm nucleated ooid beds were deposited intermittently in each
parasequence (Fig. 5, 12C).
Eight trophic guilds were recognized in samples from the Spathian Virgin Limestone.
Most molluscs represented epifaunal, semi-infaunal, and infaunal suspension feeders and
grazers. Scaphopods are infaunal carnivores and crinoids represent epifaunal, erect suspension
feeders (Bush et al. 2007).
Discussion
During the Early Triassic recovery interval, the benthic fauna were exposed to a range of
physical and chemical perturbations, two of which are the focus of this study, temperature and
oxygen stress. The Smithian Sinbad Limestone is characterized by shallow water environments
97
representing deposition within an epicontinental sea, far removed from offshore oxygen
minimum zones that were prevalent in the Early Triassic (Algeo et al., 2011a; Winguth and
Winguth, 2012). In addition, the shallow setting may have allowed for the amelioration of low
oxygen conditions brought on by increased sea surfaces temperatures through the process of
wave aeration (Beatty et al. 2008, Wallace and Wirick 1992). An extreme temperature spike
during the Late Smithian was likely caused by additional eruptions of the Siberian Traps (Payne
and Kump, 2007; Sun et al., 2012). The far-reaching effects of this volcanism included elevated
temperatures and the expansion of tropical vegetation to the poles. It also resulted in a loss of
tetrapods, fish, and icthyosaurs from many equatorial localities although fish fauna from South
China remained diverse and abundant (Sun et al., 2012, Benton et al., 2013). The paleoecology
of the benthic fauna of the Sinbad Limestone is therefore interpreted in the context of this abrupt
environmental perturbation.
Overall, the generic richness and abundance of the Sinbad Limestone was relatively high
for an Early Triassic Panthalassic community (Schubert and Bottjer, 1995; Hofmann et al.,
2012). Diversity was significantly higher in the offshore and shoal environments compared to the
shallow peritidal. Ecological complexity was limited to shallow tiers during deposition of the
Sinbad Limestone. Nine trophic guilds, from infauna to surface dwelling epifauna, are
represented by the variety of molluscs contained in the Sinbad Limestone. Rare scaphopods and
shallow infaunal bivalves such as Bakevellia represent the farthest extent of niche occupation in
this low complexity paleocommunity.
Shallow intertidal settings that experience daily variation in sea level, temperature, and
chemical parameters represent a harsh environment for colonization (Fraiser and Bottjer, 2004).
In the Sinbad Limestone, the expected stress for the shallow peritidal environment would have
98
been compounded by extreme sea surface temperatures during the intense Smithian Substage
warming, thus further limiting the richness and abundance of the benthic fauna. The fauna
inhabiting the shallow peritidal environments of the Sinbad Limestone will not be the focus of
further interpretation in this study. This is because differentiating between the effects of harsh
conditions brought on by extinction mechanisms compared to the extreme environmental settings
associated with peritidal settings is difficult in the rock record.
Changes in depositional energy along an environmental gradient did not play a strong
role in shaping the Sinbad Limestone fauna. Based on cluster analysis, the community
composition of the shoal, above storm wave base, and the offshore, below storm wave base were
very similar. In addition, there are no significant divergences from average body size between
the offshore and shoal further indicating a lack of variation between depositional environments.
A number of ecological anomalies remain that indicate that the benthic recovery was
restricted during deposition of the Sinbad Limestone. While diversity was relatively high,
evenness remained low and samples are dominated by opportunistic bivalves. In addition most
bivalves and essentially all gastropods from the Sinbad Limestone were restricted in their size
distribution to less than 10 mm in length, except one specimen of Coelostylina (Fig. 7A, 10).
These findings are in agreement with previous work on the microgastropod "Lilliput Fauna" of
the Sinbad Limestone (Fraiser and Bottjer, 2004; Fraiser et al., 2005; Nützel and Schulbert,
2005). These microgastropods have been an enigma of the Early Triassic, appearing in both the
Sinbad Limestone of the Southwestern United States and the semi-synchronous Gastropod Oolite
of the Lower Triassic Werfen Formation of Northern Italy (Fraiser et al., 2005; Nützel, 2005).
Recently Sun et al. (2012) suggested that the high temperature event at the Smithian to Spathian
stage boundary is the key to understanding these global miniaturization events (Sun et al., 2012).
99
Modern field experiments show that intertidal gastropod species metamorphose at smaller sizes
during warmer summer temperatures and under lab temperatures of 20°C will also grow to
ultimately smaller sizes (Irie and Fischer, 2009; Melatunen et al., 2013). In addition,
observations of modern climate change show that increased temperatures lead to a higher
metabolism that must be compensated for either by resource partitioning, increased nutrient
intake, or simply a size decrease (Sheridan and Bickford, 2011). Hypothesized equatorial sea
surface temperatures approaching 40°C during the Smithian are much higher than modern sea
surface temperatures and 20°C experimental temperatures. These elevated temperatures likely
generated additional environmental effects, which are observed in the modern ocean today,
including decreased nutrient and dissolved oxygen content (Sheridan and Bickford, 2011; Sun et
al. 2012). The small size of gastropods during the Smithian may have been a response to
metabolic limitations and the decreased availability of nutrients brought on by extreme
temperatures. The microgastropod fauna was diverse, representing almost 50% of the generic
diversity in the Sinbad Limestone, up to 50% of the total gastropod diversity found for the Early
Triassic, and occurred with high abundance in some samples (Fig. 8, 10B) (Nützel, 2005). High
temperatures may have played an additional role in the diversification of microgastropods by
selecting against competitors that could not adapt to extreme temperatures and subsequent
environmental effects, thereby opening ecospace for temporary colonization by microgastropods
(Frasier and Bottjer 2004). Microgastropods likely represent the "diversity-first" model
suggested by Chen and Benton (2012), where opportunists first diversify and then experience
adaptive radiation.
One example of an entire class excluded from the Sinbad Limestone, potentially by high
temperatures, are the crinoids. Crinoids experienced a bottleneck at the Permian to Triassic
100
boundary but returned to be ecologically successful in some marine communities. As early as the
Griesbachian of Oman, crinoid ossicles representing a new Triassic group are present (Twitchett
et al., 2004). Unidentified remains occur sporadically throughout the Dienerian and Smithian and
Holocrinus makes its first appearance in the Smithian of Japan (Twitchett and Oji, 2005).
Crinoid remains could therefore be anticipated in the Smithian Sinbad Limestone. The
occurrence of echinoderms as well as abundant scaphopods have been cited as evidence for the
presence of normal marine salinity during deposition of the Sinbad Limestone (Nützel and
Schulbert 2005). These references are, however, made in regard to the sporadic occurrences of
echinoid remains, mostly spines in thin section, while crinoids are completely lacking in the
Sinbad Limestone. In the fossil record, crinoids are often used as an indicator of normal marine
salinity and normal marine pH as their low buffering capabilities would not allow them to
survive in acidified waters (Nützel, 2005; Knoll et al., 2007; Clapham and Payne, 2011). Without
lithologic evidence for acidification or high salinity, extreme temperature is presented as the
most parsimonious explanation for the lack of crinoids and the sudden development of a diverse
microgastropod population. The absence of crinoids in the Sinbad Limestone contrasts strongly
with encrinites preserved within the Spathian Virgin Limestone documented herein, other
Spathian deposits in both the Tethys and Panthalassic Oceans, and global marine deposits from
the Middle Triassic (Twitchett and Oji, 2005; Greene et al., 2011).
The development of anoxic conditions following the end-Permian mass extinction likely
lagged behind other kill mechanisms such as increased temperatures and possible ocean
acidification; however, it likely served as a severe limiting factor in the benthic recovery
(Winguth and Winguth, 2012). Global studies of the Early Triassic benthic fauna show low
oxygen conditions inhibited the development of diverse and abundant benthic fauna (Wignall
101
and Twitchett, 1996) while the rebound was rapid in oxygenated settings (Twitchett et al., 2004,
Pietsch and Bottjer, 2014). In the Early Triassic, episodes of anoxia and dysoxia occurred at both
the regional and global scale. At the regional level, low oxygen conditions may have been
generated by relative sea level change and subsequent exposure to oxygen minimum zones or
deep water low oxygen incursions. One such example occurs across the Dienerian to Smithian
boundary in the Boreal Ocean where Beatty et al. (2008) observed a diverse and abundant
ichnofauna in the earliest stages of the recovery that disappears with the onset of a deepening
event which brought with it low oxygen conditions. Recent studies of the Early Triassic Boreal
Ocean demonstrate repeated, extensive anaerobic facies ranged from offshore to abyssal depths
in the Sverdrup Basin (Grasby et al., 2013). These anoxic events are associated with negative
carbon isotope excursions likely related to volcanic eruptions of the Siberian Traps (Grasby et
al., 2013; Sobolev et al. 2011). On a greater scale, studies of the lithology of accreted terranes
and modeling efforts have shown that equatorial oxygen minimum zones (OMZs) existed at 400
to 800 m depth in the Panthalassic Ocean and much, if not all, of the Tethys Ocean (Wignall and
Twitchett, 2002; Algeo et al., 2011a; Winguth and Winguth, 2012). The final carbon isotope
excursion in the latest Smithian represents the last major eruptive pulse and environmental
perturbation in the Early Triassic (Sun et al., 2012). The lasting effects of this pulse are the
persistent regional oxygen minimum zones of the Spathian Substage.
The parasequences within the Spathian Virgin Limestone repeatedly shallow upward
from deep, low-oxygen settings into shallow, oxygenated habitats. Within a single section I was
able to observe, multiple times, the transition from low to increased oxygen levels and analyze
the effects on the community recovery from the end-Permian mass extinction. As indicated by
summary studies of onshore-offshore faunal distribution during the Paleozoic, Mesozoic, and
102
Modern, deeper water, open marine environments would be expected to contain the greatest
benthic diversity under normal marine, oxygenated settings (Bambach, 1977). Parasequences in
the Virgin Limestone begin with deeper, low oxygen facies and shallow upward into wave
influenced lithologies. Each of the three parasequences were used to test the difference in faunal
composition and ecological complexity moving from low to high energy depositional
environments. Generic richness and evenness were substantially greater in high energy
depositional environments (Fig. 11). Both low and high energy environments were composed of
high dominance faunas with low Shannon-Wiener diversity indices. Two way cluster analyses
indicate that the fauna comprising all samples were highly interconnected regardless of
parasequence position or depositional environment. Disaster bivalves, such as Eumorphotis and
Promyalina were the dominant taxa. The clustering of samples independent of time and
depositional environment suggests that the underlying core fauna of opportunistic bivalves
dominated the community structure while differences in rare or unique taxa drove observed
diversity changes. Scaphopods represent an entire additional molluscan guild and this appears to
be their first recorded occurrence in the Virgin Limestone. However, in comparison to the Sinbad
Limestone, scaphopods were less ecologically important representing only up to 1% of the fossil
content in any given sample.
Although there were no significant differences between low and high energy depositional
environments, there were significant changes in diversity from lower to upper parasequences
suggesting increased diversity through geologic time (Table 6). The increase represents addition
of new taxa, especially gastropods, as rare fauna into these high dominance communities (Fig.
11). Body size of disaster bivalves Eumorphotis and Promyalina also increased by an order of
magnitude by the top of the upper parasequence (Fig. 12). The composition of stromatolite
103
deposits found in the Virgin Limestone changes from microbially dominated in the lower
parasequence to a more complex sponge, microbe, and bivalve association within higher
parasequences (Marenco et al., 2012). Strontium isotope ratios from the Lost Cabin Spring
locality show that the upper portion of the section was deposited during the late Spathian
(Marenco et al., 2012). Therefore, trends of improved diversity, body size, and stromatolite
composition moving upsection through the Virgin Limestone may be a reflection of changing
environmental conditions both regionally and globally over much of the duration of these
deposits. During this time global anoxia was likely receding as volcanic perturbations ended and
pole to equator temperature gradients were reestablished (Wignall and Twitchett, 2002; Algeo et
al., 2011a). Hautmann et al. (2013) recognized an increase in bivalve taxonomic diversity in the
Virgin Limestone but notes that the taxonomic recovery was not completed by the end of the
Spathian but instead Middle Triassic deposits represent extraordinary diversity. Improved
diversity and size through time is not evidence for a full recovery, which would be indicated by a
complex infauna and more diverse and even epifauna, but the changes discussed here depict
marked improvement observed within one stratigraphic section (Pietsch and Bottjer 2014). The
Spathian Virgin Limestone fauna was in fact continuously dominated by low diversity, high
abundance disaster bivalves of reduced size.
The presence of abundant echinoderms in the Spathian Virgin Limestone contrasts
strongly with their relative rarity in the Smithian Sinbad Limestone. In the Virgin Limestone,
crinoids and echinoids are found in a variety of depositional environments and lithologies. In
each section echinoderms may constitute up to 20% of the rock volume in molluscan-rich fossil
beds that include bivalves, gastropods, and scaphopods. Additionally, interbedded with
molluscan wackestones and packstones, abraded and micritized echinoderm grains are the
104
primary constituent of higher energy storm beds and supply the nuclei of ooid grainstones. These
two disparate styles of echinoderm deposition are manifested in alternating preservation of
autochthonous echinoderm-mollusc fauna with allochthonous storm deposits. The preservation
of carbonate rocks above fair-weather wave base was not common in this section. Storm events
and ooid beds dominated by crinoids are interpreted as an indication of a broad habitat that
ranged from the proximal offshore to shallow water environments above fair-weather wave base.
Crinoids represent an additional trophic guild, erect epifaunal suspension feeders (Bush et al.,
2007). They also represent a considerable increase in ecospace utilization and ecological
complexity for the benthic community of the Spathian. Rubrics outlining stages of recovery
following the end-Permian mass extinction classify epifaunal crinoids as indicators of a more
advanced benthic recovery compared to shallow, low diversity epifauna and infauna (Twitchett
et al., 2004, Twitchett et al., 2006; Pietsch and Bottjer, 2014).
Conclusions
Although both the Smithian and Spathian deposits discussed herein contain their own
unique faunal history, it is the contrasting dynamics between the two time intervals that shed the
most light on the Early Triassic recovery. The Spathian Virgin Limestone is younger and
therefore might be predicted to contain the most taxonomically diverse and ecologically complex
fauna. Numerically it is the Smithian Sinbad Limestone that contains a higher diversity and
lower dominance fauna than the communities found in the Spathian Virgin Limestone. While
both localities contained almost the same number of basic ecological guilds, the Virgin
Limestone contained a more complex benthic fauna with a wider variety of ecospace utilization,
from infaunal molluscs to erect, epifaunal crinoids that were not present in the Sinbad
Limestone.
105
Each time period and its unique regional conditions presented different challenges to the
recovering benthic fauna. The Sinbad Limestone was not affected by low dissolved oxygen
concentrations but was, however, deposited during an interval of intense volcanism indicated by
the largest negative carbon isotope excursion of the Early Triassic (Romano et al., 2012; Sun et
al., 2012). Subsequent environmental effects of this volcanism included extremely high
temperatures in the shallow epicontinental sea of the Sinbad Limestone that likely limited
ecological complexity. Microgastropod populations bloomed in many deposits, driven to small
sizes by metabolic limitations in response to high temperatures. They became abundant when
temperature restricted other fauna and provided niche space for this well-adapted fauna. One
such group that is missing from the Sinbad Limestone is crinoids. These echinoderms may have
been unable to survive in the high temperature Smithian Sinbad Limestone settings (Nützel,
2005; Knoll et al., 2007; Clapham and Payne, 2011; Sun et al., 2012).
Low oxygen conditions during the deposition of the Spathian Virgin Limestone did not
limit the development of complex benthic communities. A primary component of this system
was echinoderm deposits. These animals were inhabitants of a range of environments from above
fair-weather wave base to the proximal offshore. Low oxygen conditions resulted in decreased
infaunalization in proximal offshore environments indicated by laminated strata. Initially, overall
diversity and abundance were limited in these Spathian deposits. With time, however, increased
mollusc diversity and bivalve body size indicate improving environmental conditions
approaching the Middle Triassic.
Contrasts in taxonomic diversity, ecological complexity and tiering, as well as body size
between the Sinbad Limestone and Virgin Limestone highlight the dissimilarities in recovery
dynamics that were experienced by the benthic fauna throughout the Early Triassic as a result of
106
different external environmental perturbations. Previous work on the Early Triassic recovery
often focuses on global recovery dynamics or the recovery patterns of individual groups through
time. This study exposes two caveats to any generalized interpretation of the recovery following
the end-Permian mass extinction. The first is that regional differences were important drivers of
benthic recovery dynamics. The second is that punctuated deleterious environmental conditions
throughout the Early Triassic had disparate effects on the benthic fauna. It is therefore essential
that the relationships between faunas and environments in time and space need to be considered
when making interpretations about the recovery. In the Early Triassic, repeated low oxygen
events maintained low diversity faunas while extreme temperature events devastated ecological
structure.
107
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Tables and Figures
Table 1. Total generic richness, average abundance, average body size in mm, and average
Shannon-Wiener Diversity Index for bivalves and gastropods found at the two localities
representing the Sinbad Limestone; Batten and Stokes and Rod's Valley. Values are shown for
each of the three fossil bearing depositional environments in the Sinbad Limestone; peritidal,
offshore, and shoal. n= the number of samples that were used to represent each depositional
environment. See supplemental tables for generic richness, abundance, and Shannon-Wiener
Diversity Index of individual samples.
Table 2. Results of Mann Whitney U Tests on the generic diversity of bivalves and gastropods in
the Sinbad Limestone. Comparisons were made between depositional environments within each
locality. Significant differences (p<.05) are shown in bold.
Table 3. Results of Mann Whitney U Test on the generic diversity of bivalves and gastropods in
the Sinbad Limestone. Each depositional environment was compared between the two localities;
Batten and Stokes and Rod's Valley. Significant differences (p<.05) are shown in bold.
Table 4. Total generic richness, average abundance, body size in mm, and average Shannon-
Wiener Diversity Index for bivalves and gastropods found in the three parasequences sampled
from the Virgin Limestone; lower, middle, and upper. Average values are shows for the two
energy regime; low and high energy environments. n= the number of samples that were used to
represent each depositional environment. See supplemental tables for generic richness,
abundance, and Shannon Index of individual samples.
Table 5. Results of Mann Whitney U Test on the generic diversity of bivalves and gastropods in
the Virgin Limestone. Comparisons were made between low and high energy environments
114
within wach individual parasequence; lower, middle, and upper. Significant differences (p<.05)
are shown in bold.
Table 6. Results of Mann Whitney U Test on the generic diversity of bivalves and gastropods in
the Virgin Limestone. Comparisons are made between each sampled parasequence for the two
energy regimes; low and high energy environments. Significant differences (p<.05) are shown in
bold.
115
Figure 1. Three maps depict the paleogeographic context and regional study sites for the Sinbad
Limestone and Virgin Limestone and a time scale shows their position in Early Triassic time. A.
A paleogeographic map of the Early Triassic depicts the global context for the Southwest United
States localities (modified from Scotese et al. 2010). B. Map shows the study sites for the Sinbad
Limestone in the San Rafael Swell (38°59'51.0"N, 110°40'53.2"W). Two dashed lines represent
the shoreline and the shelf slope break. The Smithian Stage Sinbad Limestone of the Moenkopi
Formation represents deposition under an epicontinental sea, therefore the onshore environment
was wide (Blakey 1974, Dean 1981, Goodspeed and Lucas 2007). (Map modified from
Goodspeed and Lucas 2007). C. The Virgin Limestone at Lost Cabin Springs in the Spring
Mountains (base of section 36°05′00.0"N, 115°39′13.3"W) (Marenco et al. 2012). The relatively
deep Spathian Stage Virgin Limestone of the Moenkopi Formation and the more narrow shelf
contrast with the broad shoreface regions of the Smithian Sinbad Limestone. (Modified from
Marzolf 1993 and Woods 2009). D. Time scale for the Early Triassic and stratigraphy of the
Moenkopi Formation in Utah and Nevada (Shen et al. 2011, Lehrmann et al. 2006, Mundil et al.
2004).
Figure 2. Early Triassic oxygen and carbon isotopes represent widespread environmental
perturbations associated with volcanic eruptions. Time scale is based on dates from Shen et al.
2011, Lehrmann et al. 2006, and Mundil et al. 2004. Oxygen isotopes are derived from sea
surface and shallow water dwelling conodonts and inferred fluctuations in Early Triassic
temperature (Sun et al. 2012). The Smithian/Spathian boundary shows a sharp decline in δ
18
O
suggesting an abrupt rise in temperature. B. Composite carbon isotope trend from South China
(Payne et al. 2004), which represents a global signal. C. Carbon isotopes from the Boreal Sea
(Grasby et al. 2013) which echo Payne et al. 2004 and interpretations of these carbon isotopes
116
and other chemical and sedimentological data as intervals of oxygenated, dysoxic, anoxic, and
euxinic shallow marine environments in the Boreal Ocean (Grasby et al. 2013).
Figure 3. Stratigraphic sections with interpreted depositional environments from two sections
studied from the Sinbad Limestone. A. Batten and Stokes and B. Rod's Valley. Detailed
lithology and sedimentary structures observed in the field and petrography are included. Faunal
composition is indicated for each sample included in community analysis. Scale is in meters.
Horizontal axis reads M= mudstone, W=Wackestone, P=Packstone, G=Grainstone. C. Outcrop
exposure of fossil rich horizons. D. Petrographic image of fossil rich sample featuring a
microgastropod.
Figure 4. Microfacies of the three depositional environments sampled in the Sinbad Limestone.
A. Peritidal: Recrystallized bivalve surrounded by peloids, siliciclastic grains, and neomorphosed
micrite matrix. B. Peritidal: Secondary dolomite rhombs. C. Offshore: Microgastropod
surrounded by recrystallized, dolomitized ooids and calcite spar. D. Fossil rich deposit with
bivalves, gastropods, and scaphopods with a micrite and microspar matrix. E. Shoal: Abraded,
micritized grains and rip up clasts with interspersed siliciclastic grains and bivalve and gastropod
remains in a micrite matrix. F. Shoal: Fine grained siliciclastics and fossil fragments in micrite
matrix.
Figure 5. Stratigraphic sections from three parasequences studied from the Virgin Limestone. A.
Lower Parasequence (meters 44 to 85 in original section), B. Middle Parasequence (meters 85-
103 in original section), and C. Upper Parasequence (meters 122 to 145 in original section).
Detailed lithology and sedimentary structures observed in the field and petrography are included.
Faunal composition is indicated for each sample included in community analysis as well as other
beds that were included in initial sampling scheme and not included in analysis (See Methods).
117
Scale is in meters. Horizontal axis reads M= mudstone, W=Wackestone, P=Packstone,
G=Grainstone.
Figure 6. Histogram depicting the distribution of maximum body size represented by bivalves
and gastropods from all bulk samples used in the analysis of the Sinbad Limestone and Virgin
Limestone. Both localities exhibit a normal distributions of size frequency data which is
representative of the individual contributing samples. The normal distribution suggests reduced
taphonomic overprinting via transport or winnowing of fossil deposits as would be indicated by a
bimodal distribution (Westrop 1986)
Figure 7. Field photos and petrographic images of the fossil content of the Sinbad Limestone. A.
Gastropod genus Coelostylina 17 mm in length. B. Fossil packstone containing complete
bivalves, abraded gastropods, and other unidentified abraded and micritized fossil clasts. C.
Fossil packstone of gastropods, bivalves, and scaphopods. D. Echinoid spine.
Figure 8. Summed generic diversity through the three depositional environments sampled in the
Sinbad Limestone; Peritidal, Offshore, and Shoal. Shades of blue indicate bivalve genera, shades
of orange represent gastropod genera. The two localities are A. Batten and Stokes and B. Rod's
Valley. The two gastropod genera Omphaloptychia laevisphaera and Naticopsis utahensis are
extremely difficult to distinguish at a macroscopic level. n= total specimens identified for a given
environment.
Figure 9. Cluster diagram of the Batten and Stokes locality representing the Sinbad Limestone.
Sample numbers correspond to those in Figure 3. Notice that samples from different depositional
environments (Peritidal 1 to 4, Offshore 5 to 9, Shoal 10 to 15) cluster together based on
abundance and dominance patterns of the dominant molluscan fauna. The shading of each box
118
represents the relative contribution of a given genus in the intersecting sample. The two
gastropod genera Omphaloptychia laevisphaera and Naticopsis utahensis are extremely difficult
to distinguish at a macroscopic level.
Figure 10. Microgastropod size distribution and generic diversity. A. Histogram representing the
size distribution of the Sinbad Limestone microgastropod fauna. One specimen of Coelostylina
was found to be over 10mm in length. B. Diversity of gastropod genera found in the Sinbad
Limestone. O.laev/N. utah are the two gastropod genera Omphaloptychia laevisphaera and
Naticopsis utahensis which are extremely difficult to distinguish at a macroscopic level. Two
genera, Coelostylina and O. laevisphaera/N. utahensis are dominant overall.
Figure 11. Summed generic diversity through the three parasequences sampled in the Virgin
Limestone at Lost Cabin Springs. The two depositional environments A. Low Energy and B.
High Energy are compared. Each parasequence and energy level shows dominance by disaster
bivalves. Shades of blue indicate bivalve genera, shades of orange represent gastropod genera.
The two gastropod genera Omphaloptychia laevisphaera and Naticopsis utahensis are extremely
difficult to distinguish at a macroscopic level. n= total specimens identified for a given
environment.
Figure 12. Field photos and petrographic images of Spathian fossils. A. Field image depicting
cross sections of large, disarticulated bivalves toward the top of the Upper Parasequence. B.
Large Promyalina up to 5 cm in length dominate a bedding plane at the top of the Upper
Parasequence. C. Abraded and micritized ooids in petrographic thin section are representative of
the ooids found throughout the section which generally have only one or two micritic coatings.
Nuclei are likely echinoderm grains and show unit extinction. D. Typical mollusc packstone for
the Spathian Virgin Limestone showing gently abraded and micritized fossil grains. E. Typical
119
echinoderm packstone/grainstone for the Virgin Limestone. Ossicles are abraded and micritized.
F. Abraded fossil packstone showing bivalve fragments and scaphopods (arrow).
Figure 13. Cluster diagram of samples from the lower, middle and upper parasequences of the
Virgin Limestone. Notice that samples from different depositional environments cluster together
based on abundance and dominance patterns of the dominant molluscan fauna. (Lower
parasequence 1 to 3, middle parasequence 4 to 7, upper parasequence 8 to 14). The shading of
each box represents the relative contribution of a given genera in the intersecting sample.
120
Batten and
Stokes (n=13)
Rod’s Valley
(n=5)
Richness
Average
Abundance
Average Size
(mm)
Shannon
Diversity Index
Richness
Average Size
(mm)
Shannon
Diversity Index
Peritidal (n=4)
Offshore (n=5)
Shoal (n=5)
Peritidal (n=1)
Offshore (n=3)
Shoal (n=1)
6
19
17
22.25
62.8
91.8
6.7
5.3
7.1
.962
1.74
1.36
15
16
10
30
72.3
44
3.2
6.1
6.9
2.48
1.57
1.99
Table 1
Average
Abundance
Peritidal vs.
Offshore
Peritidal vs. Shoal
Offshore vs.Shoal
Batten and
Stokes
Rod’s Valley
p p U U
.001
.677
.001
335.5
216
318
.004
.010
.687
305
294.5
215.5
Table 2
Peritidal vs.
Peritidal
Offshore vs.
Offshore
Shoal vs. Shoal
p U
.067 268
.5997
.015
220
295
Table 3
Low Energy High Energy
Richness Abundance
Average Size
(mm)
Shannon Index Richness Abundance
Average Size
(mm)
Shannon Index
Lower (n=2)
Middle (n=1)
Upper (n=3)
Lower (n=1)
Middle (n=3)
Upper (n=4)
9
4
12
60
14
35.3
6.1
5.7
7.3
1.39
1.17
1.60
5
10
14
36
52.6
24.5
5.3
6.1
6.3
0.55
1.25
1.70
Parasequence Parasequence
Table 4
Lower vs. Middle
Lower vs. Upper
Middle vs. Upper
Low Energy High Energy
p p
U U
.276
.012
.285
157.5
194.5
157
.329
.237
.025
154.5
160
187.5
Table 5
Upper
Parasequence
Lower
Parasequence
Middle
Parasequence
p U
.925 131
.063
.420
130.5
131
Table 6
121
Griesbachian
Dienerian
Smithian
Spathian
252.2
247.2
Sinbad
Limestone
Virgin
Limestone
Moenkopi
Formation
Early Triassic
Induan Olenekian
C.
D.
Tethys
Ocean
Panthalassic
Ocean
Boreal Ocean
200 km
Basinal
Nevada Utah
Virgin
Limestone
B.
200 km
Basinal
Nevada Utah
Sinbad
Limestone
Shoreline
Shelf-Slope Break
A.
Offshore
Offshore
Figure 1
122
Δ
Estimated Temperature (°C, ice free world)
40 28 30 32 34 36 38
Gries. Dienerian Smithian Spathian
252.28
247.2
17.5 18.5 18.0 19.0 20.0 19.5 20.5
δ18O apatite (‰ VSMOW)
← warming
Δ Δ
Δ
Δ
Δ
Δ
Δ
Δ
Δ
Δ
Δ
Δ
Δ
Δ
Δ
Δ
Δ
Permian
B.
251.22
250.55
252
251
250
249
248
A.
Euxinic
Dysoxic
Oxic
C.
δ13C carb (‰)
-2 0 2 4 6 8 -32 -26 -28 -30
δ13C org (‰)
Virgin Limestone
Sinbad
Limestone
Anoxic
Figure 2
123
M W P G
1
2
3
4
5
7
8
9
10
11
12
13
14
1
2
3
4
5
6
7
8
9
10
11
2
4
M W P G
5
4
3
2
1
6
Peritidal Offshore Shoal
15
Peritidal Offshore Shoal
Dolomite
Lithology
Fine grained mudstone
Clotted, Thrombolitic
Cross Bedded Sandstone
Limestone
Oolitic Limestone
Dolomitic Limestone
Bedded Sandstone
Cross Bedded Limestone
Sedimentary Structures
Intraclasts
Peloids
Laminated Intraclasts
Bivalves
Gastropods
Echinoids
Scaphopods
Biology
A. B.
1cm
1mm
C.
D.
Figure 3
124
1000 µm
100 µm
5000 µm
1000 µm 5000 µm
5000 µm
A. B.
C. D.
E. F.
Figure 4
125
Lithology and Sedimentary Structures
Fine grained mudstone
Clotted, Thrombolitic
Limestone
Sandstone
Carbonate Grains
Intraclasts
Bivalves
Gastropods
Echinoids
Scaphopods
Biology
Crinoids
M W P G
4
m
8
m
12
m
16
m
20
m
24
m
28
m
32
m
36
m
40
m
M W P G
4
m
8
m
12
m
16
m
M W P G
4
m
8
m
12
m
16
m
20
m
22
m
Oolitic Limestone
1
2
3
A.
B.
C.
4
5
6
7
8
9
10
11
12
13
14
Figure 5
126
0 50 100 150
Virgin Limestone
Sinbad Limestone
Mollusc Size Histogram
2
4
6
8
10
12
14
16
18
20
20+
Frequency
Mollusc Size (mm)
Figure 6
127
100 µm
1 cm
1 cm
A. B. C. D.
2000 µm
Figure 7
128
Peritidal Offshore Shoal
A. Batten and Stokes
B. Rod’s Valley
Bakevellia Costatoria Entolium Eumorphotis Leptochondria Myalina Myalinella Neoschizodus
Pecten Permophorus Pernopecten Pleuronectites Promyalina Unionities Battenzyga Coelostylina Chartronella
Cylindrobullina N.depresispirus Neritaria O. homolirata O.laevisphaera/N.utahensis Strobeus Vernelia Worthenia
1 2
2
4
4
4
4
5
5 7
5
7
8
13
8
8 7
13
3
3 1
1
12
3
4
4
6
6
5
6
7
8
4
5
7
6
8
4
3
2
1
8
13
12
12
10
10
10
14
14
10
9
5
9
9
11
11
7
8
10
12
13
14
9
10
11
2
4
3
4
5
7
5
8 6
9
11
13
14
10
11
14
13
4
1 2
2
3
4
5
6
7
8
9
13
14
10
7
8
6
5
9
10
11 12
7
10
n=89 n=314 n=463
n=30 n=217 n=44
Figure 8
129
Minimum Maximum
100 75 50 25 0
0 25
2
8
1
11
14
3
5
12
15
13
6
7
9
10
Bakevellia
Unionites
Vernelia
Eumorphotis
Myalinella
Neoschizodus
Promyalina
Chartronella
Myalina
Permophorus
Coelostylina
O.laev./N.utah
Cylindrobullina
Naticopsis
Battenzyga
Pleuronectites
Leptochondria
O. homolirata
Worthenia
Strobeus
50 75 100
Peritidal Plain Font, Offshore Bold, Shoal Italic
Figure 9
130
100
50
0
Gastropod Frequency
Size Classes (mm)
Microgastropod Size in the Sinbad Limestone
1 2 3 4 5 6 7 8 9 10 10+
0 10 20 30 40 50 60
Battenzyga
Chartronella
Coelostylina
Cylindrobullina
N. depresispirus
Neritaria
O. homolirata
O. laev./N. utah.
Strobeus
Vernelia
Worthenia
Microgastropod Diversity of the Sinbad Limestone
Genera
A.
