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Paleoenvironments and paleoecology of the disaster taxon Lingula in the aftermath of the end-Permian mass extinction: Evidence from the Dinwoody Formation (Griesbachian) of southwestern Montana...
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Paleoenvironments and paleoecology of the disaster taxon Lingula in the aftermath of the end-Permian mass extinction: Evidence from the Dinwoody Formation (Griesbachian) of southwestern Montana...
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PALEOENVIRONMENTS AND PALEOECOLOGY OF THE
DISASTER TAXON LINGULA IN THE AFTERMATH OF THE
END-PERMIAN MASS EXTINCTION:
EVIDENCE FROM THE DINWOODY FORMATION
(GRIESBACHIAN) OF SOUTHWESTERN MONTANA AND
WESTERN WYOMING
B y
David Laurence Rodland
A Thesis Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF SCIENCE
(GEOLOGICAL SCIENCES)
May, 1999
Copyright 1999 David Laurence Rodland
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UMI Number: 1417215
Copyright 2004 by
Rodland, David Laurence
All rights reserved.
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UNIVERSITY O F S O U T H E R N CALIFORNIA
THE GRADUATE SCHOOL
UNIVERSITY PARK
LOS ANGELES. CALIFORNIA 9 0 0 0 7
This thesis, written by
. D^KM..L.aUX.e.ace.. Rodland....................
under the direction of hi.§. Thesis Committee,
and approved by all its members, has been pre
sented to and accepted by the Dean of The
Graduate School, in partial fulfillment of the
requirements for the degree of
Master of Science
Date November 30, 1 998
THESIS. COMMITTEE
/
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D edication:
I’d like to take the chance to dedicate this to all the survivors of the end-Permian
extinction, and to their descendants. Some of those descendants deserve particular
thanks for helping me to survive and thrive while working on this project. So:
To my family: Larry, Karin, Matt;
to the roomates who put up with me: Oliver, Bird, Yang and Laura;
to my other dear friends: Scott, Windell, Static, Maria, Karen, and Danielle;
to the guys in the lab: DJB and the whole gang;
and to everybody else who might actually find this interesting.
-- Dave
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Acknowledgem ents:
First of all, I’d like to thank my advisor, David Bottjer, who started me out on
this project and offered assistance whenever things got sticky. Secondly, to the other
professors and students of USC, who helped to teach me everything from how to run
X-ray machines to using spreadsheets, not to mention the usual details of classwork.
Thirdly, to the many people who have taught me or helped me to learn everything else,
from kindergarden through my last day of undergraduate work to today. There are too
many people to name, but I’m indebted to every single one of you, and I can only hope
I’ve managed to repay some of it along the way.
Speaking of debts, I’d also like to thank those who have contributed to this
research in a more tangible, which is to say fiscal, manner. So, many thanks go to the
USC Department of Earth Sciences for funding through the Graduate Student Research
Fund, and for the use of departmental equipment along the way. Thanks also to the
Paleontological Society for their kind financial support. And of course, I’m indebted to
my family: for the loan of a car while mine was in the shop, for contributing with the
repairs, and for all the support they’ve given me all along.
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Table of Contents
_____________________________________________________________ PAGE NUMBER
Dedication ii
Acknowledgements iii
List of Figures vi
Introduction 1
Background 3
Paleoecology in the Aftermath of Extinction 3
The End-Permian Mass Extinction 6
Magnitude and Timing 6
Proposed Causes and Associated Phenomena 6
Life in the Recovery Interval 10
Biology of Lingulide Brachiopods 12
Morphology 12
Life Habit 12
Environmental and ecological controls on distribution 13
Models for the Proliferation of Lingula 16
Nearshore Specialists 17
Dysaerobic Specialists 17
Ecological Opportunists 18
T aphonomic Bias 18
Early Triassic Deposition in the Western U.S. 20
The Dinwoody Formation 20
Methods 22
Fieldwork 22
Laboratory analysis 24
Integration of field and laboratory data 28
Results 29
Stratigraphy 29
Facies 29
Distal Mudstone Facies 29
Proximal Siltstone Facies 30
Proximal Interbedded Mudstone Facies 39
Proximal Limestone Facies 39
Ichnology 43
Distal Mudstone Facies 47
Proximal Interbedded Mudstone Facies 47
Proximal Siltstone Facies 49
Paleontology and Paleoecology 52
Brachiopods 53
Lingula 53
Crurithyris? 64
iv
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Table of Contents (continued)
PAGE NUMBER
Paleontology (continued)
Molluscs
64
Bivalvia
64
Gastropoda 65
Echinoidia 65
Osteicthyes 66
Problematica 66
Taphonomy of Lingula 66
Discussion 69
Paleoenvironmental Conditions 69
Ichnology 72
Taphonomy 73
Paleoecology 79
Conclusions 83
References 86
Appendix A: Localities 90
Appendix B: Stratigraphic Columns 93
Appendix C: Paleoecological Census Results 100
Appendix D: Cluster Analysis 108
Appendix E: Taphonomic Census Results 110
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FIGURE
List of Figures
PAGE NUMBER
1: Localities 23
2: Lingula and bivalve in distal mudstone facies 31
3: Structureless mudstone in distal mudstone facies 32
4: Parallel oriented gastropods in distal mudstone facies 33
5: Gutter cast in proximal siltstone facies 34
6: Load cast structures in proximal siltstone facies 35
7: Upper plane bed lamination and hummocky cross stratification 36
8: Current and wave ripples in proximal siltstone facies 37
9: Graded shell bed overlain by tempestite sequence 38
10: Proximal interbedded mudstone facies 40
11: Lingula in life position, proximal interbedded mudstone facies 41
12: Lingula in life position, in silty carbonate mudstone, vertical section 42
13: Cross-stratified shell bed, proximal limestone facies 44
14: Paired vertical burrows, bedding plane exposure, proximal limestone facies 46
15: Diplocraterion colony exposed in cross section, silty interbeds in mud 48
16: Arenicolites and Diplocraterion in proximal siltstone facies 50
17: Trace fossils on sole of bedding plane, proximal siltstone facies 51
18: Faunal abundance vs.lithology: Field Census Data 54
19: Faunal abundance vs.lithology: Bulk Sample Data 55
20: Faunal abundance vs. stratigraphic position, lithology: Gros Ventre 56
21: Faunal abundance vs. stratigraphic position, lithology: Blacktail Creek 57
vi
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FIGURE
List of Figures (continued)
____________________________________ PAGE NUMBER
22: Faunal abundance vs. stratigraphic position, lithology: Sandy Hollow 58
23: Faunal abundance vs. stratigraphic position, lithology: Dalys Spur 59
24: Faunal abundance vs. stratigraphic position, lithology: Hidden Pasture 60
25:Faunal abundance vs.lithology: proximal interbedded mudstone facies 61
26: Faunal abundance vs.lithology: proximal siltstone facies 62
27: Faunal abundance vs.lithology: proximal limestone facies 63
28: Disarticulated Lingula on bedding plane 68
29: Regional stratigraphy 70
30: Interpreted facies relationships for the Dinwoody Formation 71
31: Taphonomy: Lingula in life position 74
32: Taphonomy: Lingula disarticulated by storm events 75
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INTRODUCTION
The mass extinction event at the end of the Permian eliminated over 90% of
marine invertebrate species and marked the transition between Paleozoic and
Modem evolutionary faunas (Raup, 1979). Marine ecosystems were devastated by
this intense interval of extinction, and full recovery did not occur until the Middle
Triassic, after a period of five to ten million years (Schubert and Bottjer, 1995).
Earliest Triassic faunas worldwide comprise low diversity ecosystems, dominated
largely by species of the bivalve Claraia and by inarticulate brachiopods traditionally
assigned to the genus Lingula (Wignall and Hallam, 1992). Individual species of
Lingula enjoyed pandemic distributions unprecedented in the history of lingulide
brachiopods (Xu and Grant, 1992).
A number of paleoecological patterns have been described from the
aftermaths of mass extinctions, and previous workers have postulated a variety of
underlying strategies for these patterns. Terms such as 'bloom taxa', 'disaster taxa',
'Lazarus taxa', and so forth, characterize the behavior of taxa during successive
phases of the recovery interval (Bottjer, 1999). Lingula has been interpreted as a
potential disaster taxon (Schubert and Bottjer, 1995), a designation which depends
first upon the definition of the term and secondly upon the demonstration that
Lingula was quantifiably abundant during the survival interval. The abundance and
relative proportion of Lingula among other benthic invertebrates preserved in
Griesbachian strata was evaluated in the course of this study, and this work
supports this designation (sensu Bottjer, 1999).
The underlying strategies for the success of disaster taxa may be ascribed to
either opportunism within faunally depauperate, environmentally normal marine
settings, or to adaptations to environmental stresses associated with the extinction.
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Differing distribution patterns can be predicted for each of these evolutionary
strategies, and thus the two may be distinguished within the fossil record. Any
discussion of controls over paleoecological patterns during the Griesbachian must
account for the relative influence of environment and ecology over the distribution
of fauna, including Lingula. The behavior of lingulide brachiopods during this
recovery interval should also help to resolve questions over whether these animals
are specialized towards stressful nearshore settings or are ecological opportunists in
modem settings as well.
Field-based studies of paleoenvironment and paleoecology may be used to
examine the predictive power of each of several models proposed to explain the
proliferation of Lingula. The nearshore specialist model (Newell and Kummel
1942) predicts essentially modem distribution and preservation patterns for Lingula
(eg. Kowalewski, 1996). Dysaerobic specialists (disaster taxa sensu Harries et al.,
1996) should be preserved in dysaerobic, quiet-water, basinal settings and
biofacies, or be associated with indicators of pervasive anoxia in other
environments. Ecological opportunists would be expected in a variety of
environments and show a range of preservational modes. Finally, it must be
demonstrated that the unusual abundance of Lingula in this interval was not the
result of preservational biases during the Griesbachian.
The delineation of evolutionary strategies employed by taxa in the aftermath
of extinction may provide valuable insight into the interplay of environment and
biota during these intervals, and to patterns of evolution on a broad scale. Questions
which may be addressed in this manner include the temporal and spatial extent of
environmental stresses as well as the relative intensity of their influence upon
ecosystems during the recovery interval.
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PALEOECOLOGY IN THE AFTERMATH OF MASS EXTINCTION
Previous paleoecological studies indicate that different taxa play different
ecological roles throughout the survival and recovery interval after mass extinction,
depending on local conditions. Lazarus taxa disappear during the extinction interval,
only to reappear after environmental conditions have returned to normal (Jablonsld,
1986). Elvis taxa appear to have the same temporal distribution as Lazarus taxa, but
are in fact unrelated to their ostensible ancestors; they are morphologically
convergent but phylogenetically divergent (Erwin and Droser, 1993). Progenitor
taxa evolve during the extinction and radiate afterwards (Harries et al., 1996).
The definition of disaster taxa was first introduced (by Fischer and Arthur,
1977) with specific reference to work on opportunistic taxa in the fossil record (e.g.
Levinton, 1970). Since then, a multitude of differing usages have been applied for
this term, generally focusing on the coincidence of environmental stresses with the
proliferation event (e.g. Harries et al. 1996). The definition used here follows that
of Bottjer (1999), and reemphasizes the role of opportunists as originally
delineated. Disaster taxa are considered here as: taxa which have long stratigraphic
ranges above and below the extinction interval, but which occur in high abundance
during the extinction or survival intervals through opportunistic expansion within
stressed communities. It is important to note that while stressed communities are
vital to this definition, this need not be a continued environmental stress, but may
refer simply to those communities which have not yet recovered from the extinction
itself. The description of Early Triassic stromatolites as 'disaster forms' (Schubert
and Bottjer, 1992) is consistent with this usage; they represent a biosedimentary
fabric which follows the same distribution pattern as a disaster taxon. In this sense,
normal marine stromatolites became abundant in the Early Triassic through the
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extinction and geographic restriction of bioturbating organisms, rather than through
the expansion of hypersaline, marginal marine settings to which they have been
largely restricted during the Phanerozoic.
The patterns of faunal succession during the extinction, survival and
rebound intervals are thus described by the differing evolutionary strategies of taxa
(Harries et al., 1996, Bottjer 1999). During the extinction interval itself, progenitor
taxa evolve and environmental specialists proliferate opportunistically. In the
survival phase, disaster taxa and short-ranged ecological opportunists would be
particularly abundant, while progenitor taxa radiate and give rise to the fauna of the
rebound interval. In the rebound and expansion phases, Lazarus taxa begin to return
from refugia along with their imitators, Elvis taxa. Taxa which dominated the
survival interval are displaced by the influx of more specialized taxa, and find
themselves ecologically restricted to stressful settings, or driven to extinction.
Fitting these paleoecological patterns to models for the relative abundance of
taxa following distinctive evolutionary strategies may give a clearer picture of the
true conditions of the extinction and recovery. In the Lower Triassic, several taxa
show some of these patterns clearly. Griesbachian strata worldwide are
characterized by a cosmopolitan faunal assemblage depauperate in taxonomic
diversity, dominated by ‘paper pectens’ such as Claraia, and inarticulate
brachiopods traditionally assigned to the genus Lingula (Wignall and Hallam, 1992)
Despite the head start their early success may have provided these taxa in seeding
the subsequent recovery and radiation of benthic invertebrates, they were later
displaced; the communities of western North America show the slow influx of
Lazarus taxa during the later stages of recovery (Schubert and Bottjer, 1992).
Claraia has been described as a potential progenitor taxon, but the genus did not
survive past the Spathian, so it might be better characterized as an opportunist
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(Wignall and Hallam, 1997). Stromatolites have been regarded as disaster 'forms',
and Lingula as a potential disaster taxon (Schubert and Bottjer, 1992).
It should be possible to distinguish whether environmental conditions
controlled the distribution of disaster taxa, or whether they underwent ecological
release from restricted habitats. This process involves detailed examination of the
distribution of the taxon within the recovery interval relative to paleoenvironmental
conditions. A variety of methods may be used in this process, including
sedimentology and stratigraphy in the determination of facies, paleoecology and
taphonomy as tools to interpret distribution, and ichnology to examine the
interactions between fauna and sediment. In turn, the patterns observed may be
used to investigate the interplay of biogeographic factors, such as restricted habitat
availability due to environmental stresses, and patterns of ecological succession
during the recovery interval.
On the basis of trace fossils and valve morphology, the life habits of
lingulids are considered to be extremely conservative from an evolutionary
perspective, and interpretations of their paleoenvironmental and paleoecological
significance are rooted in this assumption (e.g. Rudwick, 1970; Szmuc et al., 1976;
Savazzi, 1986). Thus, the distribution of Lingula in the Early Triassic was almost
certainly due to causes extrinsic to their biology, such as changes in environment,
ecology, or preservation potential in the aftermath of the extinction. This makes
Lingula an excellent tool for distinguishing the effects of each factor in controlling
faunal distribution in the Early Triassic.
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THE END-PERMIAN MASS EXTINCTION:
Magnitude and Timing:
The first half of the Phanerozoic Eon was characterized by the evolution and
radiation of a vast array of benthic invertebrates, particularly the articulate
brachiopods, crinoids and bryozoans through the upper Paleozoic. The end of the
Permian brought a dramatic end to the dominance of the Paleozoic Fauna, marked
by the extinction of 57% of all skeletonized marine invertebrate families, and over
90% of species (Raup, 1979). The ecology and evolutionary history of the
Mesozoic was shaped by this extinction, allowing a vast array of molluscs to
dominate marine communities ever after. This is the largest, and in many ways the
most significant, mass extinction of the Phanerozoic.
