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Barnacles as mudstickers? The paleobiology, paleoecology, and stratigraphic significance of Tamiosoma gregaria in the Pancho Rico Formation, Salinas Valley, California

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Barnacles as mudstickers? The paleobiology, paleoecology, and stratigraphic significance of Tamiosoma gregaria in the Pancho Rico Formation, Salinas Valley, California

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BARNACLES AS MUDSTICKERS?
THE PALEOBIOLOGY, PALEOECOLOGY, AND STRATIGRAPHIC
SIGNIFICANCE OF TAMIOSOMA GREGARIA IN THE PANCHO RICO
FORMATION, SALINAS VALLEY, CALIFORNIA
By
Karen Elizabeth Whittlesey
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, 1998
Copyright 1998 Karen Elizabeth Whittlesey
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UN IV ER SITY O F S O U T H E R N C A U F O R N I A
T H E G R n O U A 7 E SC H O O L .
U N IV E R S IT Y »A R K
L O S A N G E L E S . C A L IF O R N IA gOOOT
This thesis, written by
J K a ^ D ...E .l i .? A b e .th _ W h it t l^ s e y ^ _________
under the direction of A _ fe C 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
______ MastSX. .fflf-..5 cience_____________
THEH& COMMITTEE
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ACKNOW LEDGEMENTS
d
I wish to acknowledge those who have made the completion of this degree
possible, through their support, whether it was financial, intellectual, or emotional.
Multiple organizations generously sponsored this research. They are the USC
Department of Earth Sciences, the Paleontological Society, and the Geological Society of
America. Without them, this study could not have been as extensive.
The faculty and staff of the Earth Sciences department have made my tin** at USC
a pleasure, and contributed greatly to the finished product Despite his retirement Donn
Gorsline served as a constant reference point throughout my stay at USC, and is truly
appreciated as both a committee member and role model. Another committee member, Bob
Douglas, was always willing to discuss the Miocene, and his extensive knowledge of
micropaleontology was essential to this thesis’ success.
Others whose support made this project worthwhile include A1 Fischer, John
McRaney, Maria Mutti, Gerald Haug, Doug Hammond, Steve Lund, Loren Smith, Rene
Kirby, Cindy Waite, Curt Abdouch, David Okaya, and the staff of Hancock Library.
Students who contributed to my success here are too numerous to list, but my labmates in
paleontology have contributed much to my intellectual development, and to my
appreciation of really bad puns. Those who assisted in the field - Kindra Loomis, Sean
Magnuson, and Margaret Keller - also are appreciated. Special thanks are also due to Joe
Barr and Eric Hovanitz for identifying minerals and helping with X-ray diffraction
analysis.
Help with this project was not limited to those from USC. Richard Pollastro from
the U.S. Geological Survey graciously analyzed samples using X-ray diffraction, and
broadened my knowledge of the subject. Steve Bohaty identified the diatoms of Pancho
Rico Formation, and with John Barron of the USGS, contributed to our understanding of
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the age of these rocks. Two more people require extra thanks. Margaret Keller not only
took time to explore my field area, but has served as an undying source of encouragement
and intellectual support Bill Newman of Scripps Institute of Oceanography served as my
biological touchstone, and my taxonomic work is a direct result of his support and
supervision. He also contributed to my biological understanding of barnacles, and
supplied a constant stream of references when I needed them.
Finally, this project could not have been accomplished without the support of my
advisor and the love of my family and friends. David Bottjer served as a model advisor,
providing a much-needed connection to reality, and having a constant supply of smiles,
jokes and suggestions on everything from research and class projects to roadside eateries.
Wendy Harrell succesfully managed to cheer me up via the internet and phone, and
contributed to the more infamous vacations of my graduate experience. This degree could
not have been completed without the love and support of my parents, who always
managed to be there if I needed them, even if it was just to take me out to dinner when I
was feeling less than sane. To all of the above people, and those who I may have failed to
mention, your contribution, no matter how small, did not go unnoticed.
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TABLE OF CONTENTS
ACKNOW LEDGEMENTS............................................................................................ ii
LIST OF FIGURES.....................................................................................................vi
LIST OF TABLES....................................................................................................... ix
ABSTRACT........................................................................................................................x
INTRODUCTION..............................................................................................................i
The Barnacle Fossil Record........................................................................... 1
Miocene Seas.......................................................................................................3
Location of A rea.............................................................................................. 5
STRATIGRAPHY............................................................................................................9
General Salinas Valley Stratigraphy............................................................... 9
Description of the Pancho Rico Formation...................................................12
Type Section......................................................................................... 16
A ge..............................................................................................................17
PALEOENVIRONMENTAL ANALYSIS.................................................................21
Paleogeography................................................................................................. 21
Depth of D eposition....................................................................................... 25
Sedimentologic Investigations....................................................................... 28
Introduction............................................................................................ 28
M ethods................................................................................................... 28
Results- Stratigraphic Section A .....................................................32
Results- Stratigraphic Section B .................................................... 42
Substratigraphy of Tamiosoma Beds...................................................45
B ioturbation............................................................................................ 55
Thin Section Analysis....................................................................... 60
X-Ray D iffraction................................................................................99
Carbon/ Carbonate Analysis.............................................................104
D iscusssion............................................................................................110
PALEOECOLOGY OF THE PANCHO RICO FORMATION..................................115
Foram inifera.......................................................................................................116
Paleobiology and Paleoecology of Tamiosoma gregaria...............................120
Evolution and Biology..................................................................... 120
Taxonom y..............................................................................................128
Systematic Description.......................................................... 134
D iscussion................................................................................. 140
Functional Morphology..................................................................... 145
E cology...................................................................................................150
B ioerosion..............................................................................................160
Past Research......................................................................... 160
Methods of Investigation .....................................................162
R esults........................................................................................164
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PALEOECOLOGY OF THE PANCHO RICO FORMATION (Continued)
Bioerosion (continued)
D iscussion................................................................................166
Taphonomy of Tamiosoma gregaria................................................. 174
Taphonomy of Pancho Rico Formation Deposits.........................................174
Site 1 : USGS Locality 903: San Lucas Shell Bed.............................178
Site 2: Wildhorse Canyon............................................................... 181
M ethods 18 I
Analysis of Data: San Lucas Shell Bed...........................................185
Analysis of Data: Wildhorse Canyon...............................................195
D iscussion.............................................................................................. 202
FUTURE RESEARCH............................................................................................... 204
CONCLUSIONS............................................................................................................208
BIBLIOGRAPHY..........................................................................................................211
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V I
LIST OF FIGURES
Figure 1. Index map of the Salinas Valley, California. Modified from Zullo (1979).....7
Figure 2. Generalized Salinas Valley stratigraphy. Modified from Durham (1974)..... 11