Frequency
B.
Figure 10
131
Bakevellia Eumorphotis Leptochondria Myalina Myalinella Neoschizodus Pecten Permophorus Pleuronectites
Promyalina Unionities Battenzyga Coelostylina N.depresispirus O.laevisphaera/N.utahensis Zygopleura
A. Low Energy
Parasequence 3
B. High Energy
Parasequence 4 Parasequence 6
A B C
C B A D
D
E
E F G H I
J K
A
A
A
B
B
B
C
C
D
E
F
H
I
J
H
I
D
E
F
D
G
H
I
J
K
B
C
C
B
D
D
E
E
I
I
C
F
H
J E
A
A
B
B
C
E
F
D
H
G
I
J
K
E
n=86 n=14 n=106
n=70 n=164 n=98
Figure 11
132
5 cm
5 cm
500 µm
500 µm
5 cm
A. B.
C. D.
E. F.
5000 µm
Figure 12
133
Minimum Maximum
5
2
8
1
11
14
3
12
13
6
7
9
10
4
Bakevellia
Unionites
Eumorphotis
Myalinella
Neoschizodus
Promyalina
Myalina
Permophorus
Coelostylina
O.laev./N.utah
Battenzyga
Pleuronectites
Leptochondria
O. homolirata
Echinoderms
Pecten
Zygopleura
100 75 50 25 0
0 25 50 75 100
Lower Parasequence Plain Font, Middle Parasequence Bold, Upper Parasequence Italic
Figure 13
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Appendix: Bivalve and gastropod data from the Virgin Limestone at Lost Cabin Spring
Row Labels
Bakevellia
Battenzyga
Coeleostylina
Eumorphotis
Leptochondria
Myalina
Myalinella
Naticopsis
Neoschizodus
Omphaloptychia
Pecten
Permophorus
Pleuronectites
Promyalina
Unionites
Zygopleura
Abundance/Avg. Size
Abundance
1 28 37 3 2 4 10 2 86
2 1 10 14 1 1 7 34
3 1 25 1 1 8 36
4 2 1 4 7 14
5 25 14 1 1 1 42
6 14 30 1 1 1 1 4 7 1 1 61
7 29 13 2 2 9 1 56
8 7 15 1 1 2 1 2 29
9 1 2 1 3 1 7 2 17
10 1 1 4 2 3 1 3 1 16
11 13 2 1 2 1 4 5 1 29
12 23 20 1 2 4 5 5 1 1 62
13 1 6 4 2 1 1 15
14 5 2 17 2 1 2 1 2 1 3 36
Size
1 7.6 6.1 2.7 5.5 5.0 6.0 4.0 6.3
2 4.0 5.8 5.9 5.0 4.0 6.4 5.8
3 3.0 4.9 6.0 4.0 7.0 5.3
4 6.0 4.0 6.0 5.7 5.7
5 9.2 5.9 4.0 6.0 5.0 7.8
6 4.4 4.7 7.0 2.0 4.0 5.0 11.3 10.4 5.0 4.0 5.7
7 4.1 4.2 2.5 15.5 5.2 4.0 4.7
8 5.1 6.1 4.0 3.0 5.0 15.0 10.5 6.2
9 2.0 3.0 13.0 8.7 13.0 8.1 2.0 7.1
10 2.0 4.0 4.3 6.5 7.7 4.0 6.0 6.0 5.4
11 6.5 4.0 5.0 11.5 4.0 7.0 22.8 9.0 9.5
12 5.4 5.6 3.0 3.0 5.0 5.4 12.8 13.0 10.0 6.1
13 6.0 4.7 5.8 10.0 8.0 8.0 6.2
14 5.2 3.5 5.8 4 2 4.5 10 15.5 5 10.3 6.3
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Appendix: Bivalve and gastropod data for the Sinbad Limestone at the Batten and Stokes Locality
Sample
Bakevellia
Battenzyga
Chartronella
Coeleostylina
Cylindrobulina
Eumorphotis
Leptochondria
Myalina
Myalinella
Naticopsis depresispirus
Neoschizodus
Omphalo. Homolirata
Omphalo/Naticopsis
Permophorus
Pleuronectites
Promyalina
Strobeus
Unionites
Vernelia
Worthenia
Abundance/Avg. Size
Abundance
1 3 4 3 10
2 1 1 1 3
3 1 9 1 60 1 2 74
4 2 2
5 2 13 1 1 2 62 1 3 1 5 3 2 96
6 7 2 1 5 15 3 6 1 1 41
7 1 12 3 10 1 3 15 1 46
8 1 2 2 5 1 3 1 2 1 18
9 11 3 34 29 2 16 3 2 10 2 1 113
10 1 1 20 3 1 1 8 35
11 2 1 1 4
12 2 1 18 2 28 8 15 1 7 5 87
13 6 5 1 16 3 96 20 3 17 2 20 2 191
14 1 70 6 1 78
15 4 26 1 21 8 3 5 68
Size
1 5.7 6 5.7 5.8
2 6 5 9 6.7
3 8 8.9 9 7.1 6 10 7.4
4 7 7.0
5 2.5 2.2 2 5 10 6.2 2 5 2 2.4 10.7 10.5 5.5
6 2.7 2.5 7 6.4 6.4 3.3 3 17 2 5.0
7 3 3.75 5 4.5 2 4.3 2.1 6 3.5
8 2 2.5 11.5 5.6 5 4.3 2 11.5 4 5.8
9 5.4 5.3 6.8 6.8 5.5 4.9 5 2.5 10.9 4 5 6.5
10 2 2 6 4.7 9 2 20 8.8
11 7 10 12 9.0
12 2.5 3 5.4 8.5 6.4 5.4 8.8 14 7.6 9 6.8
13 7.7 3.6 3 6.8 5.7 7.6 6.6 12 9.6 2 11 6.5 7.8
14 8 5.0 6.2 10 5.2
15 5.5 4.7 4 4.4 4.4 7.3 5.4 4.8
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Samples
Coelostylina
Eumorphotis
Leptochondria
Myalina
Myalinella
Neoschizodus
Omphalotychia
Pecten
Permophorus
Pleuronectites
Promyalina
Unionites
Vernelia
Neritaria
Worthenia
Pernopecten
Entolium
Cylindrobulina
Naticopsis
Bakevellia
Abundance/Average Size
Abundance
1 4 1 1 1 5 5 3 2 1 1 1 1 2 1 1 30
2 5 1 1 4 1 2 2 16
3 1 57 1 1 2 5 1 68
4 6 31 2 15 27 4 2 3 4 12 11 2 4 10 133
5 2 1 1 4 13 6 3 2 3 9 44
Size
1 2 6 1 2 3.6 3.6 3.3 7 2 2 2 3 3 2 2 3.2
2 4.6 2 3 2 3 4 2.5 3.3
3 2 5.9 18 5 2 16.2 17 6.8
4.0 6.3 6.6 2.5 9.9 7.1 7.0 8.5 11 27.8 10.7 8.2 6.0 7.0 7.7 8.3
5 2.5 2 5 7.8 5.7 8.2 4 12 9.7 7.9 6.9
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Chapter 3: Shallow water and deep water depositional settings in the Southwest United
States and differences in the taxonomic and ecological recovery
Introduction
The end-Permian mass extinction was triggered by volcanic eruptions 252 million years
ago in what is now Siberian Russia (Black et al. 2014, Konstatniov et al. 2014, Burgess et al.
2014). These volcanic eruptions resulted in global warming events recorded as oxygen isotope
perturbations in conodont apatite (Sun et al. 2012). Global warming resulted in sluggish ocean
circulation and the development of widespread oxygen minimum zones across the Early Triassic
oceans (Feng and Algeo 2014, Wignall and Twitchett 2002).
Following the initial extinction event, low oxygen and extreme temperature events likely
had different effects on the Early Triassic marine fauna. Song et al. (2014) collected modern
experimental and observational data on nine major phyla and classes of marine invertebrates and
compared their extreme temperature and oxygen sensitivity. In the geologic past, well buffered
groups like gastropods and bivalves have been shown to thrive in high temperature, low oxygen
conditions compared to more sensitive groups like cnidarians and echinoderms (Knoll et al.
2007, Clapham and Payne 2011) and their data on modern groups show the same pattern (Song
et al. 2014). Song et al. (2014) suggest a refuge zone, a region below extreme temperatures of the
surface ocean and above deep water low oxygen conditions controlled the recovery of the pelagic
and benthic fauna during the Early Triassic. They suggest the initial extinction event at the
Permian-Triassic boundary was driven by high temperatures and the second extinction, in the
earliest Triassic, was driven by subsequent low oxygen conditions. Global cooling in the
Dienerian and early Smithian allowed the refuge zone to expand and provided an environment
for the temporary recovery of ammonoids, conodonts, and forams (Song et al. 2014). The
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disparate effects of extreme temperature and low oxygen are only just being applied to
interpreting and understanding the long recovery of the benthic fauna. Here I focus on the
Smithian and Spathian intervals where a second extreme temperature event occurred (Sun et al.
2012, Cui and Kump 2014). I will explore how this additional perturbation effected the benthic
macrofauna millions of years after the initial extinction event.
I focused on the Panthalassic Ocean, which represented 85-90% of the ocean basin in the
Early Triassic but has been historically less well studied than sections in Europe and China
which represent deposition in the restricted Tethys Ocean, which represented just 10-15% of the
entire marine system during this interval (Figure 1). The Southwest United States provides an
ideal study location for the Panthalassic Ocean, or proto-Pacific Ocean. Previous work on the
Early Triassic of the Southwest United States has included analysis of strontium isotope
geochemistry for geochronology and geochemistry (Marenco et al. 2008), in depth analysis of
the microgastropod fauna in the Sinbad Limestone (Batten and Stokes 1986, Fraiser and Bottjer
2004, Nützel and Schulbert 2005), and paleoecological analysis of marine communities from the
Dienerian through the Spathian (Schubert and Bottjer 1995, Hofmann et al. 2013a, b, c).
Schubert and Bottjer (1995) provided the first comprehensive look into the benthic marine
mollusc and echinoderm fauna present in the Southwest United States and Hofmann et al. (2013
a,b,c) has focused on the Dienerian through Spathian in three subsequent publications outlining
an environmental gradient and intrinsic controls on molluscan community composition
throughout the Early Triassic recovery. His approach focused on constructing paleocommunities
using cluster analysis and calculating beta diversity between these quantitative communities.
My focus on the study and synthesis of the benthic marine invertebrate recovery of the
Southwest United States is to compile a whole community, paleoecological analysis for
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comparison to global and local environmental changes including extreme temperature and low
oxygen events. This approach favors not only quantitative analysis of the benthic macrofauna but
also ecological analysis including changes in abundance, community evenness metrics
(Shannon-Wiener Diversity Index), body size comparison, ecological guild occupation, and
tiering parameters (Ausich and Bottjer 1982, Bush et al. 2007). I also apply a recently developed
recovery rubric (Pietsch and Bottjer 2014) in order to directly compare and contrast the benthic
marine invertebrate recovery at different time points and within different environments across
the Southwest United States. This work highlights the complexity of the benthic marine
invertebrate recovery following the end-Permian mass extinction including the ecological
response to additional environmental perturbations and the variation in community ecology
between inner shelf and outer shelf benthic systems. The high resolution response of the benthic
marine fauna to high temperature and low oxygen perturbations is pertinent to the modern
ecological crisis and may inform the conservation and remediation efforts of biologists and
ecologists in response to anthropogenic climate change.
Methods
At each of the five localities, bulk samples for fossil community analysis and hand samples for
thin section analysis were made at various intervals indicated by a sample number on each
stratigraphic section (Figures 2,3,4,7,8,9). Samples were taken from a range of facies represented
at each section in order to document community changes throughout depositional environments.
Bulk samples were made from the upper 15cm of carbonate wackestone and packstone beds.
Most bulk samples were standardized at 9 liters in order to control for fossil abundance.
Additional samples in the form of 4.5 liter samples and slabs were taken from more poorly
exposed horizons to supplement stratigraphic samples. These sampling methods differ from the
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surface collecting methods employed by McGowan (2009) and contrasts with the sampling to
100 specimens or to the point of species rarefaction used by Hofmann et al. (2013a,b,c). Bulk
sampling methods captured the same span of taxonomic diversity presented in Hofmann (2013a,
c). Bulk sampling and thin section analysis allowed capture of a robust image of the community
including not only bivalve and gastropod macrofauna but also echinoderm abundance including
crinoids, echinoids, and scaphopods.
In the lab, samples were disaggregated into 2 cm
3
fragments to expose all conspicuous
fossils. For each bulk sample, specimens that were more than 50% complete and over 2 mm in
length were identified to the genus level. Specimens were indentified to the genus level using
recent publications including Hofmann et al. (2013 a,c) and Hautmann et al. (2012) as well as
Batten and Stokes (1986), Nutzel and Schulbert (2005) and the Paleobiology Database. Genus
level identifications can be made of most samples collected from the poorly preserved rocks of
the Early Triassic Southwest United States whereas species level diversity can only be obtained
from well preserved samples at lower stratigraphic resolution (Hofmann et al. 2013a, c). In
addition, genus level diversity allows the identification of all paleoecological guilds present in
the Early Triassic where species level designations would provide no advantage in ecological
analysis of paleocommunities. Each identified fossil was then consistently measured across its
widest or longest dimension. Most bivalves were measured along their length, unless the width
was the greater measurement for the particular genus as was the case for some pectenids.
Gastropods were measured from the apex to the base of the aperture.
One or more thin sections were made for each bulk sample as well as occasional
intervening stratigraphic samples in order to refine the environmental interpretation and to gather
echinoderm abundance data. Qualitative observations of the abundance of mollusc and
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echinoderm fragments as well as ooids, peloids, stylolites, micrite, and spar matrix were made.
This data was used to interpret the abundance of echinoderms as they are commonly abraded and
their small size limits their visibility in hand sample. Beds with 20% or more echinoderm grains
were treated as echinoderm storm beds. Micrite or spar matrix distinguished packstones and
grainstones and the presence of micritized and abraded grains as well as ooids throughout the
studied sections supplemented depositional environment interpretations made from field
observations.
Generic richness, abundance, Shannon-Wiener diversity index, and average size were
calculated for each sample (Table 1,2). Fossil size data was averaged across each individual
genus and then averaged for each sample. Abundance data for each location were grouped by
depositional environment and then arcsine transformed and absence data was coded as .001. In
order to test for significant changes in diversity between depositional environments both within
and between different sections, a Wilcoxin Rank test, which compares the differences in medians
between two samples, was run on transformed data (Bakus, 2007). P values less than .05 were
accepted as statistically significant and are reported, along with W values in two additional tables
(Table 3, 4).
Two way cluster analyses were generated using the statistical software program PC-
ORD. The flexible method which approximates multiple clustering strategies including nearest
and farthest neighbor as well as group average was applied with β = -0.25 (Bakus, 2007;
McCune et al., 2002). Sorensen/Bray-Curtis distance measures were used as they measure the
frequency of co-occurrence which informs community structure more than Jaccard presence-
absence measurements. Data was arcsine transformed prior to cluster analysis and absence data
was coded as .001 (McCune et al., 2002).
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Bulk samples from the Confusion Range were also processed for conodont elements.
Disaggregated carbonate samples were placed in 3L buckets in mesh baskets made of 2mm
screening material in a solution consisting of 7% acetic acid, 63% water, and 30% buffered
solution (Jeppsson et al. 1985). Samples were left to dissolve overnight or until the acetic acid
and carbonate were no longer reacting. The particulate matter remaining in the bucket was
passed through a 250µm and 150µm sieve, respectively. Samples were dried in paper filters and
then handpicked using a dissecting microscope. Conodonts were imaged with an environmental
SEM (Figure 5).
In addition to the new bulk sample data presented here, data from the Batten and Stokes
Sinbad Limestone and the high energy sections of parasequences from the Spathian Virgin
Limestone at Lost Cabin Spring are also included in the analysis. Chapter 2 detailed the high
resolution relationship between facies, community structure, and environmental perturbation for
these samples and others from the same locations. I briefly reintroduce them in this Chapter in
order to include them in the analysis. The Batten and Stokes section of the Sinbad Limestone
represents a paleocommunity from a shallow marine setting influenced by extreme temperatures
while the communities of the Virgin Limestone at Lost Cabin Spring represents the deepest shelf
environment with facies evidence for a low oxygen environment. These two data sets are
essential to make a final comparison of the variation between low and high energy environments
and extreme temperature versus low oxygen influenced paleocommunities.
Geology
The Early Triassic deposits of eastern Nevada and central Utah represent three successive
transgressive events. The first event in the Griesbachian and Dienerian resulted in the deposition
of outer and inner shelf facies of the Dinwoody Formation in northern Utah, Idaho, and Montana
143
(Carr and Paull 1983). Not until the Smithian did a second transgressive event overcome
highlands to the south allowing for the flooding of central and southern Utah and eastern
Nevada. The Smithian transgressive event was the most extensive of the Early Triassic
depositing inner shelf facies 160 to 500 km wide in eastern Utah as the Sinbad Limestone of the
Moenkopi Formation (Figure 1) (Goodspeed and Lucas 2007). The ammonoids Anasibirites
kingianus and Wasatchites of the Anasibirites tardus zone as well as rare zone four conodonts
indicate a Smithian age for these strata (Paull and Paull 1993, Lucas et al. 2007). In western
Utah, late Smithian outer shelf facies of the Thaynes Formation are recorded in the Confusion
Range indicated by the ammonoid Anasibirites (Hofmann et al. 2013c). (Figure 1). The
beginning of the Spathian is marked by regression resulting in the end of Smithian Sinbad
Limestone deposition in eastern Utah. Continued Spathian deposition in the Confusion Range is
represented by outer shelf and basinal facies dated by Columbites ammonoids and zone 6
conodonts which gradually shallow into inner shelf facies (Carr and Paull 1983). The Thaynes
Formation of the Confusion Range is correlated to the Virgin Limestone of the Moenkopi
Formation which represents the farthest extent of the middle Spathian transgression to the south
(Carr and Paull 1983). The Spathian age of the Virgin Limestone of eastern and southern Nevada
is indicated by the ammonoids Tirolites and Columbites in the Virgin Limestone found in
southern Utah, purported conodonts, as well as strontium isotope geochemistry performed on the
studied section (Poborski 1954, Carr and Paull 1983, Marenco et al. 2008, Hautmann et al.
2012). In southern Nevada the Virgin Limestone at Lost Cabin Springs spans distal offshore to
lower shoreface deposits while the section at Ute in central eastern Nevada spans upper offshore
transition to intertidal deposits (Figure 1).
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Two sections were studied from the Thaynes Formation in the Confusion Range (Figure
2, 3, 4). The Disappointment Hills section (Hofmann et al. 2013c) represents deep water
deposition of the outer shelf with some deposits potentially representing slightly more shallow
inner shelf conditions. The section begins with a fossil packstone containing microgastropods,
occasional echinoderm fragments, and conodonts. The conodonts have been identified as
Smithian aged Ellisonia triassica and Parachirognathus ethingtoni (Figure 5) (Orchard 2007,
Orchard pers comm.). This finding directly contrasts with work by Saltzman and Sedlacek
(2013) who proposed an Induan age for these deposits based on strontium isotope analysis.
Following the basal, conodont containing limestone, the section transitions to fine grained
sedimentation with occasional decimeter-scale, mixed carbonate siliciclastic beds containing
microgastropods, bivalves, and vertical bioturbation. A bed of thombolitic fabric was also
observed (Figure 2). This is followed by a 5 m thick cliff-forming, structureless limestone
composed of small bivalves, echinoderm fragments, and packed with microgastropods of about
1mm in length. The remainder of the section is measured in a hillside as mixed carbonate to
siliciclastic decimeter scale beds between regions of cover and/or fine grained muds. Five major
beds were identified in the hillside and contain well preserved bivalves, gastropod steinkerns,
and ammonoids (Figure 2). Gastropods were collected from the muds in between the
structureless beds. Additional gastropods were collected in float from mud deposits on the far
side of the wash in the same section studied by Brayard et al. (2010). Alternating muds with
coarse sand and limestone beds are interpreted as the open marine environment of the outer shelf
consisting of muds from suspension and occasional storm events (Hofmann et al. 2013c).
The Cowboy Pass section of the Thaynes Formation of the Confusion Range is exposed
in a wash (Figure 3,4). Two sections are the focus of this paper and range from outer shelf storm
145
deposits to shallow intertidal systems. The first 60 meters are composed of muds with decimeter
to meter scale, structureless, carbonate beds containing abundant Eumorphotis, occasional
additional bivalve genera, and echinoderms represented by fragments (Figure 3 A-D). The
bivalves are often positioned concave up indicating post-mortem reworking by storm or current
energy. At the top of the section, silty mudstone deposits contain complex vertical and horiztonal
burrows including lined and branching forms (Figure 3 E.F.). The structureless nature of the beds
and intervening mud deposits suggests an outer shelf depositional environment (Hofmann et al.
2013c). Following an extensive interval of cover, the section resumes but is significantly more
shallow. Ooids, teepee structures, desiccation cracks, flaser bedding, and escape structures
represent a shallow intertidal marine system (Figure 6). Ooids are composed of elongate and
rounded micritized grains (Figure 6A.). Bivalves and microgastropods occur in some of these
intertidal beds. After an additional covered interval the sequence transitions to mixed carbonate
siliciclastic wackestone and packstone deposits containing abundant crinoids, bivalves, and
echinoids as well as cross bedded sand deposits (Figure 6 D-G). Some sandstones contain
external molds of the bivalve Unionites (Figure 6G). These deposits represent storm beds from
the outer shelf characterized by transported fossil grains and bivalve colonized sand beds.
The Smithian Sinbad Limestone represents a depositional range from the offshore to
peritidal and lagoonal deposits (Figure 7). The majority of fossil deposits come from offshore
and shoal environments and are the focus of this paper. At the outcrop scale, beds of the Sinbad
Limestone are poorly differentiated and commonly structureless but do show occasional low
angle cross-stratification making microfacies analysis important for fine tuning environmental
analysis (Blakey, 1974; Goodspeed and Lucas, 2007).
146
Ooid packstones and grainstones at the base of the section interbedded with mollusc shell
hash, mud, peloids, and rip-up clasts suggests deposition in a peritidal setting (Goodspeed and
Lucas 2007). Offshore deposits are characterized by cross bedded skeletal packstone and an
increase in the importance of fossil and mud in microfacies analysis. Bivalves and gastropods
dominate fossiliferous intervals including the well-studied microgastropod lagerstätten (Batten
and Stokes, 1986; Fraiser and Bottjer 2004; Fraiser et al., 2005; Nützel and Schulbert, 2005,
Pietsch et al. 2014). An increase in ooid, peloid, and terrestrial grain content upsection represents
the higher energy shoal environment (Blakey, 1974; Goodspeed and Lucas, 2007).
The Virgin Limestone at the Lost Cabin Spring locality in the Spring Mountains of
Nevada represents a distal ramp environment (Figure 8) (Larson 1966). Parasequences,
representing repeated cycles of upward-shallowing deposits, ranging from the proximal offshore
to lower shoreface, can be traced through the stratigraphic section using sedimentary structures
and ichnofabric indices (Savrda and Bottjer 1986, Mata and Bottjer, 2011, Pietsch et al. 2014).
Each parasequence typically initiates with a generally laminated micritic mudstone facies
commonly bearing low ichnofabric indices representing low-energy, oxygen-limited offshore
deposition (Mata and Bottjer, 2011). Structureless beds of well preserved mollusc packstones
with convex up shells that result from gentle storm-generated currents represent autochthonous
deposition in the proximal offshore environment (Mata and Bottjer, 2011). Low-angle cross-
stratified crinoid packstones interbedded with shales suggest storm reworking in the offshore
transition (Mata and Bottjer, 2011). Ooid grainstones and echinoderm packstones and
grainstones contain abraded crinoid, echinoid, and occasionally mollusc material, sometimes
with micrite envelopes, indicating reworking in a high energy environment. These packstones
and grainstones are interspersed throughout the parasequences suggesting high energy storm
147
events interrupted molluscan packstone deposition. Samples from the shallow, high energy
wackestone and packstone beds are considered in this study.
The section of the Virgin Limestone found at Ute, NV unconformably lies on the
Permian Kaibab Formation (Mata and Bottjer 2011). This section represents a wide variety of
depositional environments from shallow intertidal deposits to deep-water current-influenced beds
from the offshore transition. The first 22 m are decimeter to meter scale ooid grainstones and
cross bedded sandstones. Microgastropods, bivalves, and abraded echinoderm fragments are
abundant throughout this part of the section. Ooids are nucleated on fossil fragments of molluscs
and echinoderms. They generally occur as grainstones but also appear in clotted textures from
beds with fine wavy laminations suggesting microbial deposits. Molluscs from this part of the
section are generally poorly preserved or encased in recrystallized spar and therefore not possible
to identify. Branching horizontal bioturbation occurs on bedding planes (Figure 10 C). These
high energy deposits represent deposition on shoal and lower shoreface regions of the shelf.
Upsection, directional ripples with small bivalves in trough deposits, interference ripples, and
extensive structureless mixed carbonate siliciclastic mud deposits indicate shallowing into an
intertidal system with strong terrestrial influence. The continued presence of ooid grainstones is
an indicator of a high energy, shallow water system. Microgastropod and abraded bivalve grains
make up packstone deposits in this region. In the last twenty meters of the section the number of
structureless or occasionally cross-bedded sand and carbonate deposits increases indicates a
deepening environments. Amalgamated beds of Promyalina and Eumorphotis represent
winnowing of fine material by current action in the outer shelf. Asteriacties are commonly found
on thin beds of sand in between mud horizons (Mata and Bottjer 2011).
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Taphonomic bias was considered in a few ways. First size histograms were created for
representative samples to look for bimodal size distributions which can result from winnowing
and transportation of allochthonous grains (Westrop, 1986). All of the samples used in the
analysis come from storm influenced environments but some were more influenced by wave
reworking than others. Bulk samples taken from the Virgin Limestone at Lost Cabin Spring, the
Sinbad Limestone, and the Thaynes Formation show an even distribution of body size. Bivalve
dominated storm beds sampled from the Virgin Limestone at Ute and the Thaynes Formation at
Lost Cabin Spring contain large epifaunal bivalves with shells >5cm surrounded by an epifaunal
and infaunal bivalve community with shells all <2cm. This bi-modal size distribution indicates
obvious storm influence which may have biased generic diversity or influenced average size.
Despite the difference in taphonomic setting from other bulk samples, these samples are included
as the best markers of the biological and ecological composition of the benthic fauna at their
respective horizons.
Echinoderm dominated storm beds from the Virgin Limestone at Lost Cabin Spring are
considered to represent allochthonous deposition of stereom fragments which are more easily
transported than sand grains of a similar size. These storm beds are taken to represent more
onshore, high energy deposits that were transported into deeper environments represented by
parasequences. In the other sections, echinoderm fragments do not dominate any samples and are
found sporadically in thin section as small parts of the surrounding paleocommunity.
Results
The Smithian age strata of the Thaynes Formation in the Disappointment Hills has an
overall generic richness of 12 bivalves, 9 gastropods, and one brachiopod, Obnixia (Table 1,
Figure 11). In comparison the Smithian Sinbad Formation contains 9 bivalves and 10 gastropods.
149
In the Thaynes Formation, average bivalve size was 11.3 mm and gastropods had an median
length of 6.1 mm. In the Sinbad Limestone, bivalve size was on average 7.4 mm and gastropods
had a median length of 2.0 mm (Figure 12). The Thaynes Formation contained seven ecological
guilds of molluscs, the highest observed, while the Sinbad Limestone contained six total
molluscan guilds. The Smithian stage Confusion Range Thaynes Formation did not contain
echinoderms in hand sample or in thin section. The Sinbad Limestone contained echinoid spines
in mollusc rich thin sections from offshore and shoal deposits, however these organisms don't
contribute an additional guild. Stationary suspension feeding epifauna, mobile suspension
feeding epifauna, and mobile grazing epifauna were the three most dominant guilds in both
locations (Figure 13). Finally, Shannon-Wiener diversity index, a marker of generic faunal
dominance was 1.31 in the Thaynes Formation and was on average 1.36 between the three
environments represented in the Sinbad Limestone (Table 1). A Shannon Diversity index close to
5 represents a very even community while low numbers represent extremely abundant taxa that
are the most important.
A cluster analysis of Thaynes Formation and Sinbad Limestone samples clearly expresses
the faunal differences between the two locations (Figure 14). While the composition of mollusc
taxa was similar in both the Thaynes and Sinbad, differences in abundance, dominant genera,
and unique taxa drive the clusters to separate the two localities into different branches.
Eumorphotis, Leptochondria, Promyalina, Unionites, and Coelostylina are important
constituents in both locations. The Sinbad Limestone contains unique gastropods including
Omphaloptychia homolirata, Battenzyga, and Vernelia. Hoffman et al. (2013c) introduced new
genera in the Confusion Range Thaynes Formation which I have subsequently identified
including the bivalves Confusionella, Unicardium, and the brachiopod Obnixia. I also identified
150
Sinbadiella which they named from samples in the Sinbad Limestone in the Confusion Range
bivalve fauna. Data from the Sinbad Limestone were indentified before the publication of the
Hofmann (2013c) paper and therefore taxa such as Sinbadiella would have been considered as an
unidentified bivalve and not considered in the analysis or may have been misidentified as
Neoschizodus or Unionites. A Wilcoxin Rank test shows no significant difference between the
genus diversity or genus richness of the gastropod faunas found in the Sinbad Limestone and
Thaynes Formation (richness p=.376, diversity p=.682).
The major difference between the communities in the Thaynes Formation and the Sinbad
Limestone is the body size difference. The gastropods of the Thaynes Formation measured from
five samples have a median length of 6.1 mm while gastropods from the Sinbad Formation have
a median length of 2.0 mm (Figure 12). This difference in the median size and the distribution of
gastropod body size is clearly shown in a size frequency diagram; the Sinbad Limestone
gastropods are predominantly 2 or 3 mm in total length while the Thaynes Formation gastropods
represent a wider range of body size, approaching a normal distribution. A Wilcoxin Rank test
comparing the range of gastropod length shows that the differences in these distributions is
highly significant. Bivalves do not show a substantial difference in body size between these two
locations.
In the Spathian, the overall generic richness is much lower compared to the two sections
studied from the Smithian (Table 2, Figure 15). Eighteen mollusc genera, 13 bivalves and 5
gastropods, are present in the studied sections from the Spathian Southwest United States. In the
Virgin Limestone at Lost Cabin Spring 11 bivalves and 5 gastropods were found, the Thaynes
Foramtion from the Confusion Range contained 10 bivalves, and the Virgin Limestone from Ute,
Nevada had 6 bivalves. Gastropods were observed in outcrop and thin section from Ute but they
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were poorly preserved in recrystallized grainstone making their taxonomic identification
impossible. The average bivalve size from the Virgin Limestone at Lost Cabin Spring was 6.4
mm and average gastropod size was 3.1 mm. In the Thaynes Formation from the Confusion
Range bivalves were 12.6 mm on average and the section of the Virgin Limestone at Ute showed
an average bivalve size of 20.8 mm. Wilcoxin Rank analysis of the size ranges of each formation
resulted in significant differences between each section (p<0.02). The Spathian Virgin Limestone
at Lost Cabin Spring contains five guilds occupied by bivalves and gastropods and is dominated
by stationary epifaunal suspension feeders like Eumorphotis (Figure 13). The presence of
abundant crinoids represents erect, epifaunal, suspension feeders while scaphopods represent
infaunal carnivores which when combined with the five bivalve and gastropod guilds represents
a total of seven guilds for the Spathian Virgin Limestone. The section at Ute contains two
bivalve guilds. Unidentified gastropods would likely represent a third ecological guild, mobile
epifaunal grazers, the most common life mode for Early Triassic gastropods. The presence of
echinoderm fragments at the bottom of the section and the trace fossil Asteriacites representing
ophiuroids, a semi-infaunal deposit feeder. Echinoderm fragments could be from epifaunal
echinoids, erect epifaunal crinoids, or burrowing ophiuroids. The Spathian Thaynes Formation
contained three bivalve ecological life modes. Upsection the appearance of crinoids represents an
additional guild; erect, epifaunal, suspension feeders bringing the tally to four total ecological
guilds represented by body fossils in the section. Bioturbation was observed throughout the
Confusion Range but was not ascribable to a particular life mode. Shannon-Wiener diversity
index varied between sections but was well below 2 for each location. Dominance decreased at
Lost Cabin Spring toward the latest Spathian. In the Confusion Range and at Ute changes in
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dominance did not follow a pattern but varied on a bed by bed basis. Bivalves such as
Eumorphotis and Promyalina dominated many samples from all three localities.