The timing of the end-Permian mass extinction has been constrained recendy
through detailed stratigraphic work, paleontology and radiometric dating of
boundary sections in South China. The main phase of extinction took place within
the last million years of the Permian, perhaps in less than 10,000 years, 249 million
years ago (Bowring et al., 1998). The duration of the aftermath is not as well
constrained, but the survival interval is estimated to have lasted the entire Early
Triassic, at least 7 million years in duration (Hallam and Wignall, 1997).
Proposed Causes and Associated Phenomena:
A multitude of potential killers have been proposed in the debates over the
causes of the end-Permian mass extinction (e.g. Erwin, 1993; Hallam and Wignall,
1997). Many of these proposed causal mechanisms are linked to global
environmental and geochemical changes within the extinction interval. These can be
summarized as the net result of global warming in the aftermath of the
Gondwanaland glaciation, concurrent with the eruption of massive flood basalts in
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the Siberian Traps. The precise interplay of elements is not as yet fully understood,
but several major factors have been identified. These are: global marine stratification
and possible overturn, associated with superanoxia and hypercapnia (e.g. Knoll et
al., 1996; Isozaki, 1997), large shifts in stable carbon and sulfur isotope records
(e.g. Wignall et al. 1998), a pronounced increase in fungal spores (Visscher and
Brugman, 1988), marine regression followed by Early Triassic transgression (e.g.
Pauli and Pauli, 1994), and the emission of sulfates and carbon dioxide from the
largest terrestrial flood basalt eruptions of known history (e.g. Campbell et al.
1992). Various linkages between these factors may have exacerbated the extinction
and prolonged the recovery interval (Erwin, 1993).
The Carboniferous and Early Permian were characterized by 'icehouse'
conditions of intense polar glaciation during the amalgamation of Pangaea (Fischer
1981). In contrast, the Triassic and later Mesozoic fell into 'greenhouse' conditions,
with little or no polar ice and shallow latitudinal temperature gradients (Fischer
1981). The transition between the two states is implicated in the mass extinction
through associated changes in marine circulation and biogeochemical cycles (e.g.
Hallam and Wignall, 1997). Lower Triassic strata record tropical and subtropical
faunas at high latitudes, including Lingula specimens as far north as Spitzbergen
(Wignall etal. 1998).
Upper Permian and Lower Triassic deep marine cherts indicate pervasive
anoxic conditions existed during the boundary interval, presumably due to slowed
circulation and density stratification (Isozaki, 1997). Bottom waters became
concentrated in CO2 and H2S and depleted in O2 ; all three factors are highly
antagonistic to aerobic respiration in metazoans (Knoll et al., 1996). Localized areas
of upwelling attest to the chemistry of bottom waters through unusual sediment
7
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precipitation, such as the carbonates of the Capitan Reef in Texas (Grotzinger and
Knoll, 1995).
The marine transgression of the earliest Triassic shows little evidence of
onlap onto heavily eroded Permian strata (Hallam and Wignall, 1997). Early models
suggested that regression contributed to extinction through simple species-area
relationships with diminished shelf ecospace, although this would have been
countered by exposed island flanks (Jablonski, 1986). Wignall and Hallam (1992)
suggest that the transgression brought dysaerobic waters onto shallow shelves,
further restricting available ecospace. It is not clear how transgression would
overtake larval dispersion in this model, and other authors suggest that such a
shallow dysaerobic layer would be geochemically infeasible in any case (Bowring et
al., 1998).
While pelagic marine sediments of Permian age are exceedingly rare, Isozaki
(1997) identified boundary-interval chert deposits accreted onto Japan and British
Columbia, Canada. These cherts provide a pelagic sedimentary record across the
extinction interval from both sides of the Panthalassic Ocean. A prolonged period of
anoxia stretching from the Late Permian to the Early Triassic was identified, with an
intense, 'superanoxic' pulse at the end of the Permian (Isozaki, 1997). Wignall and
Hallam (1992) have argued that these conditions extended into shallow water
settings on the basis of trace fossil and sedimentological evidence.
A number of unifying biological criteria within the Paleozoic Fauna suggests
that the group, as a whole, was particularly sensitive to CO2 poisoning, or
hypercapnia (Knoll et al., 1996). It may be worth noting at this point that a number
of alternative spellings to this term have arisen within the literature of the end-
Permian extinction; the use of ‘hypocapnia’ and ‘hypercania’ is incorrect, although
‘hypercarbia’ is a synonymous term. A number of similarities exist between the
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Late Permian and late Proterozoic oceans which may have been conducive to the
buildup of CO2 in a stratified ocean; destratification or upwelling would have
released CO2 into shallow waters and the atmosphere (Knoll et al., 1996). The
Paleozoic fauna as a whole would have been sensitive to buildup of toxic CO2
levels due to the common sessile, epibenthic mode of life and rudimentary
circulatory systems of crinoids and articulate brachiopods. Active, phosphatic,
infaunal or nektonic organisms with complex circulatory systems were far less
sensitive to hypercapnia, and appear to have survived preferentially. No evidence of
marine CO2 outgassing has been found at the Permo-Triassic boundary, but shelf-
edge seafloor carbonate precipitates have been described from Lower Triassic
sediments off the western margin of Pangaea (Woods and Bottjer, 1998). The deep-
ocean overturn hypothesis, as proposed by Knoll et al. (1996), may include an
autocyclic component, and thus periodic pulses of CO2 saturated bottom waters
may have inhibited marine recovery.
Geochemical data suggest global shifts in carbon and sulfur isotope marine
circulation and sedimentation occurred within the extinction interval. The large scale
negative d1 3 C shift was estimated to average from +2 to 0 per mil for a whole ocean
shift (Bowring et al., 1998) at the extinction boundary. This shift may be the result
of one or more major factors: global productivity collapse, the release of mantle
derived carbon from flood basalts, exposure of shelf gas-hydrates during marine
regression and erosion, changes in ocean stratification and circulation, and possible
extraterrestrial carbon sources (Bowring et al., 1998). While the sulfur record is
less understood, the sulfur-rich flood basalts of the Siberian Traps may have
injected vast quantities of SO2 into the atmosphere (Campbell et al., 1992)
Recent research on the stability of gas-hydrates on ocean shelves has led to
their potential implication in several periods of global climate change. Temperature
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and pressure strongly regulate the stability of these solids on and in the sea floor,
and climate or eustatic sea-level changes could result in the catastrophic release of
methane. A significant greenhouse gas, the release of 2500 Gtons of methane into
the atmosphere would account for the carbon isotope shifts, further aggravate global
warming trends and contribute to the inhibition of deep marine circulation (Erwin,
1993)
The implication of a bolide impact in the end-Cretaceous extinction has led
to extensive work attempting to identify similar evidence for other mass extinctions.
As of yet, no conclusive evidence has yet been presented for an impact of
significant size and appropriate age, as discussed recently by Rettallack et al.(1998).
Cometary carbon may be an ideal source for the anomalies seen in the stable isotope
record, although this hypothesis is not endorsed as a source (Bowring et al., 1998).
Life in the Recovery Interval:
Lower Triassic strata record evidence of one of the fastest marine
transgressions in Earth’s history, flooding vast areas at rates estimated up to 50 cm
of shoreline retreat per year across the exposed continental shelf of western Pangaea
(Pauli and Pauli, 1994). This rapid transgression is marked by a barely-incised
topography, suggesting that the western United States was essentially a featureless
plain before tectonic subsidence resumed under conditions similar to those of the
late Permian (Pauli and Pauli, 1994).
Griesbachian age strata are characterized by a few genera of bivalve
molluscs, including Unionites, Claraia, Promyalina and Eumorphotis, as well as
inarticulate brachiopods traditionally assigned to the genus Lingula (Wignall and
Hallam, 1997). Few of these taxa gave rise to important players in the Mesozoic
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radiations, and many were displaced by Lazarus taxa as the rebound began during
the Smithian and Spathian (Schubert and Bottjer, 1995).
The causes of low diversity in the Early Triassic have not yet been
constrained. Lingering stresses such as an expanded dysaerobic layer (Wignall and
Hallam, 1992), or episodic stresses such as pulses of hypercapnia (Knoll et al.,
1996; Woods and Bottjer, 1998) might also suppress the recovery from this mass
extinction. Paleoecological patterns of recovery in the western United States suggest
that a process of ecological succession took place within aerobic marine settings, in
which the return of Lazarus taxa and their descendants displaced unspecialized,
opportunistic taxa (Schubert and Bottjer, 1995). Deciphering the inter-relationships
between environmental stress and ecological controls in Early Triassic communities
is integral to any attempt to describe the patterns of biotic recovery.
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Biology of Lingulide Brachiopods
Morphology:
Inarticulate brachiopods have a long stratigraphic record distinguished
chiefly by an obstinate avoidance of evolutionary innovation since their origination
during the Cambrian. Lingulide brachiopods are characterized by possession of two
unhinged oblong chitinophosphatic valves of nearly equal size, a relatively large
body-to-mantle cavity ratio, a spiralophe arrangement to the lophophore and a long,
contractile pedicle (Rudwick, 1970). Ordovician lingulide brachiopods have been
described with shell ornamentation in the form of asymmetrical ridges having
steepened faces facing towards the posterior of the valve, and are consistent with
the anterior-first burrowing behaviors displayed by modem forms (Savazzi, 1986).
Life Habit:
In the modem day, Lingula is typically considered to occupy a shallow
infaunal suspension-feeding niche in fine-grained siliciclastic sediments, in patches
within intertidal, subtidal settings, often within dysaerobic, silty mud (eg. Paine,
1963). While infaunal, they may survive on the sediment surface if exhumed;
Lingula may not be capable of reburrowing, but Glottidia can do so (Thayer and
Steele-Petrovic, 1975). Lingulids use behavioral responses rather than physical
adaptations to avoid predation and competition and to maximize uptake of oxygen,
retreating into or extending partially from their burrows, or moving their burrows
laterally away from other individuals (Thayer and Steele-Petrovic, 1975).
Reproduction is performed sexually, with the release of 2,200 to 47,000 eggs per
spawning by Glottidia in modem settings (Paine, 1963). The number of spawning
episodes per year is controlled by temperature and latitude, and the planktotrophic
larvae allow rapid, wide dispersion and colonization of suitable environments
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(Hammond, 1982). Spawning takes place during spring tides, chiefly during the
full moon, and the larvae are able to maintain position in the water column actively,
tending towards the water surface (Paine, 1963). The distribution of larvae can
cover extremely wide areas, as they have been found thousands of kilometers from
shore and are capable of delaying metamorphosis until suitable substrates are
reached (Paine, 1963; Hammond, 1982).
Environmental and ecological controls on distribution:
The main environmental controls over lingulid distribution in the modem
day are temperature and salinity: while able to survive extreme fluctuations in
salinity, lingulids do not occupy consistently brackish water, nor in North America
do they extend in range further north than Virginia or Monterey, California (Paine,
1963). Lingulids are inactive at temperatures below 10°C, and can survive in
brackish water for weeks, depending on salinity (Paine, 1963). The controls on
lingulid populations appear to be ecological in nature, as they occupied open marine
shelves within the Cambrian (Droser and Li, 1997), but have been largely restricted
to marginal nearshore environments as early as the Ordovician (Patzkowski, 1995).
They are typically restricted in sediment occupation to silty or sandy bottoms with
varying degrees of mud (Emig, 1997).
Lingulids are demonstrably tolerant of fluctuations in bottom water oxygen
levels due to their use of the blood pigment hemerythrin to store oxygen (Emig,
1997). Lingula has been described as occupying burrows within sediments which
penetrate the redoxicline, but the burrow itself is typically well ventilated (Emig,
1997). Lingulids have also been described in early Paleozoic dysaerobic basinal
settings (Kammer et al, 1986). Other inarticulate brachiopods, such as Discinisca,
have been shown to be confined to deep, dysaerobic settings and remain there to
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this day (Emig, 1997). Discinisca is the dominant macrofossil in the immediate
aftermath of the Cenomanian-Turonian extinction, and has been interpreted as a
disaster taxon (Harries et al., 1996). In this interpretation Discinisca was an
environmental specialist, particularly adapted to the stressful conditions which
caused the extinction. Lingula has been been observed occurring in dysaerobic
facies along with Claraia during the Griesbachian (Wignall and Hallam, 1992).
If Lingula is an indicator of dysaerobic conditions, as Discinisca appears to
be, then the widespread occurrence of Lingula in Lower Triassic strata may parallel
Discinisca in the earliest Turonian. In this scenario, Lingula attained widespread
distribution due to continuing global stresses in the aftermath of the end-Permian
extinction, and occupied settings which contained too little oxygen for diverse
assemblages of organisms to survive. The cessation of severe anoxia later in the
Triassic resulted in reoxygenation of these settings and colonization by newly
radiating species and Lazarus taxa, driving out the relatively specialized dysaerobic
forms, including Lingula.
This interpretation is subject to argument. While lingulaceans have been
shown to occupy slope and basinal settings (Patzkowski, 1995), and lingulids were
major faunal components of dysaerobic settings prior to the Late Devonian, they are
not known from such settings in the late Paleozoic (Kammer et al. 1996), and
Lingula itself is largely restricted to oxygenated nearshore settings at present (Emig,
1997). It is not clear that Lingula could thrive under persistent anoxic conditions.
The question of whether L ingula is a nearshore specialist or an
opportunistic generalist in modem oceans has been raised by previous authors
(Paine, 1963; Emig, 1997). If it is an opportunist, it would be expected on purely
ecological reasons to be abundant in the aftermath of a mass extinction. Lingulids
are dominant nearshore, shallow water forms in stable sediments, particularly in
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well oxygenated shoreface facies (Emig, 1997). Thus, the ecological and
environmental associations of lingulids in the fossil record and present day provide
a wealth of possible models for their proliferation in the Early Triassic. As a result,
a variety of models have been supported to explain the distribution of Lingula and
other potential disaster taxa. These are explored in the following chapter.
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MODELS FOR THE PROLIFERATION OF LIN G U LA:
Lingulids are uncommon fossils, and the widespread ‘proliferation of
Lingula’ in the Early Triassic has been described as unparalleled in their long
evolutionary history (Xu and Grant, 1992). The conservative lifestyle of this clade
suggests that the causes for this proliferation were extrinsic ones, involving changes
in ecology or environment rather than evolutionary innovation.
A number of authors have proposed environmental causes for the
proliferation of Lingula. Newell and Kummel (1942) suggested that abundant
nearshore settings conducive to Lingula were created by eustatic sea level changes
on the shelves surrounding Pangaea. This model was supported by Xu and Grant
(1992) in their discussion of the pandemic distribution of Lingula. Wignall and
Hallam (1992) have noted that Lingula, along with Claraia, may have been highly
tolerant of widespread dysaerobic conditions in shallow waters during the Early
Triassic transgression. This may lend support to a model in which Lingula
proliferated as a dysaerobic specialist under similar conditions to Discinisca during
the Cenomanian-Turonian extinction (Harries et al., 1996). In contrast, Schubert
and Bottjer (1995) regarded Lingula as a potential ‘disaster taxon’, and ascribed its
success to ecological opportunism within a model of evolutionary succession.