Figure 3. Stratigraphic nomenclature of Late Miocene-Pliocene strata in the Southern
Salinas Valley. From Durham and Addicott (1965)................................................15
Figure 4 Miocene paleogeography of Central California. Modified from Addicott
(1978)............................................................................................................................. 23
Figure 5. Contour map of Wildhorse Canyon study area......................................30
Figure 6. Two end-members of diatomaceous sediment: Above bedding plane, blocky,
whiter diatomaceous mudstone. Below bedding plane, muddier diatomaceous
m udstone........................................................................................................................... 34
Figure 7. Key to symbols used in this study....................................................... 36
Figure 8. Stratigraphic section A...........................................................................38
Figure 9. Cross bedding visible in the lower portion of measured section A 40
Figure 10. Stratigraphic section B........................................................................ 44
Figure 11. The giant barnacle Tamiosoma gregaria, preserved exquisitely in situ.......48
Figure 12. Schematic drawing of Tamiosoma beds in measured section B...............50
Figure 13. Outcrop view of Tamiosoma beds......................................................... 52
Figure 14. Underside of subunit 3, as viewed from below..................................54
Figure 15. In situ burrows..........................................................................................57
Figure 16. Burrows within the diatomaceous subunit 2....................................... 59
Figure 17. Microstructure of barnacle shell.......................................................... 62
Figure 18. Barnacle shell covered with sparry calcite crystals.............................. 64
Figure 19. Phosphatic pellet, shown in plane light.............................................. 67
Figure 20. Phosphatic pellet, shown under crossed polars................................... 69
Figure 21. Phosphatic pellet, shown in plane light. Note quartz nucleus................. 71
Figure 22. A diatom serves as a nucleus for phosphate pellet creation.................... 73
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vii
Figure 23. Unidentified object, possibly fish bone. Shown in plane light.................75
Figure 24. Unidentified object, possibly fish bone. Shown under polarized light 77
Figure 25. Endobiont boring cross-cutting growth bands in barnacle shell...............79
Figure 26. Boring cross-cutting growth bands in barnacle shell............................. 81
Figure 27. Diatom and microfossil debris............................................................84
Figure 28. Centric diatom....................................................................................... 86
Figure 29. Diatom and phosphatic pellet.............................................................. 88
Figure 30. Foraminiferan test................................................................................ 90
Figure 31. Sponge spicule and microfossil debris in matrix...................................92
Figure 32. Cross section of bryozoan. Diatoms nestled in leftmost chamber............ 94
Figure 33. Possible echinoid plates. Shown in plane light.................................... 96
Figure 34. Possible echinoid plates. Shown under crossed polars........................ 98
Figure 35. X-ray diffraction pattern for sediment found at stratigraphic level of
Tamiosoma buildups (Subunit 2).......................................................................... 102
Figure 36. Albite (feldspar) fragment and two phosphatic pellets filling in crack in
barnacle shell. Shown under crossed polars.........................................................106
Figure 37. Results for carbon and carbonate analysis......................................... 108
Figure 38. Top view of a balanomorph barnacle, idealized..................................125
Figure 39. Photograph of Menesiniella aquila, and its opercular plates..................131
Figure 40. Cluster of bases of the giant barnacle, Tamiosoma gregaria.................. 133
Figure 41. Opercular plates of Tamiosoma gregaria.............................................137
Figure 42. Bases often show longitudinal lines and concentric banding.................139
Figure 43. Scallop served as anchor for juvenile barnacles to settle upon...............147
Figure 44. Barnacles serve as anchors for other individuals................................. 149
Figure 45. Kauffman and Sohl’s (1974) classification of rudist frameworks...........153
Figure 46. Illustration of “constratal” growth fabric, from Gili et al. (1995)............156
Figure 47. Tubeworm shell, found inside shell of T. gregaria.............................. 159
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viii
Figure 48. Predicted bioerosion for barnacles which are exposed to water (and boring
organisms) throughout their lives...........................................................................168
Figure 49. Observed bioerosion on Tamiosoma gregaria.................................... 171
Figure 50. Cross section of a barnacle................................................................177
Figure 51. Shell bed near San Lucas.................................................................. 180
Figure 52. Orifice length is linearly correlated with scutum length........................ 184
Figure 53. Barnacle scuta from San Lucas shell bed show a left-skewed size-frequency
distribution.................................................................................................................... 187
Figure 54. Predicted barnacle orifice lengths calculated from scutum lengths.......... 189
Figure 55. Comparison of measured and predicted scutal distributions..................192
Figure 56. When viewed as a percentage, aperture lengths of collected barnacles shows
a normal distribution................................................................................................194
Figure 57. Scutum length distribution from Wildhorse Canyon bulk samples 197
Figure 58. Predicted orifice lengths of barnacles from Tamiosoma beds, as compared to
observed scutal lengths.......................................................................................... 199
Figure 59. Predicted orifice lengths for Wildhorse Canyon differs from those in San
Lucas Shell Bed........................................................................................................ 201
Figure 60. Thin section showing microscopic growth lines in barnacle shells 206
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ix
LIST OF TABLES
Table 1. Names previously applied to the Pancho Rico Formation (Modified from
Durham and Addicott, 1964).....................................................................................13
Table 2. Biostratigraphically important megafossils from the Pancho Rico Formation
(From Durham and Addicott, 1965).........................................................................18
Table 3. Diatoms identified from the Pancho Rico Formation.................................20
Table 4. Diagenetic changes in diatomaceous rock............................................... 100
Table 5. Foraminiferal zones as defined by Long (1957) at the type area of the Pancho
Rico form ation..............................................................................................................117
Table 6. Carbonate microfossils of the Pancho Rico Formation, as discussed by Hughes
(1963)................................................................................................................................ 118
Table 7. Classification of barnacles (From Newman, 1996)................................. 122
Table 8. Classification of the Balaninae................................................................144
Table 9. Raw data for bioerosion analysis............................................................165
Table 10. Statistical comparison of predicted orifice lengths.................................165
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X
ABSTRACT
Although barnacles are a minor component of Cenozoic reef complexes, in
exceptional situations, barnacles are thought to have constructed reefs in the Neogene. To
better understand the role of barnacles as suspect reef-builders, exceptional barnacle
“reefs” were studied from the Pancho Rico Formation, in the Salinas Valley of Central
California. These deposits are dominated by the giant barnacle Tamiosoma gregaria,
include diatomaceous to locally conglomeratic facies, and are thought to record the last
marine incursion into the Salinas Basin during the latest Miocene. Because Tamiosoma
gregaria has been cited as the main reef-building barnacle in the fossil record, this study
focuses on its taxonomy, paleoecology, and stratigraphic context within these units.
Tamiosoma is found in situ in Wildhorse Canyon, in highly bioturbated
diatomaceous mudstones. Unlike most modem barnacles, Tamiosoma occupied an
ecological niche of mudsticking, much as some have hypothesized was done by rudist
bivalves in the Mesozoic. Although these have been called “reefs”, there is no evidence
that these barnacles formed any significant relief above the seafloor, and the lack of
bioerosion on the exterior of the shells suggests that most of their skeleton was located
primarily below the seafloor throughout most of their lives. In Wildhorse Canyon, shell
beds composed mostly of barnacle fragments serve to protect the in situ beds of
Tamiosoma. In the Tamiosoma beds, evidence of small-scale alternation between rapid
and slow deposition exists. The elongate shell form of the barnacles and the lack
bioerosion on the exterior of the barnacle shell are evidence of rapid deposition. In the
same stratigraphic interval, however, burrows in the diatomaceous muds which surround
the barnacles and pelletal phosphorite development indicate short-term hiatuses.