Cluster analysis displays the unique molluscs which differentiate the Virgin Limestone at
Lost Cabin Spring from the Virgin Limestone at Ute and the Thaynes Formation from the
Confusion Range (Figure 16). All three sections contained abundant and dominant Eumorphotis
and Promyalina as well as Myalinella, Unionites, Neoschizodus, and Permophorus. The Virgin
Limestone at Lost Cabin Spring is unique in that it contains both rare gastropod genera and
abundant crinoid and echinoid deposits which differentiates this section from the more sparse
echinoderm material observed at Ute. The Thaynes Formation of the Confusion Range contains
abundant crinoid material at the top of the section, in what is thought to be the later Spathian
(Hofmann et al. 2013c). Bioturbation becomes more common in the Spathian aged Thaynes
Formation toward the top of the section, following the extensive deposits of fossiliferous storm
beds (Figure 3).
Discussion
One of the major debates in the interpretation of Early Triassic benthic paleoecology is
over the level of recovery that these communities have experienced by the second half of the
era,= specifically the Smithian and Spathian Stages. McGowan et al. (2009) and Hofmann et al.
(2013a,c) have shown that the taxonomic diversity of the molluscan macrofauna is approaching
Middle Triassic values of genus and species diversity. However, Hofmann et al. (2013c) also
suggest that their cluster analysis produces paleocommunities which vary based on differences in
abundance of an otherwise similar taxonomic composition. This is supported by their analysis of
beta diversity, which is a calculation of the difference in taxonomic diversity between two
communities. Hofmann et al. (2013) calculated beta diversity using their constructed
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paleocommunities and given the compositional similarity, their value of intercommunity
diversity was very low. Schubert and Bottjer (1995) also analyzed Early Triassic
paleocommunities based on cluster analysis which were dominated by abundant taxa. The
critique is that these groups represent aberrations in abundance more than cohesive,
interdependent communities. Many "community" samples still dominated by opportunistic
bivalves well into the Spathian. My analysis of the benthic macrofauna in the Southwest United
States also finds few taxonomic differences between samples and therefore little evidence for
strongly differentiated paleocommunities within a single depositional environment or even time
period. In the Smithian Sinbad Limestone and the Smithian section of theThaynes Formation,
taxonomic diversity of bivalves and gastropods is the highest observed throughout the Early
Triassic in the Southwestern United States. Accompanying increased taxonomic diversity is a
wide variety of ecological guild representation including abundant stationary epifaunal
suspension feeders and mobile epifaunal grazers accompanied by an expansion of infaunal
bivalves, predatory gastropods, and putative chemosymbiotic bivalves (Figure 13 b,c). However,
this jump in taxonomic diversity and guild occupation is not matched by an increase in
community evenness indicated by consistently low Shannon-Wiener diversity indices (Table 1).
Although a variety of ecological niches are developed during the Smithian Stage, only 2 million
years following the main extinction pulse, the benthic macrofauna are still dominated
taxonomically and ecologically by opportunistic bivalves commonly known as "disaster taxa."
The most abundant molluscs in the Smithian Stage samples are stationary suspension feeders
including Eumorphotis, Unionites, and Myalinella. Previous work on ecological recovery rubrics
for the Early Triassic benthic macrofauna include the development of a diverse community with
widespread ecological niche occupation as prerequisites for an advanced recovery stage (Pietsch
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and Bottjer 2014). This includes a complex and diverse epifauna and infauna and the occupation
of advanced tiers and complex bioturbation Based on this recovery model, the Sinbad Limestone
and Thaynes Formation barely represent a Stage 3 recovery supported by the relatively diverse
fauna but negated by high dominance and low evenness of the benthic fauna. The Sinbad
Limestone and Thaynes Formation show relatively high taxonomic diversity compared to other
Early Triassic sections and the occupation of 7 ecological guilds but do not demonstrate evidence
for a complex community that occupied the majority of benthic habitats exhibited by
communities in the Boreal Ocean and tropics of the Early Triassic (Wignall et al. 1998,
Twitchett et al. 2004, Beatty et al. 2008). As such, I would counter arguments made on the basis
of taxonomic diversity that suggest that the Smithian benthic macrofauna was almost completely
recovered. Instead I suggest that delayed radiation into ecological niche's suggests either an
intrinsic or extrinsic limiting factor on community development during this time
In addition to the taxonomic and ecological importance of epifaunal and shallow infaunal
disaster bivalves in the Smithian Stage, gastropods also rise to dominance during this interval. In
the offshore and shoal deposits of the Sinbad Limestone as well as the outer shelf deposits of the
Thaynes Formation, epifaunal grazing gastropods such as Coelostylina, mobile suspension
feeding gastropods like Omphaloptychia, and the predatory gastropod Strobeus are major
constituents of the benthic macrofauna. Smithian gastropods are taxonomically diverse,
numerically abundant, and dominate a wide variety of shelf environments from ooid shoals in the
Sinbad Limestone to offshore, outer shelf deposits in the Confusion Range Thaynes Formation.
These two locations, although they represent vastly different environmental conditions share
many of the same genera which suggests Early Triassic gastropods were also generalists, echoing
the patterns seen in opportunistic "disaster" bivalves during the Early Triassic. While the
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taxonomic composition does not vary significantly between the two locations, the relative
abundance and body size of gastropods do vary between the shallow Sinbad Limestone and
deeper Confusion Range Thaynes Formation. In the Sinbad Limestone, gastropods can compose
between 1/4 and 1/3 of molluscan taxonomic diversity. In the mixed carbonate siliciclastic storm
beds of the Thaynes Formation in the Confusion Range, that ratio is similar. However, in the
mud deposits of the Thaynes Formation, gastropods dominate making up 90% of the molluscan
macrofauna found (Figure 11). This may be a result of soupy substrates not suitable for
colonization by stationary epifaunal bivalve macrofauna but traversable by mobile gastropods.
Beyond abundance, the most significant difference between the faunas in the shallow Sinbad
Limestone and deeper Thaynes Formation is body size. This discrepancy has already been noted
(Brayard et al. 2010) and debated (Frasier et al. 2011) and in this study, new information is
brought to bear on the interpretation of these differences. Recently, Sun et al. (2012) showed
that the earliest Triassic, at the Permian-Triassic boundary, and the late Smithian interval were
each subject to extreme equatorial sea surface temperatures indicated by oxygen isotopes
extracted from conodont apatite. These analysis indicated that equatorial temperatures in the late
Smithian could have reached up to 40°C (Sun et al. 2012). Additional modeling study support a
temperature spike at the Permian-Triassic boundary of 5-9°C with a maximum of 35°C sea
surface temperatures based on leaf stomata and paleosol proxies (Cui and Kump 2014). This
rapid increase in temperature would have had a profound effect on the benthic macrofauna living
in shallow environments within the tropics. The difference in body size between the gastropods
in the shallow Sinbad Limestone and deep water Thaynes Formation may be driven by this
environmental perturbation. Modern lab experiments and field observations indicated gastropods
are able to decrease their body size in response to increased external temperatures. Gastropods
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from an intertidal system in Okinawa were shown to metamorphose at significantly smaller body
size during warm summer months compared to gastropod cohorts that metamorphosed during
cooler winter months (Irie and Fischer 2009). In the lab, gastropods that were exposed to a 20°C
environment grew to significantly smaller total body size than those that lived in 15°C conditions
(Melatunen et al. 2013). In marine invertebrates overall, every degree of warming that occurs has
been shown to result in a 0.5-4% decrease in overall body size (Sheridon and Bickford 2011). In
ectotherms, increased external temperatures result in increased metabolism leaving the animals
the option to either take in more calories or repartition resources used in the body. In a warmer
climate, oxygen is not as readily available as a dissolved gas and nutrients do not necessarily
become more abundant. Following the end-Permian mass extinction, various hypotheses and
geochemical proxies have argued for both decreased and increased primary productivity with
variable nutrient quality and availability following the extinction event. Twitchett (2006) argues
for decreased primary productivity caused by sluggish ocean circulation, the shutdown of
upwelling currents, and loss of nutrients. Meyer et al. 2011 find a larger vertical gradient
between deep and shallow carbon deposits indicating a strong biological pump and high
productivity which could have driven euxinia and anoxia through processes of eutrophication.
Grice et al. (2005) also find biomarkers of sulfur reducing bacteria, providing additional support
for anoxic conditions in the Early Triassic. Regardless of changes in nutrient availability, thge
most common approach taken by marine invertebrates in response to increased temperatures is to
decrease in body size to compensate for increased metabolic rate. In modern times, the majority
of observed adaptations to temperature stress are in response to transient events including
metamorphosis during the summer months or exposure in a lab setting. However, gastropod
adaptation to extreme temperatures on an evolutionary time scale has also been documented. The
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endangered gastropod species Physella johnsoni lives in the hot springs of Banff National Park
in Alberta, Canada (Grasby and Lepitski 2002). In these hotsprings, water temperatures
commonly range between 30-36°C but sometimes exceed 47°C. Not only do these gastropods
thrive under these conditions but they have also been shown to suffer population declines during
the spring months when snow melt discharge cools the water temperatures in the pools. This well
adapted gastropod has been shown to have differentiated from its closest relative within the last
10,000 years (Lepitski 2002). All of this indicates a role for extreme sea surface temperatures
during the late Smithian to not only drive a decrease in gastropod body size but to act as a
selection pressure that resulted in gastropod populations that were best adapted for extreme
temperatures. In contrast with the diverse microgastropod fauna of the shallow Sinbad
Limestone, the deeper Thaynes Formation gastropods grew to overall larger body sizes (this
study and Brayard et al. 2010). The difference in size could be due to the deeper living
environment on the outer shelf which may have buffered the benthic fauna from climate
fluctuations that would have been more extreme in the shallow water shoal and inner shelf of the
Sinbad Limestone.
In addition to a dominant microgastropod fauna in the Sinbad Limestone and abundant
gastropods of larger size in the Thaynes Formation, another indicator of the effects of extreme
temperature on the benthic fauna is the absence of echinoderms. Within the Sinbad Limestone,
occasional echinoid spines indicate the presence of these epifaual grazers. Echinoderm fragments
at the base of the Smithian section of the Thaynes Formation in the Disappointment Hills also
point to sporadic echinoderms in the community, however, no diagnostic elements to identify
echinoderm class, crinoid, echinoid, etc., have been found. In contrast with these two Smithian
age deposits, the Spathian Virgin Limestone in both shallow and deep water sections as well as
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the Spathian Thaynes Formation contained abundant echinoderm deposits in the form of trace
fossil and body fossil elements representing crinoids, echinoids, and ophiuroids (Figure 6, 8, 10)
(Twitchett 2007, Fraiser and Bottjer 2009, Mata and Bottjer 2011). At this point it is still
challenging to differentiate whether echinoderms were lacking in these Smithian deposits due to
a longer recovery history from the end-Permian mass extinction compared to the mollusc fauna,
the lack of preferred substrate conditions, or if they were limited by the effects of secondary
perturbations like extreme temperature increase. Early Triassic crinoids are thought to have been
generalists in terms of habitat colonization (Hagdorn 2011). While Middle and Late Triassic
forms differentiated into a variety of niche space including planktonic and driftwood forms,
those of the Early Triassic attached by cirri to hardgrounds or soft substrate (Hagdorn 2011).
Holocrinus is well known from the Thaynes and Moenkopi Formations of the Spathian in the
Southwest United States. Crinoid remains in the Dienerian and Smithian stages are widespread
but have gone largely unidentified (Hagdorn 2011). The first appearance of Holocrinus is from
the Smithian of Japan (Twitchett and Oji 2005). Early Triassic crinoid evolution was underway
by the Smithian Stage so the lack of these animals in the Thaynes Formation and Sinbad
Limestone cannot be fully explained by delayed development of this benthic group. One
explanation for the lack of Smithian crinoids in the Southwest United States could invoke
delayed radiation from western to eastern Panthalassa. Recently, Song et al. (2014) ranked major
classes of marine benthic and pelagic fauna that commonly occurred in the Early Triassic based
on their resistance to extreme temperature and low oxygen stress. These data were based on
modern biological experiments and field observations of these groups. Echinoderms ranked third
to last for both heat sensitivity and low oxygen sensitivity which supports the hypothesis that the
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extreme temperatures, especially within the shallow water settings of the late Smithian, may
have limited their colonization of the seafloor.
By the Spathian, however, echinoderms, especially crinoids, were thriving. In the
Thaynes Formation, crinoids are represented by abundant columnals and echinoids by frequent
spines in thin section. In the Thaynes Formation, the earliest Spathian, is composed of bivalve
storm beds and is almost entirely devoid of echinoderm material which only becomes abundant
in the second set of inner and outer shelf deposits. In the Virgin Limestone at Lost Cabin
Springs, crinoids and echinoids dominate outer shelf storm deposits where they are the primary
fossil grain in allochthonous packstone deposits throughout the section. In the Virgin Limestone
at Ute, occasional echinoderm fragments occur but crinoid packstones are less common perhaps
due to the higher energy level of the section, which is predominantly composed of ooid shoals
and shallow intertidal sands and muds. In the lower shoreface sands, abundant Asteriacites, the
resting trace fossil of ophiuroids is found (Mata and Bottjer 2011).
The Spathian Stage Thaynes Formation at Cowboy Pass in the Confusion Range contains
a low diversity, high abundance bivalve fauna. The bivalve Eumorphotis, one of the four Early
Triassic disaster bivalves, is both taxonomically and ecologically dominant throughout the
section. Of the 8 beds sampled, only two contained a diverse bivalve fauna (Figure 15) the rest of
the samples were almost entirely composed of Eumorphotis. It covers entire bedding planes
spanning ten's of square meters (Figure 3A) and when found with other molluscs is still the most
numerically abundant and largest animal (Figure 3C). The Eumorphotis beds have been noted by
Hofmann et al. (2013c) who described these as Spathian Stage middle shelf colonization events
following storm deposition. Among these bivalve beds there are occasional echinoderm
fragments, the first indicators of what will become increased abundance upsection. As the
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depositional energy of these bivalve beds wanes, structureless carbonate and abraded fossil
packstones replace the Eumorphotis beds and complex bioturbation began. These burrows
include lined and branching forms suggesting a diverse and complex infauna was emerging
during the Spathian of the Confusion Range (Figure 3 E, F). The preservation of the recovery is
cut short by a sudden shallowing event that leads to intertidal deposits of ooids, flaser bedded
sands and muds, interference ripples, and desiccation structures (Figure 4, 6). This is likely the
regression observed by Carr and Paull (1983) during their work on the stratigraphy of the
Southwest United States. When open marine conditions return in the second half of the Spathian,
the paleoecological community is composed of echinoderms, including crinoids and echinoids,
infaunal bivalves like Unionites, and at the top of the section, sponge body fossils (Figure 4, 6).
I interpret these communities in two ways. First, I agree with Hofmann et al. (2013) that the
benthic recovery is escalating through these deposits as shown by the ecological variety of life
forms found in these upper Spathian deposits. The recovery rubric developed by Pietsch and
Bottjer (2014) include criteria for increased body size, decreased dominance, and increased
tiering and ecological guild occupation as indicators of improved recovery level. In the latest
Spathian deposits of the Thaynes Formation of the Confusion Range, epifaunal tiering is
indicated by crinoids and infaunalization by the bivalve Unionites as well as bioturbation.
However, the mollusc community here remains numerically dominated by the high abundance,
low diversity disaster fauna including Eumorphotis and Unionites (Figure 15) suggesting
community restzration remained just shy of a complete recovery.
The composition of the Spathian Stage Thaynes Formation is very similar to the
progression of the recovery in the Virgin Limestone at Lost Cabin Springs. The benthic fauna at
Lost Cabin Spring are dominated by disaster bivalves like Eumorphotis and other opportunistic
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taxa including Leptochondria and Pleuronectites. Body size however was much smaller in the
Virgin Limestone deposits at Lost Cabin Spring compared to the Thaynes Formation deposits.
One potential cause for this difference could be exposure to low oxygen conditions. Facies
evidence at Lost Cabin Spring including laminated mudstone deposits suggests that periodic
deepening events brought in incursions of low oxygen water masses into the offshore transition
which may be the cause for lower body size in this location compared to the Virgin Limestone at
Ute and even the Thaynes Formation in the Confusion Range (Mata and Bottjer 2011, but see
Marenco et al. 2013). The section at Ute and the Confusion Range include sandy, cross bedded,
storm deposits which indicate a shallow shoreface environment compared to Lost Cabin Springs.
These shallow water environments were located farther from deep water masses and may have
been preferentially protected from low oxygen incursions by wave aeration of the water column.
Toward the top of the Virgin Limestone at Lost Cabin Spring, facies evidence for low oxygen
influence decreases and bivalve and gastropod body size increases, reaching the size ranges
observed for the Virgin Limestone at Ute and in the Thaynes Formation in the Confusion Range.
This increase in body size is likely due to the re-oxygenation of the water column which lagged
behind the perturbations to the carbon system during the Late Smithian (Payne et al. 2004, Algeo
et al. 2011, Grasby et al. 2013). Despite facies evidence for low oxygen conditions at Lost Cabin
Spring, there is an abundant echinoderm community composed of both crinoids and echinoids.
These two groups occur in allochthonous cross bedded storm deposits and therefore might
represent echinoderm communities that lived in shallower environments where they were able to
escape frequent low oxygen water incursions. One explanation for these periodic deepening
events could be the active tectonics of the region where subsidence could allow deeper water
masses access to these more shallow environments.
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The Spathian Virgin Limestone at Ute represents a variety of depositional environments
each of which contains a corresponding benthic fauna. The basal third of the section is
dominated by ooid shoal deposits which contain a microgastropod fauna as well as bivalve storm
beds and occasional echinoderm fragments. The predominance of gastropods is very similar to
the ooid and gastropod facies observed in the Smithian Sinbad Limestone. These gastropods
were recrystallized and not able to be identified but based on qualitative thin section analysis,
they did not represent the same level of diversity seen in the Smithian. The middle third of the
section represents a shallow intertidal environment dominated by muds and occasional silt and
sand deposits with interference ripples and additional ooid deposits. This region is mainly
depauperate much like the shallow section separating the two stages of Spathian deposition in the
Confusion Range. When open marine deposition resumes the seafloor is mainly colonized by the
disaster bivalves Eumorphotis and Promyalina. Like the Eumorphotis beds in the Thaynes
Formation, these two bivalves cover bedding planes representing many ten's of square meters of
winnowed storm deposits which exclude other faunas (Figure 16). These bivalves are often very
large, up to 5cm in diameter, compared to their contemporaries from the Virgin Limestone at
Lost Cabin Spring whose shells averaged 5.9mm. The large body size of these bivalves in the
Virgin Limestone section at Ute might be a result of relatively high oxygen concentrations in a
shallow water environment. The habitable zone hypothesis of Beatty et al. (2008) showed that
trace fossil diversity and bioturbation intensity increased in regions of the shoreface where wave
activity would have mixed in atmospheric oxygen counteracting low oxygen incursions from
offshore anoxic zones. The shallow section at Ute might represent one manifestation of the
habitable zone where upper offshore transition, lower shoreface, and shoal environments were
exposed to oxygenation from the atmosphere allowing the bivalves to grow to larger body sizes,
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especially in comparison to the stunted taxa in the Virgin Limestone at Lost Cabin Spring. In
addition to bivalve beds, the other prominent fauna in the upper third of the section at Ute are
ophiuroids represented by their resting traces, Asteriacties. Ophiuroids represent an additional
indicator of moderate oxygenation of the section at Ute since under anoxic stress these animals
tend to respond by disintegrating (Nilsson and Skold 1996). While body size in the Virgin
Limestone at Ute was high, many other ecological indicators of a recovered benthic fauna are
missing. The strata are dominated by just two opportunistic bivalve genera preserved in
winnowed storm deposits. The infauna is better developed including complex structures like
Rhizocorallium and Thalassinoides (Mata and Bottjer 2011). The deposits here contrast with the
diverse fauna found at other shallow sections of the Virgin Limestone found just 100 miles away
at Hurricane, Utah (McGowan 2009, Hofmann et al. 2013a). The discrepancy in the record might
be explained by depositional environment. The mollusc rich sections at Ute represent deeper
water deposits compared to the shallow inner shoreface and intertidal section found at Hurricane
(Hofmann et al. 2013a) which may have provided a different range of habitats to support a more
diverse group of benthic molluscs. Overall, Ute represents a high dominance, low diversity
community despite a protected shallow environment in the late Early Triassic.
Conclusions
After comparing the shallow and deep water sections making up the Smithian and
Spathian Stages of the Early Triassic in the Southwest United States it is important to compare
the taxonomic diversity and ecologic composition of the two time periods and connect these
differences to their respective environmental perturbations. Figure 13 depicts the average
structure and composition of the benthic paleocommunity following extreme temperature
excursions and low oxygen environments. In comparison with the communities developed by
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Schubert and Bottjer (1995) and Hofmann et al (2013a,b,c) these groups are not determined
based on clusters of dominant fauna but instead the differences between local depositional
energy regimes and extrinsic environmental changes are highlighted. Taxonomic diversity in the
Smithian Thaynes Formation and Sinbad Limestone was notably higher than that found in the
Spathian Thaynes Formation and Virgin Limestone. I suggest that the recovery experienced by
the benthic fauna leading into the late Smithian was cut short by a second environmental
perturbation, most likely the eruption of the Siberian Traps volcanism, and the resulting extreme
temperature event documented by Sun et al. (2012) (Cui and Kump 2014). Evidence for the
effects of extreme temperatures on the benthic fauna include the miniaturization of the gastropod
fauna in shallow marine environments and the exclusion of echinoderms from these sections.
This high temperature perturbation led to a setback in taxonomic and ecological recovery that
was not overcome until the Middle Triassic. The extreme temperatures in the late Smithian likely
had lasting effects including continued global warming, sluggish ocean circulation and the
prolonged low oxygen conditions in the waters of Panthalassa (Wignall and Twitchett 2002,
Algeo et al. 2011, Grasby et al. 2013). These low oxygen conditions in the early Spathian further
delayed the recovery of the benthic macrofauna leading to stunted mollusc body size in deeper
water sections like the Virgin Limestone at Lost Cabin Spring that were directly exposed to these
conditions. Extreme conditions may have prolonged the dominance of the disaster bivalves by
providing a second opportunity following successive high temperatures and low oxygen
conditions which would have eliminated additional competitors.
Hofmann et al. (2013) suggest that depressed competition resulted in low beta diversity in
the Early Triassic recovery. I argue that despite the occasional occurrence of unique taxa
throughout the Thaynes Formation, these deposits are clearly dominated by a few opportunistic
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groups of animals, the disaster bivalves and microgastropods, and do not represent a full
recovery in the sense of taxonomic or ecological diversity. While intrinsic mechanisms like
reduced competition may explain the lack of beta diversity between samples, communities, and
sections, the lack of ecological complexity five million years following the main extinction pulse
at the Permian-Triassic boundary demands an additional explanation. I have shown that the
extreme temperature event during the late Smithian Stage caused a resurgence of microgastropod
diversity and was the most likely limiting factor in the spread of abundant echinoderms. Low
oxygen environments following the extreme temperature event likely prolonged the dominance
of the disaster bivalves delaying the full recovery of the end-Permian mass extinction until the
Middle Triassic.
166
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Tables and Figures
Table 1. Samples from the Smithian Sinbad Limestone and Smithian part of the Confusion
Range Thaynes Formation at the Disappointment Hills (DH). Generic richness of bivalves and
gastropods is indicated as well as average abundance, average body size, Shannon-Wiener
Diversity Index and the number of ecological guilds represented. The peritidal, offshore, and
shoal deposits from the Sinbad Limestone each contain 4-5 samples that are summarized here.
Individual samples are indicated in tables from Chapter 2.
Table 2. Samples from the Spathian Thaynes Formation in the Confusion Range at Cowboy Pass
(CR-CP), the Virgin Limestone at Lost Cabin Spring (LCS), and the Virgin Limestone at Ute, in
Nevada. Generic richness of bivalves and gastropods is indicated as well as average abundance,
average body size, Shannon-Wiener Diversity Index and the number of ecological guilds
represented. The lower, middle, and upper parasequence deposits from the Virgin Limestone
each contain individual samples that are summarized here. Individual samples are indicated in
tables from Chapter 2.
Table 3. Community comparisons of diversity and abundance using Wilcoxin Rank. Here p<.05
is considered to be significant and is shown in bold. There are three significant differences
between select deposits in the Sinbad Limestone and the Thaynes Formation as well as the
Spathian at Lost Cabin Spring with the Thaynes Formation from the Spathian.
Table 4. Gastropod Size from the Sinbad Limestone and the Thaynes Formation. p<.05 is
considered significant. Comparisons between bulk samples from the Sinbad Limestone and
Thaynes Formation are each significantly different and shown in bold.
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Figure 1: Three maps depict the paleogeographic context and regional study sites for the Sinbad
Limestone, Virgin Limestone, and Thaynes Formation. A time scale shows their position in
Early Triassic. A. A paleogeographic map of the Early Triassic depicts the global context for the
Southwest United States localities (modified from Scotese et al. 2010). B. Map shows the study
sites for the Sinbad Limestone in the San Rafael Swell (38°59'51.0"N, 110°40'53.2"W) and the
Thaynes Formation in the Confusion Range (39°24'42.42"N, 113°41'22.23"W and
39°20'23.55"N, 113°41'43.79"W). Two dashed lines represent the shoreline and the shelf slope
break. The Smithian Stage Sinbad Limestone of the Moenkopi Formation represents deposition
in an epicontinental sea, therefore the onshore environment was wide (Blakey 1974, Dean 1981,
Goodspeed and Lucas 2007). The Thaynes Formation represents outer shelf deposits (Carr and
Paull 1983, Hofmann et al. 2013c). (Map modified from Goodspeed and Lucas 2007). C. The
Virgin Limestone at Lost Cabin Springs in the Spring Mountains (base of section 36°05′00.0"N,
115°39′13.3"W) (Marenco et al. 2012) and the Virgin Limestone at Ute in the Muddy Mountains
(36°25′57"N, 114°36′50"W). The relatively deep Spathian Stage Virgin Limestone of the
Moenkopi Formation and the more narrow shelf contrast with the broad shoreface regions of the
Smithian Sinbad Limestone. (Modified from Marzolf 1993 and Woods 2009). The section at Ute
represents more shallow deposition ranging from the offshore transition to intertidal settings. D.
Time scale for the Early Triassic and stratigraphy of the Moenkopi Formation and Thaynes
Formation in Utah and Nevada (Shen et al. 2011, Lehrmann et al. 2006, Mundil et al. 2004) and
relative placement of the studied sections based on conodont, ammonoid, and lithostratigraphy
(references in text).
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Figure 2: The Thaynes Formation in the Confusion Range in the Disappointment Hills is
composed of bioturbated carbonates and thrombolitic limestones followed by massive, cliff
forming carbonate deposits. The majority of the section is decimeter scale fossil rich storm beds
intercalated with outer shelf mud deposits. Fossils are also found in these muds. A. The basal
limestone contains a diverse fauna including Smithian aged conodonts. B. Thrombolitic
limestone beds are found at the base of the section. The fauna of these storm beds is diverse
including C. Gastropods, D., bivalves like Confusionella, and E. Unidentified ammonoids.
Figure 3: The Thaynes Formation in the Confusion Range at Cowboy Pass is composed of
Spathian aged fossil wackestone and packstones at the base of the section which grade into cross
bedded silts and sands at the top. A. Bed 4: The fossil deposits are dominated by Eumorphotis.
B. Bed 4: Wackestones are represented by recrystallized bivalves in a micrite matrix. C. Bed 6:
Eumorphotis is accompanied by additional bivalves. D. Bed 8: Echinoderm fragments are rare
but present in later fossil beds. Bioturbation becomes more common toward the top of the section
as the abundance of body fossils decreases. E: Bed 7: A complex lined burrow. F: Uppermost
siliciclastic bed: horizontal, nondescript bioturbation.
Figure 4: The remainder of the Spathian Stage Thaynes Formation in the Confusion Range at
Cowboy Pass is represented by a variety of depositional environments. After the outer shelf
bivalve beds the section shallows into shoal and intertidal deposits represented by the formation
of intraclasts and ooids as well as mudcracks, escape structures, and interference ripples.
Upsection gradual deepening returns deposition to fossiliferous carbonate and cross-bedded sand
deposits containing various echinoderm and mollusc faunas in wackestones, packstones, and
grainstones. Unionites is preserved as external molds in sand deposits and sponge body fossils
174
are observed in echinoderm-dominated beds at the top of the section. Letters A-H refer to the
corresponding images in Figure 6.
Figure 5. Conodonts from the basal limestone layer in the Disappointment Hills section of the
Thaynes Formation in the Confusion Range. SEM images are shown on the left and light
microscope images on the right. A., C. Ellisonia triassica ranges throughout the entire Early
Triassic? B. Parachirognathus ethingtoni ranges only through the Smithian Stage. (Diagnosis
from Orchard 2007 and pers. comm.)
Figure 6: The Spathian Stage of the Thaynes Formation in the Confusion Range at Cowboy Pass
is represented by a range of depositional environments. Letters A-H correspond to Figure 4
where they indicate the approximate location of each of these images. A. shallow marine setting
including well developed ooids and micritized, abraded fossil grains. B. Flaser bedding of muds
and fine grained siliciclastics represents increased terrestrial input in an intertidal setting. C.
Desiccation cracks provides additional support for an extremely shallow environment. A return
to deeper, inner shelf settings is indicated by mollusc and echinoderm wackestones and
packstones. D. A thin section showing abundant abraded echinoderm grains surrounded by mud
and secondary spar. E. Hand sample showing a bivalve mold and echinoderm clasts. F. An
external mold of Eumorphotis in mixed carbonate siliciclastic beds. G. Unionites external molds
in a red sandstone. H. A putative sponge body fossil in echinoderm packstones at the top of the
section.
Figure 7. Stratigraphic section with interpreted depositional environments from the Batten and
Stokes Section of the Sinbad Limestone in the San Rafael Swell (approx: 38.957251, -
110.633738) (Modified from Pietsch et al. 2014). Detailed lithology and sedimentary structures
observed in the field and petrography are included. Faunal composition is indicated for each
175
sample included in community analysis. Scale is in meters. Horizontal axis reads M= mudstone,
W=Wackestone, P=Packstone, G=Grainstone. A. Petrographic image of fossil rich sample
featuring microgastropods.
Figure 8. Stratigraphic sections from three parasequences studied from the Spathian Virgin
Limestone at Lost Cabin Spring (base of section near 36°05′00.0″N, 115°39′13.3″W) (Modified
from Pietsch et al. 2014). A. Lower Parasequence (meters 44 to 85 in original section), B.
Middle Parasequence (meters 85-103 in original section), and C. Upper Parasequence (meters
122 to 145 in original section). Detailed lithology and sedimentary structures observed in the
field and petrography are included. Faunal composition is indicated for each sample included in
community analysis as well as other beds that were included in initial sampling scheme and not
included in analysis (See Methods). D. A dense packstone including echinoderms and
scaphopods. E. A echinoderm, specifically crinoid, rick packstone. Scale is in meters. Horizontal
axis reads M= mudstone, W=Wackestone, P=Packstone, G=Grainstone.
Figure 9. Stratigraphic sections from the Spathian Virgin Limestone at Ute (36°32′57″N,
114°36′50″W) Detailed lithology and sedimentary structures observed in the field and
petrography are included. Faunal composition is indicated for each sample included in
community analysis as well as other beds that were included in initial sampling scheme and not
included in analysis. Numbers 1 and 2 correspond to two samples used in quantitative analysis.
Letters A-F indicate the approximate location of images A-F in Figure 10. Scale is in meters.
Key is the same as in previous stratigraphic columns. Horizontal axis reads M= mudstone,
W=Wackestone, P=Packstone, G=Grainstone.
176
Figure 10. Sedimentary structures and benthic marine fauna from the Virgin Limestone at Ute.
The letters A-F are also shown in Figure 9 and represent the relative location of each image. A.