Each of these proposed causes imply different patterns of preservation,
based on both the lifestyle and taphonomy of modem lingulids. Thus it should be
possible to discriminate between these causes on the basis of Lingula abundance,
as well as faunal and facies associations.
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Nearshore Specialists:
If Lingula proliferated during the Early Triassic due to widespread shallow-
water nearshore conditions, the pattern of preservation should not differ appreciably
from what is observed today. The association of dense patches of Lingula in life
position among rippled beachface sands or intertidal mudflats (as per Kowalewski,
1996) would be predicted by this model. Disassociated valves and fragments should
also occur parallel to bedding, single or in groups, but less often than in situ valves
due to the fragility of the valves and their susceptibility to breakage through wave
action (Kowalewski, 1996). Banks of concentrated Lingula shells occur near the
strand line after storms at present (Kowalewski, 1996; Emig 1997) and might also
be predicted for the nearshore scenario. Valve transport, fragmentation and sorting
are expected consequences of the high wave energy of this setting.
Dysaerobic Specialists:
The dysaerobic specialist model integrates the observed association of
Lingula with dysaerobic facies (Wignall and Hallam, 1992) and the environmental
specialist model for the proliferation of disaster taxa (Harries et al., 1996). It
predicts the occurrence of Lingula in situ within distal or basinal laminated shales,
associated with euhedral pyrite crystals, pyritized faunas, or trace fossils such as
Chondrites which are diagnostic of low-oxygen conditions. Low wave energy is
characteristic of such settings, and erosional scour or transport of Lingula valves
should not occur. However, modem lingulids respond to low bottom water
oxygenation by emerging from their burrows (Thayer and Steele-Petrovic, 1975).
Paired valves on bedding planes might therefore be consistent with this model. Out-
of-habitat transport, on the other hand, should not occur in the dysaerobic
opportunist scenario.
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Ecological Opportunists:
The ‘ecological opportunist’ model predicts the occurrence of abundant
Lingula in patches among a variety of depositional settings and faunal associations.
No one preservational mode might be predicted, since a number of different habitats
might be occupied, each with differing depositional regimes. This model suggests
preservation beyond beachface and dysaerobic deep water settings, but does not
exclude either as potential habitat. Lingula is expected to live infaunally, as modem
forms do, but lingulids can survive on the sediment surface so long as predation is
low, as modem representatives of Lingula can not re-burrow after adulthood
(Thayer and Steele-Petrovic, 1975). It is not clear that all lingulid brachiopods
occupied an infaunal niche (Rudwick, 1970), and modem forms are known to be
able to attach their pedicles to pieces of shell (Davidson, 1888; Paine, 1963). In
depositional settings subject to extensive scour, lingulids may thus have filled an
epifaunal niche.
Complications arise when valve transport is considered, as Lingula valves
are very light and might be easily carried by currents to habitats beyond those which
were actually inhabited. However, the fragility of modem Lingula valves suggests
that this taphonomic process should have minimal influence on the distribution of
valves. Trace fossils, paired valves and patches of Lingula in situ should be free
from this bias. In addition, transported valves should display a degree of
fragmentation and sorting by valve size. Thus, constraints can be put on the
influence of out-of-habitat transport through careful taphonomic study.
Taphonomic Bias:
The specter of taphonomic bias has been raised regarding the abundance of
lingulide brachiopods during the Phanerozoic (Kowalewski and Flessa, 1996).
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These questions must also be addressed in order to interpret the data in an
appropriate manner. While the taphonomy of Lingula may vary depending on
environmental setting, it must be demonstrated that its relative abundance is not a
byproduct of extraordinary preservation. Often the extraordinary preservation of
delicate fossils is enhanced by rapid burial and environmental stresses such as
anoxia, which inhibit decay and bioturbation which could destroy soft body parts;
examples include the Solnhofen limestone (Allison and Briggs, 1991). In addition,
the phosphatic skeleton of Lingula would be comparatively resistant to chemical
dissolution relative to carbonate skeletal elements.
Other factors may improve the preservation of Lingula in the Griesbachian.
The importance of storm event beds in the preservation and fossilization of lingulids
parallel to bedding has been emphasized by several authors (Kowalewski, 1996;
Emig, 1997). As event beds are common throughout the Phanerozoic, the
association of Lingula with them in the Griesbachian may simply indicate the
expansion of habitat into shelf settings, such as those occupied by inarticulate
brachiopods during the Cambrian (Droser and Li, 1997).
A study of Lingula preservation mode and quality was performed to evaluate
the possibility that significant taphonomic bias exaggerated its apparent abundance.
In addition, attention was paid to the relative mode and quality of preservation of
other taxa for comparison and evaluation of potential biases.
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EARLY TRIASSIC DEPOSITION IN THE WESTERN U.S.
The Din woody Formation:
Griesbachian age deposition in the western USA appears to have been
concentrated within the reactivated Phosphoria Basin (Pauli and Pauli, 1994),
leading to the deposition of the Dinwoody Formation in southwestern Montana,
eastern Idaho, western Wyoming and northern Utah (Figure 1). The Dinwoody
Formation records the earliest Triassic transgression across a flat, featureless plain,
proceeding at the rapid pace of 35 to 50 cm per year. (Pauli and Pauli, 1994). While
the tectonic regime remained similar to that of the Permian age Sublett Basin (Pauli
and Pauli, 1994), the shelf mudstone, siltstone and limestone units of the
Dinwoody Formation are clearly distinct from the chert, phosphorite and anoxic
limestone deposits of the underlying Phosphoria Rock Complex. The contact
between the formations is unconformable, but little evidence exists to suggest
extensive erosion during the lowstand interval (Schock, 1981). Because of this
unconformity, the mass extinction event at the Permo-Triassic boundary is not
recorded (Newell and Kummel, 1942; Schock, 1981).
The Dinwoody Formation interfingers to the east with the marginal marine
red beds of the Woodside and Red Peak Formations, which eventually prograde
across the basin and overlie the Dinwoody Formation at most localities. Open
marine deposition resumed in the Smithian to Spathian, with transgression of
dysaerobic waters across the shelf during early Thaynes Formation deposition.
Thus, the Dinwoody and Woodside Formations record as single transgressive-
regressive sequence, with the bulk of the Dinwoody Formation deposited during
highstand and early regression.
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A number of previous authors have noted the prevalence of Lingula fossils
within the Dinwoody Formation compared to the underlying Phosphoria Rock
Complex and overlying Woodside and Thaynes Formations (Newell and Kummel,
1942; Kummel, 1957; Schock, 1981). Reconnaisance of the Dinwoody Formation
exposures noted in these and other previous studies indicated that Lingula was
particularly abundant in southwest Montana, along the northern margin of
deposition and coincident with the extent of limestone facies.
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METHODS
Field work:
Five sites were selected for study on the basis of Lingula abundance,
exposure and accessibility: four in southwestern Montana and one in northwestern
Wyoming. The sites at Blacktail Creek and the Gros Ventre range were identified
from the work of Newell and Kummel (1942) and revisited by Schock (1981), who
included sites at Dalys Spur, Sandy Hollow, and Hidden Pasture. This last locality
was also studied as part of a pilot project on the paleoecology of the Dinwoody
Formation (Schubert and Bottjer, 1995). A handheld Global Positioning System
receiver was used to pinpoint these localities for future work. Stratigraphic sections
were measured at each locality with attention to lithology, sedimentary structures,
trace fossils and body fossils. The localities are shown in Figure 1.
Further work at each locality included a field census based paleoecological
study, in which bedding plane assemblages were sampled with respect to the faunal
content. When possible, single bedding planes were used, although in some
horizons a limited degree of field-based quarrying and disaggregation were used
within 10 cm intervals. These planes or beds were identified in stratigraphic
context, and major sedimentological features noted along with the quality of faunal
preservation. On bedding planes, one comer of an exposed surface was selected
and identifiable shells counted systematically along a tightly meandering search path
until a standard sample of 100 preserved skeletal elements was counted and
identified to class level, or to generic level if preservation quality allowed. Each
element was marked in order to prevent recounting. In the case of fragmented
valves, only individual elements comprising more than 50% of the original valve, or
associated with molds of a whole valve, were included in order to avoid
overestimates of abundance.
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Montana
LOCALITIES
1: Sandy Hollow
2: Dalys Spur
3: Blacktail Creek
4: Hidden Pasture
5: Gros Ventre
Wyoming
Figure 1: Localities used in this study. The extent of Dinwoody Formation
deposition is shown across Idaho, Montana and Wyoming; the depocenter of the
basin is in southern Idaho. The sites studied are concentrated along the northern
margin of the basin, and do not include the ‘basinal’ outer shelf settings of previous
authors (e.g. Wignall and Hallam, 1992).
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Lingula was the only taxon for which this procedure was necessary, as all other
taxa were composed of either more mechanically durable materials (eg. aragonitic
valves) or easily disaggregated individual elements (eg. echinoid spines or fish
scales). Not all of the counted faunal elements possessed two valves, however;
gastropods possess only one, while echinoid spines and fish scales or bones
represent a small fraction of the total skeleton. Only fauna visible to the naked eye
were counted, but microscopic gastropods have been identified in bulk sample. A
grand total of 21 field counts were made, for comparison to the bulk sample data.
As the sampling procedure for the field counts was not focused on beds with
abundant Lingula, this should be a more accurate paleoecological study of the
Dinwoody Formation, and may allow some estimation of the bias of the bulk
sample collection procedure.
Laboratory Analysis:
Bulk samples of approximately 10-12 kg, filling five 10 cm x 15 cm sample
bags, were taken from single or amalgamated beds which contained common or
well preserved Lingula fossils. In sum, 23 samples were collected for laboratory
study from these five localities. The difficulty of collecting from the massive
mudstone unit, combined with the poor cohesion of this material, resulted in sub
standard volumes from a few horizons. The collection methodology poses an
inherent bias in this study, as the presence and abundance of Lingula in the field
determined the collection horizons of bulk samples. However, the data collected
here is still useful, as Lingula abundance can be correlated with both stratigraphic
and petrographic information to determine whether a strong facies control governed
the preservation of this fossil. The horizons from which these samples were taken
were noted in stratigraphic context, along with relevant sedimentological features.
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A bulk sample paleoecological study was performed, using a methodology
essentially equivalent to that of the field counting procedure. Beginning with the
first bag of rock sampled at each horizon, the first 100 skeletal elements were tallied
and marked and identified to class level or beyond, if preservation allowed. Only
one sample had fewer than 100 identifiable valves within the bulk of rock collected.
In the lab, representative samples were chosen from each bulk sample
horizon and vertical slabs cut from each. A laboratory based ichnology study was
performed using macroscopic and microscopic hand sample inspection, thin section
analysis, and X-radiography to identify sedimentary structures or burrows which
could not be identified by the naked eye. The phosphatic shells of Lingula were
easily identified in cross section by all methods. Cut slabs and hand samples were
examined under a stereoscopic microscope using reflected light, and observed
faunal and sedimentary features were noted. Thin sections were prepared from
vertical cross sections representing each bulk sample horizon, and examined
microscopically under both reflected and transmitted light. Estimates were made of
grain size and type, shell packing, and taphonomic factors (following Kidwell,
1991). A semi-quantitative measure was used to estimate the percent of sediment
composed of skeletal elements, identified to class level (taken from Kidwell, 1991).
Several techniques were explored to quantify the abundance and
preservation of Lingula, both on its own and in relationship to the other fauna at
Dalys Spur or Hidden Pasture. Each of these methods has strengths and
weaknesses, but all demonstrate the presence and relative importance of Lingula to
the overall biogenic sediment component. These methods include taxonomic shell
census of bulk samples, a taphonomic census of Lingula within hand samples, a
semi-quantitative estimate of whole fauna and Lingula shell percentages from
vertical thin sections, and Lingula valve counts from acid-etched vertical slabs. Each
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of these has been performed at a small scale to explore the feasibility and usefulness
of extrapolating the methods to field census and bulk sample analysis.
The taxonomic census used all bulk samples and field counts. This study
consisted of shell counts and identification along bedding plane exposures, sample
by sample, to a shell count of 100. Lingula is easily identifiable by the presence of
primary chitinophosphatic valves or fragments, as well as shell molds.
Identification of bivalves was attempted using photographs and drawings from
previous work (Newell and Kummel, 1942; Schubert, 1993). Bivalve
identification, however, proved to be largely impossible at the genus level from
bedding plane exposures, due to extensive micritization, sparry replacement, and
cementation of primary shell material. Only surface ornamentations could be
identified from the bulk of the bedding planes, consisting of radial or concentric
ridges along valves, and diagnostic valve outlines were largely obscured by other
shells or matrix.
A more detailed taphonomic census of Lingula exclusively was performed
according to many of the criteria for modem lingulide taphonomy set forth in
previous work (Kowalewski, 1996). These included: pairing of valves, degree of
shell blackening, decay and fragmentation, orientation of valves relative to the
substrate, and both whole valve and fragment size. Directional alignment was
measured towards the anterior margin of whole valves or molds, parallel to the long
axis of the valve, relative to an arbitrary fixed axis on the rock. For this reason,
single hand samples were used, and the alignment data from each cannot be pooled
together. Orientation relative to the bedding plane could be horizontal, concave up,
concave down, or flattened, or inclined relative to the bedding plane. The presence
of blackening on valves was noted, and both single valves and fragments were
tallied to quantify their relative abundance. Shell pairing was not noted in this
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census. Length was measured along the long axis of whole valves or molds, which
is the major direction of growth, whereas fragments were measured along their
longest axis, even when this was clearly perpendicular to the direction of growth.
No fragments less than 2 mm across were measured or tallied, as according to
previous taphonomic work on shell beds (Kidwell, 1991).
A semi-quantitative method for estimating relative valve percentage within
samples (Kidwell, 1991) was used with thin sections to investigate the volumetric
contribution of the whole fauna to the sediment as a whole, and the relative
proportion of Lingula. In order to avoid sampling bias towards high-density shell
clusters, the thin section was placed under the microscope blindly, which provided
a random sample. When only part of the thin section was visible due to this random
placement, the shortest possible adjustment was made to fill the field of view. Two
estimates were made: first, the percent contribution of all biogenic components to
the sediment was evaluated, and second, the percent contribution of Lingula alone
to the sediment. Lingula stands out in stark contrast to carbonate material, and is
well represented in this manner. This process was repeated three times per thin
section, for all horizons studied, and the results roughly averaged. As this is not
strictly quantitative as measurements go and since the proportion of valves can be
highly variable, a margin of error exists on the order of +/- 5%. However, a range
of numbers was used when the proportion of valves appeared to be variable relative
to the sediment.
Direct counting of valves on vertical slabs or thin sections cannot provide a
reliable census of bivalves, due to the degree of alteration, but the chitinophosphatic
valves and fragments of Lingula stand out in relief when etched in acid. However,
this method might not distinguish Lingula fragments from fragments of bivalve
periostracum. Vertical slabs were etched in IN HC1, rinsed and dried, and a
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measured area demarcated. This done, the slab was placed under a binocular
microscope and individual fragments tallied on the basis of their relief. This method
clearly does not work on shell molds.
Integration of field and laboratory data:
Using field sedimentology and laboratory thin section petrography together,
several main lithofacies were identified in the Dinwoody Formation. These facies
categories were used in turn to interpret the environments of deposition. The faunal
associations identified from the paleoecological studies were compared to the
lithofacies from which they were collected, in order to identify correlations or
patterns in faunal distribution. Where deemed appropriate, the interpreted
paleobiology of these organisms was used to gain insight into these patterns and the
paleoenvironmental conditions.