Two species of barnacle dominate the Pancho Rico Formation. The taxonomy of
Tamiosoma is discussed, and a new hypotype for Tamiosoma gregaria is proposed, based
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on complete specimens found in the Wildhorse Canyon barnacle beds. The taxonomy of
Menesiniella aquila is also clarified. In addition, the taphonomic properties and
occurrences of each are discussed, as illustrated by two contrasting outcrops: Wildhorse
canyon, where Tamiosoma is preserved in situ, and San Lucas, where a concentrated shell
bed of Menesiniella is preserved.
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INTRODUCTION
1
The Pancho Rico Formation provides critical insight into the geologic history of
the Salinas Valley during the latest Miocene. This formation, which was deposited
generally in shallow-water environments, differs in thickness throughout the Salinas
Valley, and varies in lithology from sandy to muddy or diatomaceous facies. The
Pancho Rico represents the final incursion of marine conditions into the Salinas Basin,
before fluctuating sea level and the tectonics of an active margin drained the Salinas
Valley, and the nonmarine Paso Robles Formation carved its way through the Pancho
Rico Formation, in places eroding strata down to Cretaceous basement (Durham, 1963).
Paleoecologically, the Pancho Rico Formation records a diverse benthic megafauna as
well as microfauna. Although typical Cenozoic marine fossils such as bivalves and
gastropods are common, the most unusual component of the megafauna is the giant
barnacle Tamiosoma gregaria. It is the purpose of this study to integrate the
paleoecology of T. gregaria into the stratigraphic context of the Pancho Rico Formation
in Wildhorse Canyon, the type area for T. gregaria. In addition, the taxonomic
affinities of Tamiosoma, never fully understood, aze discussed.
The Barnacle Fossil Record
Although barnacles are an important contributor to modem sediments (Milliman
et al., 1972), they have not been a focus of many paleontological studies, as have been
other multi-element skeletonized tax a, such as echinoderms. Barnacles, however, have
an excellent fossil record in the Cenozoic, and are useful in determining depositional
environments (Doyle et al., 1996).
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2
The barnacle fossil record dates back to the Middle Cambrian, with one
recorded lepadomorph (goose-bamacle) specimen in the Burgess shale (Collins and
Rudkin, 1981). Lepadomorph barnacles are generally not well preserved due to the lack
of fusion of their plates, but by the Mesozoic, they became abundant enough to be
stratigraphically useful (Newman and Foster, 1987). Hattin and Hirt (1991) successfully
used well preserved scalpellomorph barnacles, which are abundant in the Middle
Turanian Carlisle Shale of Kansas, to interpret the paleoenviranment of the Fairport
member, a chalk. The barnacle assemblage, in addition to the other epibionts which
colonized the shells of inoceramid bivalves, are preserved in situ. Doyle et al. (1996)
used barnacle assemblages and taphonomy to interpret a depositional sequence in
Spain. Because barnacles fuse themselves to rocks, the presence of barnacle bases on
rocks and other hardgrounds can be used to interpret the depth at which the rock rested
(Doyle et al., 1996). They are also a positive indicator of marine conditions, and are
useful in paleoenvironmental interpretation.
A few balanomorph barnacles are known from the Mesozoic, but the vast
majority do not appear until the Cenozoic Period, with the Balanoidea apparently
evolving in the Eocene (Newman, 1979). Their well-calcified shells are easily
preserved (Doyle et al., 1996) in the fossil record. In fact, Darwin (1851, p. 5) noted
their abundance, stating the following:
These Cirripedes now abound so under every zone, all over the world, that the
present period will hereafter apparently have good claim to be called the age of
Cirripedes, as the Palaeozoic period has to be called the age of Trilobites.
Since this monumental work on fossil cirripedes, the geological time scale has
undergone major revisions, and his “present period” is undoubtedly the Cenozoic, as his
“Secondary” is now called the Mesozoic.
Biogeographically, the latest Miocene is the beginning of our current time of
great endemism in the world’s oceans due to steep pole-to-equator temperature
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3
gradients (Kennett, 1977, Newman, 1979). Tamiosoma gregaria is an important part of
our understanding of barnacle taxonomy, biogeography and paleoecology.
Tamiosoma's unusual morphology is unique and represents a specific biotic response to
the unusual depositional conditions of the Salinas Basin in the latest Miocene.
Miocene Seas
The Miocene was a time of global change, from the cooler Oligocene to climates
and oceanic circulation similar to today’s oceans. Not only is it important to understand
this transition, but paleoceanographic techniques such as stable isotope analysis,
paleomagnetism, and a well-established chronostratigraphy help to recreate circulation
and other environmental patterns to levels impossible for any other geologic period.
Although ice has been present in the poles at many times throughout geologic history, it
was during the Middle Miocene that it became widespread and permanent (Sclater et
al., 1985). This growth of the ice sheet was caused by the intensification of the circum-
Antarctic current (Sclater et al., 1985). The accumulation of ice created associated
oceanographic and paleoclimatic changes, including increased thermal gradients in
surface waters, as well as an increased gradient from the poles to the equator (Kennett,
1985). Widespread diatomite deposition in the Pacific margin was initiated at this time
(Ingle, 1981), due to pronounced coastal upwelling. Indicators of coastal upwelling, as
demonstrated off the coast of Peru, include high organic carbon and phosphate
abundances, and high biogenic opal concentrations (Scheidegger and Krissek, 1983).
Increased thermal gradients in the world’s oceans were caused by numerous
tectonic factors. Prior to the Neogene, the water circulation around Antarctica, due to
the close proximity of South America and Antarctica. The Miocene opening of the
Drake Passage, however, caused the circum-Antarctic current to intensify, and the pole-
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4
to-equator thermal gradient became steeper as cooler waters stayed near the poles
(Sclater et al., 1985). Kennett (1977) argued that the increased temperature gradient
would divide the Earth into latitudinal climate zones. Thus, the biota of the world’s
oceans would become more endemic, as habitats diversified (Newman and Foster,
1987). This study suggests that barnacle biodiversity may be higher than previously
documented, supporting this hypothesis.
Another tectonic factor which contributed to these oceanographic changes was
the closure of the Tethyan seaway, including the straits of Gibraltar and the isthmus of
Panama. Although the exact dates of closure are debated, this closure helped increase
the temperature gradient in the ocean basins and created distinct water masses of the
oceans (Sclater et al., 1985).
During the Latest Miocene, this very cold water mass expanded, encompassing
New Zealand and California (Sclater et al., 1985). At this time, oceanic circulation
increased, the climate became cooler. There was a carbon isotope shift associated with
this cooling, and the Mediterranean ocean dried up, a time referred to as the Messinian
salinity crisis (Sclater et al., 1985).
Ingle (1981) reported remarkable coincidence between Miocene-Pliocene
stratigraphic successions around the Pacific Rim. He noted that in California, Mexico,
Japan, and Korea, diatomites are a common lithology of Miocene rocks (1981). This
lithologic succession can be described in the following sequence:
1) Deposition of Oligocene-lower Miocene volcanic rocks and continental and/or
neritic marine clashes
2) Middle-upper Miocene laminated diatomites
3) Latest Miocene- earliest Pliocene carbonate-poor mudstone facies
4) Plio-Pleistocene terrigenous and clastic wedges.