Microgastropod packstone from the ooid shoal B. Echinoderm fragments not identified to the
class level. C. Branching horizontal bioturbation. D. Interference ripples represent shallowing
into intertidal/lagoonal system. E. Asteriacites, the resting trace of an ophiuroid, on a cm scale
sand bed from the lower shoreface. F. Amalgamated, winnowed, Eumorphotis and Promyalina
bivalve beds.
Figure 11. Molluscan diversity in the Smithian Sinbad Limestone and Smithian Thaynes
Formation from the Disappointment Hills in the Confusion Range. Shades of blue indicate
bivalve genera, shades of orange represent gastropod genera. The two localities are A. Batten
and Stokes and B. Disappointment Hills, Confusion Range. The two gastropod genera
Omphaloptychia laevisphaera and Naticopsis utahensis are extremely difficult to distinguish at a
macroscopic level. n= total specimens identified for a given environment.
Figure 12. Microgastropod size distribution and generic diversity. A. Histogram representing the
size distribution of gastropods from the Batten and Stokes section of the Smithian Sinbad
Limestone microgastropod fauna (in black) and the Smithian Disappointment Hills section of the
Thaynes Formation gastropods from the Confusion Range (in grey). Whisker plots show the
median and 1st and 3rd quartiles of gastropod size distribution and the results of a Wilcoxin
Rank test on gastropod size distribution.
Figure 13. Six boxes represent six common communities found in the Early Triassic of the
Southwest United States. A. Smithian Sinbad Limestone, Offshore Facies. B. Smithian Thaynes
Formation from the Disappointment Hills of the Confusion Range mud beds dominated by
gastropods. C. Smithian Thaynes Formation from the Disappointment Hills of the Confusion
177
Range storm beds with a more diverse mollusc fauna. D. Spathian Thaynes Formation from
Cowboy Pass representing a diverse bivalve storm bed from the Eumorphotis beds section. E.
The Spathian Virgin Limestone from Lost Cabin Spring representing the upper parasequence. F.
The Spathian Virgin Limestone from Ute representing a Promyalina and Eumorphotis dominated
storm bed.
Figure 14. Cluster diagram containing samples from the three depositional environments
represented at the Batten and Stokes locality of the Sinbad Limestone; Peri=Peritidal,
Off=Offshore, and Shoal. These cluster separately from the storm and mud beds sampled from
the Disappointment Hills of the Thaynes Formation in the Confusion Range (CR-DH). Mud 2/3=
the muds sampled between Beds 2 and 3, shown in Figure 11. Mud>3 represents the muds
sampled above Bed 3. The shading of each box represents the relative contribution of a given
genus in the intersecting sample. The two gastropod genera Omphaloptychia laevisphaera and
Naticopsis utahensis are extremely difficult to distinguish at a macroscopic level.
Figure 15. Molluscan diversity in the Spathian Limestone at Lost Cabin Spring and Ute and the
Spathian Thaynes Formation from Cowboy Pass in the Confusion Range. Shades of blue indicate
bivalve genera, shades of orange represent gastropod genera. The three localities are A. Spathian
Thaynes Formation from Cowboy Pass in the Confusion Range. Samples 3 and 6 correspond to
beds 3 and 6 in Figure 3, samples F and G correspond to letters F and G in Figure 4 and 6. B.
The Spathian Virgin Limestone at Lost Cabin Spring. The three pie charts represent high energy
depositional environments from each of the three parasequences of Figure 8. C. The Spathian
Virgin Limestone at Ute. The two pie charts represent samples 1 and 2 shown in Figure 9. The
two gastropod genera Omphaloptychia laevisphaera and Naticopsis utahensis are extremely
178
difficult to distinguish at a macroscopic level. n= total specimens identified for a given
environment.
Figure 16. Cluster diagram containing A. Spathian Thaynes Formation from Cowboy Pass in the
Confusion Range (CR-CP), the Spathian Virgin Limestone at Lost Cabin Spring (LCS) and the
Spathian Virgin Limestone at Ute (Ute). The Spathian Virgin Limestone at Lost Cabin Spring
clusters separately from the Eumorphotis dominated beds at Ute and in the Thaynes Formation in
the Confusion Range. The shading of each box represents the relative contribution of a given
genus in the intersecting sample. The two gastropod genera Omphaloptychia laevisphaera and
Naticopsis utahensis are extremely difficult to distinguish at a macroscopic level.
179
Ecological
Guilds
Richness
Average
Abundance
Average Size
(mm)
Shannon
Index
Sinbad
Peritidal(n=4)
Sinbad
Offshore(n=5)
Sinbad
Shoal(n=5)
DH 1
DH2
DH 3
6
19
17
22.25
62.8
91.8
6.7
5.3
7.1
.962
1.74
1.36
Table 1.
9
39
119
1.89
1.2
1.8
8.1
7.2
9.5
6
7
12
DH between
2&3
60 1.4 7.4 6
DH Muds
above 3
179 .35 6.9 8
6
5
3
4
3
5
3
4
Richness Abundance
Average Size
(mm)
Shannon
Index
Ute 1
Ute 2
Sample
34
32
33
13 1.28
.43 2
6
LCS Lower
(n=1)
5 36 5.3 0.55
CR-CP
6
CR-CP
3
122 1.48 12.3
15.2 1.22 4 17
9
LCS Middle
(n=3)
LCS Upper
(n=4)
10
14
52.6
24.5
6.1
6.3
1.25
1.70
Ecological
Guild
2
1
2
3
2
4
5
Table 2.
180
Table 3.
Table 4.
Sample Sample p W
Sinbad Thaynes 2.2 x10-16 4091.5
Sinbad
Omphaloptychia
Thaynes
Omphaloptychia
1.436X10-
14 291.5
CR-CP CR-DH 6.51x10-8 3252
Sample Sample p W
CR-DH sum Sinbad Shoal 0.8865 566.5
CR-DH sum
Sinbad
Offshore 0.8623 592
Sinbad
Offshore CR-CP BB3 0.0062 773.5
LCS Upper CR-CP BB3 0.1121 684.5
LCS Upper
Sinbad
Offshore 0.1595 471
LCS Upper Sinbad Shoal 0.2834 497.5
LCS Upper CD-DH sum 0.2266 486
LCS Lower CD-DH sum 0.029 417.5
LCS Middle CD-DH sum 0.0003 327
181
Griesbachian
Dienerian
Smithian
Spathian
252.2
247.2
Sinbad
Limestone
Virgin
Limestone
Moenkopi
Formation
Early Triassic
Induan Olenekian
D.
Thaynes
Formation
Disappoint-
ment Hills
Cowboy
Pass
Tethys
Ocean
Panthalassic
Ocean
Boreal Ocean
A.
200 km
Basinal
Nevada Utah
LCS Virgin
Limestone
200 km
Basinal
Nevada Utah
Sinbad
Shoreline
Shelf-Slope Break
Offshore
Offshore
Thaynes
Onshore
Ute Virgin
Limestone
B. C.
Figure 1
182
M W P G
5
10
15
20
25
30
35
Lithology
Biology
Outer Shelf
3
2
1
1000 um
1 mm
A.
A
B
C
D
C.
1 cm
1 cm
E
E.
Calcareous shale/marl
Clotted, Thrombolitic
Sandy Limestone
Limestone
Bivalves
Gastropods
Conodont
Echinoderm Stereom
Bioturbation
B.
D.
5000 µm
Figure 2
183
2 cm
C.
5 mm
B.
M W P G
5/
10
15/
20
25
30/
40
45
50/
60
Outer Shelf Muds & Storm Deposits
3
7
6
4
5
8
65
70
75
80
85
2
1
Lithology
Fine grained mudstone
Limestone
Sedimentary Structures
Bivalves
Biology
Shallowing into inner shelf
Silty mudstone
Fossiliferous Limestone
Cross Bedding
Bioturbation
Echinoderm Stereom
1 cm
A.
200 µm
D.
E.
1 cm
1 cm
F.
F
E D
8
B
C
A
Figure 3
184
M W P G
5
10/
20
25
30
35/
45
50
55/
65
70
75
80
85
M W P G
90
95
100
105
110
115
120
125
130
135
E
B
C
A
D
F
G
H
Sedimentary Structures/
Carbonate Grains
Intraclasts
Teepee/
escape
Planar
Laminations
Ooids
Mud Cracks
Flaser
Bedding
Ripples
Brachiopod
Bivalve
Crinoid
Echinoid
Bioturbation
Biology
Carbonaceous
shale
Breccia
Fossiliferous Limestone
Shaley silt
Sandy Limestone
Wavy bedded Sandstone
Mudstone
Cross bedded Sandstone
Limestone
Lithology
Figure 4
185
250 μm
200 μm
500 μm
A.
B.
C.
Figure 5
186
1mm
10 cm
10 cm
B.
C.
5 mm
D.
E.
2 cm
G.
1 cm
A.
1 mm
F.
1 cm
2 cm
H.
Figure 6
187
M W P G
1
2
3
4
5
7
8
9
10
11
12
13
14
1
m
2
m
3
m
4
m
5
m
6
m
7
m
8
m
9
m
10
m
11
m
6
Peritidal Offshore Shoal
15
Dolomite
Lithology and Sedimentary Structures
Fine grained mudstone
Clotted, Thrombolitic
Cross Bedded Sandstone
Limestone
Oolitic Limestone
Dolomitic Limestone
Bedded Sandstone
Cross Bedded Limestone
Carbonate Grains
Intraclasts
Peloids
Laminated Intraclasts
Bivalves
Gastropods
Echinoids
Scaphopods
Biology
A.
A.
2000 µm
Figure 7
188
M W P G
4
m
8
m
12
m
16
m
20
m
24
m
28
m
32
m
36
m
40
m
M W P G
4
m
8
m
12
m
16
m
M W P G
4
m
8
m
12
m
16
m
20
m
22
m
Lithology and Sedimentary Structures
Fine grained mudstone
Clotted, Thrombolitic
Limestone
Sandstone
Oolitic Limestone
Carbonate Grains
Intraclasts
Bivalves
Gastropods
Echinoids
Scaphopods
Biology
Crinoids
1
2
3
A.
B.
C.
4
5
6
7
8
9
10
11
12
13
14
5 cm
5 mm
D.
E.
Figure 8
189
M W P G
2
4
6
8
10
12
14
16
18
20
22
M W P G
24
26
28
30
32
34
36
38
40
42
44
46
56
M W P
48
50
52
54
58
60
62
64
66
68
G
A
B
C
D
E
F
1
2
Figure 9
190
1 cm
200 µm 1 cm
10 cm
1 cm 5 cm
A.
B.
C.
D.
E.
F.
Figure 10
191
Peritidal Offshore Shoal
A. Batten and Stokes: Smithian Sinbad Limestone
n=89 n=314 n=463
6
10
15
7
9
3
5
4
3
2
8
12
14
15
19 11
12
13
6
7
8
9
10
1
9
1
23
4
5
12
10
14
12
11
9
9
7
8
6
15
19
B. Disappointment Hills, Confusion Range: Smithian Thaynes Formation
Bed 1
5
6
14
15
19
10
2
4
6
10
4
18
19
17
15
12
14
A
Bed 3
6
6
10
11
2
4
4
Bed 2
1
4
6
6
7
10
11
13
Muds Between 2 and 3 Muds above Bed 3
9
6
7
9
3
19
n=39
n=60
n=8 n=119
n=179
1
2
4
4 5
5 7 8
8 7
13 3
3 1
6
6
10
10
14
9
9
11
2
15 18
12 13
16
Promyalina
17 19
A
Bakevellia Confusionella Costatoria Crittendenia Entolium Eumorphotis Leptochondria Myalina Myalinella Neoschizodus Permophorus Pernopecten Pleuronectites
Promyasidella Sinbadiella Unionities Unicardium Abrekopsis Battenzyga Coelostylina Chartronella Cylindrobullina Laubopsis Neritaria O. homolirata
O.laevisphaera/N.utahensis Polygyrina Strobeus Vernelia Worthenia Obnixia
12 11
Pecten
14
Zygopleura
Figure 11
192
Body Size (mm)
Thaynes Formation
Sinbad Limestone
60
10
20
30
40
50
Gastropod Frequency
n=114
n=237
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Median=2 Median=6.1
Wilcoxin Rank
p=2.2x10-16, W=4091.5
0 5 10 15 20
Sinbad Thaynes
Figure 12
193
E. Spathian Virgin Limestone
Lost Cabin Spring
B. Smithian Thaynes Frm.
Muds
A. Sinbad Oshore
D Spathian Thaynes Frm.
Bivalve Beds
F. Spathian Virgin Limestone
Ute
C. Smithian Thaynes Frm.
Storm Beds
mobile
epifaunal
suspension
feeder
mobile
epifaunal
carnivore
stationary
semi-infaunal
suspension
feeder
stationary
infaunal
suspension
feeder
mobile
epifaunal
grazer
stationary
epifaunal
suspension
feeder
erect,
epifaunal
suspension
feeder
Infaunal
deposit
feeder
Figure 13
194
Figure 14
Bakevellia
Unionites
Eumorphotis
Myalinella
Neoschizodus
Promyalina
Myalina
Permophorus
Coelostylina
O.laev./N.utah
Battenzyga
Pleuronectites
Leptochondria
Abrekopsis
Vernelia
Chartronella
Cylindrobulina
Worthenia
Strobeus
Confusionella
Promyasidella
100 75 50 25 0
0 25 50 75 100
Minimum Maximum
CR-DH Bed 1
CR-DH Mud>3
CR-DH Bed 2
CR-DH Mud 2/3
CR-DH Bed 3
Sinbad Peri
Sinbad O
Sinbad Shoal
O. homolirata
Laubopsis
Polygyrina
Crittendenia
Obnixia
Sinbadiella
Unicardium
195
1
2
4
4 5
5 7 8
8 7
13 3
3 1
6
6
10
10
14
9
9
11
2
15 18
12 13
16
Promyalina
17 19
A
Bakevellia Confusionella Costatoria Crittendenia Entolium Eumorphotis Leptochondria Myalina Myalinella Neoschizodus Permophorus Pernopecten Pleuronectites
Promyasidella Sinbadiella Unionities Unicardium Abrekopsis Battenzyga Coelostylina Chartronella Cylindrobullina Laubopsis Neritaria O. homolirata
O.laevisphaera/N.utahensis Polygyrina Strobeus Vernelia Worthenia Obnixia
12 11
Pecten
14
Zygopleura
Ute 1 Ute 2
n=32 n=33
6
15
19
8
10
12
7
6
C. Ute: Spathian Virgin Limestone
B. Lost Cabin Spring: Spathian Virgin Limestone
6
7
8
9
14
7
6
8
9
14
1
10 12
1514
n=70 n=164 n=98
1
2
6
3
7
8
9
9
12
11
14
15
19
14
Lower Parasequence Middle Parasequence Upper Parasequence
1
2
9
8
10
12
19
Bed 3
6
9
15
19
Bed 6
n=122 n=17
A. Confusion Range, Cowboy Pass: Spathian Thaynes Formation
19
7
10
6
F. Unionites G. Eumorphotis
n=20 n=29
15
16
Figure 15
196
Bakevellia
Unionites
Eumorphotis
Myalinella
Neoschizodus
Promyalina
Myalina
Permophorus
Coelostylina
O.laev./N.utah
Battenzyga
Pleuronectites
Leptochondria
Abrekopsis
Pecten
Zygopleura
Confusionella
Promyasidella
100 75 50 25 0
0 25 50 75 100
Minimum Maximum
CR-CP Bed 6
Ute 1
Ute 3
CR-CP Bed 3
LCS Lower
LCS Middle
LCS Upper
Figure 16
197
Appendix: Bivalve Data for the Thaynes Formation of the Confusion Range
Bakevellia
Claraia
Confusionella
Crittendenia
Eumorphotis
Leptochondria
Myalina
Myalinella
Neoschizodus
Permophorus
Pleuronectites
Promyalina
Promyasidella
Sinbadiella
Unicardium
Unionites
Abundance
CR-CP Bed 3 42 6 2 4 5 3 2 5 53
CR-CP Bed 6 5 2 2 8
CR-CP Unionites 2 1 17
CR-CP Eumorphotis 29
CR-DH 1 2 1 1 1
CR-DH 2 2 1 25
CR-DH 2/3 2 1
CR-DH 3 42 9 27 4 1 1 2 5 4 22
CR-DH >3 2 5 2 1
Size
CR-CP Bed 3 12.2 11.0 15.5 15.5 8.4 9.3 13.0 11.8 11.7
CR-CP Bed 6 28.4 9.5 11.0 11.8
CR-CP Unionites 13.5 8.0 13.4
CR-CP Eumorphotis 13.7
CR-DH 1 9.5 16.0 13.0 16.0
CR-DH 2 8.5 9.0 7.9
CR-DH 2/3 9.5 11.0
CR-DH 3 9.2 13.1 8.4 11.0 9.0 23.0 12.5 13.2 15.5 14.4
CR-DH >3 9.0 19.4 5.0 11.0
198
Appendix: Brachiopod and Gastropod Data for the Thaynes Formation of the Confusion Range
Obnixia
Abrekopsis
Chartronella
Cylindrobulina
Laubopsis
Polygyrina
Strobeus
Worthenia
Abundance/Avg. Size
Abundance
CR-CP Bed 3 122
CR-CP Bed 6 17
CR-CP Unionites 20
CR-CP Eumorphotis 29
CR-DH 1 1 2 8
CR-DH 2 1 6 1 2 38
CR-DH 2/3 1 9 20 1 23 2 59
CR-DH 3 1 1 119
CR-DH >3 10
Size
CR-CP Bed 3 11.9
CR-CP Bed 6 16.3
CR-CP Unionites 13.2
CR-CP Eumorphotis 13.7
CR-DH 1 2.0 3.0 9.0
CR-DH 2 10.0 5.7 4.0 4.0 7.4
CR-DH 2/3 4.0 4.3 5.8 4.0 5.8 3 5.6
CR-DH 3 13.0 6.0 10.9
CR-DH >3 13.6
199
Appendix: Bivalve data for the Virgin Limestone at Ute, Nevada
Samples
Eumorphotis
Myalinella
Neoschizodus
Permophorus
Promyalina
Unionites
Abundance
1 17 7 1 1 1 5
2 28 5
Size
1 3.5 10.7 11 17 20 16
2 35.8 32.8
200
Chapter 4: The Early Triassic paleoecology in the Tethys Ocean. Rapid resurgence and
delayed diversity tied to local depositional environments and global climate change.
Introduction
The pattern of the Early Triassic recovery from the end-Permian mass extinction was not
the same between the two major ocean basins of the Early Triassic; Panthalassa and Tethys. The
literature survey and accompanying recovery rubric developed in Chapter 1 indicate a complex
benthic paleoecological recovery that varied through time and between localities. This analysis
was conducted at the stage level, with each Early Triassic stage representing 500,000 to 2.5
million years. The relatively low resolution of stages makes it challenging to make precise
comparisons between localities across the globe. Higher resolution records are more easily
correlated to geochemical and sedimentological records of climate change and to other
paleoecological records resulting in the most informative synthesis of the benthic recovery from
the end-Permian mass extinction. To this end, a high resolution lithological, geochemical, and
paleoecological data set was collected from the Early Triassic Werfen Formation of the Italian
Dolomites for detailed comparison with the benthic communities studied in the Southwest
United States (Figure 1).
The Italian Werfen Formation was deposited in Paleo-Tethys and contains a record that
extends from the Permian-Triassic boundary throughout the entire Early Triassic interval. The
Tethys Ocean represented 10% of the entire ocean volume in the Early Triassic but contains the
majority of the Permian-Triassic boundary sections studied today as well as Early Triassic
recovery records. Carbon isotope analysis in South China, Iran, and Italy have been interpreted
as an indication that the Tethys Ocean was a high-productivity region leading to basin wide
201
stratification and overturn events throughout the Early Triassic (Horacek et al. 2007). Sensitivity
studies indicate that eutrophication and an increased biological pump are required to drive
stagnation events (Winguth and Winguth 2012). The addition of a tectonic sill at the boundary of
the enclosed Tethys basin with the vast Panthalassic Ocean would have further driven stagnation
and low oxygen concentrations (Winguth and Winguth 2012). Eutrophication and stagnation
would lead to anoxic conditions throughout much of the Tethys Ocean potentially punctuated by
basin overturn events. Wignall and Twitchett (1996) studied sections in the Italian Dolomites
using Thorium/Uranium analysis and concluded that low oxygen conditions waxed and waned
throughout the early stages of the Early Triassic.
The Italian section is one of the most complete records of the end-Permian mass
extinction and Early Triassic recovery available anywhere in the world. The Permian-Triassic
boundary is well exposed in many localities throughout the Italian Dolomites (Figure 2)
(Broglio-Loriga 1983, Perri 2003, Farabegoli et al. 2007, Posenato 2008b). Initial work on the
section constrained it in time through lithostratigraphic and biostratigraphic comparison both
throughout the Dolomites and to neighboring sections in Hungary (Neri and Posenato 1985,
Broglio-Loriga 1990, Perri and Farabegoli 2003, and Posenato 2008b). More recently, Brandner
et al. (2009) redefined some of the lithostratigraphic boundaries of the Early Triassic Werfen
Formation (see discussion in the text). Later work on the Werfen Formation began to investigate
the fossil record and ecological development throughout the Early Triassic. These studies
focused on turnover at stage boundaries and between the Members of the Werfen. Posenato
(2008a) analyzed bivalve diversification and body size changes. Hofmann et al (2011) described
a diverse and complex trace fossil community from the early Siusi Member of Late Griesbachian
202
and Early Dienerian age. Twitchett (1999) applied an early version of his paleoecological
recovery rubric to the benthic communities of the Werfen Formation.
In this chapter I will further explore the benthic paleoecological recovery in the Werfen
Formation taking into account previous analysis of these sections. Using a combined
sedimentological, geochemical, and paleoecological approach, I will address how benthic life
responded to volatile ancient climate change events recorded in the Werfen Formation. In
contrast with previous work on the Werfen Formation, I will present a higher resolution
sampling scheme that takes into account transitions along changing environmental gradients in
the different Members of the Werfen Formation. I will study macrofossils collected in the field
and also meso-scale organisms that can only be discerned through thin section analysis in order
to address how macro and meso-scale communities converged or differed in their recovery
patterns. I will define which organisms had a lasting role in the restructured benthic fauna. I will
apply the modified recovery rubric to each of the major stages of the recovery. Finally, I will use
the high resolution analysis of the benthic paleoecological recovery in deep and shallow water
sections of the Southwest United States to synthesize how changes in global climate and
environmental perturbations impacted the benthic fauna in both Tethys and Panthalassa.
Methods
Throughout the Werfen Formation at the U'omo and Bulla localities in the Italian
Dolomites bulk samples for benthic community analysis and thin sections for microfossil and
microfacies analysis were collected at various intervals indicated by sample numbers on the
stratigraphic section (Figures 4-16). Samples were taken from fossil bearing mixed carbonate-
siliciclastic wackestones, packstones, and grainstones in a range of depositional environments in
order to capture variation across the Early Triassic shoreface. Bulk samples were made from the
203
upper 15 cm of fossil bearing horizons. Bulk samples varied in volume because sample
transportation back to the United States from Italy was a challenge. Sample volume varied from
1L to 2L to 4L (Table 1, Table 2). These differences in sample volume are noted in data table
and bias abundance data. Based on our results, diversity data does not seem to be effected by
increased sample size (Table 1, Table 2). As previously discussed, bulk sampling offers the
benefits of capturing a wide size range of benthic organisms when compared to surficial
sampling (Chapter 2, 3, McGowan 2009, Hofmann et al. 2013a, b, c). Bulk samples were
analyzed in a stratigraphic context; each sample was considered independently but also grouped
by geological member. Thin section analysis allowed for the inclusion microscopic gastropod
abundance and size as well as other microscopic components of the benthic fauna including
ostracods and echinoderm fragments for a robust analysis of Early Triassic communities.
In the field, samples were disaggregated into 2 cm
3
fragments to expose all conspicuous
fossils. For each bulk sample, specimens that were more than 50% complete and over 2 mm in
length were identified to the genus level. Specimens were identified to the genus level using
recent publications and the Paleobiology Database. (Neri and Posenato 1985, Broglio-Loriga
1986, Broglio-Loriga et al. 1990, Hips and Pelikán 2002, Posenato et al. 2005, Hautmann et al.
2008. Hautmann et al. 2011, Hautmann et al. 2013, Wasmer et al. 2012). Genus level
identifications were made so as to not add bias to trends in diversity from taxonomic interests
units. The study of certain biological groups as index fossils for geologic correlations can result
in taxonomic over-splitting of these groups and inflated levels of species level diversity. In the
Werfen Formation, bivalves, specifically the genera Claraia and Eumorphotis are used as
biostratigraphic indicators throughout the Italian Dolomites and corresponding sections from
Hungary (Broglio-Loriga 1983, Twitchett 1999, Posenato 2008b). Paleoecological guild
204
diversity was calculated by placing each genus into one of seven possible guilds facultatively
mobile infaunal suspension feeder, facultatively mobile epifaunal suspension feeder, stationary
grazing epifauna, mobile grazing epifauna, stationary epifaunal suspension feeder, stationary
infaunal suspension feeder, stationary low level epifaunal suspension feeder, stationary semi-
infaunal suspension feeder.
Size data from hand samples was measured to the nearest mm and in thin section,
gastropod size data was measured using the program ImageJ. Each genus was consistently
measured across its widest or longest dimension. Most bivalves were measured along their
length, unless the width was the greater measurement for the particular genus as was the case for
some pectinids. Gastropods were measured from the apex to the base of the aperture. Gastropods
were counted as one individual in diversity and abundance analysis. Through taphonomic
processes, each bivalve individual contributes two equal valves to the fossil record. For each
individual sample, for every two bivalve valves that were the same length, only one was
considered in the analysis to conservatively estimate the number of individuals present in the
community.
One or more thin sections were made for each bulk sample as well as occasional
intervening stratigraphic samples in order to refine the environmental interpretation and to search
for microfossil presence/absence data and to determine a samples viability for reliable
geochemistry. Microfacies analysis yielded ooids, peloids, stylolites, micrite, and spar matrix
which was used to more precisely determine depositional energy level. Samples were not
included in geochemical analysis if they exhibited extensive recrystallization, dolomite, or
stylolites or if they were primarily siliciclastic. Qualitative observations were made of the
205
relative abundance of mollusc fragments as well as the presence of echinoderm stereom,
foraminifera, microconchids, ostracods, and microbial fabric.
Generic richness, bivalve and gastropod abundance and average size within a sample,
Shannon-Wiener Diversity Index, and ecological guild diversity were calculated for each sample
(Table 1, Table 2). Fossil size data was averaged for each individual genus. Abundance data for
each sample was grouped by sample and then arcsine transformed and absence data was coded as
.001.
Stratigraphic members of the Werfen Formation were interpreted based on key
lithostratigraphic and biostratigraphic data indicated by previous authors working on the Early
Triassic of the Italian Dolomites (Broglio-Loriga 1983, 1990, Wignall and Twitchett 1996,
Twitchett 1999, Galfetti et al. 2007, Posenato 2008, Brandner 2009). The top of the Siusi
Member was recognized by an abrupt shallowing event recognized by other authors and
indicated in our section by the presence of channels and gutter casts. (Figure 9). The Campil
Member is recognized by its increased siliciclastic content and significant sedimentary structures
including desiccation cracks and interference ripples.
After analysis of samples via thin section, a small amount of powder was drilled from
micrite or fossil rich samples. Samples with carbonate mud or fossil clasts were preferred for
analysis while those showing extensive recrystallization, dolomite, or stylolites in thin section
were not included in the analysis. Bulk carbonate isotopes were analyzed using an MC-ICP-MS
Thermo-Finnegan Neptune in the lab of Dr. Lowell Stott at USC (Figure 3). Oxygen isotope data
was also collected in order to test for diagenetic alteration (Figure 3a). Results were compared to
previous geochemical studies of the U’omo and Bulla sections of the Werfen Formation in order
to improve temporal placement of paleontological samples (Horacek et al. 2007) (Figure 3).
206
Geology
The Werfen Formation in the Italian Dolomites of Italy has been well studied in the Val
Gardena and Val Badia (Broglio-Loriga et al. 1990) including detailed analysis of the
stratigraphy and the benthic fauna including descriptions of microgastropod communities and
trace fossils (Figure 2) (Neri and Posenato 1985, Broglio-Loriga et al, 1990, Wignall and
Twitchett 1996, Twitchett 1999, Perri 2003, Farabegoli et al. 2007, Posenato et al 2008a,b.,
Brandner et al. 2009, Hofmann et al. 2011). These studies provide a broad baseline to perform a
comprehensive paleoecological community analysis of the benthic marine macrofauna. The
Werfen Formation is composed of nine members and represents a mixed carbonate siliciclastic
shelf system from outer shelf deposits to sub-aerially exposed supratidal environments. The
Permian/Triassic boundary is thought to be recorded in the first member of the Werfen
Formation, the Tesero Oolite Member (Farabegoli et al. 2007) (Figure 2). The remainder of the
section contains two, third order, parasequence with a sequence boundary occurring at the
Induan/Olenekian Boundary (Posenato 2008b).
The base of the Werfen Formation is indicated by the Tesero Oolite which was not
sampled in this study but is usually composed of ooids (Farabegoli et al. 2007). The measured
section begins in the Upper Mazzin as carbon isotopes shift from -2‰ to +2‰ (Horacek et al.
2007) (Figure 3) The Upper Mazzin member contains well developed, laminated ooids (Figure 5)
and occasional stromatolite beds with ooids forming the matrix between the microbial fingers
(Figure 17 D, E). Upsection, 0.5 to 1 m ooid grainstones occur among silts, marly limestones,
and bioclastic wackestones. This depositional regime represents a subtidal shoal environment
below fair weather wave base. The Mazzin Member at U’omo has previously been interpreted as
a low oxygen setting based on abundant pyrite framboids and depleted Thorium/Uranium ratios
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(Wignall and Twitchett 1996, Twitchett 1999). The dominance of the paper pectin Claraia and
limited, planar bioturbation serve as additional indicators of low oxygen conditions in this
environment (Wignall and Twitchett 1996, Twitchett 1999). In this study, we find the oxidized
remains of previous pyrite framboids but also evidence for oxygenated conditions (Figure 17B).
Abundant ostracods in microbialite and micrite deposits indicate oxygen availability as these
crustaceans are not tolerant of oxygen concentrations below 20% (Figure 17F) (Song et al.
2014). The Late Griesbachian-Upper Mazzin sampled for this study appears to be a high-energy,
well-oxygenated environment with abundant ooids and microbialite deposits. The termination of
the positive isotope excursion indicates the transition from the Griesbachian Mazzin Member to
the Siusi Member of the Werfen Formation at U’omo (
3) (Horacek et al. 2007). A shallowing event leads to the deposition of the sub-aerially exposed,
peritidal Andraz Horizon which was not directly observed in the section at U'omo. There is
however, evidence for extreme shallowing and wave activity including abundant rip up clasts
and cross bedded sands (Brandner et al. 2009).
The majority of the Siusi Member is composed of storm deposits consisting of cross
bedded silts and sands as well as carbonate wackestone and packstone deposits containing a
mollusc fauna (Figure 6, Figure 7, Figure 18E). At the base of the section, channelization of silts
and the occasional presence of ooid rich facies represent deposition ranging from tidal flat to
shoal, well above fair weather wave base (Figure 18A, C). The base of the Siusi Member also
exhibits depleted Th/U ratios and dominant Claraia (Wignall and Twitchett 1996, Posenato
2008b). Above thirty meters, the section contains cross bedded silty limestones, carbonate
wackestones, and massive carbonate deposits indicating deposition seaward of the shoal, below
fair weather wave base but above storm wave base. Upsection, increased siliciclastics, primarily
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silts interbedded with carbonate wackestones and packstones and extensive cross bedding
represent shallowing into the offshore transition. The presence of muddy carbonates excludes the
possibility of deposition in the lower shoreface. However, in the last 10 meters of the Siusi
Member, an increase in siliciclastic input and sedimentary structures indicating a high energy
environment, indicate a further shallowing into the lower shoreface. The microfacies of the Siusi
Member show abundant gastropods and bivalves preserved with a micrite envelope and
occasional dolomite rhombs and pyrite (Figure 18F, Figure 19B). Geochemically the upper Siusi
Member at U’omo represents an interval of “chaotic carbon” marked by 3‰ excursions ranging
from 0‰ to +3‰ (Figure 3). The transition from the Siusi Member into the Campil Member is
placed at the last +3‰ excursion at the top of the chaotic carbon interval before the more gradual
positive excursion, the termination of which marks the Dienerian-Smithian Substage boundary
which occurs within the Campil Member (Figure 3) (Horacek et al. 2007).