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RESULTS
Stratigraphy:
Previous workers have described the depositional history of the Dinwoody
Formation as representative of the Griesbachian marine transgression across a flat
plain followed by early regression off the shelf (Newell and Kummel, 1942;
Kummel, 1957). Four major facies are represented in the Dinwoody localities
studied herein: distal mudstone, proximal interbedded mudstone, limestone and
siltstone. The early transgression and highstand is largely recorded in the distal
mudstone facies, although a basal siltstone can be found in western Wyoming. The
regression is represented by the proximal siltstone, limestone and interbedded
mudstone facies, interfingering with and overlain by the marginal marine Woodside
or Red Peak Formations. Stratigraphic columns are presented in Appendix B.
Facies:
A spectrum between distal and proximal shelf settings is reflected in the four
facies identified in this study. Whereas a distinct break separates distal and proximal
settings, facies of the proximal shelf intergrade and interdigitate to some degree
between three end member lithologies. Distinctive variations in sediment structures
and the inferred flow regime for each of these lithologies support the distinction of
these facies from one another. Individual units cannot be correlated from one
locality to the next, thus making their use as informal members untenable (Carr and
Pauli, 1983; Pauli and Pauli, 1994).
Distal Mudstone Facies:
The distal mudstone facies comprises the lower Dinwoody Formation, and
is the finest grained sediment of the formation. Carbonate is present in the
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mudstone, as it reacts strongly with acid. The distal mudstone is grey or yellow to
brown in color on freshly exposed surfaces, and weathers to a dark grey. No
laminations finer than 1 mm were observed, although a coarser set between 0.1cm
and 1 cm thick are common. Infaunal organisms such as small Lingula specimens
have been found (Figure 2), and occasional massive beds seem consistent with
mixground bioturbation (Figure 3. Thin limestone event beds occur interbedded
within the distal mudstone facies and include microgastropods oriented parallel to
one another with apices pointed in the same direction (Figure 4).
Proximal Siltstone Facies:
The proximal siltstone facies consists of medium to thickly bedded
siltstones, amalgamated or interbedded with minor mudstone beds, with varying but
relatively minor proportions of bioclastic sediment, with minor bioturbation.
Sedimentary structures within the proximal siltstone facies include shell pavements,
gutter casts (Figure 5), load cast pillow structures (Figure 6), plane-bed
laminations, hummocky and ripple cross-strata (Figure 7), as well as bedding plane
oscillation and current ripples (Figure 8). Occasionally a perfect tempestite sequence
can be observed, starting with a basal plane-bedded siltstone, overlain in turn by
hummocky and ripple cross stratification in silt and topped with mud (Figure 9). In
rare instances a graded shell bed underlies the silt beds, illustrating the wane of
strong flow conditions. Thickly bedded siltstone beds commonly appear to deform
the underlying beds, and indicate episodically rapid deposition over soft
sediments. These are illustrated here as pillow structures from the Gros Ventre as
well as Hidden Pasture localities. Thin bedded siltstone beds may also be folded
and squeezed into the overlying thick siltstone beds.
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FIGURE 2: small Lingula valve and unidentified bivalve in distal mudstone facies.
Lingula is the small specimen indicated by the arrow, and has split parallel to shell
layering. Note relative size difference between bivalve, Lingula.
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FIGURE 3: Distal mudstone facies. Thinly bedded carbonate mudstone and
interbedded, structureless limestone. The lack of primary bedding structures is
suggestive of mixground bioturbation.
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FIGURE 4: Parallel oriented gastropods in a limestone interbed within the distal
mudstone facies. Current flow was right to left in this photo. Specimens are
indicated by arrows, and aligned parallel to current direction.
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FIGURE 5: Gutter cast truncating cross-stratification in proximal siltstone facies.
This is indicative of high flow regime conditions, causing erosional scour in the
underlying hummocky cross-stratification in the siltstone bed. Contact indicated by
arrows.
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FIGURE 6: Load casting structures in proximal siltstone facies. Rapid deposition of
siltstone event beds over unconsolidated beds caused soft-sediment deformation and
formed ball and pillow structures. Shown on bedding surface (A) and in cross
section (B).
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FIGURE 7: Upper plane bed lamination and low angle hummocky cross
stratification in the proximal siltstone facies.
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FIGURE 8: Unidirectional current ripples (A) and bidirectional oscillatory wave
ripples (B) in proximal siltstone facies. Inferred flow directions shown by arrows.
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FIGURE 9: Graded limestone shell bed overlain by rippled siltstone, mudstone
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Proximal Interbedded Mudstone Facies:
Several factors distinguish the proximal mudstone facies from the distal
equivalent, including a higher proportion of quartz silt and frequent interbeds of
limestone and siltstone. Beds are frequently thin, 2-3cm thick, and individual beds
may contain common, evenly spaced vertical burrows through the thickness of the
bed (Figure 10). The highest diversity of vertical trace fossils are found in this
setting at Sandy Hollow, including the only specimens of Lingula in life position
found during this study (Figures 11 and 12). Sedimentary structures in the proximal
mudstone facies are less frequently preserved compared to the siltstone facies.
Lingula shell pavements, ripples and gutter casts in carbonate cemented siltstones
indicate energetic conditions existed at least episodically, while silt infillings within
vertical burrows indicate the muddy substrate was firm at the time of burial.
Proximal Limestone Facies:
A number of carbonate lithologies fall into the limestone facies, including
rudstone, floatstone and wackestone (following Embry and Klovan, 1972) The
major skeletal elements of the proximal limestones are bivalve shells, but
gastropods, articulate brachiopods and echinoids are also present and contribute in
various degrees to the sediment. Minor diagenesis has altered the carbonates of the
Dinwoody Formation, with aragonitic skeletal elements completely micritized or
replaced by sparry calcite, while calcitic fossils display original minerals and
layering.
Wackestone is rare in the sites studied but represents a unique lithology
determined by the local biota. Micritized, disarticulated bivalves are only found in
the base of these beds, and are overlain by a mix of silt and skeletal debris. Intact
articulate brachiopods and echinoid spines are the most readily apparent constituents
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FIGURE 10: Proximal interbedded mudstone facies
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FIGURE 11: Lingula in life position, occupying a siltstone bed within the proximal
interbedded mudstone facies at Sandy Hollow. Valve and mold of two separate
individuals. Position indicated by arrows.
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FIGURE 12: Lingula in life position associated with vertical trace fossils in a silty
carbonate mudstone bed. Proximal interbedded mudstone facies. Note abandoned
burrow directly underlying the fossilized specimen indicated by the arrow.
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of the biogenic sediments, but close examination reveals abundant microgastropod
steinkems as well.
Both Lmgu/a-dominated and mixed-fauna floatstone beds have been
identified. Lingula-dominated floatstone consists of common valves in a micritic
matrix; in contrast, distinctive sedimentary structures occur in mixed-fauna
floatstone. Typically, the mixed-fauna floatstone consists of loosely packed Lingula
and bivalve shells, with some other faunal elements, in an angular quartz silt matrix.
A degree of reverse grading may be present in the shelly material, and shell
pavements sometimes cap fine-grained sediments and smaller fossils.
In outcrop, weathering has obscured most primary bedding structures
beyond a generally wavy or curved texture subparallel to bedding in the rudstone
facies. Beds are rarely of even thickness, and stylolitic bedding is common,
particularly where shell preservation is poor. Micritized shell molds dominate the
sediment, although in many cases Lingula is common. Shells are almost universally
disarticulated and disassociated, but in many cases practically intact and unabraded
on bedding plane exposures. Orientation is random in some beds, parallel to
bedding in others, but articulated bivalve shells never appear in life position. Shells
are often nested or imbricated when orientation is random, and shell size is highly
variable, with small Lingula valves and gastropod shells interspersed with much
larger bivalves. Cross-stratified shell beds attest to high energy flow (Figure 13).
Ichnology:
The use of trace fossils as indicators of paleoenvironmental conditions has
been well established. As with body fossils, new behavioral patterns which result in
new traces arise through the stratigraphic record, and may expand or contract their
niches across the seafloor. Specific life strategies can be deduced from many
43
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FIGURE 13: Cross-stratified limestone shell bed, proximal limestone facies.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
trackways, however, and some inferences made from the assemblages preserved.
In particular, paleooxygenation has been strongly correlated to the diameter, depth,
tiering, and type of trace fossil made in a given setting (eg. Savrda et al., 1986).
Thus, the trace fossils of the early Triassic represent valuable clues to the conditions
of the time.
By definition, trace fossils are produced through the activities of organisms,
and any factor which affects the trace-makers must therefore influence trace fossil
production and preservation. Mass extinction has been considered as one likely
influence on the number of trace-makers, and the severity of the end-Permian
extinction is reflected in the effects it had on infaunal tiering (e.g. Bottjer and
Ausich, 1986). Ichnofabric, a measure of the net alteration of sediment structures
by bioturbation, has been correlated with ecological factors such as changes in
infaunal tiering throughout the Phanerozoic (Droser and Bottjer, 1986). Thus, the
relative utilization of infaunal ecospace may be semi-quantitatively measured
through the use of ichnofabric indices (Droser and Bottjer, 1986).
Lithologic characteristics constrain bioturbation to some degree within the
Dinwoody Formation, based on grain size, consistency, water content, and
burrowing mode. The shelly facies of the Dinwoody shows little indication of
burrowing, although congregations of vertical pits were observed rarely on bedding
surfaces (Figure 14). Given the small size of organisms relative to individual shells,
this facies would not be particularly hospitable to weakly burrowing infauna. In
contrast, the siltstone and mudstone beds include patches of moderately bioturbated
sediment.
45
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FIGURE 14: Paired, vertical trace fossils in proximal limestone facies. These
appear to be the trace fossil Diplocraterion aligned parallel to one another. Arrows
indicate a pair of burrow openings. Lingula fragments common on bedding surface.
46
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Distal Mudstone:
Extensive soil cover and weathering hindered study of the mudstone facies
in the Dinwoody Formation, but some observations can be made from the field and
from the few bulk samples collected. The distal mudstone facies was examined at
Sandy Hollow, where the upper half of it is well exposed, and at Blacktail Creek
from pits dug through weathered clays and spaced every 2 meters.
Bioturbation was minimal or absent in the distal mudstone facies. Previous
authors have described this facies as being laminated at a millimeter scale (e.g.
Schock, 1981), but the fine, submillimeter-scale laminations of a black shale were
not observed in this study. Instead, distal mudstone beds are either structureless or
weakly laminated, typically grey to tan in color, and show no indications of
macroscopic burrows. Microscopic burrows, mixground bioturbation or meiofauna
have not been excluded, however.
Proximal Interbedded Mudstone:
Burrows are fairly common in the interbedded mudstone units. Lingula was
observed in life position only in silty beds of this facies, but this lithology is
preferred by modem lingulids. It is associated with three-dimensional meandering
burrows in silty muds, possibly made by deposit-feeding worms. Bed thickness
appears to constrain the depth of burrow penetration, the result of frequent, thin
event beds. These event beds were colonized by the makers of the trace fossil
Diplocraterion parallelum at Sandy Hollow (Figure 15). The burrows are typically
evenly spaced and extend only through the depth of single beds. Size is variable,
but Diplocraterion was measured at up to 13 cm in depth. Diplocraterion occurs
uncommonly, but where present it is evenly spaced in moderately dense
Al
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FIGURE 15: Diplocraterion parallelum colony within a single bedding plane in the
proximal interbedded mudstone facies. Arrows indicate individual burrows.
48
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congregations, none of which appear to penetrate the underlying bed. These often
occur in carbonate rich, hummocky cross-stratified event beds.
Proximal Siltstone:
The majority of burrows are found in rippled and hummocky beds rather
than plane-bed or structureless siltstones. These may be due to extremes of
deposition rate and flow regime as opposed to oxygenation state, since these
structures all attest to episodes of scour and deposition under high flow regimes.
The bulk of trace fossils observed were at two sites, Sandy Hollow and
Hidden Pasture. The most common identifiable trace fossils are Diplocraterion
parallelum and Arenicolites. Arenicolites occurs less frequently and penetrated
bedding, often in conjunction with single Diplocraterion traces at Hidden Pasture
(Figure 16). Densely packed vertical burrow openings (possibly Diplocraterion)
were observed at several horizons and localities, reaching up to 30 / 10cm2 at
Blacktail Geek.
Horizontal trackways were also noted at Sandy Hollow and Hidden Pasture,
some identifiable as Planolites, but others of less certain affinity. Gastropod trails
are a likely source for some tracks observed. Almond-shaped sole traces may be
bivalve resting traces, such as Lockeia (Figure 17).
The lower siltstone beds at the Gros Ventre site show extensive
bioturbation, with mud fill and haloes around three-dimensional burrows, but the
individual tracks are impossible to discern. Despite thick siltstone beds in the upper
Dinwoody Formation at this locality, trace fossils are otherwise rare.
49
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FIGURE 16: Arenicolites and Diplocraterion in proximal siltstone facies.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 17: Trace fossils on sole of a bedding plane in proximal siltstone facies.
Horizontal tubes and traces are considered as Planolites, almond-shaped burrows
may be Lockeia (shown by arrow).
51
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Paleontology and Paleoecology:
A variety of taxa are represented in the benthic faunal fossil assemblage of
the Dinwoody Formation. These include the inarticulate brachiopod Lingula,
pelecypods such as Claraia, Unionites and Eumorphotis, articulate brachiopods
referred by previous authors to Crurithyris, as well as unidentified genera of
gastropods, echinoids and fish. While this study was not intended as a detailed
paleoecological inquiry into the whole fauna of the Dinwoody Formation, several
methods were used to quantify the abundance of Lingula relative to other fossils.
These include stratigraphic presence / absence data, field counts of bedding plane
and thin bed faunal occurrences, bulk sample counts of faunal occurrence and
qualitative estimation of sediment composition within thin sections. This data was
recorded relative to stratigraphic position and lithology for analysis of trends within
the distribution of various taxa. While the data collected is non-parametric, and
resists attempts at normalization, this is typical of ecological data (Fraser, N. pers.
comm., 1998). Limited statistical and cluster analysis of the data collected was used
to further examine potential trends in faunal associations.
Due to a variety of taphonomic processes, some taxa were much easier to
identify than others in all methods of counting. Lingula valves are chitinophosphatic
in composition and are therefore resistant to chemical alteration in comparison to the
many aragonitic valves of pelecypods and gastropods, as were the few fragments of
fish bone or scales. Skeletal elements composed of calcite, such as the spines of
echinoids or the valves of articulate brachiopods, were generally well preserved
relative to aragonitic valves, and some even preserved original shell layers in thin
section. In contrast, aragonite altered quickly within the sediment, leading to
dissolution, micritization, sparry replacement and cementation. This makes
identification of taxa at levels lower than class difficult, but where possible
52
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specimens were compared to published photographs to identify fossils to genus
level.
The master census lists of faunal distributions relative to locality,
stratigraphic position, facies and lithology are presented in Appendix C. This also
includes thin section data on presence or absence of taxa, and the inferred guild
structure of each site, including trace fossils as indicators of non -Lingula infauna.
The data is presented as a series of graphs versus lithology and stratigraphic
position in Figures 18-27 in the following pages. Cluster analysis of each grouping
of faunal data was performed, and the results are presented within Appendix D.