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5
The rocks of the Salinas Valley also follow this pattern, with the succession of
the Vaqueros, Monterey, Pancho Rico, and Paso Robles Formations. The Pancho Rico
Formation is interesting in that it retains a high percentage of diatomaceous sediment as
well as having a unique barnacle fauna. It is, however, mudstone, and is indeed
carbonate poor, with the majority of carbonates derived from the very durable low-
magnesium calcite shells of barnacles and other benthic organisms. Thus, the Pancho
Rico Formation is both representative of the latest Miocene and yet individual enough
to demand a closer look at the circumstances of its deposition, and the resulting
lithologies.
Location of Study Area
This study was conducted on the eastern side of the Salinas Valley, near King
City, as shown in Figure 1. The field area, characterized by low, rounded, vegetated
hills, is pan of the larger landform of Gabilan Mesa, a rectangular upland measuring
approximately 110 km long, and 20 km wide. Wildhorse Canyon is one of the major
drainages which cut through the high hills, though it is not one of the four which
completely dissect the mesa (Tinsley and Dohrenwend, 1979).
The Salinas Valley is pan of a larger tectonic feature known as the Salinian
block. This tectonic unit is characterized by a basement of granitic and high-grade
metamorphic rocks, which are distinctly different than the Franciscan block to the west
(Durham, 1974). The Salinian block is bounded by the San Andreas Fault on the west,
and the Jolon-Rinconada and Sur-Nacimiento fault zones to the east.
Though the Pancho Rico Formation extends from the San Andreas Fault to the
foothills of the Santa Lucia Range (Addicott, 1978), the majority of outcrops are located
on the eastern side of the valley. Wildhorse Canyon, where reef-like buildups of
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6
Figure 1: Index map of the Salinas Valley, California. Wildhorse Canyon study site
marked with open box. Adapted from Zullo (1979).
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k \V
C Q \ • Study A tm
San Luis
San Mguel
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Tamiosoma gregaria are found, serves as the main site for this study. In the Wildhorse
Canyon area, the Pancho Rico Formation contains only a small portion of sand-sized or
coarser particles; in general the rocks are diatomaceous mudstone. The vast majority of
rock which Wildhorse Canyon dissects is Pancho Rico Formation, with the Paso Robles
Formation capping the tops of the rolling hills.
As a contrast paleoecologically and taphonomically, a shell bed outside the town
of San Lucas was also sampled for this study. This shell bed is also part of the Pancho
Rico Formation, although the poor exposure of the formation in this area prohibits
stratigraphic correlation with Wildhorse Canyon.
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9
STRATIGRAPHY
General Salinas Valley Stratigraphy
The stratigraphy of the Salinas basin has been complicated by the basinal nature of the
strata found in the valley. As Durham and Addicott (1965) noted, lateral heterogeneity
of formations such as the Pancho Rico Formation has hampered description, as has the
vegetated nature of the Salinas Valley, making widespread correlation difficult. The
overall geology of the basin, as described by Durham (1974), is shown in Figure 2.
Mesozoic granitic basement rock floors most of the valley, and was exposed to erosion
and deposition in different areas, at different times. The oldest sedimentary rock in the
Salinas Valley is the Eocene Reliz Canyon Formation, overlain by the Oligocene (?)
Berry Formation (Durham, 1974). The Monterey Formation, ubiquitous to much of the
California Miocene, is divided into three members in the Salinas Valley- the Sandholt,
Hames, and Buttle Members (Durham, 1974). Interfingering with the Monterey are the
Tierra Redondo Formation and Santa Margarita Formation. The Santa Margarita is a
shallower, sandier Miocene facies than the Monterey, containing a diverse benthic
megafauna including the giant oyster Crassostrea titan. Tamiosoma gregaria has also
been collected from the Santa Margarita Formation. The Pancho Rico Formation
overlies the Monterey and Santa Margarita Formations, possibly interfingering with the
Santa Margarita in its southern extent Subaerial erosion and the deposition of the Paso
Robles Formation, a nonmarine unit, carved through many of the formations. It overlies
erosional surfaces above the Pancho Rico Formation and the Monterey in some places,
and cuts through to granitic basement in other places. The basal conglomerate which
marks the lowermost extent of the Paso Robles formation commonly contains cobbles
of Monterey Formation, including localities in Wildhorse Canyon. As Long (1957)
discussed, the deposition of the Paso Robles was controlled by the draining of the
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10
Figure 2: Generalized Salinas Valley stratigraphy. Modified from Durham (1974).
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Paso Robles Formation (Pliocene-Pleistocene?)
Ffencho Rico Formation
) B utttefcfem ber
“ Mcnterey
H am e^sm ber
FormatioUjMio
SandhcMt Member
Santa
Margarita
Formation
(Miocene)
, x x x x x x x‘
Tierra Redondo
Formatior(Mio.)
Vaqueros Formation (Mtocene)
» / / / / / / / / ,
. X X X X X X S X X
X X X N X X X X X X
. X X X X X X X X X X X
/ ✓ / / / / / / / / / ,
rx x x x x x x x x x x
V / / / / / / / / / /,
x x x x x x x x x x x x
' / / / / / / / / / / / ,
X X X X X X X X X X X X
> / / / / / / / / / / / ,
x x x x x x x x x x x x
NT////////////,
x x x x x x x x x x x x x x
Ber ryFormat ion(OI igocene?)
X X X X X X X
x x x xx xxx
/ / / / / / / /
»->^ XX XX X\ X\
/ / / / / / / / / / / '////// / V / / / /V / // // // // // ^/
x x x x x x x x x x x x x x x x x x x x x x x x x x
' / / / / / / / / / ^ / / / / / / / / / / / / / / / ^
v v v V V v v v VV V V V V V V V V v v v v v v v v
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12
Salinas Basin in the latest Miocene and Pliocene. Durham (1974) noted that the
deposition of the Paso Robles Formation may have persisted into the Pleistocene.
Differential uplift and topographic highs likely controlled the distribution of the Paso
Robles Formation, resulting in the highly variable lower boundary.
The strata now classified as the Pancho Rico Formation have been classified
under a multitude of names, as seen in Table 1.
Because of the lateral facies changes in the Salinas Valley, all formations in the
area, and not just the Pancho Rico Formation, have been identified under different
names. Durham and Addicott (1965) concisely summarized these changes, as seen in
Figure 3. This change in nomenclature must be taken into account when observing
museum specimens - fossils collected at the same locality which may bear much
different formational identification, creating a challenge for paleontologists. In this
study, this complication was removed by studying only those specimens collected by
the author, although published specimens were examined for taxonomic comparison.