The next member of the Werfen Formation has classically been recognized as the
Gastropod Oolite Member (Broglio-Loriga 1990, Twitchett 1999, Posenato 2008b), but its
validity as a Member versus a facies has more recently been called into question (Brandner et al.
2009). The gastropod oolite is historically characterized by well developed, radial ooids and
abundant small gastropods; however, this facies occurs prominently throughout much of the
Early Triassic Werfen Formation, including the Siusi and Campil Members, making its diagnosis
as a unique member in and of itself, problematic. Brandner et al. (2009) propose, and I support, a
new stratigraphic arrangement, which places the Dienerian Siusi Member in direct contact with
the overlying Smithian Campil Member. This transition should no longer be marked by the
Gastropod Oolite Member, represented by a few meters of gastropod and ooid dominated
peritidal environments, but rather by a dramatic transition from mollusc wackestones and
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packstones with occasional siliciclastics of the Siusi Member to dominant siliciclastic input from
a terrigenous source in the Campil Member (Brandner et al. 2009). Regardless of the Member
represented by these facies, the gastropod and ooid dominated beds of the Early Triassic Werfen
Formation signal a transition to a high energy, inner shoreface environment where wave energy
resulted in abraded grains coated in radial fabric and intraclasts.
The Smithian Campil Member is dominated terrigenous material in the form of red
sandstones and siltstones as well as ooid grainstones and bivalve packstones (Figure 8-11)
(Broglio-Loriga 1990, Posenato 2008b). Gutter casts and water escape structures represent a very
shallow intertidal environment at the base of the Campil Member (Figure 20A). The remainder
of the Campil Member is dominated silts and sands as well as carbonate wackestones and
packstones. The presence of muds indicates a low energy environment prevailed during the
deposition of the Campil Member, likely representing lagoonal and intertidal environments
indicated by occasional cross bedding. Extreme shallowing and subaerial exposure in the Campil
Member is represented by interference ripples, desiccation cracks, and putative rip up clasts
(Figure 20G). Diplocraterion are represented in the siliciclastic environment with ichnofabric
index ranging from ii1-2 and trace diameter no greater than 5mm (Figure 20F) (Twitchett 1999).
In thin section, some deposits in the Campil Member is represented by abraded and coated grains
separated by fringing cements indicting deposition in a high energy, marine environment (Figure
20D). Multiple shallowing events are represented throughout the Campil Member indicating
fluctuation between lagoonal, intertidal, and supratidal conditions. The middle Campil Member
and the top of the studied section was deposited in a peritidal environment represented by
abundant rip up clasts interbedded with gastropod packstones as well as cross bedded siliciclastic
deposits.
210
At Bulla, the Permian-Triassic boundary is present in the Tesero Oolite at the base of the
section. The overlying Mazzin Member of the Werfen Formation is dominated by low energy
carbonate mud deposits with occasional half meter thick wackestone and packstone deposits
approximately every 5 meters (Figure 12, Figure 13). From meter 30 to 40, there is an increase in
the thickness and frequency of wackestone and packstone deposits. There is also an increase in
the percentage of siliciclastic material in the top 5 meters of the section. The base of the section,
dominated by structure-less mudstones, likely represents a proximal offshore environment below
fair weather and storm wave base. Clotted textures represent microbial deposits (Figure 22A,
Figure 23A). The section shallows upward reaching the offshore transitional environment, below
fair weather wave base but above storm weather wave base. Finally the top of the section
represents a shallower offshore environment with increased siliciclastic input, but lacking any
significant sedimentary structures, likely the offshore transitional environment below storm wave
base. This transition at the top of the Mazzin Member to increased depositional energy and
siliciclastic input is echoed by the U’omo section previously described. The extremely shallow
Andraz Member was observed at Bulla but not sampled for this study. The Griesbachian aged
Mazzin Member records a positive isotope excursion that begins at -2‰ and reaches to +1‰ by
the top of the section and the transition into the supratidal Andraz Member (Figure 3).
At Bulla, the Siusi Member varies from the lower shoreface indicated by hummocky
cross stratification, gutter casts, and wave ripples to the offshore, below storm wave base (Figure
14, Figure 15) (Brandner et al. 2009). The Siusi Member at Bulla begins with low energy muds
and microbialites containing ostracods, mollusc hash and pyrite framboids (Figure 24A, Figure
25A). This contrasts with the section at U’omo where the base of the Siusi contained 30 meters
of higher energy facies representing deposition in a shoal or intertidal environment. Upsection,
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mollusc rich packstones and interbedded with carbonate muds indicating an offshore
depositional environment, below fair weather wave base. At meter 20, ooids indicate increased
energy likely offshore of an ancient shoal (Figure 25B). Mudstones, wackestones, and
packstones, occasionally bioturbated, continue for the next 38 meters to the boundary with the
Campil Member. These mud rich deposits lack sedimentary structures, indicating continued
deposition below fair weather wave base. This low energy environment, below fair weather wave
base, is congruent with observations made at the U’omo section. Similar to the section at U’omo,
there is a dramatic transition into the Campil Member which is a siliciclastic rich, high energy,
shallow water environment. Complex bioturbators like Diplocraterion are found in coarse,
channelized sand deposits interbedded with finer silt and clays (Figure 25D). Microbialites
remain prevalent in the form of Kinneaya found near the top of the section (Figure 25E).
Geochemically, the Dienerian aged Siusi Member contains no dramatic shifts in carbon isotope
space. After the positive excursion in the late Griesbachian Mazzin Member, the Siusi Member
shows 1‰ fluctuations throughout the Dienerian, reflective of the ‘chaotic carbon’ observed in
the Dienerian at U’omo. While not as dramatic, the most positive isotope excursions can be
correlated to previous work by Horacek et al. (2007) and to the geochemistry and
lithostratigraphy at U’omo (Figure 3).
The same Campil Member facies are represented in the sections at Bulla and U’omo
(Figure 16, Figures 8 to 11).
Results
These data highlight diversity, abundance, evenness, body size, and ecological guild
occupation for the benthic macrofauna of the Early Triassic Werfen Formation. The studied
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section at U’omo begins in the Griesbachian Mazzin Member where one bulk sample of a
wackestone was collected (Table 1) (Figure 26). This sample contains a fauna comprised of
Unionites, a pectin, and the brachiopod Lingularia. The average bivalve size was 8.3 mm and for
Lingularia average body size was 2.8mm. Shannon-Wiener Diversity Index was low due to the
dominance of Unionites and Lingula. This sample contains two guilds, facultatively mobile,
infaunal Unionites and Lingularia and facultatively mobile, epifaunal, suspension feeding, pectin
(Table 1). In thin section, microconchids were prevalent (Figure 17C). In addition, ostracods
dominated carbonate mudstone deposits (Figure 17F).
The Siusi Member was more thoroughly sampled at U'omo and is constrained
biostratigraphically by Claraia aurita found in many of the bulk samples. The Siusi Member
represents the Late Griesbachian and Early Dienerian (Twitchett 1999, Posenato 2008b).
Diversity is higher in the Siusi and includes many bivalves and the gastropod Pseudomurchsonia
as well as two other unidentified gastropod genera (Figure 26). Bivalve diversity ranged from 6-
13 genera per sample, higher than the values indicated for the Siusi by Posenato (2008a). The
bivalves Unionites and Neoschizodus dominated and Claraia and Leptochondria were constant
members of the benthic fauna. Average bivalve body size ranged from 8.2 to 14.5mm while the
gastropod Pseudomurchsonia was between 2 and 7.2mm on average. Shannon-Wiener Diversity
Index was higher than the sample described from the Mazzin and ranged from 1.19 to 2.18.
Finally, guild diversity increased in the Dienerian represented by Bakevellia, a semi-infaunal
suspension feeder, and free-swimming Entolium at the very top of the Siusi Member. These two
genera are not a major faunal constituent at this time but become numerically important in the
Smithian Campil Member. In thin section, foraminifera and microconchids are the dominant
microfauna (Figure 18B). Foraminifera are in many meters of section below Sample 1 of the
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Siusi Member and microconchids are also present in these thin sections. Foraminifera included
the genus Rectocornuspira and Earlandia. Echinoderm stereom was also observed in two of the
thin sectioned samples (Figure 18G).
The Late Dienerian Smithian Campil Member now includes the shallow marine deposits
that were previously considered part of the Gastropod Oolite. In the Early Campil Member,
benthic marine guild diversity is maintained by infaunal Unionites, semi-infaunal Bakevellia,
free-swimming Entolium and a suite of epifaunal suspension feeders including Eumorphotis and
Leptochondria (Figure 27, Figure 28). Guild diversity does not see an improvement like that
observed between the Griesbachian Mazzin Member and Dienerian Siusi Member. In fact,
bivalve diversity and evenness remained stagnant and in some samples dramatically decreased in
the Campil Member with 7 of the 13 samples containing five or fewer genera per sample and a
Shannon-Wiener Diversity Index ranging from 0.40 to 2.01 at the highest. The low evenness
represented by the majority of samples in the Campil Member is the result of the dominance of
the two bivalves Unionites and Neoschizodus. The Early Campil member is relatively devoid of
microfossils containing ostracods in Sample 8 but lacking, foraminifera, microconchids, and
echinoderm stereom.
The extremely shallow deposits of the Smithian Stage Middle Campil are numerically
dominated by small gastropods that are primarily observed in thin section. Bivalve diversity
ranges from 5 to 10 genera and does not represent an improvement over the magnitude of benthic
diversity reported from the Dienerian age Siusi Member (Figure 28). The gastropod fauna
remains dominated by Pseudomurchsonia and/or Coelostylina. Both of these genera are a
"garbage bin" for poorly preserved high-spired gastropods in the Early Triassic with
Pseudomurchsonia being more commonly identified in the Tethys region. There is an increase in
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community evenness has increased in Smithian Campil Member and ranges from 1.4 to 2.06.
Five guilds are present in an individual sample from the Smithian Campil Member including
infaunal Unionites and Neoschizodus, semi-infaunal Bakevellia, epifaunal suspension feeding
bivalves like Eumorphotis, epifaunal grazers including Abrekopsis and Coelostylina, and free-
swimming Entolium and other pectins. Guild diversity reached an acme in the Smithian Campil
Member not seen since the Dienerian Siusi Member and infaunal tiering increased indicated by
prevalent Diplocraterion and Rhizocorallium (Figure 20F) (Twitchett 1999). The Smithian aged
Campil Member contains one sample of microconchids (Figure 21F) and more echinoderm
stereom (Figure 21G).
At the Bulla Section, three samples were made of the Griesbachian ages Mazzin Member
(Figure 29). Based on lithostratigraphy and geochemistry, these samples form the Late
Griesbachian Mazzin (Figure 3, Figure 13). Richness and abundance were relatively high in the
Griesbachian Mazzin Member ranging from 8 to 11 genera of mollusks. Bivalve size ranged
from 5.5mm to 10.1mm on average and gastropods from Sample 1 reached a maximum average
size of 2.5mm. Shannon-Wiener Diversity Index was relatively high ranging from 1.45 to 1.82.
Guild diversity of these three samples was also high including 4 to 6 guilds at the most including
some combination of facultatively mobile infaunal suspension feeders, facultatively mobile
epifaunal suspension feeders, mobile grazing epifauna, stationary epifaunal suspension feeders,
stationary low-epifaunal suspension feeders, stationary infaunal suspension feeders, and
stationary semi-infaunal suspension feeders. Foraminifera were the only microfossil observed in
the Griesbachian Mazzin Member at Bulla from Sample 3 at the top of the section (Figure 23B).
In the Late Griesbachian to Dienerian aged Siusi Member at the Bulla Section, samples
were taken from mixed carbonate-siliciclastic beds representing a range of depositional energy
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from the lower shoreface to below storm wave base (Figure 30). Generic richness ranged from 5
to 17 genera of mollusks. Bivalve size, on average, fell between 5.6 and 11.6mm while gastropod
size ranged from 2.7 to 4.0mm. Shannon-Wiener Diversity was similar to that seen for the
Mazzin Member at Bulla and ranged from 1.12 to 2.4 while guild diversity included the same
guilds described from the Mazzin Member at Bulla in combinations of 2 to 6 guilds in a given
sample. Microconchids and foraminifera were important constituents of the microfauna in the
Dienerian Siusi Member at Bulla (Figure 24B). They often co-occurred in a given thin section.
Discussion
In addition to the benthic paleocommunity data presented here, trace fossil analysis and
benthic diversity and community data from previous researchers will be used to interpret how
environmental change affected the benthic fauna in Italy through time. A global narrative of the
recovery will begin to emerge from comparisons between Italy, which represents the Tethys
Ocean, with sections from the Southwest United States, which represent life and conditions in
the Panthalassic Ocean. Previous authors who have made global comparisons have focused on
general recovery trends using recovery rubrics or more commonly, have worked on one
taxonomic group (Fraiser and Bottjer 2004, Fraiser et al. 2005, Chen et al. 2005, Nützel and
Schulbert 2005, Chen and Benton 2012). While these studies have added valuable detail to the
understanding of the end-Permian mass extinction, the purpose of this work is to perform high
resolution paleocommunity analysis in the context of environmental perturbations.
The Griesbachian Stage is represented by the Mazzin Member and lower Siusi Member.
In the Early Griesbachian, oxygen isotope data from conodonts indicates that equatorial sea
surface temperatures were warming throughout the Early and Middle Griesbachian, approaching
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40°C (Sun et al. 2012). By the Late Griesbachian, temperatures had cooled again to 34°C (Sun et
al. 2012). On a more regional scale, using Th/U ratios, Twitchett and Wignall (1996) suggest that
the Griesbachian Mazzin Member was influenced by a low oxygen environment while the
middle and upper Siusi do not show geologic evidence for low oxygen conditions.
At the U’omo section, thin section analysis of samples from the Middle Griesbachian
show the predominance of microbialites at the beginning of the Early Triassic. In an environment
just outboard of the ooid shoal, microbes bind and trap ooids as well as microconchids who
likely lived in-situ among the ancient microbial deposits (He et al. 2012). Previous work has
indicated that microbialites are one of the first ‘disaster taxa’ of the Early Triassic (Baud et al,
2006, Pruss et al. 2007), taking over the seafloor in the absence of metazoan life. With more
research, the co-habitation of microbialites and microorganisms has come to light. In this case,
microconchids lived among the microbial deposits and may have functioned as another Early
Triassic opportunistic group, colonizing other disaster taxa, specifically microbial mounds and
flat-clam Claraia on the seafloor (He et al. 2012). The bulk sample from the Mazzin Member at
U’omo contains a low diversity fauna also dominated by disaster taxa, Unionites and Lingularia.
At Bulla, the Griesbachian Mazzin displays a different picture of benthic life during the
earliest Triassic. Microbial laminations and thrombolytic textures are prevalent here as well
though microconchids were not observed. Mollusc samples from the Mazzin at Bulla represent
the Late Griesbachian and are extremely diverse and abundant but are still dominated by disaster
bivalves and other Early Triassic opportunistic fauna. Foraminifera were also found in the latest
Griesbachian. Their absence in the majority of Griesbachian sediments may be an indicator of
extreme sea surface temperatures excluding this group (Figure 31). Their re-appearance in the
latest Griesbachian corresponds with the geochemical cooling trend and may represent the first
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re-appearance of foraminifera following the extinction boundary in Italy. The presence of
ostracods in the late Griesbachian is another likely indicator of improved environmental
conditions, suggesting that oxygen saturation was above 20% (Figure 31) (Song et al. 2014).
In the Early Dienerian, equatorial sea surface temperatures were cooling, reaching
temperatures as low as 30°C by the Late Dienerian (Sun et al. 2012). Carbon isotopes recorded in
the Werfen Formation were very variable throughout the Dienerian, representing an interval of
‘chaotic carbon’ during which multiple positive and negative excursions ranged from 0‰ to
+4‰ while trending toward a positive +6‰ excursion at the Dienerian-Smithian boundary
(Figure 3) (Payne et al. 2004, Horacek et al. 2007).
The Early Dienerian exhibits a rapid rebound in mollusc diversity and abundance,
compared to the Middle Griesbachian Mazzin sample from the section at U’omo. The difference
in taxonomic diversity and paleoecological complexity between the Mazzin and Siusi Members
of the Werfen Formation has often been attributed to low oxygen conditions which prevailed in
the early Griesbachian Mazzin and were ameliorated by the Siusi Member. An increase in
taxonomic diversity between the Mazzin and Siusi Members is accompanied by an increase in
ecological guild occupation, increased body size, and an increase in the depth and complexity of
bioturbation. Increased ecological niche occupation is represented by semi-infaunal and free
swimming bivalves. Hoffman et al. (2011) documented an increase in the complexity and
diversity of tracemakers in the Late Griesbachian to Early Dienerian of the Siusi Member in the
Werfen Formation. He finds evidence for Thalassinoides, Rhizocorallium, and Ophiomorpha in
association with the bivalve Claraia clarai and conodonts Hadrodontiana aequabilis and
Isarcicella staeschei which indicate a late Griesbachian to early Dienerian age (Figure 2)
(Twitchett 1999, Posenato 2008b., Hofmann et al. 2011).These tracemakers were uncommon in
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other global locations during this interval. He interpreted the complex trace makers as evidence
for oxygenated sediment in the Italian Basin. This is further supported by echinoderm fragments
found throughout the Early Dienerian of both studied sections, indicating a well-oxygenated,
normal-salinity marine environment which led to the rapid restructuring of the benthic fauna in
the Italian Basin.
Foraminifera have their last appearance in the Siusi Member before the transition to the
Campil Member. The rapid rebound of foraminifera and their high abundance in Griesbachian
and Dienerian aged rock, but their absence from later Early Triassic substages suggests these
organisms may have functioned as opportunistic disaster taxa in the Early Triassic by using their
low oxygen tolerance to thrive in prevailing low oxygen conditions directly after the end-
Permian mass extinction (Song et al. 2014). The foraminifers’ genera observed in the Mazzin
and Siusi Members did not become part of the lasting paleocommunity during the restructuring
of the benthic system.
Using the modified recovery rubric (Pietsch and Bottjer 2014, Chapter 1), the Siusi Member
likely represents a recovery Stage 3. More than 10 genera are present and no one genus
dominates more than 25% of the entire Siusi Member. Bivalve size is greater than 10mm in some
samples and trace fossil diversity is high and complex including trace fossils like common
Thalassinoides, all indicators of an advanced, Stage 3 recovery. The lack of epifaunal crinoids
and a lack of reef deposits of any kind during the Siusi Member limit its designation to a Stage 3,
an incomplete recovery. The rapid return to a complex community within the Dienerian Stage
following the end-Permian mass extinction is comparatively unique across the Early Triassic
globe. In the Tethys Ocean, Dienerian age deposits in Pakistan show evidence for low oxygen
conditions and a low diversity fauna, a trend echoed in other Tethyan strata from Hungary which
219
does not show an improvement in benthic fauna until the Spathian Stage. Both the basin and
platform margin settings of the Early Triassic in South China do not progress past a recovery
stage 2 during the Dienerian (Pietsch and Bottjer 2014, Chapter 1). In Panthalassa, Hofmann et
al. (2013) find low diversity, high dominance communities during the Dienerian in the
Southwest United States. The Dienerian recovery in Italy is most comparable to the dense
infaunalization and complex trace fauna from British Columbia and Spitsbergen in the habitable
zone regions formed in the Boreal Ocean (Beatty et al. 2008, Zonneveld et al. 2010, Hofmann et
al. 2011).
The geochemical record in Iran, South China, and Italy as well as modeling of the Tethys
Ocean suggest that low oxygen conditions lingered well into the Dienerian, contrasting with the
Th/U data from Wignall and Twitchett (1996). Basin wide stratification events are indicated by
carbon isotope perturbations in the Tethys Ocean. Negative carbon isotope excursions likely
represent light carbon emplacement from continued eruptions of the Siberian Traps (Sun et al.
2012, Grasby et al. 2013). Positive excursions, like the one from the Dienerian into the Smithian,
represent removal of light, organic, carbon from the water column. Significant deposition of
black shales supports this hypothesis (Algeo et al. 2010). An increased biological pump with
increased nutrient inputs can drive organic carbon burial at a cost to oxygen availability in the
water column (Horacek et al. 2007, Winguth and Winguth 2012). The earlier Mazzin Member
was deeper and therefore more heavily influenced by low oxygen conditions. The recovery in the
benthic fauna seen in the Dienerian Siusi Member could represent a decrease in biological pump
activity and a return of oxygen saturated waters to shallow marine environments. However,
geochemical analysis and the sedimentological and biological indicators at other localities
throughout the Tethys Ocean indicate that low oxygen environments prevailed. The recovery in
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the Siusi Member is more likely driven by relative shallowing into the offshore transition,
influenced by storm wave activity which would have oxygenated the water column by
atmospheric injection. This unique depositional setting within an oxygenated habitable zone
helped to buffer the benthic fauna from offshore, low oxygen water masses (Beatty et al. 2008).
The rapid recovery of the benthic fauna in the Dienerian Siusi Member distinguishes it from
many other low diversity and low complexity communities in both Tethys and Panthalassa.
The Late Dienerian is represented by the Lower Campil Member at U’omo. In the Late
Dienerian at both the Bulla and U’omo sections, no echinoderms, microconchids, foraminifera,
ostracods, or microbialites were present. Overall, the benthic fauna seems depleted compared to
the Early Dienerian Lower Siusi Member.
During the Early Smithian Campil Member, equatorial sea surface temperatures began to
rise again to reach their maximum in the Late Smithian (Sun et al. 2012). Carbon isotopes show
a negative excursion throughout the Smithian Substage, starting at the highest value, +8‰ at the
Dienerian-Smithian boundary and decreasing to -2‰ by the Smithian-Spathian boundary (Figure
3). This excursion is thought to be driven by an additional eruptive pulse from the Siberian Traps
(Payne et al. 2004) which may have also driven the increase in sea surface temperatures
represented by depleted oxygen isotopes throughout this interval (Sun et al. 2012). There is no
known geochemical or lithological evidence for low oxygen conditions in the Campil Member.
The Campil is dominated by a few infaunal bivalves including Unionites and
Neoschizodus, the gastropod Pseudomurchsonia, as well as uncommon trace fossils including
Diplocraterion. Additional bivalve genera identified by Farabegoli et al. (2007) include
Avichlamys, Scythentolium, and shallow infaunal Costatoria subrotunda. By the Middle
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Smithian Campil Member, echinoderm fragments were present in some thin sections and
microconchids were found in one sample, while foraminifera and ostracods were not present.
Crinoids and other echinoderms do not appear in this section and are not documented until the
Cencenighe Member of the Spathian which does not outcrop at U'omo. However, Twitchett
(1999) indicates the presence of ophiuroids during this interval based on their trace fossil
Asteriacites. Ophiuroids indicate a minimum level of oxygen as under total anoxia they will
leave their infaunal habitat for the surface and disintegrate, thereby not leaving much of a trace
fossil record (Nilsson and Rosenburg 1994).
The loss of diversity and increase in dominance is often attributed to the abrupt facies
change seen across the Dienerian-Smithian boundary. From the Late Dienerian into the Smithian,
there is extensive local and global evidence for a shift toward increased siliciclastic input into
Early Triassic ocean basins. Korte et al. (2003) document an increase in the Sr
86
/Sr
87
ratio which
indicates increased riverine input of weathered sediments compared to hydrothermal input of
strontium into the ocean basin. During the Early Triassic, the continents had already formed
Pangea suggesting that no new orogenic belts were available for weathering and increased
sediment supply (Korte et al. 2003). Instead, increased siliciclastic sedimentation and weathering
of continental strontium is hypothesized to have originated from enhanced erosion following the
loss of terrestrial vegetation at the end-Permian mass extinction boundary (Korte et al. 2003).
Rapid input of CO
2
into the atmosphere would have resulted in rapid warming and acid rain
which lead to widespread loss of vegetation, which loosened the hold of sediment on the
continent, resulting in a massive erosional event at the end-Permian mass extinction event (Korte
et al. 2003). Later, in the Smithian, humid phases in the otherwise arid climate of the Early
Triassic would have allowed for enhanced weathering of the continents and increased sediment
222
supply. Physical evidence for this comes from braided river deposits dated to the Smithian which
indicate enhanced sediment load and transport during this stage (Korte et al. 2003).
Evidence for increased siliciclastic sediment input into the Smithian oceans includes
thick sedimentary sequences from around the Tethys Ocean Basin as well as in the Panthalassic
Ocean. This so called ‘Campil Event’ was first noticed by Twitchett (1999) in the Werfen
Formation in Italy. The increase in siliciclastics was accompanied by a reversal in the benthic
marine recovery evidenced by decreased diversity and a decrease in ichnofabric index
throughout the Campil Member (Twitchett 1999). He traced this phenomenon from Lombardy in
the west to Slovenia in the east and Hungary to the north. Galfetti et al. (2007) separately
documented this phenomena but for outer platform environments in Tibet and China.
Sedimentation switched from carbonates filled with ammonoids to siliciclastic dominated
sedimentation in the Early Smithian (Galfetti et al. 2007). Pollen indicative of humid conditions
further supports the hypothesis that wetter environments induced weathering and transport of
continental deposits throughout the Smithian (Galfetti 2007). By the Spathian, after the negative
carbon isotope perturbation, carbonate sedimentation resumed in the Tibetan and Chinese
sections (Galfetti et al. 2007).
Twitchett (1999) hypothesized that increased run off resulted in siliciclastic deposition
which brought with it increased freshwater, resulting in reduced salinity and increased “stress”
on some organisms, leading to decreased diversity and size (Twitchett 1999). In this study, the
lack of echinoderm fragments from the Late Dienerian through the Early to Middle Smithian
support a loss of stenohaline conditions via increased freshwater input during this interval.
Increased sediment flux into the Smithian oceans would have presented biological challenges to
other marine groups including suspension feeders. Increased sedimentation means these
223
organisms must spend time removing inorganic particles in their search for food, complicating
the extraction of nutrient rich material (Algeo and Twitchett 2010). Grazers could also suffer as
huge sediment loads could bury them and their food sources (Algeo and Twitchett 2010). The
loss of taxonomic and ecological diversity in the Late Dienerian and Early Smithian is likely tied
to the variety of “stress” provided by increased continental weathering including increased
freshwater input and a high sediment load detrimental to the benthic fauna. Increased
infaunalization of the bivalve fauna, dominated by the genera Neoschizodus and Unionites is
likely driven by constantly shifting sediments and a high siliciclastic sedimentation rate.
Increased sea surface temperatures during the Smithian would have provided an
additional challenge for the benthic fauna. Taxonomic groups sensitive to extreme temperatures
include foraminifera and echinoderms, both of which are missing from the majority of Smithian
aged deposits. Bivalves and especially gastropods are more metabolically resilient to extreme
temperature perturbations (Song et al. 2014). Although the deposits studied here do not represent
the extreme temperature acme in the Late Smithian, warming throughout the Campil Member is
possibly reflected in the steady turnover in the benthic fauna from a bivalve and foraminifera
dominated assemblage to a gastropod dominated fauna.
A similarly shallow depositional environment for comparison to the Smithian Campil
Member is represented by the Late Smithian Sinbad Limestone of the Southwest United States
(Pietsch et al. 2014, Chapter 2). This section contains both shallow shoal deposits as well as
offshore, storm wave dominated mixed carbonate-siliciclastic deposits. In contrast with the low
diversity, high dominance deposits of the Smithian Campil Member of the Werfen Formation,
the Sinbad Limestone of the Moenkopi contains a relatively diverse bivalve and gastropod fauna,
dominated by a few genera but containing a variety of ecological guilds including mobile and
224
stationary epifauna and infauna. Differences in lithology and depositional environment most
likely drive the difference in community ecology between the Smithian Campil Member and the
Smithian Sinbad Limestone. However, more detailed analysis on the regional geochemistry
might provide insight into additional climate or variations in ocean chemistry between these two
sections.
The lack of diversification and a reduction in paleoecological complexity during the
Campil Member can be summarized in two steps. First, the abrupt depositional transition limited
colonization of this environment by the predominantly epifaunal benthic fauna which are not
suited to shifting sediments in a storm dominated, sandy environment. A few mobile epifaunal
gastropods and infaunal bivalves therefore became dominant. Before the benthic fauna could
adapt to these new conditions the extreme temperature event in the Late Smithian resulted in a
setback in an ecological set back that lasted until environmental conditions improved in the
Spathian.
The extreme temperature perturbation at the end of the Smithian Stage raised equatorial
sea surface temperatures into the neighborhood of 40°C (Sun et al. 2012, Cui and Kump 2014).
Both the Southwest United States and Italy were in tropical paleolatitudes at this time. This
abrupt temperature rise is thought to be the result of additional volcanic eruptions of the Siberian
Traps. Paleoecological effects of the rapid warming event include a decrease in shallow marine
gastropod body size and the lack of a diverse and abundant echinoderm fauna in the Southwest
United States (Chapter 2, 3). Similar to the Sinbad Limestone, the Campil Member of the Werfen
Formation contains a gastropod fauna that exhibits reduced body size in the Smithian Stage
(Fraiser and Bottjer 2004, Nützel and Schulbert 2005, Nützel 2005). Microgastropods are not
limited to the Smithian Stage and are found throughout the Dienerian Siusi Member and
225
Spathian Val Badia Member suggesting that some environmental or intrinsic control beyond
rapid temperature rise is controlling gastropod body size in the Werfen Formation and in
localities across the globe (Fraiser and Bottjer 2004, Fraiser et al. 2005). Abundant
microgastropods across the globe and throughout the Early Triassic may be responding to a
plethora of environmental changes including repeated stratification and overturn events in the
Tethys Ocean or changes in nutrient availability (Horacek et al. 2007). It is likely that the
extreme temperature excursion at the end-Permian mass extinction boundary drove the initial
miniaturization of the Early Triassic gastropod fauna. The second extreme temperature event in
the Late Smithian resulted in vacated niche space from groups like temperature sensitive
echinoderms (Song et al. 2014). Microgastropods became even more diverse and abundant
during the Smithian thanks to their adaptation to extreme temperatures. Microgastropods
continue to find success throughout the Early Triassic even during intervals of global cooling
because in addition to being heat tolerant, smaller body size also contributes advantages to low
oxygen conditions and low nutrient availability. This adaptation made gastropods an extremely
competitive fauna throughout the Early Triassic.
Based on the literature, the Spathian Val Badia and Cencenighe Members and the Middle
Triassic Anisian, show increased epifaunal tiering as well as increased complexity and diversity
of trace fossils (Twitchett 1999, Pietsch and Bottjer 2014).
The incremental increase in diversity and complexity is comparable to both the Thaynes
Formation and Virgin Limestone of the Moenkopi Formation in the Southwest United States. In
the case of the Thaynes Formation in the Confusion Range, the Early Spathian is dominated by
Eumorphotis in a storm influenced outer shelf environment. It is not until later in the Spathian
that echinoderms, sponges, and an increase in the abundance and evenness of other mollusc
226
genera that the Thaynes Formation begins to develop a complex benthic community. This pattern
of increasing diversity and complexity throughout the 3 million years of the Spathian Stage is
also exhibited in the Virgin Limestone at Lost Cabin Spring. In the Early Spathian, at the base of
the section, benthic community complexity is low with only a few ecological guilds represented.
In addition, low diversity, high abundance, opportunistic bivalves dominate samples in the
Virgin Limestone at Lost Cabin Spring until the latest Spathian when body size increases and a
more diverse and even community emerges (Chapter 2). In contrast with the Werfen and
Thaynes Formations, the Spathian of the Virgin Limestone at Lost Cabin Spring contains a rich
echinoderm fauna throughout the Spathian including abundant crinoids and echinoids in
allochthonous storm beds.
The differences in recovery between the stratigraphic sections in the Southwest United
States and the Werfen Formation in Italy relate information about the pace of recovery in the
Panthalassic Ocean compared to the Tethys Ocean. However, local depositional environments
tend to overwhelm overarching trends in the benthic recovery between the Early Triassic
deposits of the Southwest United States and Italian Dolomites. Based on these analyses, the
recovery between Panthalassa and Tethys appears to be offset and was predominantly controlled
by local environmental conditions except during extreme environmental perturbations.
Conclusions
The Werfen Formation provides a high resolution view of the Early Triassic recovery in
the Tethys Ocean basin for comparison to trends in the Panthalassic Ocean. Directly following
the end-Permian mass extinction, low oxygen conditions in the early Griesbachian allowed for
the take-over of the seafloor by disaster taxa. The Middle Griesbachian was dominated by
227
microbialites and microconchids as well as ‘disaster taxa’ and foraminifera. This trend is echoed
by Griesbachian deposits from across the globe. By the Dienerian, long lived ecological groups
were being established in this locally oxygenated environment. The Siusi Member of the Werfen
Formation appears to have been a refugia of benthic diversity and complexity compared to the
depauperate and purportedly low oxygen environments in other regions of the Tethys Ocean
basin. Some disaster taxa, specifically foraminifera, had their last appearance in the Dienerian.