Brachiopods:
Lingula:
Previous authors have described all inarticulate brachiopods from the
Dinwoody Formation as Lingula borealis Bittner (eg. Newell and Kummel, 1942).
While other species of Lingula are known from south China during the
Griesbachian, only the species Lingula borealis has been described as pandemic in
distribution. It has not been described from the underlying Phosphoria Rock
Complex (see Schock, 1981) but rare specimens have been noted in the overlying
Thaynes Formation (Newell and Kummel, 1942). The proliferation of Lingula thus
appears to be constrained here, as elsewhere, to the Griesbachian.
Lingula specimens were identified in all bulk samples collected and in all but
a few of the field based census counts. They have been identified in all lithologies,
and are abundant in most samples, although they are highly variable in proportion
of the fauna represented. No throughgoing stratigraphic control over the distribution
of Lingula was noted, counter to the assertion of a ‘ Lingula zone’ (Newell and
Kummel, 1942). The taphonomy of Lingula is discussed in a separate chapter.
53
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lithology. Presented in stratigraphic order from left to right, where left represents
the base of each measured section.
54
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55
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6
Gros Ventre Faunal Abundance vs. Strat. Pos.
% fauna
bivalve
Lingula
taxon
meters
D Lingula H bivalve
Figure 20: Paleoecology of the Gros Ventre locality. Relative faunal abundance is
plotted relative to stratigraphic position in meters from the uppermost exposure of
the underlying Phosphoria Rock Complex.
56
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Blacktail Creek Faunal Abundance vs. Strat. Pos.
% fauna
fish
articulate
echinoid
gastropod
bivalve
Lingula
taxon
C M C9 O
meters
D Lingula bivalve gastropod l a echinoid articulate □ fish
Figure 21: Paleoecology of the Blacktail Creek locality. Relative faunal abundance
is plotted relative to stratigraphic position in meters from the uppermost exposure of
the underlying Phosphoria Rock Complex.
57
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Sandy Hollow Faunal Abundance vs. Strat. Pos.
% fauna
meters
gastropod
bivalve taxon
Lingula
106
G Lingula B bivalve B gastropod
Figure 22: Paleoecology of the Sandy Hollow locality. Relative faunal abundance is
plotted relative to stratigraphic position in meters from the uppermost exposure of
the underlying Phosphoria Rock Complex.
58
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Dalys Spur faunal abundance vs. Strat.Pos.
% fauna
gastropod taxon
bivalve
\ Lingula
meters
D Lingula B bivalve B gastropod H echinoid M articulate
Figure 23: Paleoecology of the Dalys Spur locality. Relative faunal abundance is
plotted relative to stratigraphic position in meters from the uppermost exposure of
the underlying Phosphoria Rock Complex.
59
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Hidden Pasture faunal abundance vs. Strat.Pos.
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Figure 24: Paleoecology of the Hidden Pasture locality. Relative faunal abundance
is plotted relative to stratigraphic position in meters from the uppermost exposure of
the underlying Phosphoria Rock Complex.
60
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Faunal abundance in Mixed Proximal Facies
I Lingula □ bivalve
Figure 25: Relative abundance of Lingula, and bivalves in interbedded siltstone,
shell bed limestone and mudstone lithologies within the Proximal Interbedded
Mudstone Facies. No other fauna were counted within this facies.
61
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Faunal Abundance in Proximal Siltstone Facies
■ Lingula D bivalve
Figure 26: Relative faunal abundance within the Proximal Siltstone Facies, by
lithology.
62
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Figure 27: Relative Faunal Abundance in various lithologies of the Proximal
Limestone Facies. Includes wackestone, rudstone, and floatstone lithologies.
63
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Crurithyris?:
The articulate brachiopods of the lower Dinwoody Formation at the Blacktail
Creek locality have been assigned to Crurithyris, which like Lingula was
widespread during the Griesbachian (Schock, 1981; Xu and Grant, 1992). These
articulate brachiopods are locally prolific at this site within an interval of 5 m, and
are found in association with microgastropods and the largest echinoid spines
observed in the Dinwoody Formation. Original calcite is preserved, and the valves
are almost always articulated, which is typical for articulate brachiopods, as the
hinge must be broken to disarticulate the valves. No signs of boring or encrustation
were observed, although like many valves in the Dinwoody Formation, some
specimens appear to have been crushed. This damage is interpreted to have taken
place post-burial, due to compaction of the sediment. Articulate brachiopods are
rarely observed elsewhere in the Dinwoody Formation.
M ollusca:
Bivalvia:
The bivalve molluscs are the dominant biogenic sediment producers of the
Dinwoody Formation, and the limestone facies is comprised chiefly of pelecypod
shell beds. These beds typically display signs of sparry recrystallization and
micritization of valves, suggesting an original aragonitic composition. Identification
of genera was severely hampered by this process, however, specimens of Claraia,
Unionites and Eumorphotis were clearly identified in some samples, and myalinid-
promyalinid bivalves were also present.
No facies or lithologic control appears to exist for the pelecypods, which
were found in a variety of preservational modes, from single specimens in the distal
mudstone to bedding plane assemblages to amalgamated shell beds. Size range is
64
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highly variable, from specimens a few mm across in thin section to individual
valves up to 3 cm across the longest axis on bedding planes.
Gastropoda:
Microgastropods were relatively rare in comparison to the bivalves and
inarticulate brachiopods, but occasionally dominated census counts, as in the distal
mudstone facies at Sandy Hollow. However, the small size of these valves may
bias their relative contribution, as microgastropods were identified frequently in thin
sections of the limestone facies. Original shell material appears to have been
aragonite, as most specimens are preserved as casts or have been recrystallized,
while a few individuals were replaced by pyrite in densely packed shell beds from
Blacktail Creek. The pyritization process must have been limited, as only a single
large valve of uncertain bivalve affinity (approximately 1 cm across the longest axis)
was observed with pyrite precipitated along the valve margins.
Echinodermata:
Echinoidia:
Macroscopic echinoid spines were restricted to a single locality, the Blacktail
Creek site, associated with articulate brachiopods. However, microscopic echinoid
spines were identified from a total of three sites in this study, indicating a greater
distribution than otherwise suggested. Spines were frequently broken although
recrystallization appears uncommon. One partially articulated specimen was noted in
float at Blacktail Creek but was not collected; thus the affinites of these spines
remain unclear. The reasons for the limited size of most Dinwoody echinoids has
yet to be determined, but might parallel the trend of ‘microgastropods’.
65
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Osteicthyes:
Rare remains of fish were noted during this study, mostly disarticulated
phosphatic scales, but one bone was observed as well. These are not benthic
invertebrates, but indicate that nektonic life did exist in the Dinwoody Basin.
Problematica:
One thin section from Dalys Spur included calcitic tubular elements of
probable biotic origin but unclear affinity. These may be serpulid worm tubes, as
these have been reported by previous authors (Pauli and Pauli, 1994).
Taphonomy of Lingula'.
The taphonomy of lingulide brachiopods differs from the post-mortem
behavior of bivalve molluscs and other benthic organisms, in large part due to
compositional differences. The valves of all lingulide brachiopods are composed of
a mix of apatite and chitin, with organic material approaching 50% of dry weight
(Kowalewski, 1996). Besides changing the chemical properties of the valves in
comparison to the aragonitic or calcitic valves of most fossilizable benthic
organisms, these shells are extremely thin (0.2-0.3mm) and fragile. Thus far, the
most detailed modem study of lingulide taphonomy is from macrotidal mud flats
(Kowalewski, 1996) but the general patterns observed in that study are applicable to
the Mesozoic and Cenozoic fossil record. The valves of Lingula borealis are
similarly thin (0.1mm), and may follow the patterns delineated by Kowalewski
(1996). Modifications to this scheme may be necessary to account for differences in
paleoenvironmental conditions in the Dinwoody basin relative to the modem
mudflats of Baja California. Lingulids may be preserved most commonly in event
beds throughout the fossil record (Emig, 1997). Attention must also be paid to
66
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occurrences of inarticulate brachiopods in the fossil record. While specimens from
intertidal settings follow Kowalewski’s (1996) modes of preservation (eg.
Ferguson, 1963), inarticulate brachiopods have been described with different
preservational patterns in mid-shelf Cambrian settings (Droser and Li, 1997).
Of the five modes of Lingula occurrence laid out by Kowalewski (1996),
Lingula borealis only occurs in two: paired, in situ vertical valves, and single,
horizontal valves. Single, disarticulated horizontal valves are the predominant mode
of occurrence in the Dinwoody formation, accounting for 2125 specimens (Figure
28). In comparison, a meager six individuals were identified in life position in the
field and from bulk samples. These modes of occurrence provide valuable
information on the paleoenvironmental conditions in which Lingula lived, died, and
was preserved. A detailed taphonomic census was performed on the Lingula
specimens collected from Dalys Spur, and the results are provided in Appendix E.
67
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DISCUSSION:
Paleoenvironmental Conditions:
Newell and Kummel (1942) and Kummel (1957) interpreted the Dinwoody
Formation as representing the basal Triassic transgression and early regression,
overlying the Permian age Phosphoria Rock Complex and overlain by prograding
red-beds of the Woodside Formation. This is reflected in the generalized
stratigraphic column in Figure 29, and facies distribution shown in Figure 30. The
Dinwoody Formation itself was deposited on an open, shallow shelf, with suspect
sedimentological structures considered evidence of storm influence (Newell and
Kummel, 1942). A broad spectrum of sedimentological evidence supports the
interpretation of the Dinwoody Formation as a storm-dominated shelf. Deposition
was episodic and sometimes extremely rapid, as evidenced by load deformation in
silty sediments, or compressional breakage and stylolitic bedding in shell beds.
Graded shell beds, waning-flow siliciclastic sedimentation from upper plane bed to
ripple cross-stratification to mud, hummocky cross stratification, and gutter casts
are all classic indicators of storm erosion and deposition. The opportunistic infaunal
colonization of shifting substrates represented by single-bed occurrences of
Arenicolites and Diplocraterion are also consistent with episodic, storm-driven
sedimentation. Burrows exceeding 10 cm in depth are recorded in some sites, and
are evenly spaced in colonies occupying some horizons. Bioturbation did not play a
major role in reworking coarse sediment, but water oxygenation was apparently
sufficient for episodes of dense infaunal colonization.
The distal mudstone is regarded as a low energy, distal and possibly
dysaerobic facies representing maximum transgression, but limestone interbeds
with oriented gastropods indicate low energy storm or current influence in deeper
69
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Age Formational Boundaries Lowstand Highstand
Thaynes Formation
r oodside Formation
Dinwoody Formation
unconformit
Phosphoria
Rock
Complex
Figure 29: Regional stratigraphy for the study area. Relative sea level is plotted
according to the transgression-regression curve suggested by Newell and Kummel
(Newell and Kummel, 1942) and followed by subsequent workers.
70
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Facies Relationships in the Dinwoody Basin
West East
Sea Level
Unconformity
Phosphoria Rock Complex
Figure 30: Interpreted facies distribution in the Dinwoody Basin during the
Griesbachian transgressive-regressive sequence.
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waters. The proximal mudstone facies includes frequent silty and shelly interbeds
and relatively high bioturbation consistent with shallower waters influenced more
by storms and biota than the quiet water conditions of the distal mudstone facies.
Irregular siltstone beds with rippled bedding surfaces, and planar, hummocky, and
cross-stratified laminae represent storm deposition well above storm wave-base,
possibly near fairweather wave-base, with a high influx of terrigenous, angular silt
relative to other facies. The lack of fragmentation and size sorting among bivalve
shells suggests that limestone shell beds accumulated within habitat and underwent
minimal transport. This facies was frequently energetic enough to winnow
terrigenous silt. Irregular bed thickness, cross-stratification, rare graded shell beds
and reverse grading under shell pavements supports the assertion of high flow
regime. The lack of fossils in life position suggests that periodic storms were the
dominant factor in erosion and deposition in all facies save the proximal interbedded
mudstone. Siltstone, limestone and mudstone facies occur both in units multiple
meters in thickness and interbedded with each other at the centimeter scale,
suggesting lateral patchiness in facies distribution. The interplay of these lithologies
is typical of mid-to-inner shelf or mixed carbonate-siliciclastic ramp settings, in
which storm scour and deposition clearly dominated sediment distribution (e.g.
Seilacher and Aigner, 1991; Einsele and Seilacher, 1991).
Ichnology:
The trace fossils of the Dinwoody Formation, while relatively uncommon
compared to late Paleozoic or later Mesozoic sediments, indicate that conditions
adequate for dense infaunal colonization occurred at least episodically. Arenicolites
is traditionally regarded as an opportunistic colonist of storm beds, and this
interpretation is consistent with the facies evidence. Colonies of Diplocraterion
72
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parallelum along single beds also attest to the opportunistic invasion of fresh
sediments. The presence of load casts and similar structures are also consistent with
the inference of relatively unstable sediments, although these traces do not typically
occur in massive, load-cast beds. In short, episodic storm deposition and
subsequent invasion by opportunistic fauna are clearly indicated.
The bulk of the infauna may be soft-bodied or relatively weak burrowers
such as Lingula, based on the general absence of extensive burrowing in massive or
shelly beds. No indications of boring organisms have been observed thus far,
although articulate brachiopods and byssate bivalves must have attached to hard
substrates. Some variety in life mode is apparent: Lingulichnus and Diplocraterion
are associated with suspension feeding, while Arenicolites may be a deposit feeder
trace, and gastropod trackways indicate mobile grazing and/or predatory epifauna.
Taphonomy:
Two modes of Lingula preservation have been observed in the Dinwoody
Formation, and models describing each mode were constructed from modern
analogues and the fossil record. These are described in Figures 31 and 32.
Taphonomic evidence provides a number of lines of argument against the
interpretation of pervasive dysaerobic conditions in the northern Dinwoody Basin.
Mass mortality horizons of infaunal organisms associated with overlying black
shales are frequently cited as evidence of anoxia, but none have been observed in
the Dinwoody Formation thus far. However, Lingula is capable of escape
burrowing and increasing its exposure to oxygenated water by extending the body
beyond the burrow, and these factors may account for the relative lack of specimens
found in situ.
73
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Figure 31: Lingula in life position. This is the dominant preservational mode in the
present day. Lingula lives infaunally (A), and establishes itself in life position
through a unique burrowing technique, forming a U-shaped burrow (B). Lingula
may remain within its burrow after death (C). Subsequent burial will preserve these
specimens in situ. When oxygen levels are low, Lingula responds by extending its
body from the burrow (D). Adult individuals exhumed by scour may survive on the
sediment surface (E). Valve ‘blackening’ has only been described for individuals
preserved in situ, and has been linked to pyritization in the fossil record (Ferguson,
1963, Kowalewski 1996).
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Figure 32: Preservation of disarticulated Lingula valves parallel to bedding planes.
Lingula may be removed from its burrow during life or post-mortem by erosional
scour or extension of the pedicle (2). The valves are quickly disarticulated and
fragmented once decay begins and are scattered parallel to bedding during
redeposition (3).