Description of the Pancho Rico Formation
The Pancho Rico Formation consists of marine sandstones and mudstones, locally
conglomeratic or diatomaceous. Although the majority of the formation is sandy
(generally very fine-grained), in some areas the rock can be quite diatomaceous, and
may be considered diatomite. However the presence of conspicuous amounts of clastic
material makes the majority of the white, lightweight rocks, such as those found in
Wildhorse Canyon, by definition diatomaceous mudstone (Durham 1974). These strata
generally overlie the Monterey Formation, though in the northern portion of the Salinas
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Table 1: Names applied by previous authors to rocks now recognized as Pancho Rico
Formation (Modified from Durham and Addicott, 1964).
Unit Name Authors)
San Pablo Formation Eldridge (1901 p. 408-410)
Santa Margarita Formation Hamlin (1904, p. 15, 18); Pack and English (1915,
p. 133), English (1918, p. 229,231); Reed, (1925, p.
539); Kleinpell (1930, p. 30); Clark (1930, p. 781,
782); Talafieiro (1943, p. 459,460)
Santa Margarita Sandstone Bramlette and Daviess (1944)
Jacalitos < & Etchegoin Fms. English (l9l8, p.231)
Jacalitos Horizon Clark (1930, p. 767)
Etchegoin Formation Talafieiro (1943, p. 460) Baldwin (1950, upper half
of Pancho Rico)
King City Formation Clark (1940)
Poncho Rico Formation Reed (1925, p. 591); Clark (1940)
Pancho Rico Formation Bramlette and Daviess (1944)
Unnamed Formation Durham, (1963,017-19)
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14
Figure 3: Stratigraphic nomenclature of Late Miocene-Pliocene strata in the Southern
Salinas Valley. From Durham and Addicott (1965).
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15
o o
5 O
UO'ltUAj
2 o
* 2
ueiifwMj uu«f«yi
93
•* z
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16
Valley, they overlie crystalline basement where the Monterey is absent. The Pancho
Rico Formation is locally fossiliferous with shallow marine assemblages dominant.
The unit constitutes the majority of Gabilan Mesa north of San Ardo, and
quickly disappears south of that area. The Pancho Rico Formation is difficult to
distinguish in well logs from the older Santa Margarita Formation, so that the
subsurface extent of the unit is not well understood (Durham 1974). Long (1957)
attempted to correlate the formations through the use of well logs; while exact
determination of formational boundaries is weak, his interpreted lower portion of the
Pancho Rico is more homogenous than the Santa Margarita Formation below it.
In the Adelaida quadrangle, on the west side of the Salinas Valley, the formation
is thin, measuring no more than 17 m (Smith and Durham, 1968). This area is one of the
few which has a conformable sequence containing the Pancho Rico Formation,
although deposition was limited in this region. Long (1957) interpreted the center of
deposition to be about 4.8 Km northwest of Lonoak (N36 16' 40", W120 56'32"), where
the thickness of the formation totals over a thousand meters, most of which is fine
grained. On the east side of the valley, the lower contact of the Pancho Rico is not
present in outcrop, and must be ascertained through subsurface exploration.
Type Section
The type section of the Pancho Rico Formation (“Poncho Rico Formation”) was
first described by Reed (1933) based on outcrops on the slopes of Pancho Rico Valley,
through which Pancho Rico Creek flows toward the town of San Ardo. This area is
located at 36*05 N latitude, 125*45 W longitude in the San Ardo and Priest Valley
quadrangles (Long 1957). Long (1957) noted that the type section does not encompass
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17
the entire formation in outcrop view, and formally described the unit based on oil
company cores and well logs. Durham and Addicott (1965) also recognized the strata
exposed along Pancho Rico Creek as the type section of the Pancho Rico Formation,
but noted that the lateral heterogeneity of the formation must be taken into account.
Aye
Age dating of the Pancho Rico Formation has traditionally been accomplished
by either megapaleontology or micropaleontology, but rarely both. Unfortunately, as is
common in the Late Miocene-Early Pliocene basins, the faunas of the Pancho Rico
contain no distinct biostratigraphic markers. Long (1957) recognized that the majority
of the megafossils in the Pancho Rico Formation are characteristic of the Miocene and
Pliocene, and as such, are not very useful for biostratigraphy. However, he recognized
two megafossils which he considered fairly reliable indicators of a Dehnontian age. The
last occurrence of Astrodapsis salinasensis was considered a defining biostratigraphic
indicator of the end of the Delmontian (Kleinpell, 1938). Surprisingly (and without
further supporting evidence or bibliographic citation), the presence of Tamiosoma
gregaria was the second indicator of a Delmontian age (Long, 1957). Long (1957)
concluded that the lower zone of the Pancho Rico Formation was Delmontian in age,
based on the presence of Tamiosoma gregaria and Astrodapsis salinasensis. The
presence of the foraminifera Eponides exigua and Pullenia malkinae in the upper
portion of the formation led Long (1957) to conclude that deposition of the Pancho Rico
Formation continued into the Pliocene.
Durham and Addicott (1965) came to the opposite conclusion- that the Pancho
Rico Formation is of Pliocene age, based on the presence of taxa in Table 2. However,
Addicott (1977) revised the age to latest Miocene based on foraminiferal evidence.
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18
Table 2: Biostratigraphically important megafossils from the Pancho Rico Formation
From Durham and Addicott (1965).
Gastropods Bivalves fechinoids
Calliostoma coalingense
(Arnold)
Area cf. A. santamariense
Reinhart
Astrodapsis amoldi Kew
Calliostoma etchegoinense
(Nomland)
Anadara camuloensis
(Osmont)
Astrodapsis
fernandoensis Pack
Turritella cooperi forma nova
Nomland
Mytilus coalingensis Arnold
Turritella gonostoma
hemphilli Applin in Merriam
Ostrea atwoodi Gabb
Turritella vanvlecki Arnold Lyropecten terminus
(Arnold)
Bittum casmaliense Bartsch Lyropecten cerrosensis
(Gabb)
Thais collomi Carson Patinopecten lohri (Hertlein) tirachiopod
Thais etchegoinensis (Arnold),
ribbed form
Cyclocardia califomica
(Dali)
Terebratalia arnoldi
Hertlein and Grant
Calicanthus cf. C. humerosus
(Gabb)
Macoma afftnis Nomland
Calicanthu kettlemanensis
(Arnold)
Spisula mercedensis Packard
Nassarius coalingensis
(Arnold)
M y a arenaria Linne
Nassarius grammatus (Dali)
Clavus coalingensis (Arnold)
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19
The development of the California Provincial biochronology by Barron (1981)
provided more accuracy in the correlation of all California basins. However, the Pancho
Rico has received only passing attention by micropaleontologists, despite well
preserved diatoms in portions of the formation. Womardt (1967) produced the only
published list of diatoms and silicoflagellates of the Pancho Rico Formation. This
sample was taken from a locality about 16 km northeast of King City and contained no
diagnostic taxa, but represented a similar assemblage to that of the lower Sisquoc
Formation (Womardt, 1967). He interpreted this flora as of Early Pliocene age
("Repettian" benthic foraminiferal stage). No stratigraphic data was included in
Womardt’s (1967) paper.