In the late Dienerian to Smithian, chaotic environmental conditions indicated by volatile carbon
isotopes and a high temperature excursion by the Late Smithian, reset the progress of recovery.
Infaunal mollusks and high temperature resilient gastropods survived this interval of extreme
sedimentation and temperature fluctuations. More environmentally sensitive taxa were restricted
until the Spathian Stage and a cooler, carbonate dominated environment. Across the globe,
Smithian aged sediments in the Sinbad Limestone also reflect gastropod miniaturization and
dominance during a time of extreme sea surface temperatures (Sun et al. 2012). However, the
difference in siliciclastic versus mixed-carbonate siliciclastic sedimentation allowed the Sinbad
Limestone benthic fauna to flourish while diversity and ecological complexity were more limited
in the sandy Smithian Campil Member of the Werfen Formation.
228
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Tables and Figures
Table 1: Sample volume, generic richness, bivalve and gastropod abundance and average size
within a sample, Shannon-Wiener Diversity Index, and ecological guild diversity for the 19
samples considered in the analysis of the Early Triassic Werfen Formation at U'omo.
Table 2: Sample volume, generic richness, bivalve and gastropod abundance and average size
within a sample, Shannon-Wiener Diversity Index, and ecological guild diversity for the 11
samples considered in the analysis of the Early Triassic Werfen Formation at Bulla.
233
Figure 1. Paleogeographic and temporal context for the Italian Werfen Formation. A. The Early
Triassic globe showing the continents in Pangea. A star marks the location of modern day Italy
(Modified from Scotese 2010). The extent of Siberian Trap volcanism is also indicated (Reichow
et al. 2007, Saunders and Reichow 2009). B. Modern day Italy showing the study location of
U'omo in the Italian Dolomites. Box shows the area represented in D. C. Early Triassic time
scale depicts the four major stages that make up the Early Triassic. The nine members of the
Werfen Formation are listed in stratigraphic order. Dashed lines represent approximate
boundaries that are poorly constrained by bivalve biostratigraphy performed at U'omo and
conodont stratigraphy performed at another exposure of the Werfen Formation in a neighboring
valley of the Dolomites region. D. Ancient structure and relative depth of the Early Triassic
basin where deposition of the studied sections occurred (modified from Posenato 2008b). E. A
photo showing the entire section of U'omo from the Mazzin in the hillside to the Val Badia at the
top of the hill. Ski lift supports for scale. The mountains in the background are Middle Triassic in
age.
Figure 2. The biostratigraphy from the Werfen Formation modified and updated from Twitchett
(1999). The Early Triassic timescale depicts four stages spanning five million years. Modeled
after Lehrnmann et al. (2006). The dates for the boundaries are found in the following
publications; end-Permian: Mundil et al. (2004), Shen et al. (2011). Dienerian-Smithian
boundary Galfetti et al. (2007). Smithian-Spathian boundary: Ovtcharova et al. (2006), Galfetti et
al. (2007). End Spathian/Early-Middle Triassic boundary: Ovtcharova et al. (2006), Galfetti et al.
(2007), Brayard et al. (2009). Lithostratigraphy follows Brandner et al. (2009) by considering
deposits that were formerly separated as the Gastropod Oolite as part of the Campil Member.
The microgastropod facies cannot be used to distinguish this member which is truly transitional
234
between the Suisi and Campil members. Bivalve biostratigraphy is based on Broglio-Lorgia et al.
(1990) and Posenato (2008b). Conodonts are from Perri (1991), Perri and Farabegoli (2003), and
Posenato (2008b). Ammonoids are from Broglio-Lorgia (1986).
Figure 3. Carbon isotopes in the Early Triassic. A. δ13C and δ18O cross plot from the
geochemical data from this study. A lack of correlation indicates low values of diagenesis in the
samples used. 3 samples with higher δ18O are likely influenced by meteoric diagenesis but are
relatively rare in this data set. B. Bulk carbonate carbon isotope curve for the Bulla Section from
this study. C. Carbonate carbon isotope curve from the Bulla section and U’omo section from
Horacek et al. 2007. Used here to correlate between the two sections measured in this study. D.
Bulk carbonate carbon isotope curve from the U’omo section measured in this study. E. Carbon
isotope curve modified from Payne et al. 2004 showing the global average of carbon isotope
excursions from around the globe. Tied to biostratigraphy, this curve provides an age context for
the geochemistry of the Italian Werfen Formation.
Figure 4. Key for the Stratigraphic Sections of the Early Triassic Werfen Formation at U'omo in
the Italian Dolomites. Lithological Symbols, Sedimentary Structures, Carbonate Grains, and
Biology are included.
Figure 5. The Stratigraphic Section of the Tesero Oolite, stromatolites and Mazzin Member of
the Werfen Formation at U'omo in the Italian Dolomites. Faunal composition is indicated for
each sample included in community analysis. Scale is in meters. Detailed lithology and
sedimentary structures observed in the field and petrography are included. Horizontal axis reads
Sh/M= Shale/Carbonate mudstone, Si/W=Siltstone/Carbonate Wackestone,
Sa/P=Sandstone/Carbonate Packstone, C/G=Conglomerate/Carbonate Grainstone.
235
Figure 6. The Stratigraphic Section of the Siusi Member of the Werfen Formation at U'omo in
the Italian Dolomites. Faunal composition is indicated for each sample included in community
analysis. Scale is in meters. Detailed lithology and sedimentary structures observed in the field
and petrography are included. Horizontal axis reads Sh/M= Shale/Carbonate mudstone,
Si/W=Siltstone/Carbonate Wackestone, Sa/P=Sandstone/Carbonate Packstone,
C/G=Conglomerate/Carbonate Grainstone.
Figure 7. The Stratigraphic Section for the remainder of the Siusi Member of the Werfen
Formation at U'omo in the Italian Dolomites. Faunal composition is indicated for each sample
included in community analysis. Scale is in meters. Detailed lithology and sedimentary structures
observed in the field and petrography are included. Horizontal axis reads Sh/M=
Shale/Carbonate mudstone, Si/W=Siltstone/Carbonate Wackestone, Sa/P=Sandstone/Carbonate
Packstone, C/G=Conglomerate/Carbonate Grainstone.
Figure 8. The Stratigraphic Section of the Dienerian aged Campil Member of the Werfen
Formation at U'omo in the Italian Dolomites. Faunal composition is indicated for each sample
included in community analysis. Scale is in meters. Detailed lithology and sedimentary structures
observed in the field and petrography are included. Horizontal axis reads Sh/M=
Shale/Carbonate mudstone, Si/W=Siltstone/Carbonate Wackestone, Sa/P=Sandstone/Carbonate
Packstone, C/G=Conglomerate/Carbonate Grainstone.
Figure 9. The Stratigraphic Section for the remainder of the Dienerian aged Campil Member of
the Werfen Formation at U'omo in the Italian Dolomites. Faunal composition is indicated for
each sample included in community analysis. Scale is in meters. Detailed lithology and
sedimentary structures observed in the field and petrography are included. Horizontal axis reads
236
Sh/M= Shale/Carbonate mudstone, Si/W=Siltstone/Carbonate Wackestone,
Sa/P=Sandstone/Carbonate Packstone, C/G=Conglomerate/Carbonate Grainstone.
Figure 10. The Stratigraphic Section of the Smithian aged Campil Member of the Werfen
Formation at U'omo in the Italian Dolomites. Faunal composition is indicated for each sample
included in community analysis. Scale is in meters. Detailed lithology and sedimentary structures
observed in the field and petrography are included. Horizontal axis reads Sh/M=
Shale/Carbonate mudstone, Si/W=Siltstone/Carbonate Wackestone, Sa/P=Sandstone/Carbonate
Packstone, C/G=Conglomerate/Carbonate Grainstone.
Figure 11. The Stratigraphic Section for the remainder of the Smithian aged Campil Member of
the Werfen Formation at U'omo in the Italian Dolomites. Faunal composition is indicated for
each sample included in community analysis. Scale is in meters. Detailed lithology and
sedimentary structures observed in the field and petrography are included. Horizontal axis reads
Sh/M= Shale/Carbonate mudstone, Si/W=Siltstone/Carbonate Wackestone,
Sa/P=Sandstone/Carbonate Packstone, C/G=Conglomerate/Carbonate Grainstone.
Figure 12. The Stratigraphic Section of the Griesbachian aged Mazzin Member of the Werfen
Formation at Bulla in the Italian Dolomites. Faunal composition is indicated for each sample
included in community analysis. Scale is in meters. Detailed lithology and sedimentary structures
observed in the field and petrography are included. Horizontal axis reads Sh/M=
Shale/Carbonate mudstone, Si/W=Siltstone/Carbonate Wackestone, Sa/P=Sandstone/Carbonate
Packstone, C/G=Conglomerate/Carbonate Grainstone.
Figure 13. The Stratigraphic Section for the remainder of the Griesbachian aged Mazzin Member
of the Werfen Formation at Bulla in the Italian Dolomites. Faunal composition is indicated for
237
each sample included in community analysis. Scale is in meters. Detailed lithology and
sedimentary structures observed in the field and petrography are included. Horizontal axis reads
Sh/M= Shale/Carbonate mudstone, Si/W=Siltstone/Carbonate Wackestone,
Sa/P=Sandstone/Carbonate Packstone, C/G=Conglomerate/Carbonate Grainstone.
Figure 14. The Stratigraphic Section of the Siusi Member of the Werfen Formation at Bulla in
the Italian Dolomites. Faunal composition is indicated for each sample included in community
analysis. Scale is in meters. Detailed lithology and sedimentary structures observed in the field
and petrography are included. Horizontal axis reads Sh/M= Shale/Carbonate mudstone,
Si/W=Siltstone/Carbonate Wackestone, Sa/P=Sandstone/Carbonate Packstone,
C/G=Conglomerate/Carbonate Grainstone.
Figure 15. The Stratigraphic Section for the remainder of the Siusi Member of the Werfen
Formation at Bulla in the Italian Dolomites. Faunal composition is indicated for each sample
included in community analysis. Scale is in meters. Detailed lithology and sedimentary structures
observed in the field and petrography are included. Horizontal axis reads Sh/M=
Shale/Carbonate mudstone, Si/W=Siltstone/Carbonate Wackestone, Sa/P=Sandstone/Carbonate
Packstone, C/G=Conglomerate/Carbonate Grainstone.
Figure 16. The Stratigraphic Section of the Campil Member of the Werfen Formation at Bulla in
the Italian Dolomites. Faunal composition is indicated for each sample included in community
analysis. Scale is in meters. Detailed lithology and sedimentary structures observed in the field
and petrography are included. Horizontal axis reads Sh/M= Shale/Carbonate mudstone,
Si/W=Siltstone/Carbonate Wackestone, Sa/P=Sandstone/Carbonate Packstone,
C/G=Conglomerate/Carbonate Grainstone.
238
Figure 17. Thin Sections of the Mazzin Member of the Werfen Formation at the Uomo Section.
A. Peloidal texture and possible mollusc fragments with spar (Meter 2). B. Micrite rich deposits
separate ooid layers. Micrite deposits contain oxidized framboids that may have been pyrite rich
in the past (Meter 3-5). C. Microconchids in micrite (Meter 9). D. Microbial laminations and
clotted texture, ooids bound among micrite layers (Meter 9). E. Stromatolite mounds in outcrop
corresponding to microbial laminations in D (Meter 9). F. Nodular wackestone containing
abundant ostracodes (Meter 13).
Figure 18. Thin section and field images of the Siusi Member of the Werfen Formation at the
Uomo Section. A. Ooid dominated sample containing bivalves showing shelter porosity. Possible
microbial fabric supporting the ooid grains (Meter 31.5). B. Abundant foraminifera are found in
TS1 (Meter 31.5). C. Outcrop view of ooid grainstone (Meter 31.5). D. Large Unionites and
Claraia (Meter 36). E. Mixed carbonate-siliciclastic interbeds in outcrop (Meter 41). F.
Gastropod containing foraminifera (Meter 44). G. Replaced high-spired gastropod and
echinoderm stereom (unit extinction under polarized light) (Meter 48).
Figure 19. Additional thin section and field images of the Siusi Member of the Werfen
Formation at the Uomo Section. A. Intraclastic conglomerate (Meter 56). B. Bioclastic packstone
containing bivalves and gastropods (Meter 61). C. Gastropod rich carbonate packstone
interbedded with fissile silts (Meter 65). D. Horizontal bioturbation (Meter 80).
Figure 20. Field photos and petrographic images of the Campil Member in the Dienerian Stage at
the U’omo Section. A. Gutter casts from the base of the section represent shallowing into the
siliciclastic dominated system (Meter 84). B. Bivalve bedding plane (Meter 92). C. Bedding
plane bioturbation right below sample 1. D. Thin section 1 shows radial ooids from a shallow,
239
high energy environment. E. Microgastropods, genus Pseudomurchsonia. F. Diplocraterion
found throughout the Campil Member. G. Ripples and mudcracks together indicate a extremely
shallow, subaerially exposed environment (Meter 113). H. Interbedded carbonates and sands
(Meter 176).
Figure 21. Thin section and field images of the Smithian Campil Member of the Werfen
Formation at the Uomo Section. A. Interbedded cross bedded fine sands and carbonate
wackestones (Meter 176). B. Rip up clasts in thin section (Meter 190). C. Wrinkle marks or
Kinneaya (Meter 202). D. Potential water escape structures (Meter 206). E. Microgastropod rich
packstone (Meter 191). F. Microconchid identified by the laminar structure of the tube walls
(Meter 202). G. Echinoderm sterom (Meter 206).
Figure 22. Field images of the Mazzin Member of the Werfen Formation at the Bulla Section. A.
Clotted, laminated microbial texture from the base of the Mazzin Member (Meter 1). B.
Horizontal bioturbation at Sample 1 (Meter 15).
Figure 23. Thin Sections of the Mazzin Member of the Werfen Formation at the Bulla Section.
A. Clotted microbial texture from Thin Section 1 includes dolomite rhombs, peloids, and
possibly micritized ooids (Meter 5). B. Sample 3 is dominated by foraminifera in thin section.
Figure 24. Thin Sections of the Siusi Member of the Werfen Formation at the Bulla Section. A.
Microbial layers with siliciclastic grains and ooids bound in clotted micrite layers (Meter 1). B.
Microconchid from meter 39. C. Pollen grain (Meter 55).
Figure 25. Field images of the Siusi and Campil Members of the Werfen Formation at the Bulla
Section. A. Clotted texture of mudstone (Meter 2). B. Pink ooids indicating increased
depositional energy, approaching the shoal (Meters 20-21). C. Microgastropods (<1cm) (Meter
240
41). D. Interbedded muds and sands of the base of the Campil Member. Coarse sands are rich in
Diplocraterion. A channel in the sands toward the top of the image indicates a shallow water
environment (Meter 62-63). E. Kinneaya structure in fine grained siliciclastics, interpreted as
microbial in origin (Meter 63-64).
Figure 26. Generic diversity, abundance, and size frequency of Sample 1 from the Griesbachian
Mazzin Member at U'omo and Samples 1 to 5 from the Dienerian Siusi Member. Each color
represents one mollusc or brachiopod genus as indicated for the corresponding key for each
graph. The x-axis represents size in mm, and the height of each bar represents the count of
bivalves or gastropods of a given genus that represent that size class.
Figure 27. Generic diversity, abundance, and size frequency of Sample 1 to 8 from the Dienerian
Stage Campil Member at U'omo. Each color represents one mollusc or brachiopod genus as
indicated for the corresponding key for each graph. The x-axis represents size in mm, and the
height of each bar represents the count of bivalves or gastropods of a given genus that represent
that size class. Note the dominance of the bivalve Unionites in many of the samples followed by
the gastropods Coelostylina and Pseudomurchsonia.
Figure 28. Generic diversity, abundance, and size frequency of Sample 1 to 5 from the Smithian
Campil Member at U'omo. Each color represents one mollusc or brachiopod genus as indicated
for the corresponding key for each graph. The x-axis represents size in mm, and the height of
each bar represents the count of bivalves or gastropods of a given genus that represent that size
class. Diversity increases from the Dienieran Stage Campil Member and includes a variety of
mollusc guilds.
241
Figure 29. Generic diversity, abundance, and size frequency of Sample 1 to 3 from the
Griesbachian Mazzin Member at Bulla. Each color represents one mollusc or brachiopod genus
as indicated for the corresponding key for each graph. The x-axis represents size in mm, and the
height of each bar represents the count of bivalves or gastropods of a given genus that represent
that size class.
Figure 30. Generic diversity, abundance, and size frequency of Sample 1 to 8 from the Dienerian
Siusi Member at Bulla. Each color represents one mollusc or brachiopod genus as indicated for
the corresponding key for each graph. The x-axis represents size in mm, and the height of each
bar represents the count of bivalves or gastropods of a given genus that represent that size class.
Figure 31. Environmental Sensitivity of benthic macro and microfauna observed in the Early
Triassic Werfen Formation adopted from Song et al. 2014. Foraminifera are tolerant of
extremely low oxygen conditions, down to 3% but are not tolerant of temperatures above 35°C.
Ostracods in contrast as not tolerant of low oxygen conditions below 20% but can withstand
extreme water temperatures approaching 55°C. These environmental tolerances can be used to
interpret Early Triassic environmental conditions based on the presence or absence and
combinations of these benthic organisms.
242
Table 1.
Member Sample
Number
Sample
Volume
Genus
Richness
Bivalve
Abundance
Bivalve
Size
Gastropod
Abundance
Gastropod
Size
Shannon -Wiener
Diversity Index
Guild
Diversity
Mazzin 1 2L 3 11 8.3 0 n/a 0.83 3
Siusi 1 2L 13 106 8.2 1 2.0 2.08 4
Siusi 2 2L 14 89 9.5 86 4.2 2.18 6
Siusi 3 2L 9 51 14.5 0 n/a 1.31 3
Siusi 4 2L 9 68 12.7 5 7.2 1.43 6
Siusi 5 2L 9 33 9.5 61 4.3 1.19 6
Campil-Dienerian 1 2L 10 57 7.2 16 4.8 1.69 4
Campil-Dienerian 2 2L 3 11 9.6 30 3.2 0.71 3
Campil-Dienerian 3 2L 2 40 20.2 0 n/a 0.46 1
Campil-Dienerian 4 2L 5 57 13.1 0 n/a 1.23 3
Campil-Dienerian 5 2L 4 19 26.2 0 n/a 0.41 3
Campil-Dienerian 6 1L 3 21 16.3 0 n/a 0.50 2
Campil-Dienerian 7 2L 8 33 4.4 2 2.5 1.79 4
Campil-Dienerian 8 2L 4 9 8.9 1 5.5 1.17 3
Campil-Smithian 1 4L 8 52 9.7 1 3.3 1.40 4
Campil-Smithian 2 4L 12 43 9.2 3 5.0 2.06 5
Campil-Smithian 3 2L 9 57 10.1 11 3.3 1.86 4
Campil-Smithian 4 1L 6 14 7.1 0 n/a 1.57 3
Campil-Smithian 5 1L 5 7 4.3 2 3.4 1.53 4
243
Table 2.
Member Sample
Number
Sample
Volume
Genus
Richness
Bivalve
Abundance
Bivalve
Size
Gastropod
Abundance
Gastropod
Size
Shannon -Wiener
Diversity Index
Guild
Diversity
Mazzin 1 4L 11 88 10.1 14 2.5 1.82 6
Mazzin 2 0.5L 11 68 5.5 0 n/a 1.45 6
Mazzin 3 4L 8 41 8.4 0 n/a 1.52 4
Siusi 1 4L 5 19 11.6 3 4.0 1.25 3
Siusi 2 2L 9 29 10.9 3 2.7 1.88 5
Siusi 3 1L 6 23 9.7 0 n/a 1.54 3
Siusi 4 6L 17 220 10.7 87 3.5 2.4 6
Siusi 5 4L 6 16 11.4 11 3.2 1.56 4
Siusi 6 2L 5 25 5.6 3 2.7 1.12 2
Siusi 7 1L 8 15 6.8 12 3.6 1.70 6
Siusi 8 4L 6 18 9.6 0 n/a 1.43 4
244
Tethys
Ocean
Panthalassic
Ocean
Boreal Ocean
Griesbachian
Dienerian
Smithian
Spathian
252.2
247.2
Early Triassic
Induan Olenekian
C. Werfen Formation
Mazzin
Andras
Suisi
Campil
Val Badia
Cencenighe
San Lucano
Tesero
Venice
Bolzano
Switzerland
Austria
Italy
200km
Onshore Basin Onshore
U’omo
D.
A. B.
E.
Siberian
Traps
Figure 1
245
E.
mulitformis
Cencenighe
Member
San Lucano
Member
C. clarai
C. aurita
Tesero Oolite
Mazzin Member
Andraz Member
Suisi Member
Campil Member
Val Badia
Member
Lingularia
C. wangi-
griesbachi
Claraia Eumorphotis
E.
hinnitidea
E. telleri
E. kittli
Costatoria
costata
anceps
obliqua
aequabilis
isarcica
parvus
triang-
ularis
Gries. Dienerian Smithian Spathian
Bivalves
Cono-
donts
Litho-
stratigraphic
Members
Tirolites
cassianus
Dinarites
dalmatinus
Ammonoids
250.55
251.22
247.2
252.2
praeparvus
staeschei
lobata
Permian
Stages
Gastropod Oolite
Figure 2
246
δ13C carb (‰)
-2 0 2 4 6 8
δ13C carb (‰)
-2 0 2 4 6
δ13C carb (‰)
-2 0 2
δ13C carb (‰)
-2 0 2 4 6 -4
-2 0 2 4 6 -4
δ13C carb (‰)
Mazzin Siusi
50
100
150
200
250
40
100
A.
B.
C.
D.
R² = 0.075
-9.000
-8.000
-7.000
-6.000
-5.000
-4.000
-3.000
-2.000
-1.000
0.000
-4.000 -2.000 0.000 2.000 4.000 6.000 8.000
δ18O
δ13C
E.
Siusi Mazzin Campil (Dienerian) Campil (Smithian)
Gries Dienerian Smithian Spathian
Figure 3
247
Th/U
Thorium to
Uranium Ratio
Pyrite Framboid
Chemistry
Congolomerate
Mixed Carbonate/
Siliciclastic
Lithology
Sandstone
Limestone
Dolomite
Marl
Siltstone
Shale
Ooid Limestone
Bioturbation
Claraia
Gastropod
Stromatolite
Diplocraterion
Rhizocorallium
Biology
Thalassinoides
Skolithos
Planolites
Ripples
Cross Bedding
Planar Laminations
Ooids
Herringbone
Mud Cracks
Sedimentary Structures
Bioclastic Limestone/
Packstone
Chert
Bivalve
Brachiopod
Crinoid
Muddy carbonate
Fossiliferous Rock/
Wackestone
Echinoid
C Org
High Organic
Carbon Content
ii #
Ichnofabric Index
Master Key
Conodont
Echinoderm
Stereom
Carbonate Grains
Intraclasts
Peloids
Laminated Intraclasts
Fossil Hash
TS 2 6
Thin Section Number Sample Number
Microconchid
Foraminifera
Figure 4
248
15 m
cover
13
m
10
m
Sh/M Si/W Sn/P C/G
1
5
m
0
m
Sh/M Si/W Sn/P C/G
TS 1
TS 2
TS 3-5
Figure 5
249
60
m
55
m
Sh/M Si/W Sn/P C/G
TS 4
TS 5
6
40
m
35
m
30
m
Sh/M Si/W Sn/P C/G
TS 1
45
m
50
m
ii 3
Sh/M Si/W Sn/P C/G
TS 2
TS 3
Figure 6
250
65
m
70
m
Sh/M Si/W Sn/P C/G
2
75
m
80
m
3
TS 6
TS 7
4
Sh/M Si/W Sn/P C/G
Figure 7
251
85
m
90
m
Sh/M Si/W Sn/P C/G
1
95
m
100
m
105
m
2
Sh/M Si/W Sn/P C/G
109
m
118
m
115
m
113
m
35 meters of
cover
3
4
5
6
Sh/M Si/W Sn/P C/G
Figure 8
252
153
m
164
m
155
m
165
m
7
8
Sh/M Si/W Sn/P C/G
Figure 9
253
175
m
170
m
Sh/M Si/W Sn/P C/G
1
180
m
185
m
2
TS 1
Sh/M Si/W Sn/P C/G
190
m
195
m
200
m
3
TS 2
TS 3
TS 4
Sh/M Si/W Sn/P C/G
Figure 10
254
205
m
4
5
TS 5
Sh/M Si/W Sn/P C/G
210
m
215
m
Sh/M Si/W Sn/P C/G
220
m
Sh/M Si/W Sn/P C/G
Figure 11
255
25
m
30
m
Sh/M Si/W Sn/P C/G Sh/M Si/W Sn/P C/G
0
m
5
m
10
m
PTB
TS 1
15
m
20
m
Sh/M Si/W Sn/P C/G
11
m
1
Figure 12
256
Sh/M Si/W Sn/P C/G
35
m
2
40
m
Andraz Member
Sh/M Si/W Sn/P C/G
3
Figure 13
257
Sh/M Si/W Sn/P C/G
0
m
5
m
10
m
TS 1
11
m
15
m
20
m
Sh/M Si/W Sn/P C/G
1
25
m
30
m
21
m
Sh/M Si/W Sn/P C/G
2
3
Figure 14
258
Sh/M Si/W Sn/P C/G
TS 2
35
m
40
m
31
m
4
45
m
Sh/M Si/W Sn/P C/G
5 6 7
Sh/M Si/W Sn/P C/G
55
m
8
50
m
58
m
Figure 15
259
60
m
TS 1
65
m
Sh/M Si/W Sn/P C/G
70
m
Sh/M Si/W Sn/P C/G
Figure 16
260
500µm
A.
50µm
B.
200µm
C.
D.
5mm
5mm
F.
1 cm
E.
Figure 17
261
10 cm
1 cm
A.
B.
F.
200µm
1 cm
C.
D.
1 cm
E.
200µm
1 mm
G.
Figure 18
262
1 cm
A.
2mm
B.
C.
1 cm
D.
Figure 19
263
5 cm
A.
1 cm
B.
1 cm
C.
1 mm
D.
5 mm
E.
5 cm
2 cm
5 cm
F.
G.
H.
Figure 20
264
1 cm
A.
5 mm
B.
1 cm
1 cm
500µm
200µm
200µm
C.
E.
D.
F
G
Figure 21
265
1 cm
A.
1 cm
B.
Figure 22
266
1 mm
A.
1 mm
B.
Figure 23
267
5 mm
A.
100µm
B.
C.
100µm
Figure 24
268
1 cm
A.
1 cm
B.
1 cm
C.
D.
1 cm
E.
Figure 25
269
Griesbachian Mazzin
Dienerian Siusi
Sample 1 Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
Dienerian Siusi
Figure 26
270
Sample 1
Sample 2
Sample 3 Sample 4
Sample 5 Sample 6
Sample 7
Sample 8
Figure 27
271
Sample 1 Sample 2
Sample 3
Sample 4
Sample 5
Figure 28
272
Sample 1
Sample 2
Sample 3
Figure 29
273
Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
Sample 6
Sample 7 Sample 8
Figure 30
274
Oxygen Saturation 20% 3%
Temperature
55°C
35°C
20°C
Figure 31
275
Appendix: U'omo section sample names and geochemistry results
for carbon and oxygen isotopes used in Figure 3
Meters
Sample 1st
Visit d13C carb Replicates
Sample
2nd Visit d13C carb
Third Run duplicates
and new samples
219 002 -0.649424089
218 003
217.5 U5
217 004
213 005 -0.804693408
211 006 -0.000331896
209 007
207 008
206 U18 -0.124713709 -0.333
205.6 009
205 U19 -0.172376539 -0.272278289
204
202.5 U22 -0.29366362 010
200 U24, THU24b 011 1.973089392
199 012
198 013 2.122071164
194 U31 014 2.575803598
193 015
192 THU33 016 3.151506347
190 THU35 017
189.5 U36
189 018 3.051778272
188 019 3.230158007
187 020 3.194810001
186 021 3.782787306
184 022
183 U42
182 023
181 024
180 025 4.725279813
175 026 4.546484389
173 U52 027 5.498575648
170 028 5.023975952
169 030 5.116164511
167 U58 031 5.625932266 5.623
166 032 5.698976668
164 033 4.587450729
155 U70 4.134637644 034 4.052452952 3.724
153 035 4.047798832
118 036 2.507164202
117 037 1.83444091
276
116.5 U109? 038
116 U110 1.863078513 039 2.295095987
Meters
Sample 1st
Visit d13C carb Replicates
Sample
2nd Visit d13C carb
Third Run duplicates
and new samples
115 040 2.687061766
114 041
113 U113
112 042 1.755871384
108 043
106.5 U119A
106 U119B 1.821636156 044
101 U125 045
100 046
95 U132 -0.166953753 047 0.178
94.5 048 1.688499484
94 049
93.5 THU133 050
92 051
91 052
90.5 U136 0.889091129 053 2.858044729 0.713
90 054
89 055
88 U139 056 2.450
87 057 2.747048345
86.5 058 2.935653712
86 059 0.704454541
85 060 1.242162904
84 061 0.410976831
83.5 U144 2.483236261 062 0.547824014 0.365
82.5 063 -0.145412415
82 064
81 U147 1.868247048 1.688
78.5 065 0.192757901
78 066 0.158473417
77 U151 067 0.25519028 1.935
76 068 2.081335908
75 069 0.951443681
74.5 070
74 071
73.5 072
72.5 073 2.63756262
72 U155 1.635796191 074 1.193
71.5 075 1.20707083
71 076
70 077
69 078
277
68.5 U159
68 079 1.864749846
Meters
Sample 1st
Visit d13C carb Replicates
Sample
2nd Visit d13C carb
Third Run duplicates
and new samples
66 080 2.153322905
65 081
64.5 082
64 U164 083 1.785434314
63 084 2.031089494
62.5 085
61 U167 1.356892713
60.5 086 1.523369475
60 087 1.451094936
58 THU 170 088 1.357004885 1.200
56.5 TH U172 -2.103571425 -2.067747737 089 1.574738228 0.678
55.5 090 0.542998464
54.5 091 0.925114974
53.5 092
52.5 093
51.5 094 1.385763737
50.5 095
49.5 096 1.419223877
49 U179 -2.225426223 097 1.102984584
48 098
46.5 099 1.554813294
46 099.5
45.5 100 1.331663444
45 101 1.397959962
44 THU183 102 1.78320047 1.048
43 103
42 104 1.515930711
41 105 1.201254105
40 106 1.217732644
38 107 1.573285989
36.5 108 1.483508964
35 109 1.221459414
32.5 111 1.088425303
31 THU 195 112 1.334765029
30 113 1.550684541
29 114 1.463907917
13 U1stW -0.989054168 115 1.030176642 -1.425
11 116 1.1458099
9.5 117 1.046283918 -1.655
8 118 1.200826865
6 119 1.172540156
4.5 120 -1.208015415
278
4 UMO 0.471484057 0.311
3 121 -0.624859419
279
Appendix: Bulla section sample names and geochemistry results for
carbon and oxygen isotopes used in Figure 3
Sample Name d13C carb d18O Meter
6-25-15 BCTop -0.927 -7.260 57
6-24-15 BS19-1 1.097 -6.800 57
6-24-15 BS19-2 1.110 -6.414 57
6-24-15 BS21 0.560 -7.141 57
6-24-15 BS37 0.823 -7.061 57
6-24-15 BS47 1.489 -6.745 57
6-24-15 BS51 0.746 -8.111 57
6-25-15 BS53 1.302 -6.904 57
6-25-15 BS57 0.526 -6.366 57
6-25-15 BS61 1.038 -3.005 57
6-25-15 BM1 0.720 -6.512 41
6-25-15 BM3 0.843 -6.719 39.5
6-25-15 BM4 -1.085 -7.338 38
6-25-15 BM9 -0.581 -5.781 33.5
6-25-15 BM20 -1.991 -6.891 23
6-25-15 BM27 -2.241 -5.984 15
280
Appendix: U'omo Section. Summary of diversity and abundance data before equal valve division of bivalves.