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No soft-bodied fauna have been preserved within the Dinwoody Formation,
through selective mineralization nor any other means. Evidence for their presence
exists in the form of trace fossils, including the fleshy pedicle of Lingula, but there
is no extraordinary preservation of the fauna. 'Konservat-Lagerstatten' are typically
related to stratified water bodies with limited ventilation, and their presence would
support the interpretation of anoxia (Allison and Briggs, 1991). In contrast, the
strangest aspect of Dinwoody Formation taphonomy, rare Lingula shell beds, are
similar in occurrence to inarticulate brachiopod shell beds of the Lower Cambrian, a
time interval in which they were ecologically dominant (see Droser and Li, 1997).
Pyrite formation is a common aspect of low-oxygen sediments, and can
commonly preserve aragonitic and calcitic skeletal elements after dissolution. While
sulfate reduction associated with pyritization is not conducive to carbonate
dissolution, pyrite can replace carbonate skeletons along surfaces rich in organic
matter (periostracal coatings, etc). While pyrite is present in the Dinwoody
Formation sediments, it occurs mainly as scattered framboids throughout the
sediment, with an extremely patchy distribution; framboids may be common on one
face of a vertically cut slab but be entirely absent from the other. Pyritized fossils
are generally absent; only one macroscopic specimen has been noted, a small shell
tentatively identified as a bivalve. Microscopic gastropods have been completely
replaced with pyrite in the same sample, as seen under thin section. The minimal
size and limited extent of pyritized fossils may attest to poor development of a
sulfate-reducing layer within the sediment. While pyrite lags have been interpreted
as evidence of euxinic bottom waters in shallow settings (e.g. Wignall and Hallam,
1992) these may instead be further evidence of scour below the redoxicline and
subsequent redeposition, in which the densest grains settled fastest.
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No trend towards extensive faunal phosphatization was observed within the
Dinwoody Formation. Under dysaerobic to anoxic conditions carbonate skeletons
may dissolve while phosphate precipitates, leading to intricate preservation of
fossils by phosphatization, particularly in regard to phosphatic skeletal elements, as
possessed byLingula. (Allison and Briggs, 1991).
The trend towards disarticulated valves exposed on bedding planes attests to
scour and erosion of Lingula from burrows post-mortem before redeposition. Signs
of transport have been noted, including frequent breakage of valves, exposure of
blackened valves, and bedding-plane assemblages associated with high flow regime
sedimentary structures. These indicate mixing and flow of the water column, which
is not consistent with the stagnation commonly associated with anoxia.
Nonetheless, transported specimens typically show high fidelity to local faunas
(Kidwell, 1991), and the fragility of lingulide valves further constrains dead
specimens to the near vicinity of their habitat in life (Kowalewski, 1996).
In modern samples, only articulated valves found in situ have been
described as 'blackened', which is to say that some had undergone a change in
coloration from tan with brown stripes to a black or bluish hue (Kowalewski,
1996). This change in coloration was accompanied by a drastic loss of mechanical
strength, leaving the valve with the consistency of 'wet tissue paper' and rendering
it too fragile for transport (Kowalewski, 1996). In the Dinwoody samples,
blackened valves make up a significant proportion of single horizontal valves
preserved, and none of the articulated, in situ specimens. However, given the lesser
degree of flow energy expected from shelf settings as well as frequent episodes of
scour, the removal of blackened Lingula valve fragments from life position is not
unexpected, and such fragments may have survived mechanical abrasion at the
sediment water interface for a greater length of time than in shallower waters.
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The exact nature of the blackening process has not been examined in detail,
but has been interpreted as the result of decay of organic material within the valve
and linked to eventual pyritization of specimens preserved in life position
(Kowalewski, 1996). In contrast, none of the single, horizontal specimens
identified in the tidal flat studied by Kowalewski (1996) showed signs of
blackening, but this may be due to the intensity of mechanical degradation in this
setting. In any case, on the basis of this selectivity the blackening process cannot be
a late stage diagenetic effect, but must illustrate fundamental differences in the early
decay and remineralization of the lingulide valves. This pattern is also seen in the
fossil record (Ferguson, 1963). This may correlate with oxygenation state, as
pyritization is not described from well-ventilated conditions, but could occur during
early decay and diagenesis within sediments below the redoxicline. Valves retaining
the original coloration are known to occur in surface modes in the modem day (e.g.
Kowalewski, 1996), and presumably the preservation of this color pattern reflects
similar bottom-water oxygenation conditions. Under anoxic or dysaerobic bottom-
water conditions, blackening (or the lack thereof) should affect all valves equally,
regardless of articulation and orientation. Instead, some valves in this study show
coloration patterns similar to modem specimens, including the tan to brown stripes
parallel to growth bands. The presence of color bands in original shell material has
been linked to relative food abundance in modem lingulides, as starved individuals
do not display this striped pattern (Paine, 1963).
While blackening of original shell material may have occurred after scour
and reburial, the mix of blackened and original shell noted in this study argues that
this process took place largely before these events. Blackened valves from below
the redoxicline may have been scoured from the sediment by storm flow, and then
mixed in the water column with unblackened valves from the overlying sediment or
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the sediment water interface before redeposition took place. This author can not
devise a convincing argument for selective blackening of some but not all original
shells after reburial.
No sign of predation (e.g. drill holes, repair scars) or influence by biotic
taphonomic factors such as boring organisms was noted within the Lingula valves
studied. This suggests little or no biotic interaction between Lingula and the other
taxa of the Dinwoody Formation.
Paleoecology:
There is little or no strong lithologic or facies-level correlation of taxa
evident from the data collected. This is shown in Figures 25 through 27, plotting
faunal abundance versus lithology by facies. Dominance varies between bivalve
molluscs and Lingula in practically all lithologies, although gastropod, echinoid and
articulate brachiopod skeletal elements occasionally occur in significant numbers.
Limited data was collected from mudstone lithologies due to poor exposure, but
Lingula dominated the proximal interbedded mudstone, while gastropods were the
main faunal element observed in the single horizon counted from the distal
mudstone facies. Similar patterns are seen between both bulk sample and field count
data when plotted as stacked histograms (Figures 18-19). No trends were observed
in stratigraphic position versus relative abundance of Lingula and other fauna either,
as shown for each locality in Figures 20 through 24.
The lack of lithologic control over Lingula distribution could represent either
wide distributions in life or preservation after widespread post-mortem transport.
Several lines of evidence suggest deposition within habitat. Lingula is found in life
position within burrows in the siltstone interbeds of the proximal interbedded
mudstone facies, and these cannot have been transported. However, this facies may
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represent relatively quiet-water deposition in proximal settings, with minimal
erosional scour. Reworked Lingula valves occur in a variety of preservational
modes in the Dinwoody Formation, typically as single, disarticulated and often
fragmented valves, parallel or at an angle to bedding, in a wide variety of
lithologies. This suggests that erosional scour, transport, and redeposition may
have played a major role in their post-mortem redistribution. Several factors limit
this possibility, as discussed in the chapter on taphonomy. The chitinophosphatic
valves of Lingula are very fragile, and easily destroyed by mechanical abrasion
(Kowalewski, 1996). Although heavily altered by diagenesis, the shells of bivalves
rarely appear to be fractured or broken. The co-occurrence of Lingula valves under
10 mm long on bedding planes with those of bivalves up to 30 mm or longer is not
consistent with co-transport. However, bivalves were frequently preserved
concave-down, as opposed to the random orientations of Lingula, suggesting that
Lingula was sometimes deposited on top of or preserved beneath winnowed shell
pavements.
The presence of Lingula in life position in the proximal interbedded
mudstone facies argues against transport from high-energy shoreface settings as the
principal means of distributing Lingula across the shelf. These interbedded silts and
muds are interpreted to represent storm beds in an otherwise quiet, muddy mid-to-
proximal shelf setting. Modem lingulids can occupy depths up to 143m in modem
shelves (Jones and Bernard, 1963), but are better known from shallow waters with
little competition for space or food. Lingula has not been reported from the marginal
marine Woodside Formation overlying the Dinwoody, which further argues against
a strict nearshore model.
The extensive association of Lingula with storm beds, disarticulated and
parallel to bedding planes, is inconsistent with the predictions for a dysaerobic
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opportunist. Erosional scour and transport of these organisms from low-energy,
thinly bedded shales to mixed-fauna shell beds and rippled siltstones is unrealistic.
It is possible that the proximal settings were themselves dysaerobic, but the
implications of low oxygenation in a shallow, mixed water column are highly
unusual. Geochemical study is the most likely route to determine probable bottom-
water oxygenation states, but the occasional dense bedding plane trace fossil
assemblages indicate at least episodic bottom water ventilation.
The predictions of the ‘ecological opportunist’ model appear to best describe
the distribution of Lingula in the Dinwoody Formation. Lingula fossils occur across
a spectrum of lithologies, from proximal limestone shell beds to distal mudstone
beds, with little lithologic control over relative faunal abundance. While transport
and redistribution clearly played a role in the ultimate dispersal of Lingula valves,
these small, thin skeletal elements may be found in mud as well as shell beds
composed primarily of bivalves centimeters across. Waning flow may have
controlled the deposition of siliclastic sediments, but the absence of size sorting and
fragmentation in biogenic particles indicates weak or minimal transport. In situ
fossils from the proximal interbedded mudstone facies indicate Lingula colonization
in a lithology commonly occupied by modem species, in apparently unstressed
settings, but not confined to either dysaerobic or shoreface conditions. Lingula
should not be expected to forsake normally hospitable environments if it were acting
as an ecological opportunist.
Ecological opportunism by morphologically simple, generalist taxa appears
to be a hallmark of Early Triassic faunas (Schubert and Bottjer, 1995). In the
desolation after the end-Permian mass extinction, the Dinwoody shelf most likely
would have provided abundant, unclaimed ecospace for any survivor taxon with
wide larval dispersion. Modem lingulides release large numbers of planktotrophic
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larvae, which have been collected from plankton nets thousands of kilometers from
land (Hammond, 1982). While diversity remained low, significant biomass
developed among bivalves and brachiopods, but minimal competition may have
existed between taxa (Schubert and Bottjer, 1995). Widely dispersing populations
can maintain genetic connections over large expanses of ocean, and this might
hamper evolutionary radiation. Thus, as more specialized forms with low
dispersion rates ‘trickled back’, they may have outcompeted the opportunists and
continued to evolve, setting off the Mesozoic radiation. This follows closely the
‘evolutionary succession’ model for the aftermath of the end-Permian extinction
(Schubert and Bottjer, 1995).
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CO NCLUSIONS:
The inarticulate brachiopod Lingula borealis Bittner occurs in high but
highly variable abundance throughout a variety of storm-dominated shelf facies in
the Griesbachian age Dinwoody Formation. This pattern is fully consistent with the
designation of Lingula as a disaster taxon by previous authors, and a number of
lines of inference may be used to deduce the evolutionary strategies behind the
success of this taxon in the aftermath of the end-Permian mass extinction.
Two preservational modes were defined for Lingula in the Dinwoody
Formation in relation to modem taphonomic studies of lingulide brachiopods. The
relative abundance of individuals between these modes is more consistent with
Cambrian inarticulate brachiopod preservation in mid-shelf settings than with the
modem intertidal zone. These changes in preservation style can be correlated with
differences in facies, flow and depositional regimes, and suggest that changes in
lingulide taphonomy through time may be controlled by ecological restriction of
habitat. The rarity of Lingula specimens in situ is interpreted as an indicator of the
intensity of shallow sediment scour in a storm dominated setting, although it may
also indicate an epifaunal life habit for some individuals.
No indications of intertidal or shoreface deposition within the Dinwoody
Formation have been noted either in this study or by previous authors. While it is
possible that extensive transport of Lingula from marginal marine settings is
responsible for this pattern, a number of arguments against this interpretation exist.
Not only are no Lingula specimens reported from the overlying marginal marine
sediments of the Woodside or Red Peak Formations, but the fragility of Lingula
valves, particularly blackened valves, precludes extensive transport. A wide variety
of size ranges between whole valves and fragments indicates minimal transport took
place, as does the association of Lingula valves with much larger pelecypod valves.
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Thus, the evidence presented here is not considered consistent with the
interpretation of Lingula as a nearshore specialist.
A number of arguments have been presented here against the interpretation
of pervasive, highly dysaerobic conditions in proximal facies of the Dinwoody
Formation. These include the presence of many sedimentary structures indicative of
episodically high flow regime conditions, a lack of low-oxygen diagnostic trace
fossils and occasionally dense associations of vertical trace fossils in fine-grained
sediments, a variety of taphonomic arguments and the presence of high biomass
represented in the common shell beds of the limestone facies.
The apparent expansion of Lingula into proximal to mid-shelf environments
in the Griesbachian appears to represent the return of a generalist to a setting from
which it has been excluded since the Ordovician. This may be due to the relaxation
of ecological pressures of competition in the aftermath of mass extinction. The high
dispersive capabilities of modem lingulids would allow them to rapidly colonize
open ecospace in a manner similar to the patterns seen in the Griesbachian. It may
be that this dispersive power limits the speciation of lingulide brachiopods, as they
can maintain genetic connections over vast extents of ocean.
Ecological opportunism by taxa with high larval dispersion may thus
represent the most successful strategy in the immediate aftermath of mass
extinction. This is fully consistent with models of ecological succession at
evolutionary scales proposed by Schubert and Bottjer (1995), wherein ‘r-selected’
generalists rapidly and opportunistically occupy vacant ecospace until displaced by
more specialized taxa arising from evolutionary radiation and innovation or Lazarus
taxa returning from refugia.
Lingering environmental stresses may well have constrained the rates of
rebound after the end-Permian mass extinction, but this may be through
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biogeographic effects rather than natural selection towards specialized taxa. In this
scenario, factors such as deep water anoxia or hypercapnia may be too successful at
exterminating faunas to allow even specialized taxa to thrive, and only those taxa
capable of circumventing these conditions may have thrived. This issue, of course,
requires more research to determine the interplay between these factors.
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APPENDIX A: LOCALITIES
Hidden Pasture:
44° 40.658’ N, 112° 47.466’ W elevation: 7237 feet (2207 m)
This locality is southwest of the town of Dell in Beaverhead County,
Montana, and accessible from the south by a trail from Sheep Creek Road.
Exposures are found on the western flank of a valley to the west of Dixon
Mountain, and include the underlying Phosphoria Rock Complex and overlying,
ridge-forming Thaynes Formation. The Woodside Formation is largely covered, but
where exposed appears as white, quartz-rich amalgamated siltstone storm beds with
gutter casts.
Dalys Spur (Grasshopper Creek):
45° 05.851’ N, 112° 46.884’ W elevation: 5515 feet (1682 m)
The Dinwoody Formation is exposed to the north and south of a cliff-
forming exposure of the Phosphoria Rock Complex immediately to the west of
Interstate 15 in Beaverhead County, Montana. These exposures are on private land
and it is always advisable to ask permission to work on the site; the ranch house is
easily accessible from the Grasshopper Creek exit from 1-15. The Dinwoody
Formation forms the ridge immediately to the south of the ranch house, but a
significant portion of the lower Dinwoody Formation is covered by valley fill. The
formation is immediately overlain by white siltstone beds of similar aspect to those
overlying the Dinwoody Formation at Hidden Pasture, and these are assigned here
to the Woodside Formation, as are the red beds directly upsection.