In this study, diatom samples were taken throughout the Wildhorse Canyon
stratigraphic section and analyzed by Steve Bohaty of the University of Nebraska at
Lincoln. Although the majority of samples contain numerous but poorly preserved
diatoms, site 12, located at the same stratigraphic level as the Tamiosoma gregaria
“reef’, contains many well-preserved diatoms, although poorly preserved diatoms were
by far more common. Although neither species are present, the assemblage can be
assigned to the Thalassiosira hyalinopsis or T. praeoestruppii Partial Range Zones of
Dumont and Barron (1995), which is similar to the assemblages previously described by
Dumont and Barron (1995) and Barron and Baldauf (1986) for the Sisquoc Formation
(S. Bohaty and J. Barron, pers. comm. 1997). This assemblage can thus be assigned a
latest Miocene age. This correlates to the Messinian European stage, as well as the
Delmontian benthic foraminiferal stage. As such, it is 6.2-5.5 MY old, according to the
Cande and Kent (1995) and Berggren et al. (1995) time scales.
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20
Table 3: Diatoms identified from Pancho Rico Formation, from samples collected
during this study, at site 12 (S. Bohaty and J. Barron, pers. comm., 1997).
Thalassiosira antiqua
(Common)
T. nativa (California var.
Schrader)
T. multipora
(of Whiting and Schrader)
T. leptopus
(sensu Baron and Baldauf
(1986)
T. symbolophora T. sp. small form
(recorded by Dumont in
Dumont & Barron, 1995)
Delphineis sachalinensis D. simbriskiana Coscinodiscus subtilis
Actinocyclus ehrenbergi
\ 2x.tenella
Nitzchia rolandi Lithodesmium minisculum
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21
PALEOENVIRONMENTAL ANALYSIS
Paleogeography
Long (1957), in a study of the Pancho Rico Formation, provided no
paleogeographic maps, but he inferred that the formation was deposited in an area quite
similar to that of the present Salinas Valley, with the Gabilan High bounding the sea on
the east As the coast ranges were being lifted up tectonically, the basin shallowed, and
the axis of the basin shifted eastward. Long (1957) estimated that at no time did the
basin exceed a depth of 100 m. This tectonic uplift created the time-transgressive
nonmarine Paso Robles Formation, whose deposition eroded through the marine
sediments as fluvial sedimentation began to dominate the Salinas Valley, first in the
west, then gradually eastward as the basin drained.
Durham and Addicott (1965) interpret the shallow-water megafossil
assemblages of the Pancho Rico Formation as “suggestive of the modem Californian
mollusc an province, as recognized by Valentine (1961), which extends from Point
Conception (lat 34.5*N) to Cedros Island Qat. 28*N).” Based on the warm-water
affinities of the megafossil assemblages, Durham and Addicott (1965) suggested that
the marine basin which filled the Salinas Valley may have had a southern connection to
the Pacific, connecting with the Santa Maria basin (Fig. 4), as well as a connection to
the San Joaquin Basin. This connection must have crossed the present Santa Lucia
range to fill the Salinas Valley. An alternate theory presented by Durham and Addicott
(1965), which does not involve the southern connection, is that the fauna represents a
relict assemblage, left over from warmer Miocene waters.
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22
Figure 4: Miocene paleogeography of Central California. Modified from Addicott
(1978) (Paleogeographic interpretation remains unchanged from Addicott’s original).
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23
1 2 2
121
120
Caiz
alinga
Monterey
Bay
lomeray ■
King
onoak
B akersfield • -
36
Late Miocene
0 50
Kilometers
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24
Although King City is interpreted to have been at the edge of the basin by
Durham and Addicott (1965), the presence of the Pancho Rico Formation on both sides
of the Salinas Valley is indicative of a more widespread center of deposition in the
Latest Miocene, such as that suggested by Long (1957).
The Salinas Valley is an area which has been controlled by active tectonics since
at least the Miocene (Long 1957). Faulting, associated with the San Andreas fault zone
to the Northeast, has separated coeval strata, resulting in a marked disparity in rock
types on the two sides of the valley (Hughes 1963). This, combined with the vegetated
nature of the area, makes precise stratigraphic correlation and paleogeographic
reconstructions a daunting task, and one which will be only superficially discussed in
this paper. Fault motion on the San Andreas Fault has played a significant role in
moving the strata of the Salinas Valley farther north than the latitudes where they were
deposited. This motion must be taken into account when paleoecological analysis is
considered. In addition to the San Andreas, parallel faults in the Salinas Valley, such as
those in the Jolon-Rinconada Fault Zone, are also responsible for offset of strata. Smith
and Durham (1968) noted the proximity of two very different stratigraphic sequences of
rocks along this fault, in the Adelaida Quadrangle. Each sequence is different due to the
nature of faulting, which brought the two conformable yet unlike sequences which
contain all of the major formations of the Salinas Valley into close proximity.
As Durham (1965) documented, the Pancho Rico and Santa Margarita
Formations, both deposited in the Miocene, as well as at least part of the Paso Robles
Formation, which is most likely Pliocene, are offset across the Jolon fault by
approximately 18 Km of right-lateral motion, based on the southern limits of the two
formations. While this method is hampered by the lack of knowledge of the
formations’ exact extent, it does provide a rough estimate of post-Pliocene fault motion
along the faults of the Salinas Valley. This offset has created difficulty in correlating
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25
the strata on either side of the valley, as the lithologies vary a great deal. Hughes (1963)
noted this dichotomy in his paper “The Two Sides of Salinas”. Unlike Durham (1965),
who recognized Pancho Rico strata south of Lockwood, Hughes recognized no strata
younger than the Miocene Monterey Formation on the west, as opposed to numerous
late Miocene-early Pliocene rocks found on the east side of the Valley. Despite offset
between sides, the east side of the Salinas Valley is relatively undeformed, and Pancho
Rico strata are nearly horizontal.
Depth of Deposition
Topographic highs throughout the Salinas Basin resulted in a wide variety of
depths, and lithologies, throughout the Pancho Rico Formation (Long, 1957).
Conglomerates and sands are characteristic of nearshore facies, while shales and
diatomites may indicate deeper portions of the basin, and those removed from
terrigenous input (Long, 1957). These rapid facies changes throughout the formation
have served to confuse the stratigraphic nomenclature of the Pancho Rico Formation.
Long (1957) estimated that the Salinas Basin was filled with less than 100 m of
water at its deepest point, based on shallow-water fossil faunas, the scarceness of
foraminifera, and the relative abundance of the foraminiferal genus Elphiduim. While
fossil faunas at some localities, such as those near San Lucas, do indeed point to the
shallow deposition mentioned, those in Wildhorse Canyon are not as easy to interpret,
and most likely represent a deeper facies of the basin than that near San Lucas. The
barnacle Tamiosoma gregaria represents an ecology no longer present in the modem
oceans, and as such cannot be used to interpret depth, though the other fossils found in
association with it, such as gastropods and echinoids, are generally found in shallow
subtidal waters.
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26
Diatoms may provide a rough estimate of depth of deposition. Because diatoms
are photosynthetic plants, they are capable of living in the photic zone only
(approximately the upper 30 m of the water column)(Barron, 1987). Barron (1987)
noted that if one allows for downslope transport of diatom frustules, it may be possible
to estimate the relative distance from the strand line by using the ratio of shallow-water
benthic diatoms to planktonic diatoms. In deep water, the only source of diatom s is
from surface waters, and the ratio of benthic to planktic approaches zero. In shallow
waters, diatoms can come from the bottom or the water column. However, most of the
diatoms in the Pancho Rico Formation are poorly preserved, thus a ratio is difficult to
determine. Benthics are present, but not dominant (J. Barron, pers. comm., 1997).