Summary of thin section analysis where color indicates the presence of a given micro or macrofauna in a sample
Sample Member/Ti Diver Abu Pyrite Iron Bivalve Gastro Echino Tubew Foram Ostraco Microbial SIze
M15
M5
M177
M164
M133
M125 Medium
M109
M105
M18 Smith/Camp Small
M19 Smith/Camp 5 9
M22 Smith/Camp 6 14 Small
M24B Smith/Camp
M31 Smith/Camp 9 73 Large
M33 Smith/Camp
M35 Smith/Camp
M36 Smith/Camp
M42 Smith/Camp 12 50 Large
M52 Smith/Camp 8 59 Large
M58 Dien/Cam 4 10 Medium
M70 Dien/Cam 8 51 Coat grainsMedium
M109 Dien/Cam 3 6 Medium
M110 Dien/Cam 3 28 Slabs
M113 Dien/Cam 4 21 Medium
M119A Dien/Cam 5 59 Medium
M119B Dien/Cam 2 61 Medium
M132 Dien/Cam 3 45 ooids Medium
M136 Dien/Cam Medium
M139 Dien/Cam Medium
M062 Dien/Cam
M144 Dien/Cam 10 107 Medium
M147B Dien/Suis Medium
M147A Dien/Suis 9 105 Medium
M065 Dien/Suis
M149 Dien/Suis Slabs
M151 Dien/Suis 9 77 Mediun?
M155 Dien/Suis 14 226 Medium
M167 Dien/Suis 13 173 Medium
M089 Dien/Suis
M170 Dien/Suis
M179 Dien/Suis
M097 Dien/Suis
M183 Dien/Suis
M195 Dien/Suis ooids
1st Gries/Maz 3 27 Medium
US2 Gries/Maz
US1 Gries/Maz ooids, clotted
UOOS Gries/Maz ooids and Strom
UTO1 Gries/Maz
UMO Gries/Maz
281
Appendix: Bulla Section. Summary of diversity and abundance data before equal valve division of bivalves.
Summary of thin section analysis where color indicates the presence of a given micro or macrofauna in a sample
Sampl Member Di Abu Pyrite Iron Bivalv Gastro Echino Tubew Foram Ostr Microbial/O Size
M57 Suisi Small Bulk
M1 Suisi Thin
M4 Mazzin Small (1/2
M10 Mazzin Slabs
M6 Suisi Very small
M13 Suisi Very small
M19 Suisi Slab
M1 Suisi Large Bulk
M5 Suisi 6 18 Large Bulk
M19-1 Suisi 8 29 Small Bulk
M19-2 Suisi 5 44 Medium
M19-4 Suisi 6 27 Large Bulk
M21 Suisi 17 447 Very Large
M29 Suisi ThinSectio
M31 Suisi 6 24 Small bulk
M37 Suisi 9 33 Medium
M43 Suisi 5 22 Large Bulk
M51 Suisi 3 7 Medium
M61 Suisi ? ? Microbial Handsampl
M1 Mazzin 8 45 Microbial? Large Bulk
M3 Mazzin 11 84 Very small
M6 Mazzin Pellets/Thrombolitic ThinSectio
M27 Mazzin 11 155 Bulk
282
Chapter 5: Rapid and resilient pelagic ecological development interpreted from the shell
shapes of Early Triassic Boreal Ocean ammonoid species
Introduction
The end-Permian mass extinction was the most severe taxonomic and ecological
extinction event in the history of life on Earth (McGhee et al. 2012). The benthic macrofauna
were devastated by this extinction and exhibited varied trends in the path to recovery that varied
based on ocean basin, depositional environment, and additional extinction perturbations. The
pelagic fauna including marine invertebrates and vertebrates were also affected by this event and
exhibited loss of taxonomic diversity and changes in trophic structure (Brayard et al. 2009,
Stanley et al. 2009, Chen and Benton 2012, Scheyer et al. 2014). The major extinction
mechanisms at the extinction boundary including ocean acidification and extreme temperature
would have had an acute affect on animals living in the water column (Clapham and Payne 2011,
Hinojosa et al. 2012, but see Kershaw et al. 2012, Sun et al. 2012, Romano et al. 2013).
Following the initial extinction event, persistent expanded oxygen minimum zones, intense
temperature increases, and sudden incursions of shallow shelf anoxia are interpreted to have
lasted during the ensuing recovery interval which is divided into the four stages of the Early
Triassic epoch.
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Classically, the progression of the recovery of the benthic and pelagic fauna were
considered to be decoupled. The benthic recovery was thought to have been a slow process
taking up to five million years, until the Middle Triassic, for pre-extinction levels of taxonomic
diversity and complex ecological systems to be re-established (Schubert and Bottjer 1995, Knoll
et al. 2007, Chen and Benton 2012). One prediction for the re-diversification of trophic systems
following a mass extinction event suggests that taxonomic diversity will increase to fill
sequentially higher, more complex trophic levels (Sole et al. 2012). The pelagic taxa, specifically
ammonoids and conodonts, showed a rapid resurgence of taxonomic diversity closely following
the Permian-Triassic boundary, within 1-1.5 million years (Brayard et al. 2009, Stanley 2009,
Whiteside and Ward 2011). Ammonoids and conodonts then experienced additional radiations
and crashes in taxonomic diversity during the rest of the recovery interval, finally stabilizing in
the Middle Triassic (Brayard et al. 2009, Whiteside and Ward 2011). These trends in diversity
have been shown for different data sets and across the globe and in some cases were tied to the
carbon isotope record of the Early Triassic but an ecological consequence to these rapid changes
had not been thoroughly explored. Recently, work on the benthic macrofauna has found evidence
for rapid recovery events within the first million years of the end-Permian environmental
perturbations (Chapter 1-4, Twitchett et al. 2004, Kershaw et al. 2007, Crasquin et al. 2007,
Beatty et al. 2008, Brayard et al. 2011, Payne et al. 2011). Two studies of pelagic ammonoid
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diversity and inferred paleoecology were undertaken in order to complement the paleoecological
analysis of the benthic macrofauna. One focuses on the pelagic environment of the Boreal Ocean
and the second tests for a latitudinal gradient for shell morphology and inferred paleoecology.
Ammonoids were a key member of the Paleozoic and Mesozoic trophic web. They acted
as both prey to vertebrates and even shell crushing invertebrates and as predators to other
ammonoids, isopods, and other invertebrates (Kruta 2011, and Keupp 2012). Based on their jaws
and what is known about their musculature and digestive tracks, their closest extant relatives are
thought to be the modern coleoid cephalopods including octopus, squid, and cuttlefish (Kroger
2011) although this diversification event likely occurred in the Devonian (Jacobs and Landman
2007 and references therein). Modern squid show a wide range of metabolic activity,
independent of size, between fast moving, jet propelled ammonoids, and slow swimming squids
(Seibel 2007). This variation of metabolism and oxygen consumption across the range of modern
coleoid mobility groups can be applied to extinct ammonoids. Hydrodynamic modeling efforts
show that evolute shells, where each successive whorl of the ammonoid coils like a snake, and
inflated, wide shells experienced more turbulence and drag forces compared to streamlined,
narrow, involute shells, where the final, body whorl, wraps around each successive coil (Jacobs
1996). Fossil analysis find evidence for jet propulsion by extinct ammonoids including muscle
and mantle attachment scars and neutrally bouyant shells (Doguzhaeva and Mutvei 1996,
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Hammer and Bucher 2006, Klug et al. 2007). In addition, other studies have examined the affects
of external shell morphology including orientation and ornamentation on the efficiency of
ammonoid swimming (Ward 1980, Saunders 1986, Hammer and Bucher 2006). Jet propulsion,
even by modern squid, is an inefficient means of transportation and can require twice as much
energy as that exhibited by fish (ODor 1991, Seibel 2007). Measurements of ammonoid shell
shape and modeled directed swimming ability can then be used to make inferences about the
metabolic demands exhibited by these ancient, extinct animals.
In order to study the hypothetical swimming ability and metabolism of extinct
ammonoids a new quantitative morphospace, Westermann Morphospace, was used to analyze
ammonoid shell shape (Ritterbush and Bottjer 2012). Previous analysis of ammonoid shell shape
included bivariate and multivariate approaches, usually a principal components analysis
(Dommergues 2001, McGowan 2004, Yacobucci 2004, Klug and Korn 2004, Lug 2005,
Saunders 2008, Monnet 2008, Dera 2010, Whiteside and Ward 2011, Korn 2012, and Brosee et
al. 2013). These types of ordinations and analysis are dependent upon the data included while
Westermann Morphospace is a fixed frame analysis which allows the user to plot ammonoid
morphometrics and compare shell morphology and hypothetical swimming ability between any
data set regardless of shape occupation and sample size. Ammonoids that exhibit the most
extreme variation of occupied morphology plot at each corner of the ternary diagram; involute
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oxycones, evolute serpenticones, and inflated sphaerocones (Figure 2) (Westermann 1996,
Ritterbush and Bottjer 2012). The boundaries between each shape category are delineated by
Westermann's (1996) original hypotheses of ammonoid swimming ability based on facies and
morphological data. Further work on ammonoid shell morphology and hydrodynamics will
refine these distinctions.
Westermann Morphospace was used to study Early Triassic ammonoid ecology in the
Boreal Ocean as well as across a latitudinal gradient during the Smithian Stage. The Boreal
Ocean was the most proximal basin to the eruptions of the Siberian Traps and at the same time
exhibits some of the most rapid recovery trends. Trace fossil diversity and complexity increased
rapidly in the Griesbachian and Dienerian of British Columbia and Spitsbergen and sensitive
taxa such as bryozoans showed an early resurgence here causing the Boreal Ocean to often be
labeled a refugia following the extinction event (Wignall et al. 1998, Beatty et al. 2008, Baud et
al. 2008, Zonneveld et al. 2010). However, the pelagic record in the Boreal Ocean shows
taxonomic "booms and busts" (Stanley et al 2009, Whiteside and Ward 2011). The Boreal Ocean
record which spans the entire Early Triassic can be tested to answer two major questions about
the ecology of ammonoids, and more broadly, the recovery of the pelagic system as a whole. The
first is whether the pelagic recovery followed the Sole et al. (2012) model which would suggest
that the ammonoid ecological recovery would progress from initial colonization by shell shapes
287
that exhibit low metabolic demand to increased metabolic demand, represented by directed
swimming, involute shell shapes over time. Second, the additional environmental perturbations
that devastated benthic diversity and re-set their ecological recovery exhibited deleterious affects
on ammonoid taxonomic diversity. Westermann Morphospace was used to decipher patterns of
ammonoid morphospace occupation to determine what ecological effect extreme temperature
and low oxygen incursions had on shell shape occupation and inferred metabolism.
Ammonoid morphological occupation in temperate and tropical locations was compared
to trends in Boreal ammonoid shape occupation. Previous work on ammonoids from the poles to
the tropics has shown evidence for rapid diversification, increased endemism, differences in
disparity vs. diversity, and latitudinal stratification (Brayard et al. 2006, Brayard et al. 2007,
Bruhwiler et al. 2008, Brayard et al. 2009, Zakharov 2011, Ware et al. 2011, Brayard et al. 2013,
and Brosse et al. 2013). If the global latitudinal gradient observed in ammonoid taxonomic
assemblage similarity had an ecological consequence, then the relative distribution of shell shape
categories that allowed ammonoid success should have differed by region along the same
latitudinal gradient.
Changes in ammonoid shell shape through time and space will be interpreted through the
lens of Early Triassic environmental perturbations including extreme temperature changes, low
oxygen regions, and anoxic intervals. Changes in ammonoid morphology and inferred swimming
288
ability and metabolic expenditures and trophic ecology will then be compared to the patterns of
benthic ecological recovery observed from the Italian Werfen Formation and the Smithian and
Spathian Stages of the Southwest United States
Materials
Ammonoid morphological data was gathered from publications and monographs
available online and at the University of Southern California Rare Books collection (Table 1). In
addition, Tozer (1994) described and measured all Canadian Triassic ammonoid species at high
stratigraphic resolution. Chronometric dates allow for global correlation at the 100,000 year
timescale to other regions and geochemical proxies for the Early Triassic. In the Spring of 2012,
Kathleen Ritterbush and I gathered morphological data from his prepared specimens sorted by
species and locality which was discussed but not figured in his final volume and are housed at
the Canadian Geological Survey in Vancouver, British Columbia.
Tozer's type collection was made in a few localities found on present day Axel Hieberg
Island in Arctic Circle and the Greyling and Toad formations from the Canadian Cordillera. The
provenance of ammonoids through the Early Triassic changes significantly starting with the
Arctic as the primary source of Griesbachian and Dienerian ammonoids and the Cordillera as the
predominant locality for Smithian and Spathian collections. However, the increase in Cordilleran
ammonoids does not drive the Early Triassic species diversity curve. Species diversity in the
289
Arctic and Cordillera both correlate significantly with overall Boreal species diversity while the
percent of species sourced from the Cordillera or the Arctic do not. The increasing percentage of
Cordilleran ammonoids in each Early Triassic stage does not correlate with diversity patterns.
Overall, the diversity from these two regions is additive resulting in the observed Boreal Ocean
diversity trend.
Methods
The Westermann Morphospace method was scrutinized in two ways. First, the stratigraphic data
set of Tozer from the Canadian Geological Survey was used to examine Early Triassic
intraspecies variation in morphology. The first test was to examine whether intraspecies variation
posed a problem for interpretation when single, type specimens are used to examine patterns of
morphological variation. Each species represented by at least three specimens was included in a
photographic analysis (n=190 specimens) to allow shape characterization in Westermann
Morphospace (measurement of diameter, umbilical diameter, aperture height, and width, all in
one plane). Of these, 70 specimens representing five species were chosen for analysis because
they had more than five specimens per species. The type specimens from Tozer (1994) were
plotted in Westermann Morphospace for comparison to the 70 additional intraspecies specimens.
Intraspecies specimens from the Canadian Geological Survey were measured digitally with
ImageJ, plotted in Westermann Morphospace, and the results were tallied and tested against the
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morphological distribution of the type specimen data set with a chi-squared test in R (R Core
Development Team, 2010).
Second, for comparison to the majority of ammonoid paleobiological and morphological
analysis, the Early Triassic Boreal Ammonoid collection was also plotted using a principal
components analysis (PCA). A Wilcoxon Rank Sum test was used to determine if ammonoid
groups determined by the PCA resembled those created through Westermann Morphospace
analysis.
Westermann Morphospace was used to test for ecological consequences of changes in
ammonoid diversity and Boreal Ocean environment through the Early Triassic at the highest
resolution available. Type specimen measurement data from Tozer (1994) were plotted in
Westermann Morphospace (using the R functions published in supplement for Ritterbush and
Bottjer 2012) and the number of species within each of four morphotypes (oxycone,
serpenticone, intermediate, and spherocone) were tallied for each time bin manually using
Adobe Illustrator. The four morphotypes (serpenticone, oxycone, spherocone, intermediate) are
interpreted to facilitate drifting, swimming, vertical migration, and weak swimming,
respectively. Once ammonoids were classified by one of four basics shell shapes, statistical tests
were run to evaluate whether shape, and hypothesized hydrodynamic and metabolic rates, varied
through time according to the trophic development model of Sole et al. (2012). Chi square tests
291
were used to test for significant change in shape expression between the four stages of the Early
Triassic. The role of species diversity in the development of morphological disparity was tested
for each of the 14 subzones using Spearman Rank Coefficient to test for significant correlations
in species richness within each shape category and overall species diversity within each substage
in the Boreal Ocean. The high temporal resolution allows comparison to geochemical and
sedimentological proxies for environmental conditions and interpretation of any selectivity in the
success or elimination of specific shell shape categories during “booms and busts” in taxonomic
diversity. Changes in diversity within taxonomic suborders were also tested for significant
change between stratigraphic stages and were compared to changes in diversity of the four shape
categories.
The Smithian Stage was further studied to test for an ecological consequence to the
latitudinal gradient observed by Brayard et al. (2006, 2007). Boreal Ocean ammonoids from the
Smithian Stage collections of Tozer (1994) were compared to Temperate Tethys, Tropical
Tethys Ocean, and Tropical Panthalassa. The Smithian stage was chosen because it had the most
accessible species-level ammonoid collections in each region, the tropics are poorly represented
in the Griesbachian and Dienerian (Brosse et al. 2013) and because the taxonomic latitudinal
gradients to be tested for an ecological signal were well developed by this stage of the Early
Triassic (Brayard et al. 2007). First, differences in taxonomic diversity were tested between the
292
four study locations to see if comparisons in diversity within shell morphology could be
rigorously compared. Chi-square tests were used to compare global diversity of ammonoids to
the number of cosmopolitan and endemic species in the Boreal Ocean, Temperate Tethys, and
Tropical Tethys. Measurement data for type specimens was compiled from references listed for
the Smithian Stage in Brayard et al. (2006) (Table 1), the data were plotted in Westermann
Morphospace, and the number of species in each shape category was tallied and compared. Chi-
square tests were used to evaluate changes in species richness for each shape category between
each of the four different regions.
Early Triassic ammonoid taxonomy is being re-evaluated so additional tests looked at whether
potential taxonomic over-splitting in older references could create trends that would accentuate
or diminish significant changes in the taxonomic diversity within shape categories through time
and space.
The data set from the Early Triassic was compared to a previous analysis of shape
occupation of ammonoids from the Early Jurassic to see whether or not morphological response
of ammonoids to mass extinction events was a repeatable phenomena. The end-Triassic mass
extinction was also likely triggered by the emplacement of a large igneous province, CAMP,
during the breakup of Pangea resulting in many of the same extinction mechanisms observed
during the end-Permian (Greene et al. 2012, Blackburn et al. 2013). The end-Triassic mass
293
extinction 201.3 Mya eliminated all but two ammonoid genera, and the following Hettangian
Stage of the Jurassic lasted about 1.6 million years, about the same duration as the Griesbachian
– Smithian Stages of the Early Triassic (Schoene et al. 2010; Schaltegger et al. 2008). For
ammonoids the extinction severity, taxonomic diversification and initial recovery interval for
both global biological crises are similar. Hettangian ammonoids from Nevada (Guex 1995) were
examined in Westermann Morphospace by Ritterbush and Bottjer (2012). Chi-square was used
to test for differences in species richness for each shape cateogory in the Jurassic Hettangian
compared to the Early Triassic Griesbachian through Smithian stages.
Results
Intraspecies variation did not produce Westermann Morphospace distributions
significantly different than the morphological distribution of ammonoid species type specimens
alone. The 22 specimens of Kashmirites warreni were expected to plot as serpenticones, seven
Wasachites macconnelli were expected to plot as oxycones, and the remaining 41 specimens (of
three different species) were expected to plot in the intermediate zone. The results for each
species are shown in Figure 3. The final count of 23 serpenticones, 32 intermediate and 15
oxycones shapes was not significantly different (X
2
=2.26, p=0.1331; chi squared test in R) than
the original distribution based on the type specimens.
294
Repeated “booms and busts” in ammonoid diversity across the Early Triassic did not
result in significant differences in the distribution of shell shapes until the final Spathian Stage of
the Early Triassic, where there were significantly more species with oxyconic shells (p=0.016;
Figure 4). The Spathian ammonoids are also significantly different when considered in PCA
(p=0.016; Figure 5). Throughout the Early Triassic, species richness within each shape category
increases with (and correlates significantly to) overall Boreal Ocean species diversity (Figure 6,
Table 2). Figure 6 shows bar graphs of ammonoid species shell shape (Fig. 6A) and taxonomic
suborder (Fig. 6B) in each subzone of the Early Triassic. Species diversity within each shape
category correlates significantly to the diversity of multiple suborders, though the diversification
of Spathian Ceratina represent the only significant change in higher taxonomic makeup
throughout the interval. Finally, species shell shape categorizations are compared to proxies for
regional environmental conditions from recent studies (Figure 7).
During the Smithian stage, the shapes of ammonoid shells differed significantly by
latitude. There is no significant difference in cosmopolitan genus diversity through time at the
three sites compared (Figure 8, Table 3), but both diversity and endemism increased throughout
the Early Triassic. Smithian species from the Boreal Ocean were compared to Smithian species
from the Western US (tropical Panthalassic Ocean), South China (tropical Tethys Ocean), and
Pakistan (high-latitude Tethys Ocean) (Figure 9). Streamlined oxycones are significantly more
295
speciose at both low latitude sites compared to the Boreal Ocean (Table 4). The Pakistan, high
latitude Tethys Ocean, site is an intermediate to the two latitudinal end-members; the ratio of
oxycones is statistically indistinguishable from either tropical region or the Boreal realm.
Early Jurassic Hettangian species shape distributions are compared to the above listed
analyses of Early Triassic shape distributions (Figure 10, Table 5). The Hettangian collection
contains significantly (p=1.15*10
-5
) more serpenticones than those ammonoids occurring in the
Boreal Ocean during the Griesbachian through Smithian Stages. The Hettangian data are from a
lower latitude setting than Early Triassic Boreal ammonoids, but their difference in shell shape
representation is the opposite of the trend between tropical and high latitude regions in the Early
Triassic.
Discussion
The statistical investigation of the Westermann Morphospace method supports its use to
examine pelagic ecology. The type collection of Tozer (1994) shows statistically
indistinguishable groups of ammonoids in both Westermann Morpospace projection and a
principal components analysis. Hydrodynamic efficiency data from Jacobs (1992) adds
quantitativev evidence for a link between shell shape, life mode, and metabolic demand. Extinct
ammonoids can be compared to the physiology of their extant relatives the coleoid squid and
more distant cousin, the Nautilus. For coleoid squid, metabolic demand correlates to the type of
296
locomotion; fast moving squid show metabolic rates orders of magnitude greater than slow
swimmers (Seibel et al. 2007). Using this information, the hydrodynamic efficiency of each
ammonoid shell shape is tied to an inferred metabolic rate providing a way to make inferences
about ancient cephalopod trophic relationships. According to hydrodynamic modeling data,
ammonoid locomotion within a streamlined shell (oxycone) is significantly more efficient at
higher speeds. Locomotion within non-streamlined shells (serpenticones, spherocones) is more
efficient at low speeds. If moving very slowly, ammonoids within an oxyconic shell would have
higher metabolic demand than slowly moving ammonoids in shells of other shapes. Once it was
moving quickly, an ammonoid in an oxyconic shell would be more metabolically efficient. Using
this information it is possible to make the interpretation that ammonoids with streamlined
oxyconic shells were active, directed swimmers with a high metabolic rate. Serpenticonic and
spheroconic and intermediate forms were less efficient swimmers and likely occupied life modes
including drifting planktonic, demersal, or vertical migrant with an overall lower metabolic rate.
This hypothesis is the basis of further interpretation of extinct ammonoid shell morphology and
inferred mobility and metabolic rate. While there are many inferences in this interpretation it is
consistent with other previous authors' suggestions of ammonoid mobility and metabolism (Dera
et al. 2010; Whiteside and Ward 2011). Shell ornamentation and aperture shape were not
297
considered as influences on hypothetical mobility in accordance with other ammonoid
paleobiologists (Brosse et al. 2013 but see McGowan 2004).
The test for intraspecies variation of ammonoid shell shapes from the Tozer stratigraphic
collection compared to type specimens from Tozer (1994) did not show a statistically
significantly different occupation of the four morphological groups displayed in Westermann
Morphospace. Type specimens were deemed appropriate to be used for the rest of the study as
differences in the sample sizes of intraspecies groups would have skewed statistical analysis. The
use of a single type specimen or a single genus representative is common in modern analysis
(McGowan 2004, Dera et al. 2010). This result should not be interpreted to suggest that
intraspecies variation no longer needs to be considered. Its affect on ammonoid shell shape has
been well documented for other groups in other time periods and localities and should be tested
before Westermann Morphospace analysis is applied to type specimen samples (Dagys et al.
1993). Differences in intraspecies variation are important in groups whose shells plot toward the
center of Westermann Morphospace indicating an intermediate life mode. With further tests and
refinement of the hydrodynamic interpretations inherent in the morphospace, intraspecies
variation within these groups could result in one species which represents a wide variety of
mobility and metabolic interpretations. These concerns, however, could not be shown to
significantly affect the current investigation.
298
Boreal Ocean ammonoid shell shape was investigated using Westermann Morphospace to
examine how environmental perturbations affected ammonoid shell shape and to see whether
increases in taxonomic diversity were accompanied by increases in trophic complexity marked
by additional occupation of streamlined, mobile, high metabolism life modes (Sole et al. 2002).
Directly following the extinction event, earliest Triassic Boreal ammonoids occupied a wide
range of potential ammonoid morphotypes including representatives of all four morphological
groups. This was a surprising result as it went against the expectation that increasing diversity
over time would lead to the occupation of higher metabolic regimes. Instead, within the 500,000
years following the initial extinction boundary, both streamlined, actively mobile ammonoids
with potentially high metabolic demand co-existed with non-streamlined, low metabolism
ammonoids. An additional interpretation that can be made of the high shape disparity but
relatively low diversity of earliest Triassic ammonoids is that ecological opportunity existed for
each of these groups and that nutrients or potentially, prey, were available for swimming, high
metabolism groups. Brosse et al. (2013) examined this pattern for another Early Triassic data set
and concluded the high disparity was linked to hold over taxa from the late Permian. However, I
would maintain that the resources still had to be present to support these high metabolism
groups.
299
Diverse ammonoid shape occupation continued throughout the rest of the Griesbachian
through the Smithian and the species richness of each morphotype correlated significantly with
overall diversity trends in Boreal Ammonoids. Streamlined oxycones were persistently present in
each diverse subzone of the Early Triassic in the Boreal Ocean and were formed from a variety
of ammonoid suborders. At the end of the Early Triassic, during the 3.25 million year interval
represented by the Spathian Stage, Boreal ammonoids produced significantly more oxyconic
shapes than they had in the preceding 1.75 million years represented by the Griesbachian-
Smithian Stages. The increase in oxycones during this time is limited by the lower resolution of
the subzones representing the Spathian Stage. However, the increase in oxycones with inferred
high metabolism fits with Sole et al (2002) that higher trophic levels will develop over time as
diversity increases following a mass extinction event. Ammonoids diversified to a peak during
the Spathian and this corresponded to a rise in species occupying higher trophic levels. This is
partially the result of taxonomic turnover as the suborder Pinacocertina declined at the Smithian-
Spathian boundary and Ceratitina arose which lead to the increased production of oxycones.
Ammonoid shell shape occupation in the Early Triassic and inferred mobility and
metabolism can be used to analyze the affects of environmental perturbations on the pelagic
fauna. During the Early Triassic in the Boreal Ocean, extreme temperature perturbations, low
oxygen bottom water, and euxinia were present (Sun et al. 2012, Grasby et al. 2013). The
300
modern ecological response of coleoid squid and Nautilus to climate change and oxygen
availability can be used to infer how ammonoid cephalopods would have responded to these
changes. Repeated crashes in Early Triassic Boreal Ocean ammonoid diversity did not show
evidence for selection against any one morphological group over another. This suggests that the
intrinsic or extrinsic causes of diversity crashes and radiations were not selective for a particular
morphology, hydrodynamic regime, or metabolic grade of ammonoid. Abrupt loses of taxonomic
diversity are correlated with global and local carbon isotope excursions (Figure 7) (Payne et al.
2004, Grasby et al. 2013). These fluctuations, associated with sedimentary indicators, are
interpreted as low oxygen water mass incursion events. These sudden events are the likely cause
for the more minor drops in Boreal Ocean ammonoid diversity. Studies of ammonoids’ extant
relatives show increased temperature as a single factor that severely limits cephalopods with both
high and low metabolic demands in both coleoids (Rosa et al. 2008) and nautiloids (Dunstan et
al. 2011). Recent data on oxygen isotopes from conodont apatite suggest sea surface
temperatures at the equator reached lethal levels up to 40°C during the Late Smithian (Sun et al.
2012, Romano et al. 2012, Cui and Kump 2014). This resulted in a breakdown of sea surface
temperature gradients with increased temperatures reaching higher latitudes (Brayard et al. 2006,
Brayard et al. 2007). The abrupt rise in temperatures in the Late Smithian corresponds to the
greatest loss of taxonomic diversity in the Boreal Ocean which occurred between the Smithian
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and Spathian Stages (Figure 4,7). The slight offset in timing between the Late Smithian
perturbation and Smithian-Spathian taxonomic loss might be due to the lower resolution of
ammonoid subzones delineating the late Smithian versus the Smithian-Spathian boundary.
The taxonomic decline of Boreal Ocean ammonoids corresponding to the abrupt rise in
equatorial sea surface temperature had a non-selective affect on ammonoid morphology
suggesting that both swimming and non-swimming ammonoids were equally affected by the rise
in temperatures.
The affects of low oxygen incursion events and extreme temperatures on pelagic
ammonoids in the Early Triassic can be compared to the ecological effects on the benthic fauna.
In response to low oxygen incursions, the best comparison to the pelagic groups can be made
with the benthic fauna seen at Lost Cabin Spring. Here, the development of stratigraphic
parasequences resulted in sudden deepening events that brought low oxygen water masses onto
the shelf. During these intervals, taxonomic diversity dropped, microbial stromatolites were
dominant and there was extremely low (ii1-2) bioturbation of the sediment. As low oxygen
conditions waned and the section shallowed, high domainance, low diversity benthic
communities of bivalves, occasional gastropods, and abundant echinoderms returned (Chapter
2). The pelagic ammonoids of the Boreal Ocean exhibited an ecologically non-selective loss
during low oxygen water mass incursions which occurred sporadically throughout the Early
302
Triassic (Figure 7). These similarities suggest that both benthic and pelagic groups experienced
non-selective loss of taxonomic diversity following sudden low oxygen events. In contrast, the
pelagic taxa did not show an ecological consequence to the booms and busts in taxonomic
diversity brought on by low oxygen events and occupied all niches equally. The benthic
ecological recovery was severely limited by sudden low oxygen events resulting in the high
dominance low diversity fauna seen in the Virgin Limestone at Lost Cabin Spring. While the
benthic recovery was limited following each low oxygen incursion, overall ecological
complexity was relatively high compared to other Early Triassic benthic communities and
included epifaunal echinoderms. In the Boreal Ocean, the Early Triassic infauna showed diverse
and complex development of trace fossils directly following the end-Permian extinction event
that was cut short by the development of low oxygen water masses or a deepening event at the
end of the Dienerian (Beatty et al. 2008, Zonneveld et al. 2010). This loss of diversity echos the
changes seen in ammonoid diversity but while ecological complexity was lost following the low
oxygen event, ammonoid ecology was resilient and rebounded to occupy multiple morphologies
and hypothetical niche space.
The late Smithian temperature rise had a non-selective effect on ammonoid morphology
and hypothetical ecology. Other marine pelagic groups including reptiles, fish, and terrestrial
tetrapods may have also experienced taxonomic diversity losses during this interval (Sun et al.
303
2012, but see Goudemand et al. 2013). The benthic fauna exhibited a notable decrease in the
body size of gastropods which was recognized in the Sinbad Limestone of the Moenkopi
Formation (Frasier and Bottjer 2004, Nützel 2005) as well as the Werfen Formation in Italy
(Twitchett 1999, 2007). This decrease is likely the result of a trade off where growth rate is
decreased to compensate for increased metabolic demand due to increased temperatures
(Sheridan and Bickford 2011). The pelagic fauna did not show a change in morphological shape
occupation or hypothetical ecology in response to the temperature perturbation. The mechanism
behind the difference in benthic and pelagic ecological response to high temperature
perturbations and even low oxygen environments may based on the variation in metabolic rate.
Knoll et al. (2007) and Clapham and Payne (2011) each show that higher metabolism, better
"buffered" groups thrive after the end-Permian mass extinction. As active swimmers in the water
column, the average ammonoid likely has a higher metabolism than the majority of the stationary
epifauna that prevailed in the benthos during this time. Higher metabolism lead to a faster
recovery and more rapid rebound from additional environmental change.
The shell shapes of ammonoids of the Boreal Ocean differed significantly, during the
Smithian Stage, from the shell shapes of ammonoid species at lower latitudes. Brayard et al.
(2006, 2007) have shown that ammonoid taxonomic diversity became increasingly latitudinally
stratified throughout the Early Triassic and by the Smithian, endemic populations were
304
established. Westermann Morphospace was used to show that significantly more oxycones
existed in the tropics than at the poles (Figure 9). This is supported by recent findings by Brayard
and Escarguel (2012) which showed on average narrower and more involute shells in the
Western USA than the Canadian Arctic.
The significant increase of taxonomic diversity of oxycones in low latitude tropical
regions corresponds to the development of oxygen minimum zones around the equator (Figure 9)
(Wignall and Twitchett 2002, Algeo et al. 2011, Winguth and Winguth 2012). These regions
were vertically limited between 400-800 meters depth leaving more oxygenated surface and deep
waters. Modern cephalopods show adapatations to low oxygen environments (Staples et al. 2003,
Kroger et al. 2011) Recently, high metabolism coleoid squid, specifically the Humboldt squid,
have adapted to expanded OMZs in the eastern Pacific Ocean (Bazziano et al. 2010). Here, squid
use low oxygen zones to hide from vertebrate predators and then emerge to feed and replenish
their oxygen supply at night. Persistent shallow ocean OMZ in the Early Triassic created an
ecological opportunity for ably swimming, high metabolism oxycone ammonoids. Adaptation to
oxygen minimum zones comes at an additional cost making modern cephalopods more sensitive
to hypercapnia and temperature spikes (Rosa et al. 2008) which would result in more intense
extinction during the rapid temperature rise at the end of the Smithian Stage. The benthic fauna
shows a few groups that were also resilient to low oxygen conditions. Song et al. (2014) recently
305
ranked common invertebrate groups from the Early Triassic according to low oxygen tolerance
and the molluscs, bivalves and gastropods, showed the greatest resilience. This has been
observed by the dominance of opportunistic bivalves including the ubiquitous Claraia which
colonized low oxygen environments of the Early Triassic (Hallam and Wignall 1992, Schubert
and Bottjer 1995).