90
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Sandy Hollow:
45° 26.368’ N, 112° 35.125’ W elevation: 4866 feet (1484 m)
Again, this is an exposure of the Dinwoody Formation on private land
associated with a farm to the north of the Beaverhead River; exposures appear to
continue to the north on BLM land. The locality occurs along Burma Road in
Madison County, Montana, between the towns of Glen and Twin Falls?. Burma
Road parallels the Beaverhead River to the east of 1-15. The Dinwoody Formation
is underlain by and deformed by a thrust fault; thus the stratigraphic section
measured is a composite. The lowermost Dinwoody Formation is again covered by
valley fill, but the distal mudstone facies is best exposed at this locality along a steep
face immediately north of the road. The Dinwoody Formation is overlain here by a
white, structureless limestone unit devoid of fossils and of unknown affinity, and
that is in turn overlain by fluvial conglomerates. This may represent the
progradation of a fluvial / lacustrine system over the Dinwoody Formation,
bypassing or incising the Woodside Formation at this locality. All Lingula
specimens found in situ come from the proximal interbedded mudstone facies,
which has excellent exposures at this locality relative to other sites.
Blacktail Creek:
44° 45.129’ N, 112° 17.845’ W elevation: 8340 feet (2543 m)
The Blacktail Member of the Phosphoria Rock Complex was distinguished
from the Dinwoody Formation at this locality, largely from paleontological criteria
(Schock, 1981). The exposure occurs along the western face of the roadcut on
Blacktail Creek Road in the Snowcrest Range of Montana, approximately 40 miles
91
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(65 km) southwest of Dillon and several miles to the north of Lima Reservoir. The
Blacktail Member displays a diverse, late Permian fossil assemblage, yet within
meters, in a poorly exposed gray carbonate mudstone unit, the only fossils to be
found in float are specimens of Lingula. The most diverse macrofaunal assemblage
of the Dinwoody Formation noted in this study can be found here, including a few
beds dominated by articulate brachiopods, echinoid spines and microgastropods.
The middle of the section is poorly exposed due to soil coverage, as is much of the
overlying Woodside Formation.
Gros Ventre:
43° 38.169’ N, 110° 34.000’ W elevation: 7128 feet (2174 m)
This is the only exposure of the Dinwoody Formation from Wyoming
included in this study, occurring immediately to the east of the Grand Tetons
National Park. The locality is in the western Gros Ventre Range, west of the town
of Kelly and approximately 2 miles (3.2 km) east of the Gros Ventre Landslide, the
largest documented landslide prior to 1980. Located on a ridge north of the road and
the Gros Ventre River, this is the thinnest of the measured sections in this study. It
overlies a roadcut into the Phosphoria Rock Complex, which exposes spectacular
examples of the chertified vertical trace fossil Skolithos grandis. In comparison to
this gigantic trace fossil, infaunal tiering depths could only decrease, but the
transgressive siltstone at the base of the Dinwoody Formation appears to be well
homogenized by bioturbation in bulk samples, reaching ii4-ii5 . The Dinwoody
Formation is overlain here by red beds of the Red Peak Formation.
92
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P erm ian Triassic
A ppendix B: Stratigraphic Columns
Generalized
Age
Stratigraphy Modified From Newell and Kummel, 1942
Highstand
with interpreted sea-level curves
Formational Boundaries Lowstand
Thaynes Formation
Dinwoody Formation
oodside Formation
unconformit
Phosphona
Rock
Complex
93
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Stratigraphy of the Dinwoody Formation
Gros Ventre Range
60m
contact with Red Peak Formation
covered by soil
m
Key
Limestone |Q Lingula
Siltstone Bivalves
\------ 1 Covered ^ ttace fossils
1 1 Section
45cm shell bed limestone
8.65m amalgamated siltstone storm beds
HCS, upper plane bed laminae
wave ripples, load casts
GV 7, GRx 4
r/W 5.9m interbedded HCS siltstone, limestone
shell beds and pavements GV 5, GV 6
GV 3, GV 4, GRx 3
35cm HCS siltstone with small shells
30cm rippled siltstone with small shells
GRx 2
1 0 m
0m
GV 1, GRx 1
1 .2 1 m discontinuous bioturbated siltstone beds
HCS, reworked granules from PRC
mud siltstone limestone
94
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90m
80m
70m
Stratigraphy of the Dinwoody Formation
Blacktail Creek
60cm shell bed
Qp) 1.25m shell beds
50cm thin, rippled siltstonebeds
" r ’ J E N 1.5 m silty, thin bedded shell beds
'vlr 2.3m amalgamated shell bed
50m
40m
30m
1 .8 rti structureless carbonate bed with laminated
siltstone rip-up clasts up to 4cm in length
4.8m silty amalgamated shell beds
BC 7
1 .8 m interbedded siltstone, shell beds
2.25m siltstone with thin shell pavements
some densely packed vertical burrows
1.12m shell beds with large bivalves B C 6
65cm silty shell bed
Key
Limestone
BCx4
□
Siltstone
Mudstone
Covered
Section
Q Lingula
Bivalves
/Echinoids
Crurithyris?
m m
1. lm shell-rich wackestone BC 4, BC 5, BCx2
2.2m shell beds and wackestones BCx 3
1.3m amalgamated shell beds overlain by
25cm of thin siltstone beds ®C 3
25cm silty shell beds
45cm silty shell beds
BC 2, BCx 1
1 2 .0 m thinly bedded to structureless mudstone
Lingula common in float
mud siltstone limestone
95
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Stratigraphy of the Dinwoody Formation
Dalys Spur
1 .0 m thin, silty shell beds
1.25m silty shell beds
©
W
70m
60m
50m
extensive covered section
65cm silty shell beds
70cm silty shell beds
5.0m thinly interbedded mudstone, shell beds
1 .1 m silty shell beds
2 .1 m silty shell beds
3.15m amalgamated shell beds
45cm shell bed with high % Lingula DSx 2
5.4m amalgamated and single shell beds
with minor mudstone interbeds
3.35m interbedded shell beds and mudstone
1.5m interbedded plane bed, HCS siltstone
and silty shell beds DSx 3, DS 4
DS 3, DSx 4
4.7m amalgamated shell beds, with mudstone
interbeds, high % Lingula DS 2
4.6m interbedded shell beds, HCS siltstone,
mudstone, rare vertical burrows
65cm silty shell beds, high % Lingula
1.5m interbedded shell beds and mudstone
DS 1, DSx 1
1.95m amalgamated shell beds
Key
Limestone
iQl Lingula
Siltstone
Bivalves
□
Covered
Section
trace fossils
siltstone limestone
96
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1 0 0 m
Stratigraphy of the Dinwoody Formation
Sandy Hollow (composite section)
section continues into massive white limestone and fluvial conglomerates
2 .6 m amalgamated shell beds gjj 4
80cm interbedded mudstone, limestone
with gutter casts
90m
80m
70m
60m
50m
40m
30m
2 0 m
0m
6 4 ^
©
0
0 W
0 W
&
0
T z z z m
Thrust fault through
roadcut, measured section.
limestone
5.0m amalgamated shell beds
3.8m interbeddded silty limestones, mudstone
Diplocraterion colonies SH 3
1 .0 m shell bed
1 lm interbedded carbonate mudstone
thin silty carbonate beds
common vertical trace fossils
ii 2 - ii 3
95cm shell bed limestone, vertical burrows
1 . 0 m intebedded siltstone, mudstone cxTy 4
1,35m silty shell bed SH 2, SHx 3
3.1m interbedded shell beds, thin horizons
of carbonate mudstone
1.5m amalgamated shell bed
2.25m interbedded silty shell beds, mudstone
90cm siltstone with common shells
8.45m amalgamated shell beds
SHI, SHx 2
2 0 m thin bedded carbonate mudstone with
limestone interbeds increasing in
frequency upsection.
Includes parallel oriented gastropods,
Lingula floatstone in micrite at top
SHxl
K ey
T i me. st nne.
|Q| Lingula
Siltstone
Bivalves
H U H Mudstone
^ Gastropods
■ | Covered
1 1 Section
trace fossils
97
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1 0 0 m
W
M & W K & W f f i / / / / / / / / / / / > (
Stratigraphy of the Dinwoody Formation
Hidden Pasture
1.5m silty shell beds
HP3, HPx 4
2 .0 m silty shell beds, excellent preservation
of pectenid bivalves
3.5m amalgamated shell beds
1 .0 m shell beds
1.0m HCS siltstone
1 .0 m shell beds
1.0m shell beds, thin silt interbeds HPx 3
1 .0 m shell beds HPx 2
2.0m HCS siltstone, thin graded shell beds,
wave ripples, Arenicolites, ii 2 - ii 3
70cm HCS siltstone
2.5m amalgamated shell beds HP 2
10.3m interbedded HCS siltstone,
shell beds, mudstone
1 .0 m shell beds, thin siltstone interbeds
3.2m interbedded HCS siltstone, HP 1
cross-stratified shell beds HPx 1
1 .0 m silty amalgamated shell beds
2 0 cm planar siltstone over shell bed
60cm interbedded limestone and mudstone
Key
Limestone |Q| Lingula
Siltstone Bivalves
I-------- 1 < T overed W & trace fossils
1 1 Section
mud siltstone limestone
98
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Stratigraphy of the Dinwoody Formation
Hidden Pasture (continued)
Key
Limestone @ Lingula
| % % % | Siltstone Bivalves
■ I Covered
1 I Section
trace fossils
130m
120m
100m
Om
lowest exposure of Woodside Formation
Lingula in float
60cm white, amalgamated shell beds HPx 5
1 .0m white, amalgamated shell beds
1 .0m white, amalgamated shell beds
M y 4.0m amalgamated HCS siltstone beds
shells common towards top
large load cast pillow structures
1.3m amalgamated shell beds
1.5m silty shell beds
mud siltstone limestone
AP
99
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APPENDIX C: Faunal distribution tables
Master Census:
This table includes total counts of individual taxa per sampling horizon, the sum
number of taxa per horizon, and the stratigraphic position of each sample as measured
from the uppermost exposure of the Phosphoria Rock Complex. These are ordered
according to stratigraphic position, from lowermost samples to uppermost The
corresponding lithology for each sample is noted, as is the facies it belongs within.
Facies are abbreviated as follows: DM is distal mudstone, PM is proximal interbedded
mudstone, SS is proximal siltstone and LS is proximal limestone. Abbreviations for
lithologic criteria are as follows: IB is interbedded, LS is limestone, MUD is mudstone,
SILT is siltstone, BIOT. is bioturbated, SH BED is shell bed.
The abbreviations for each census site are broken down as follows: the first two
letters designate the locality. Ie; BC is Blacktail Creek, DS is Dalys Spur (Grasshopper
Creek), GV or GR is Gros Ventre Roadcut, HP is Hidden Pasture, and SH is Sandy
Hollow. The suffix ‘x’ is used to indicate that the sample was a field count; the lack of
this letter indicates the data is from bulk sample count. The numeric designation simply
indicates order of collection. The final suffix ‘q’ indicates that a field count was taken
from quarrying and disaggregation within a 10 cm horizon, rather than from bedding
surface counts. Stratigraphic position of these samples is indicated in Appendix B.
For ease of comparison between bulk sample counts and field counts, the same
table is broken down by sampling method in the following tables.
Thin section fauna:
The approximate abundance or absence of each taxon is noted in the same
context as the bulk sample data from which these thin sections were taken.