Although this small portion of benthics might be indicative of deeper water, the
invertebrate macrofauna as published by Durham and Addicott, (1965) is indicative of
waters shallower than 100 m Thus, it is likely that the diatom component of the fauna is
not a reliable indicator of depth of deposition as Barron (1987) warned.
Nowhere in the studied sections of the Pancho Rico Formation are there any
laminated rocks, such as those ubiquitously found in the Monterey Formation. This is
undoubtedly due to the shallower nature of the strata. Bioturbation was sufficient in
most portions in the basin to completely remove most of the stratification features. Such
bioturbation results in an ichnofabric of 6, representing complete homogenization of
sediments. Discrete burrows found amongst the barnacle shells in Wildhorse canyon
indicate preservation of burrows in addition to the complete homogenization. Such an
ichnofabric can be represented as 6/2 to 6/3 as described by Droser and Bottjer (1990).
Portions of the measured section in Wildhorse Canyon contain evidence of
tidally-dominated deposition. This area is characterized by fine-grained sands which
preserve low-angle cross-bedding. These layers contain in situ burrows which are
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27
interpreted to be made by bivalves. This section is the shallowest of all lithologies seen
in Wildhorse Canyon, and represents an ichnofabric index of 3 to 4.
However deep the water was at the time of deposition, it is clear that the area
was removed from terrigenous input, based on the lack of sand-sized or greater grains
(Prothero 1990). The basin also shows evidence of being well oxygenated, with none
of the characteristics (such as laminations) of an anoxic or dysoxic basin (Savrda,
1995).
The most appropriate model for the environment in which Tamiosoma
flourished involves a gently sloping, mid-shelf portion of seafloor, well removed from
the shore. This area was unaffected by river and stream drainages which flowed down
into the basin, making the area habitable for organisms that preferred a quite seafloor.
The shallow-water faunas indicate that despite the limited input to the area, the water
depth was probably less than 50 meters.
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28
Sedimentologic Investigations
Introduction
The strata found in Wildhorse Canyon are critical to our understanding of the
Pancho Rico Formation, and Late Miocene paleoecology in the Salinas Basin. In most
of the area, vegetation covers the fine-grained rocks of the Pancho Rico Formation.
Stratigraphic methods used in this study include meter-scale characterization of the area
using traditional stratigraphic measurement, carbon-carbonate analysis of samples from
throughout this area, and small-scale analysis on the strata which contain Tamiosoma
gregaria. In addition to field characterization of these rocks, X-ray diffraction analysis
of the diatomaceous mudstone and thin section analysis of the overlying shell bed were
also utilized.
Methods
Two stratigraphic sections were measured in Wildhorse Canyon. Both are
located in T20S, R 9E, Sec. 9. This area straddles the San Lucas and Nattrass Valley
7.5 minute quadrangles. A ranch road winds through a small tributary of the canyon;
recent road cuts reveal much of the upper portion of the formation, as exposed in this
area. Both stratigraphic sections were measured from the same starting point along this
road. This point has no significance to the formational boundaries, and is based solely
on exposure of the rock. Section A, as seen in Figure 5, follows the road. Because of
the presence of roadcuts, more strata are exposed along the road, and thus this reveals a
more continuous record of deposition. Section B veers north up a small canyon in
which barnacles are abundant, and continues through the overlying Paso Robles
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29
Figure 5: Contour map of Wildhorse Canyon study area. Outcrops of in situ Tamiosoma
gregaria are indicated by arrows.
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F
1 km
CONTOUR INTERVAL 40 FEET
Wildhorse Canyon Area: T 20S, R 9E, Section 9
| T. gregaria localities
Measured Stratigraphic Sections
-- Ftaads
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31
Formation. This area, while limited in outcrop where the rocks are softer, has more
pronounced shell bed development, and barnacle beds dominated by both species are
both extensive and spectacularly weathered in relief.
Strata were measured using a 1.5 m Jacob’s staff. Despite the active tectonic
environment, the strata which make up Gabilan Mesa are generally undeformed and
have a 1.5* to 3* southwesterly dip. Lithologic contacts generally parallel topographic
contours (Tinsley and Dohrenwend, 1979). At least one small fault has been observed
by the author. Based on comparison of adjacent shell beds, the offset appears to be
approximately 1 m of vertical displacement Because of this essentially undeformed
environment strata were measured without compensating for tilting. This methodology
thus incorporates a small degree of error based on the lack of compensation, however,
the error resulting from such a method is smaller than other errors. These include
slipping o f the Jacob’s staff or geologist on the steep, vegetation-covered hills, sighting
error, unnoticed slumping, and small faults. Shell beds, of critical importance to this
study, are not continuous throughout the measured sections. Although measured
section “A”‘s placement is bound by the position of the road, in measured section “B”,
beds were often traced horizontally, when possible to accommodate exposed rock, as
well as laterally discontinuous shell beds. Most shell beds measure less than 4 m in
lateral extent, and 1/2 m in thickness.
Sediment samples were collected from well-exposed outcrops, which are
irregularly spaced. Clean bags were used to contain the samples, as well as clean tools.
Care was taken to avoid conspicuous organic matter, such as roots, however there is no
guarantee that these were not present in minute amounts in some of the samples.
Portions of each sample were sent to the University of Nebraska at Lincoln for analysis
of the diatom faunas. The remaining part of each sample was pulverized for use in XRD
and carbon/carbonate analysis.
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Results- Stratigraphic Section A
32
The vast majority of rocks in both measured secdons are diatomaceous, with
the exception of a few sandy intervals. Thus, any fluctuations (except where noted) are
subtle, and must be taken in that context The most common variation is the mud
content, with color and fissility changing from whitish with a blocky conchiodal-type
fracture pattern to grayer, with a more crumbly (but generally blocky as well) fracture.
Figure 6 shows two extreme end members - a muddy, organic-rich layer containing
abundant barnacle debris, and a blocky, more diatomaceous interval above. The
stratigraphy of measured section A is graphically represented on Figure 8, using the
symbols explained in Figure 7.
Near the base of stratigraphic section A is a sandy interval, no more than 5
meters in thickness. This interval is characterized by very fine-grained, golden sands,
with faint cross-bedding preserved (Fig. 9). The sediment at the base of the cross beds
is not coarser than that at the top of the layer. The sandy interval continues up section
with a layer which contains in situ burrows. These burrows measure 5-10 cm in length,
and are slightly elliptical in cross-section. Although the organisms which created the
burrows are not preserved, it is presumed that infaunal clams made them. As aragonite
is rarely preserved in the area, it is quite feasible that the clam’s shell was dissolved.