Early Triassic ammonoids produced significantly different shell shapes than ammonoids
recovering from the end-Triassic mass extinction. The Early Triassic ammonoids rapidly
occupied a diverse range of morphological shapes and hypothetical life modes in comparison to
the Jurassic fauna which were dominated by evolute serpenticones. This difference could be
driven by phylogeny. Following the end-Permian mass extinction, the Boreal Ammonoids were
dominated by the suborder Ceratina (Treatise 1957) including three superfamilies; Medlicottidae,
Xenodiscidae, and Otoceratidae. Otoceratidae was a “dead clade walking” but before Otoceratids
went extinct at the end of the Griesbachian, other ammonoid clades had begun to produce species
with oxyconic shells. The suborder Phylloceratina appeared at the very end of Early Triassic and
produced the only two genera to survive the end-Triassic mass extinction well into the first
Jurassic Hettangian stage. Three superfamilies also developed in the earliest Jurassic and had the
capability to produce ammonoid species with oxyconic shells. The similarity in the
306
diversification of ammonoid lineages following each major extinction event suggests that the
morphological differences observed were likely driven by extrinsic environmental differences.
The rapid occupation of streamlined oxycones following the end-Permian mass extinction
suggests that ecological niche space was available for active, high metabolism ammonoids. In
the earliest Jurassic, cosmopolitan and large serpenticonic ammonoids indicate a low energy,
potentially planktonic life mode, suggesting the ecological opportunities following the end-
Permian mass extinction were not available for the pelagic fauna following the end-Triassic mass
extinction. Ammonoids during this time exhibit a low energy, possibly r-selected, planktonic life
mode. The repeated environmental disturbances in the Early Triassic may have served to
stimulate ecological expansion as whole tiers of niches were repopulated in a short time interval.
Conclusions
Ammonoids produced a wide variety of shell shapes directly following the end-Permian
mass extinction and continued to do so for 1.7 million years through the first three stages of the
Early Triassic recovery. The taxonomic and morphological radiation of ammonoids during this
interval suggests that the high metabolic demands of streamlined, swimming ammonoids could
be supported in the post-extinction environment of the Boreal Ocean. This contrasts directly with
the findings for the Early Jurassic when evolute serpenticones were the dominant morphology.
By the Spathian Stage, significantly more oxycones were produced which follows the trend
307
predicted by Sole et al. (2002) that increase diversity will be matched by increased trophic
complexity during the recovery from mass extinction events.
The recovery was not a linear process and taxonomic booms and busts in diversity were
accompanied by the radiation of all four shell morphologies suggesting that each diversity loss
was tied to a non-selective environmental pressure. Low oxygen incursions and high temperature
events in the Boreal Ocean are the extrinsic events correlated with taxonomic changes. In the
benthic fauna, decreased oxygen and rapid temperature rise correspond to ecological changes
including low diversity, high dominance communities and decreased body size, respectively.
Corresponding changes in the pelagic taxa are not observed suggesting they were taxonomically
sensitive but ecologically resilient to environmental changes, perhaps due to more robust
metabolism.
Tropical settings contained significantly more streamlined oxycones which is interpreted
as an ecological adaptation to OMZs in the tropics of the Early Triassic providing a place to hide
from predators much like modern coleoid squid use OMZs in the Pacific Ocean today. The
benthic fauna contain a few "disaster bivalves" specifically Claraia, that may also have taken
advantage of low oxygen zones becoming numerically dominant across the shoreface when other
benthic groups could not survive.
308
Comparison to modern coleoid squid and nautiloids provides the basis of ecolgical
interpretations made on ammonoid morphology and inferred metabolism and ecology. Overall,
Boreal Ocean ammonoids exhibited rapid and resilient occupation of niche space following the
end-Permian mass extinction and additional environmental perturbations in the Early Triassic
recovery interval. Significant differences in ammonoid shape occupation exist between different
regions and across different extinction boundaries which require further quantification and
exploration.
309
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Tables and Figures
Table 1. List of references used for analysis of Early Triassic ammonoid morphology by locality
for the latitudinal analysis. Author, Year, and Publication volume are included.
Table 2. Morphologic Categorization of Boreal Ammonoid Species by Subzone.
Table 3. Genus Diversity and Endemism
Table 4. Morphologic Categorization of Smithian Species by Region
Table 5. Morphologic Categorization of Ammonoid Species
Figure 1. Early Triassic timescale depicts four stages spanning five million years. Modeled after
Lehrnmann et al. 2006. The dates for the boundaries are found in the following publications;
end-Permian: Mundil et al. 2004, Shen et al. 2011. Dienerian-Smithian boundary Galfetti et al.
2007. Early Spathian Ovtcharova et al. 2006, Galfetti et al. 2007. End Spathian/Early-Middle
Triassic boundary: Ovtcharova et al. 2006, Galfetti et al. 2007, Brayard et al. 2009.
Figure 2. Westermann Morphospace modified by Ritterbush and Bottjer 2012. Each corner of the
ternary diagram represents one of three extreme ammonoid morphologies and intermediate forms
fall in between the green dotted lines.
Figure 3: Intraspecies variation. For each species, the type specimen from Tozer (1994) is plotted
as an enlarged version of one of five shapes representing the 5 species tested. The specimens
from Tozer’s stratigraphic collections at the Canadian Geological Survey in Vancouver, British
Columbia are plotted as small versions of each of five shapes. Shell shape variation of the
intraspecies distribution was not significantly different the type specimens alone (X
2
=2.26,
degrees of freedom=1, p=0.13). Meekoceras gracilitatis (square, with hypotype), Wasachites
316
deleeni (circle, with holotype), Anawasachites tardus (cross, with topotype), Kashmirites
warreni (triangle, with holotype), Wasachites macconnelli (diamond, with holotype).
Figure 4. A., B. Westermann Morphospace sorts ammonoids into shape categories based shape
parameters. Each datapoint represents the type specimens for a Boreal Ocean species during the
(A) Griesbachian and Dienerian and (B) Smithian and Spathian stages. Symbols indicate
subzones. Griesbachian (filled shapes): Concavum (squares), Boreal (dots), Commune
(triangles), Strigatus (diamonds). Dienerian (open shapes): Candidus (squares), Sverdrupi 1
(dots), 2 (triangles), 2/3 (diamonds), 3 (cross). Smithian (filled shapes): Hedenstroemi (square),
Romunderi (dots), Tardus (triangles). Spathian (open shapes): Pilaticus (square), Subrobustus
(dots). C. Histograms show the number of Boreal Ocean species in each shape category through
the Early Triassic (black = oxycone, dark grey = serpenticone, medium grey = intermediate, light
grey = spherocone). Pie charts represent the proportion of the four morphologies in the A.
Griesbachian and Dienerian. B. Smithian and Spathian. Colors are as follows: Red: Oxycones,
Light Grey: Serpenticones, Dark Grey: Intermediate, White: Spherocones. C. Barplot showing
the number of species within each shape category through time; bar width indicates the duration
of each subzone within the four stages of the early Triassic. Shell shape occupation was
consistent from the Griesbachian through the Smithian. There is not a significant change until the
Spathian Stage, which has significantly more species of oxycones (X
2
=5.78, degrees of
freedom=1, p=0.016).
Figure 5: Ammonoid shell shapes are represented in both Westermann Morphospace (top) and
Principal Components Analysis (PCA; bottom plot). Black dots represent Griesbachian,
Dienerian and Smithian ammonoids species. Open circles represent Spathian ammonoid species.
Both Westermann Morphospace and the PCA display an increase in oxycones (and spherocones)
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in the Spathian. Spathian ammonoid species have significantly higher values of Principal
Component Two (Wilcoxon Rank Sum Test in R: p=0.016).
Figure 6. A. The relative distribution of Boreal Ocean ammonoid species within each shape
category (top bar plot) is fairly stable and does not change significantly until the increase of
oxycones during the Spathian Stage (Table 2). B. Barplot of species within each suborder
demonstrates that there is not a simple relationship between higher taxonomy and ammonoid
shell shape in this case. For example, the Spathian increase in oxycones coincides with the
appearance of the suborder Ceratina, but oxycones were produced by multiple suborders
throughout the Early Triassic interval.
Figure 7. Early Triassic Boreal ammonoid diversity is compared to oxygen and carbon isotope
data and their interpreted environmental implications. A. Trends in Boreal ammonoid shape
space occupation at the subzone scale (this study): key as in Figure 4. B. Oxygen isotopes
derived from sea surface and shallow water dwelling conodonts and inferred fluctuations in
Early Triassic temperature (Sun et al. 2012). The Smithian/Spathian boundary shows a sharp
decline in δ
18
O suggesting an abrupt rise in temperatures. C. Composite carbon isotope trend
from South China (Payne et al. 2004), which represents a global signal. D. Carbon isotopes from
the Boreal Sea (Grasby et al. 2013) which echo Payne et al. 2004 and E. Interpretations of these
carbon isotopes and other chemical and sedimentological data as shallow water oxygenated,
dysoxic, anoxic, and euxinic intervals (Grasby et al. 2013).
Figure 8: Barplots of genus diversity, all data from genus counts in Brayard et al., (2006). A.
Global genus diversity counts. Each vertical bar represents genus diversity during an Early
Triassic stage (G=Griesbachian, D=Dienerian, Sm=Smithian, S=Spathian). Global diversity
318
consists of genera found within more than one region added to genera uniquely occurring within
a single region. Endemism increases throughout the Early Triassic. B. The diversity of
ammonoids within three regions through the Early Triassic, including the unique genera. C.
When unique endemic genera are removed, there is no statistically significant difference (X
2
=
6.85, degrees of freedom=6, p=0.34; see Table 3) in diversity pattern through time between any
of the three sites or the global diversity pattern.
Figure 9. The Early Triassic globe contained three major ocean basins, the Panthalassic, Tethys,
and Boreal Oceans. Enduring shallow oxygen minimum zones existed throughout equatorial
Panthalassa and much of the Tethys (Horacek et al. 2007, Algeo et al. 2010, Algeo et al. 2011,
Winguth and Winguth 2012). Westermann Morphospace diagrams represent ammonoid shape
occupation during the Smithian stage at four relative paleogeographic locations indicated.
Tropical (South China, American Southwest) settings featured significantly more species of
oxycones than the Boreal Ocean (Table 4). Pie charts show the proportion of oxycones, which
were likely directed swimmers, in red and the other three morphologies, which were likely
inefficient swimmers, in grey. The second high latitude site, in the Tethys (Pakistan), represents
a midpoint in the latitudinal gradient of relative oxycone diversity.
Figure 10: Earliest Triassic and Jurassic ammonoids compared in Westermann Morphospace. A.
Triassic (Griesbachian – Smithian stages, ~1.7 Ma) from the Boreal Ocean based on Tozer’s
(1994) type specimens (n=84). B. The complete Jurassic Hettangian Stage (~1.8 Ma) record of
ammonoids from Nevada, based on type and largest available specimens (n=47) from Guex
(1995). There are significantly more serpenticone species in the earliest Jurassic (X
2
=19.2,
degrees of freedom=1, p=1.15*10-5; Table 5). If shell shapes expressed by earliest Triassic and
319
earliest Jurassic species were the same, the difference in latitude would predict the opposite trend
(see Fig. 9).
320
Table 1.
Locality Author Year Publication
South China Tien 1933 Acta Palaeontologica Sinica Vol. 13(1)
Guo 1982 Acta Palaeontologica Sinica Vol. 21(5)
Tong et al. 2004 Acta Palaeontologica Sinica Vol. 43(2)
Brühwiler et al. 2008 Paleontology 51(5)
Salt Ranges Griesbach 1880 Records of the Geological Society of India 13(2)
Waagen 1895 Palaeontologica Indica 13
Diener 1913 Palaeontologica Indica 5
Kummel 1970 University of Kansas- Department of Geology Special
Publication 4
Brühwiler et al. 2011 Swiss Journal of Palaeontology 130
Southwest
US
Smith 1932 USGS Professional Paper 167
321
Table 2.
Subzone
Concavum
Boreal
Commune
Strigatus
Candidus
Sverdrupi_I
Sverdrupi_II
Sverdrupi_II_III
Sverdrupi_III
Hedenstroemi
Romunderi
Tardus
Pilaticus
Subrobustus
p value;
correlation to total
Grisbachian -
Smithian
Spathian
oxycones 1 1 0 2 2 1 1 2 2 1 4 4 4 7 0.0035 18 3
serpenticones 0 0 3 1 3 0 1 1 0 0 7 3 0 3 8.39E-06 38 3
intermediate 1 2 3 2 7 0 1 2 1 0 5 14 1 2 0.0035 20 11
sphaerocones 0 0 0 1 1 0 0 0 0 0 6 1 0 5 0.00013 8 5
total 2 3 6 6 13 1 3 5 3 1 22 22 5 17
84 22
Stage Griesbachian Dienerian Smithian Spath.
322
Table 3.
Cosmopolitan
Global
Total
Global
Endemic Global
British
Columbia
South
China Pakistan
Griesbachian 11 1 10 8 8 5
Dienerian 23 8 15 14 14 12
Smithian 61 19 42 21 30 19
Spathian 93 44 42 17 25 10
323
Table 4.
Boreal US West S China Pakistan
Serpenticones 10 14 24 30
Intermediates 19 24 29 48
Oxycones 8 27 36 32
Sphaerocones 6 4 10 4
p value
0.02265 0.03549 0.2248
The p values are for chi square tests of the number of oxycones vs all other species,
compared to the Boreal ammonoids.
324
Table 5.
Hettangian
G-Sm
Boreal
Smithian
US West
Smithian
S China
Smithian
Pakistan
Serpenticones 28 16 14 24 30
Other morphs 19 66 55 75 84
p value when
compared to
Hettangian
1.15E-05 3.70E-05 6.88E-05 0.000136
325
Gries. Dien. Smith. Spathian 252.28 247.2
Figure 1
326
Figure 2
327
deleeni
tardus
macconnelli
gracilitatis
warreni
Spherocone Serpenticone
Oxycone
Figure 3
328
deleeni
tardus
macconnelli
gracilitatis
warreni
0 5 10
Species
Gries. Dien. Smithian Spathian
252.28 251.22 250.55
Concavum
Boreal
Commune
Strigatus
Griesbachian Dienerian
Candidus
Sverdrupi 1
Sverdrupi 2
Sverdrupi 2/3
Sverdrupi 3
Plicatus
Subrobustus
Spathian
Hedenstromi
Romunderi
Tardus
Smithian
247.2
0 5 10 0 5 10 0 5 10 15
Oxycone
Serpenticone
Intermediate
Spherocone
Spherocone Serpenticone
Oxycone
Spherocone Serpenticone
Oxycone
A.
B.
C.
Figure 4
329
deleeni
tardus
macconnelli
gracilitatis
warreni
Oxycone
Spherocone Serpenticone
-0.2 0.0 0.2 0.4
0.8 0.6 0.4 0.2 0.0 -0.2 -0.4
Principal Component 2 (28.4% of total variance)
Principal Component 1 (59.49% of variance)
Griesbachian,
Dienerian, and
Smithian
Spathian
A.
B.
w
U
Th
Figure 5
330
Boreal Suborders by Subzone
Ceratina
Sageceratina
Phylloceratina
Pinacoceratina
Paraceltina
Otoceras
Oxycone
Serpenticone
Intermediate
Spherocone
0 5 10 15 20
Species
0 5 10 15 20
Species
Griesbachian Dienerian Smithian Spathian
252.28 252 251.22 250.55 250 249 248 247.2
Griesbachian Dienerian Smithian Spathian
252.28 252 251.22 250.55 250 249 248 247.2
A.
B.
Figure 6
331
247mm
Δ
Estimated Temperature (°C, ice free world)
40 28 30 32 34 36 38
Gries. Dienerian Smithian Spathian
252.28
247.2
17.5 18.5 18.0 19.0 20.0 19.5 20.5
δ18Oapatite (‰ VSMOW) ← warming
Δ Δ
Δ
Δ
Δ
Δ
Δ
Δ
Δ
Δ
Δ
Δ
Δ
Δ
Δ
Δ
Δ
Permian
B.
251.22
250.55
252
251
250
249
248
A.
Euxinic
Anoxic
Dysoxic
Oxic
C. D. E.
δ13C carb (‰)
-2 0 2 4 6 8
E
E
E
E
E
Boreal Ammonoid Speices Diversity
0 5 10 15 20 -32 -26 -28 -30
δ13C org (‰)
Figure 7
332
0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100
Genus Diversity Genus Diversity Genus Diversity
Total Global Diversity
Total Global Diversity
Genera occuring in
more than one region
Genera occuring in
more than one region
Genera unique
to a single region
Boreal Ocean
(Canada)
Tropical Tethys
(South China)
Temperate Tethys
(Pakistan)
Boreal Ocean
(Canada)
Tropical Tethys
(South China)
Temperate Tethys
(Pakistan)
G D Sm S
G D Sm S
G D Sm S G D Sm S G D Sm S
G D Sm S G D Sm S G D Sm S
G D Sm S G D Sm S G D Sm S
A.
C.
B.
Figure 8
333
B
Low oxygen
zones
Panthalassic
Ocean
Boreal Ocean
Tethys
Ocean
A
C
D
Figure 9
A. Boreal B. Tethys Tropical
C. Panthalassa Tropical D. Tethys Temperate
334
Oxycone
Spherocone Serpenticone
B. Hettangian (Guex 1995)
A. Griesbachian-Smithian
(Tozer 1994)
Intermediate
Intermediate
Oxycone
Spherocone Serpenticone
Figure 10
335
Chapter 6: Concluding thoughts on ecological recovery dynamics of the benthic and pelagic
fauna in response to extreme temperature events and low oxygen environments developed
during the Early Triassic
Introduction
The Early Triassic recovery from the end-Permian mass extinction was complex.
Environmental perturbations that began at the mass extinction boundary continued to affect the
benthic and pelagic fauna for the remainder of the Early Triassic. The interaction between the
benthic and pelagic macrofauna as a community was heavily dependent on locality. Each stage
of the Early Triassic presented unique environmental challenges including ocean acidification,
low oxygen incursions, oxygen minimum zones, and extreme temperature events. The severity of
these perturbations to the benthic fauna varied with depositional environment. The pelagic
response varied with latitude and corresponding environmental gradients. The synthesis provided
here is the first of its kind and serves to unite the ecological response of the benthic and pelagic
invertebrates of the Early Triassic to one another to create a broad picture of the steps by which
the marine community was re-structured following the end-Permian mass extinction.
Discussion
The development and application of the modified recovery rubric presented in Chapter 1
serves as a review of what is known about the benthic recovery following the end-Permian mass
extinction. It provides a mechanism by which paleoecologists can evaluate ecological change in
the benthic fauna by quantifying parameters such as dominance and body size into recovery
stages. The recovery varied between ocean basins and depositional environments through time.
In the Griesbachian and Dienerian, an oxygenated habitable zone developed in shallow water
marine environments in the Boreal Ocean as well as in the Tethys Ocean at localities including
Italy, Pakistan, and Oman. The habitable zone promoted the rapid recovery of a diverse benthic
336
fauna and complex bioturbation. Local deepening events led to the flooding of the shelves and
carbonate banks by pervasive low oxygen water in the Dienerian leading to the end of these
rapid, post-extinction resurgences in diversity. Localities in Neo-Tethys, including Oman and
Pakistan, seem to have experienced the onset of anoxia later than the other localities across the
Early Triassic globe, suggesting there may have been oxygenated refugia for the benthic fauna
directly after the initial extinction event.
The quantitative recovery rubric developed for the Early Triassic benthic fauna was then
applied to paleocommunities through time and space, focusing on the differences among
depositional environments in the Southwest United States in comparison with Early Triassic
sections in Italy. Chapter 2 provides a detailed study of two sections from the Southwest United
States, the Smithian stage Sinbad Limestone and the Spathian stage Virgin Limestone. A high
resolution analysis of the Sinbad Limestone describes the changes in the benthic fauna between
three major depositional environments. There were two primary ecological responses to extreme
temperatures including the miniaturization of the gastropod fauna and the exclusion of a diverse
or abundant echinoderm fauna. The Virgin Limestone at Lost Cabin Spring depicts a high
resolution record where repeated low oxygen perturbations continually reset the benthic recovery
resulting in a low diversity fauna compared to the Sinbad Limestone. In the Virgin Limestone,
echinoderms thrived indicating resilience to low oxygen conditions was greater than extreme
temperature events. Small body size and low diversity were indicators of continued
environmental stress at Lost Cabin Spring throughout the majority of the Spathian stage.
In Chapter 3 the study of the benthic recovery in the Southwest United States was
expanded to include the deeper water Smithian and Spathian-aged Thaynes Formation and the
shallow water Spathian Virgin Limestone at Ute. With the addition of these two localities, the
337
variation of extrinsic environmental perturbations could be compared between depositional
environment and through time. The deeper water deposits of the Smithian Thaynes Formation
were possibly buffered from extreme sea surface temperatures by a thick overlying water mass
compared to the shallow water Sinbad Limestone. The direct result is the difference in body size
between the two sections; gastropods near the surface decreased their body size in response to
extreme temperature rise while deeper water faunas were significantly larger because they were
living in protected cooler offshore temperatures. Both onshore and offshore environments did not
contain diverse and abundant echinoderm fauna until the Spathian suggesting that their radiation
was delayed due to extreme temperatures. Shallow and deep water environments during the
Spathian each exhibited dominance of high abundance, low diversity opportunistic bivalve fauna
including, but not limited to, the classic disaster bivalves; Eumorphotis, Promyalina, Unionities,
and Claraia. In the deeper water Virgin Limestone at Lost Cabin Spring, repeated low oxygen
events reset environmental conditions leading to the return of a complex stunted benthic
epifauna. At Ute, despite deposition in a less volatile, shallow water environment, the benthic
fauna did not show an increase in complexity and remained dominated by a low diversity bivalve
fauna with sporadic echinoderms. The benthic fauna of the Thaynes Limestone is also dominated
by low diversity benthic fauna. Taxonomic diversity increased into the Spathian, as indicated in
this work and others, while community evenness and complexity was slow to recover,
representing a disconnect driven by additional environmental stress. Persistent deleterious
conditions allowed for the resurgence of dominant, opportunistic taxa throughout the recovery
interval. Body size, diversity, and evenness increased by the end of the Spathian when evidence
for low oxygen conditions declined.
338
In comparison to the deposits in the Southwest United States, the benthic recovery in the
Werfen Formation of the Italian Dolomites was initially rapid. By the Dienerian-aged Suisi
Member, diversity, infaunalization, and paleoecological complexity reached a recovery Stage 3.
However, a sudden shallowing event into the Smithian stage Campil Member in addition to the
global extreme temperature rise at the end of the Smithian resulted in a reset in delay in
taxonomic diversification until the Spathian. Ecological complexity was more slowly rebuilt
during the final Spathian stage. Two extreme temperature events; one at the end-Permian mass
extinction and one in the late Smithian, generated and maintained a diverse fauna of
microgastropods throughout this section and across the Early Triassic globe.
The benthic fauna responded in dynamic ways to the different environmental
perturbations of the Early Triassic. Extreme temperature perturbations at the Permian-Triassic
boundary likely generated low body size paleocommunities that were perpetuated into the late
Early Triassic by additional extreme temperature events at subsequent eruptions of the Siberian
Traps. These disturbances to the Earth system prolonged the recovery in the Spathian by
resetting ecological recovery in the Late Smithian and excluding the development of an
echinoderm fauna. Extreme temperatures also likely maintained or strengthened remaining low
oxygen environments reinvigorating the dominance of opportunistic, low diversity
paleocommunities in the Spathian. Relaxation of low oxygen conditions by the end of the
Spathian resulted in increased body size, diversity, and a more taxonomically even benthic
community.
In Chapter 5, the pelagic response to high temperature and low oxygen is evaluated for
comparison to the variable benthic recovery dynamics. Boreal ammonoids experienced
radiations and losses of taxonomic diversity throughout the entire Early Triassic. These were
339
decoupled from morphological changes through time. Directly following the end-Permian mass
extinction, streamlined shell shapes which represent swimming, high metabolic demand
ammonoids rapidly re-appeared and were maintained despite taxonomic overturn. In contrast
with the ammonoid recovery from the end-Triassic mass extinction, the Early Triassic Boreal
Ocean was able to support rapid ecological diversification of both swimming, high metabolism
and non-swimming, low metabolism ammonoids. During the Smithian, the tropics had a greater
proportion of swimming oxycones compared to the Boreal Ocean which was interpreted as the
adaption of swimming ammonoids to well-developed equatorial oxygen minimum zones.
Conclusions
The benthic and pelagic faunas did not respond taxonomically or ecologically in the same
way to extrinsic environmental perturbations. In the benthic fauna some groups, like
echinoderms, were totally excluded while other taxonomic groups, the bivalves and gastropods
prevailed. However, molluscs showed signs of stress including high dominance faunas and
severely reduced body size. The cephalopod ammonoids experienced a resilient recovery from
the extreme temperature event in the Late Smithian with both swimming and non-swimming
ammonoids exhibiting an equal recovery following the drop in taxonomic diversity. Low oxygen
environments were deleterious to the benthic fauna resulting in low diversity paleocommunities
often dominated by opportunistic bivalves. Streamlined, swimming ammonoids were able to
adapt to persistent low oxygen conditions that developed around the equator during the Early
Triassic. Sudden incursions of low oxygen conditions reset pelagic taxonomic diversity in the
Boreal Ocean but did not result in any ecological selection against ammonoid shell shape and
inferred metabolism. Therefore, while the benthic fauna experienced ecological set-backs from
repeated low oxygen perturbations, the pelagic fauna were more ecologically resilient.
340
The complexity observed throughout the five million years of the Early Triassic benthic
and pelagic recovery documented herein redefines the way that paleoecologists have viewed the
extinction recovery for decades. What was once considered to be a slow and step-like process
over millions of years is shown to be a dynamic response to sudden and extreme environmental
variables, changes in depositional environment, and variations between ocean basins and across a
latitudinal gradient. The complexity of the recovery pattern presented here will be used to
encourage future investigation of recovery dynamics through careful consideration of the local
and global environmental context. Future studies can make use of the quantified recovery rubric
and global comparisons can make use of this high resolution data set. In addition, as the modern
world descends into a sixth mass extinction, brought on by the rapid onset of climate change
from anthropogenic sources, the complexity of this extinction recovery can serve as a rubric by
which to gauge future remediation efforts.
341
Abstract (if available)
Abstract
The end-Permian mass extinction, 252 million years ago, represents the greatest loss of biodiversity in the history of life on the planet. The extinction was likely driven by greenhouse and toxic gas emissions and an extreme temperature rise triggered by volcanic eruption of the Siberian Traps (located today in Russia) followed by stagnant oceans and low oxygen conditions. Baking and burning of coal, carbonate, and evaporite deposits during Siberian Traps emplacement released additional greenhouse and toxic gases into the atmosphere. Volatile environmental conditions continued into the 5 million-year-long Early Triassic interval where additional carbon isotope excursions are interpreted to represent subsequent volcanic eruptions. Additional eruptive events prompted extreme temperature perturbations of the surface ocean, reaching up to 40°C, reducing temperature gradient driven ocean circulation, and propagating oxygen minimum zones. Following the initial mass extinction event, the marine ecosystem experienced additional community restructuring events in response to extreme climate change intervals. Currently, anthropogenic greenhouse gas production is outpacing the rate of atmospheric carbon input modeled for the end-Permian mass extinction, signaling that environmental change in the modern world may be even more devastating to today’s ecosystems than the biotic crisis 252 million years ago. ❧ New insights into high resolution paleoecological, sedimentological, and geochemical data in the Early Triassic highlight a complex ecological response that varies across ocean basin, water depth, and latitudinal gradient. Intrinsic conditions including metabolic rate and hypoxic or high temperature tolerance determined which marine organisms thrived or died as environmental conditions shifted. A modified recovery rubric was used as an evaluation mechanism to compare and contrast the restructuring of sea floor communities throughout the Early Triassic. In both the Panthalassic and Tethys Ocean basins opportunistic or “disaster taxa” were the most abundant members of the marine community directly following the mass extinction and in low oxygen environments. The periodic resurgence of low oxygen conditions throughout the Early Triassic resulted in decreased diversity and abundance of seafloor-dwelling fauna and the dominance of “disaster” groups like microbialites, microconchids, and flat bivalves. During intervals of extreme sea surface temperature, miniature gastropods prevailed, possibly due to their metabolic flexibility, whereas echinoderms were often excluded by these deleterious conditions. The pace and pattern of recovery varied among ocean basins. The marine organisms of the Tethys Ocean, represented by deposits from the Italian Dolomites, experienced a more rapid recovery, including increased taxonomic diversity and ecological complexity within the first 1 million years following the initial extinction event. This was likely due to well-oxygenated conditions in the shallow marine environments of this region. Additional set-backs to the recovery of the sea-floor dwelling fauna included increased humidity and terrestrial run-off about 2 million years after the end-Permian mass extinction which inundated the seafloor and smothered diverse communities. ❧ In the open ocean, ammonoid cephalopods showed booms and busts in taxonomic diversity corresponding to carbon isotope excursions throughout the Early Triassic. In the polar, Boreal Ocean, ammonoids quickly diversified to fill a variety of ecological niche space represented by Westermann Morphospace including hypothetical planktonic forms, vertical migrants, and fast-moving predators. During each loss of taxonomic diversity, there was no selection against a particular shell shape, and new ammonoid communities rapidly repopulated shape disparity. In the tropics, a significantly higher proportion of ammonoid species diversity occupied streamlined shell shapes, likely representing rapid swimming abilities and higher metabolic rates. These potential predators may have taken advantage of expansive equatorial oxygen minimum zones as their modern relatives, coleoid squid, do today. ❧ The marine fauna of the Early Triassic exhibited a dynamic response to sudden and extreme environmental change. In the aftermath of the mass extinction event, communities were predominantly reconstructed by organisms tolerant to hyperthermals and hypoxic conditions. At the equator, extreme sea surface temperature and oxygen minimum zones selected for high temperature tolerant microgastropods and fast-moving ammonoids. The recovery from the end-Permian mass extinction was not slow and step-like. Instead, the invertebrate communities of the Early Triassic were highly malleable, changing their composition rapidly in response to new challenges from continuous environmental perturbations.
Linked assets
University of Southern California Dissertations and Theses
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Asset Metadata
Creator
Pietsch, Carlie
(author)
Core Title
Ecological recovery dynamics of the benthic and pelagic fauna in response to extreme temperature events and low oxygen environments developed during the early Triassic
School
College of Letters, Arts and Sciences
Degree
Doctor of Philosophy
Degree Program
Geological Sciences
Publication Date
08/04/2015
Defense Date
03/06/2015
Publisher
University of Southern California
(original),
University of Southern California. Libraries
(digital)
Tag
benthic,early Triassic,end-Permian,high temperature,hyperthermal,low oxygen,mass extinction,molluscs,OAI-PMH Harvest,pelagic,recovery,restructuring
Format
application/pdf
(imt)
Language
English
Contributor
Electronically uploaded by the author
(provenance)
Advisor
Bottjer, David J. (
committee member
), Caron, David A. (
committee member
), Corsetti, Frank A. (
committee member
)
Creator Email
carlie.pietsch@gmail.com,cpietsch@usc.edu
Permanent Link (DOI)
https://doi.org/10.25549/usctheses-c3-627058
Unique identifier
UC11303605
Identifier
etd-PietschCar-3791.pdf (filename),usctheses-c3-627058 (legacy record id)
Legacy Identifier
etd-PietschCar-3791.pdf
Dmrecord
627058
Document Type
Dissertation
Format
application/pdf (imt)
Rights
Pietsch, Carlie
Type
texts
Source
University of Southern California
(contributing entity),
University of Southern California Dissertations and Theses
(collection)
Access Conditions
The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...
Repository Name
University of Southern California Digital Library
Repository Location
USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA
Tags
benthic
early Triassic
end-Permian
high temperature
hyperthermal
low oxygen
mass extinction
molluscs
pelagic
restructuring