100
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MASTER CENSUS DATA, ALL LOCALITIES
Census Site FA C IE S Lithology strat.pos. Lingula bivalve gastropod echinoid articulate fish
BC 2 L S rudstone 12.25 m 82 16 0 0 0 2
BCx 1 LS rudstone 13.5 m 95 4 1 0 0 0
BC 3 LS rudstone 19.5 m 53 46 1 0 0 0
BCx 3 L S rudstone 22 m 6 0 0 2 92 0
BC 4 L S rudstone 24 m 37 60 2 1 0 0
BC 5 L S wackestone 25 m 4 29 3 16 48 0
BCx 2 L S rudstone 25 m 0 0 0 5 95 0
BCx 4 L S rudstone 28 m 47 53 0 0 0 0
BC 6 L S wackestone 53.5 m 44 56 0 0 0 0
BC 7 L S rudstone 61 m 1 99 0 0 0 0
SHx 1 q D M ib Is/mud 31 m 7 2 91 0 0 0
SHx2q LS floatstone 44 m 99 1 0 0 0 0
SH 1 L S floatstone 44 m 65 35 0 0 0 0
SH 2 SS siltstone 72 m 83 17 0 0 0 0
SHx 3 SS siltstone 72 m 87 13 0 0 0 0
SHx 4q F M ib silt/mud 73 m 94 6 0 0 0 0
SH 3 P M ib silt/mud 92 m 88 12 0 0 0 0
SH 4 SB biot. siltstone 106 m 11 89 0 0 0 0
GV 1 SS biot. siltstone 1.5 m 14 86 0 0 0 0
GRx 1 q SB biot. siltstone 1.5 m 100 0 0 0 0 0
GRx2q SB siltstone 17.75 m 81 19 0 0 0 0
GV 3 IS floatstone 21.5 m 30 70 0 0 0 0
GV 4 L S floatstone 24.25 m 62 38 0 0 0 0
GRx3q SS siltstone 24.5 m 3 97 0 0 0 0
GV 5 SS biot. siltstone 25 m 7 93 0 0 0 0
GV 6 SS siltstone 25.25 m 0 100 0 0 0 0
GV 7 SS siltstone 26 m 0 100 0 0 0 0
GRx 4 L S floatstone 29 m 3 97 0 0 0 0
DSx 1 L S rudstone 61.5 m 3 96 0 0 1 0
DS 1 L S rudstone 62 m 22 39 0 0 0 0
DS 2 L S rudstone 83 m 91 9 0 0 0 0
DSx 4 L S rudstone 84.5 m 74 26 0 0 0 0
DS3 L S rudstone 85 m 99 1 0 0 0 0
DSx 3 q SB ib silt/sh bed 89.5 m 96 4 0 0 0 0
DS4 SB siltstone 90 m 82 18 0 0 0 0
DSx 2 L S rudstone 104 m 91 3 6 0 0 0
HP 1 SB ib silt/sh bed 45 m 72 28 0 0 0 0
HPx 1 S6 ib silt/sh bed 45 m 65 35 0 0 0 0
HP 2 L S rudstone 61 m 98 2 0 0 0 0
HPx 2 SS HCS siltstone 72 m 97 3 0 0 0 0
HPx 3 IS ib silt/sh bed 74 m 9 91 0 0 0 0
HP 3 IS floatstone 92 m 15 85 0 0 0 0
HPx 4 L S floatstone 92.5 m 6 44 50 0 0 0
HPx 5 L S rudstone 121 m 2 89 0 0 0 9
A LL FAUNA: 4361 CENSUSTOTAL 2125 1811 154 24 236 11
101
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MASTER CENSUS DATA, BULK SAMPLES
Sample Position FA CIES Lithology Lingula bivalve gastropod echinoid articulate fish
BC 2 12.25m LS rudstone 82 1 6 0 0 0 2
BC3 19.5m LS rudstone 53 46 1 0 0 0
BC 4 24m LS rudstone 37 60 2 1 0 0
BC 5 25m LS wackestone 4 29 3 16 48 0
BC 6 53.5m LS wackestone 44 56 0 0 0 0
BC 7 61m LS rudstone 1 99 0 0 0 0
SH 1 44m LS floatstone 65 35 0 0 0 0
SH 2 72m SS siltstone 83 1 7 0 0 0 0
SH 3 92m P M ib silt/mud 88 12 0 0 0 0
SH 4 106m SS biot. siltstone 1 1 89 0 0 0 0
GV 1 1.5m SS biot. siltstone 14 86 0 0 0 0
GV 3 21.5m LS floatstone 30 70 0 0 0 0
GV 4 24.25m LS floatstone 62 38 0 0 0 0
GV 5 25m SS biot. siltstone 7 93 0 0 0 0
GV 6 25.25m SS siltstone 0 100 0 0 0 0
GV 7 26m SS siltstone 0 100 0 0 0 0
DS 1 62m LS rudstone 22 39 0 0 0 0
DS 2 83m LS rudstone 91 9 0 0 0 0
DS 3 85m LS rudstone 99 1 0 0 0 0
DS 4 90m SS siltstone 82 18 0 0 0 0
HP 1 45m SS ib silt/sh bed 72 28 0 0 0 0
HP 2 61 m LS rudstone 98 2 0 0 0 0
HP 3 92m LS floatstone 15 85 0 0 0 0
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MASTER CENSUS DATA, FIELD COUNTS
Sample Position FA CIES Lingula bivalve gastropod echinoid articulate fish Lithology
BCx 1 13.5m LS 95 4 1 0 0 0 rudstone
BCx 2 25m LS 0 0 0 5 95 0 rudstone
BCx 3 22m LS 6 0 0 2 92 0 rudstone
BCx 4 28m LS 47 53 0 0 0 0 rudstone
SHx 1 31m D M 7 2 91 0 0 0 ib Is/mud
SHx 2 44m LS 99 1 0 0 0 0 floatstone
SHx 3 72m SS 87 13 0 0 0 0 siltstone
SHx 4 73m F M 94 6 0 0 0 0 ib silt/mud
GRx 1 1.5m SS 100 0 0 0 0 0 biot. siltstone
GRx 2 17.75m SS 81 19 0 0 0 0 siltstone
GRx 3 24.5m SS 3 97 0 0 0 0 siltstone
GRx 4 29m LS 3 97 0 0 0 0 floatstone
DSx 1 61.5m LS 3 96 0 0 1 0 rudstone
DSx 2 104m LS 91 3 6 0 0 0 rudstone
DSx 3 89.5m SS 96 4 0 0 0 0 ib silt/sh bed
DSx 4 84.5m LS 74 26 0 0 0 0 rudstone
HPx 1 45m SS 65 35 0 0 0 0 ib silt/sh bed
HPx 2 72m SS 97 3 0 0 0 0 HCS siltstone
HPx 3 74m LS 9 91 0 0 0 0 ib silt/sh bed
HPx 4 92.5m LS 6 44 50 0 0 0 floatstone
HPx 5 121m LS 2 89 0 0 0 9 rudstone
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MASTER CENSUS DATA: ABUNDANCE BY LITHOLOGY
Census Site M ETERS FA CIES Lithology Lingula bivalve gastropod echinoid articulate fish
BC 6 53.5 LS wackestone 44 56 0 0 0 0
BC 5 25 LS wackestone 4 29 3 1 6 48 0
AVERAGE (% ) 24 42.5 1.5 8 24 0
DS3 85 LS rudstone 99 1 0 0 0 0
HP 2 61 LS rudstone 98 2 0 0 0 0
BCx 1 13.5 LS rudstone 95 4 1 0 0 0
DS 2 83 LS rudstone 91 9 0 0 0 0
DSx 2 104 LS rudstone 91 3 6 0 0 0
BC2 12.25 LS rudstone 82 1 6 0 0 0 2
DSx 4 84.5 LS rudstone 74 26 0 0 0 0
BC 3 19.5 LS rudstone 53 46 1 0 0 0
BCx 4 28 LS rudstone 47 53 0 0 0 0
BC4 24 LS rudstone 37 60 2 1 0 0
DS 1 62 LS rudstone 22 39 0 0 0 0
BCx 3 22 LS rudstone 6 0 0 2 92 0
DSx 1 61.5 LS rudstone 3 96 0 0 1 0
HPx 5 121 LS rudstone 2 89 0 0 0 9
BC 7 61 LS rudstone 1 99 0 0 0 0
BCx 2 25 LS rudstone 0 0 0 5 95 0
AVERAGE (% ) 50.06 33.94 0.625 0.5 11.75 0.7
SHx 2 q 44 LS floatstone 99 1 0 0 0 0
SH 1 44 LS floatstone 65 35 0 0 0 0
GV 4 24.25 LS floatstone 62 38 0 0 0 0
GV 3 21.5 LS floatstone 30 70 0 0 0 0
HP 3 92 LS floatstone 15 85 0 0 0 0
HPx 4 92.5 LS floatstone 6 44 50 0 0 0
GRx 4 29 LS floatstone 3 97 0 0 0 0
AVERAGE (% ) 40 52.86 7.14286 0 0 0
PROXIM AL LIMESTONE AVERAGE (% ) 45.16 39.92 2.52 0.96 9.44 0.4
104
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MASTER CENSUS DATA: ABUNDANCE BY LITHOLOGY (cont.)
HPx 2 72 SS siltstone 97 3 0 0 0 0
SHx 3 72 SS siltstone 87 13 0 0 0 0
SH 2 72 SS siltstone 83 1 7 0 0 0 0
DS 4 90 SS siltstone 82 1 8 0 0 0 0
GRx2q 1 7,75 SS siltstone 81 1 9 0 0 0 0
GRx3q 24.5 SS siltstone 3 97 0 0 0 0
GV 6 25.25 SS siltstone 0 100 0 0 0 0
GV 7 26 SS siltstone 0 100 0 0 0 0
AVERAGE (% ) 54.13 45.88 0 0 0 0
GRx 1 q 1.5 SS biot. siltstone 100 0 0 0 0 0
GV 1 1.5 SS biot. siltstone 14 86 0 0 0 0
SH 4 106 SS biot. siltstone 1 1 89 0 0 0 0
GV5 25 SS biot. siltstone 7 93 0 0 0 0
AVERAGE (%) 33 67 0 0 0 0
PROXIM AL SILTSTONE AVERAGE (%) 40.04 59.96 0 0 0 0
DSx 3 q 89.5 PL/S ib silt/sh bed 96 4 0 0 0 0
HP 1 45 PL/S ib silt/sh bed 72 28 0 0 0 0
HPx 1 45 PL/S ib silt/sh bed 65 35 0 0 0 0
HPx 3 74 PL/S ib silt/sh bed 9 91 0 0 0 0
M IX ED PROXIM AL AVERAGE (% ) 60.5 39.5 0 0 0 0
SHx 4 q 73 F M ib silt/mud 94 6 0 0 0 0
SH 3 92 P M ib silt/mud 88 12 0 0 0 0
PROXIM AL MUDSTONE AVERAGE (% ) 91 9 0 0 0 0
SHx 1 q 31 D M ib Is/mud 7 2 91 0 0 0
DINW OODY FORM ATION CENSUS TOTAL 2125 1811 154 24 236 11
WHOLE FORMATION AVERAGE (% ) 48.3 41.16 3.5 0.5455 5.363636 0.3
105
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MASTER CENSUS DATA: THIN SECTION FAUNA
Site/Sample lithology # taxa Lingula bivalve gastropod echinoid articulate biogenic %
Gros Ventre 1 siltstone 2 <1% <0.05 5%
Gros Ventre 3 siltstone 2 <5% 10% 15%
Gros Ventre 4 siltstone 3 10% 40% <1% 50%
Gros Ventre 5 siltstone 2 <1% <0.05 5%
Gros Ventre 7 siltstone 2 <1% <1% 1%
Blacktait 2 rudstone 3 5% 55% <1% 60%
Blacktail 3 rudstone 3 5% 50% 5% 60%
Blacktail 4 rudstone 4 <5% 40% 5% 5% 55%
Blacktail 6 wacke 4 10% 5% present <15% 25%
Blacktail 7 rudstone 3 <1% 50% present 50%
Dalys Spur 1 rudstone 3 5% 55% present 60%
Dalys Spur 2 rudstone 2 5% 45% 50%
Dalys Spur 3 rudstone 2 5% 55% 60%
Dalys Spur 4 siltstone 1 15% 15%
Sandy 1 floatstone 3 10% 10% present 20%
Sandy 3 mudstone 1 1% 1%
Sandy 4 siltstone 2 1% <10% 10%
Hidden 2 rudstone 2 5% 65% 70%
Hidden 3 floatstone 3 <1% 30% <20% 50%
106
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MASTER CENSUS: GUILDS REPRESENTED IN THIN SECTION
GUILDS REPRESENTED IN THE DINW OODY FORM ATION
Site/Sample lithology infauna semi infauna/epifauna vagile benthos
Gros Ventre 1 silt 2 1 0
Gros Ventre 3 silt 1 0 0
Gros Ventre 4 silt 1 1 1
Gros Ventre 5 silt 1 1 0
Gros Ventre 7 silt 2 1 0
Blacktail 2 rudstone 1 1 0
Blacktail 3 rudstone 1 1 1
Blacktail 4 rudstone 1 1 2
Blacktail 5 wacke 1 2 2
Blacktail 6 wacke 1 1 2
Blacktail 7 rudstone 1 1 1
Dalys Spur 1 rudstone 1 J J 1
Dalys Spur 2 rudstone 1 1 0
Dalys Spur 3 rudstone 1 1 0
Dalys Spur 4 silt 1 1
Sandy 1 float 1 2 0
Sandy 3 mudstone 2 0 0
Sandy 4 silt 2 1 0
Hidden 1 silt 1 0 0
Hidden 2 rudstone 1 1 0
Hidden 3 float 1 1 1
INCLUDES: IN CLU D ES: INCLUDES:
inarticulata bivalvia gastropoda
trace fossils articulata echinoidia
107
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APPENDIX D: Cluster Analysis
Cluster analysis was performed in an attempt to identify biofacies within the
Dinwoody Formation, using the Master Census data on faunal abundance. This was
broken down into Field and Bulk Sample Clusters in order to look at possible
differences due to the focus of one method (bulk sampling) on Lingula-rich strata.
Significant differences were in fact noted between the two methods. A
correlation between gastropods, fish, echinoids and articulate brachiopods appears in
the bulk sample data, but there is no correlation at all between Lingula and other taxa.
The field data is far less conclusive despite, or perhaps because of, its stronger focus on
non-Lingula fauna. It seems likely that the small populations of these taxa in bulk
sample artificially increased the appearance of correlation. This is particularly evident in
the case of fish, which were only observed at two horizons, yet are strongly correlated
with echinoids in all clusters.
108
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APPENDIX D: Cluster Analysis Results
Bulk Sample Cluster
STATISTICA
CLUSTER
STATS
(Dlink/Dmax)*100
Variable 1--10--20--30-- 40--50-- 60-- 70-- 80--90—
-100
Tree
Variable
T.TN(STTT.A
LINGULA
GASTROPO —1
GASTROPO
FISH -H
FISH
ECHINOID --1 ----1
ECHINOID
APTTf'TTT.A
ARTICULA -------1 ----
U T V A T V F
B lV iU jV fc
Field Data Cluster
STATISTICS Tree
CLUSTER
STATS
(Dlink/Dmax)*100 Variable
Variable 1--10--20-- 30--40--50--60-- 70--80--90-- 100
LINGULA
BIVALVE
GASTROPO
ECHINOID
FISH
ARTICULA
LINGULA
BIVALVE
GASTROPO
ECHINOID
FISH
ARTICULA
STATISTICS Tree
CLUSTER
STATS
(Dlink/Dmax)*100 Variable
Variable 1--10--20-- 30--40-- 50-- 60--70-- 80-- 90---100
LINGULA
BIVALVE
GASTROPO
ECHINOID
FISH
ARTICULA
LINGULA
BIVALVE
GASTROPO
ECHINOID
FISH
ARTICULA
Combined Cluster
109
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APPENDIX E: Taphonomy of Lingula
The preservation of Lingula was intensively studied from bulk samples
taken from the Dalys Spur locality. Among the factors measured were whether
valves were whole, molds of whole valves, or fragmented; whether valves were
composed of original shell material or had undergone valve ‘blackening’; the length
of each valve or valve fragment along its longest axis, and the orientation of each
valve with respect to horizontal.
The data is presented in the following table, in outline form. Each sample
was taken from a single rock (in order to allow consistent orientation
measurements), identified from the sample horizon (e.g., DS 2 is Dalys Spur 2.x)
and designated sample bag. Five sample bags were taken from each horizon, so bag
5 would be Dalys Spur 2.5.
Each category is set up in outline form, with subsidiary characteristics tallied
as a breakdown of the initial category. I.e., for 6 whole valves in Dalys Spur 2.5, 2
have the original shell material, while 4 are molds, and the average length of all 6 is
8.2mm. This format allows rapid subdivision of each category and comparison
between different categories.
110
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
APPENDIX E: Taphonomy of Lingula
Dalys Spur 2.1
whole valves, original material: 4
horizontal, concave up: 2
horizontal, concave down: 2
average length: 5.25mm
maximum length: 7mm
minimum length: 3mm
fragmental valves: 18
original material: 15
horizontal, concave up: 6
horizontal, concave down: 3
horzontal, flattened: 6
blackened: 3
horizontal, concave up: 2
horizontal, concave down: 1
average length: 3.2mm
maximum length: 6mm
minimum length: 2mm
Dalys Spur 2.5
whole valves: 6
with original material: 2
molds: 4
horizontal, concave up: 2
horizontal, concave down: 4
average length: 8.2mm
maximum length: 13mm
minimum length: 4mm
fragmental valves: 12
original material: 8
horizontal, concave up: 2
horizontal, concave down: 3
horizontal, flattened: 3
blackened: 4
horizontal, concave up: 1
horizontal, concave down: 0
horizontal, flattened: 3
average length: 3.7mm
maximum length: 10mm
minimum length: 2mm
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Appendix E continued
Dalys Spur 3.3
whole valves: 5
original material: 3
molds: 2
horizontal, concave up: 2
horizontal, concave down: 2
inclined: 1
average length: 4.8mm
maximum length: 6mm
minimum length: 4mm
fragmental valves: 40
original material: 35
horizontal, concave up: 10
horizontal, concave down: 13
horizontal, flattened: 2
inclined: 10
blackened: 5
horizontal, concave up: 2
horizontal, concave down: 2
horizontal, flattened: 0
inclined: 1
average length: 4.5mm
maximum length: 12
minimum length: 2
Dalys Spur 4.3
whole valves: 5
original material: 4
molds: 1
horizontal, concave up: 2
horizontal, concave down: 3
average length: 7mm
maximum length: 10mm
minimum length: 4mm
fragmental valves, original material: 24
horizontal, concave up: 5
horizontal, concave down: 10
flattened: 8
inclined: 1
average length: 4.7mm
maximum length: 13mm
minimum length: 2mm
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Appendix E continued
Dalys Spur 4.5
whole valves: 11
original material: 7
horizontal, concave up: 1
horizontal, concave down: 6
molds: 4
horizontal, concave down: 4
average length: 7.1mm
maximum length: 12mm
minimum length: 3mm
fragmental valves: 35
original material: 32
horizontal, concave up: 9
horizontal, concave down: 16
flattened: 7
blackened: 3
horizontal, concave down: 1
flattened: 2
average length: 3mm
maximum length: 7mm
minimum length: 2m
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
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Rodland, David Laurence
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Paleoenvironments and paleoecology of the disaster taxon Lingula in the aftermath of the end-Permian mass extinction: Evidence from the Dinwoody Formation (Griesbachian) of southwestern Montana...
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