Gypsum is present at one interval. It is abundant as veins which intersect
bedding planes, and is clearly a diagenetic feature. These veins appear in one distinct
horizon, which is approximately 15-20 m thick. Also present in this horizon are
multiple beds which contain abundant shelly debris, including the wall plates of small
barnacles, which are constructed of low-magnesium calcite. These beds are also darker
in color, and contain less diatomaceous material. None of the shells appear to be
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33
Figure 6: Two end-members of diatomaceous sediment: Above bedding plane, blocky,
whiter diatomaceous mudstone. Below bedding plane, muddier diatomaceous
mudstone. Note vertically oriented, sediment-filled barnacle cluster in this rock. Lens
cap for scale.
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Figure 7: Key to symbols used in this study.
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KEY
$ Fossils (Undifferentiated)
0 Clam? Burrow
Oyster
Oyster Fragements
Scallop
^ Scallop Fragments
O Small Barnacle
© Small Bamade Fragments
0 Large Barnacle
0 Large Barnacle Fragments
\ J Diatomite
— Middy
Gtain size:
M - Mud
FS- Rne Sand
CS- Coarse Sand
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Figure 8: Stratigraphic section A. Matrix grain size indicated by scale at bottom.
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N O T DfOJED
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39
Figure 9: Cross bedding visible in the lower portion of measured section A. Colored
bands on pole are 10 cm thick.
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40
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41
articulated. The only discrete bedding planes visible in the area separate these beds
from the surrounding diatomaceous material. These may be event beds, such as storms,
which brought in shell material and organics from shallower water. Event beds may
have many sedimentologic and taphonomic signatures. Features which may be
indicative of event beds include a simple internal structure and disarticulation of shell
material (Kidwell, 1991). They are commonly associated with a discontinuity surface,
indicating abrupt erosion of the sediment, and can be found within a less fossiliferous
body. However, it is possible that these were caused by episodic sediment bypassing
caused by relative sea level or local tectonic changes (Kidwell, 1991). Although the
shell material bears damage which may be a sign of exposure at the seafloor, it is more
likely that it is an event bed. The most notable reason to indicate an event bed is the
return to an identical depositional regime above the shelly layer.
The vast majority of sediment in measured section A is diatomaceous, and
continues to be diatomaceous throughout the interval. A small number of Tamiosoma
specimens are present at 65 m above the base of the section. These are all poorly
preserved due to the mechanical grading of the road- Some appear to be undisturbed in
the sediment, oriented upright, while others show evidence of disturbance before burial,
and are not oriented vertically. No significant development of a biological framework
was established. All are found in small clusters, or as individuals. Although other shells
are preserved in this layer, shell beds are not well developed.
The very top of this section becomes quite sandy, is golden in color, and shows
evidence of significant soil development Thus, it is likely that the sand is not part of the
original depositional sequence, but is a product of weathering of higher stratigraphic
units.
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Results- Stratigraphic Section B
42
By nature of the topography, much of stratigraphic section B (Fig. 10) is
covered with vegetation, and the fine-grained strata that make up the formation are
poorly exposed. However, even when covered, burrows from terrestrial vertebrates such
as ground squirrels provide clues as to the lithology of the underlying rock. The debris
found near the burrow openings is most commonly diatomaceous mudstone, which is
by far, the vast majority of the rock present in the field area.
Those rocks which are visible provide critical insight into the paleoecology and
deposition of the Pancho Rico Formation. Because both stratigraphic sections start at
the same point along the road, both sections have a muddy interval near the base,
containing articulated oysters, scallop shells, and fragments of small barnacles. Section
“B” veers left off the road, up into the small canyon which contains the barnacle “reef.”
The lower portion of this canyon, over 50 m of rock, is heavily vegetated, although
burrows by small vertebrates provide evidence of the fine-grained nature of this rock.
At approximately 65 m above the base of the section, small (2-3 m in length),
discontinuous shell beds are present in the rock. These are comprised mainly of
barnacle fragments. Bases of Tamiosoma gregaria are present, as well as wall plates of
both Tamiosoma gregaria and smaller barnacles. In most barnacles, the base is a disk
which serves as a point of attachment to the substrate, as well as the point to which all
wall plates attach. In Tamiosoma, however, the bases are large, measuring up to 30 cm
in height, and are modified to serve as anchors in the soft substrate. The bases range in
orientation from vertical to horizontal; none have been observed inverted. These beds
are well cemented, and weather out prominently.
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43
Figure 10: Stratigraphic section B. Matrix grain size indicated by scale at bottom.
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Meters
11 0 l° OoPasaRobIesJFm. o ^ o q
NOTEXPOSBD
1 00
I
90
Possible
— —w - ^ ^ ^ j l r ^ N o r m a F a u l t :
Offset- 1M
80
70
60
A Laterally
B Extensive
c Tamiosombeds
1 0
NOT EXPOSED
(DOMNATED
BY FINE-GRAINED
DIATOMACEOUS
ROCKS)
M FS CSP
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45
Most prominent in the canyon are thcTamiosoma gregaria beds, which contain
in situ barnacles. The majority are preserved in their original vertical position. Because
Tamiosoma gregaria is the focus of this study, these beds have received a great deal of
attention, and are of critical importance to the understanding of Tamiosoma gregaria
and their stratigraphic significance. The substratigraphy of these beds is discussed in
the section below.
Moving upsection, the dominance of a diatomaceous lithology in the area
continues, however shelly deposits become more common. Scallops, small barnacle
fragments and in situ molds of clams are more common upsection, and large barnacles
are less common, appearing only as isolated individuals. The diatomaceous nature of
the formation continues through the top of the section, which is capped by the
nonmarine Paso Robles Formation.
Substratiyraphv of Tamiosoma Beds
Fossils are not evenly distributed in the Pancho Rico Formation. Most are found
in concentrated, laterally discontinuous beds. Such is the case with Tamiosoma
gregaria, which is found mainly at one stratigraphic level.
Weathered-out bases of Tamiosoma gregaria can be found scattered throughout
the ranchland of Wildhorse Canyon, collecting at the bottom of washes. However, at
two localities, Tamiosoma is exquisitely preserved in life position.
Evidence that the deposit is in situ includes:
* The majority of large specimens of Tamiosoma gregaria are oriented
vertically in the rock.
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46
♦Whole specimens, including the fragile parietes of T. gregaria are commonly
present, although they quickly disarticulate when exposed to the elements.
♦ Terga and scuta (sometimes all four, and sometimes oriented with respect to
each other) are present in some specimens.
♦ Molds of fragile bivalves are preserved with both valves, in life position.
This preservation appears to be facilitated by the presence of shell beds, ranging
in thickness from 15-50 cm. Because of their well cemented nature, these shell beds
weather as ledges. These overhangs formed by the shell beds provide protection to the
in situ barnacles beneath from the erosion which is so prevalent in the area. The western
outcrop is small, and shell beds are less developed, making the substratigraphy of that
outcrop less straightforward. Figure 11 illustrates the preservation of T. gregaria at this
western outcrop. The eastern outcrop is extensive, measuring over 100 m in length, and
is easily divided into three subunits: 1, 2, and 3, as illustrated in Figure 12 and Figure
13. As seen in Figure 14, the barnacles are exquisitely preserved in situ, with the
overhanging shell beds holding many suspended in mid air.
Subunit 1 is composed similarly to the shell beds which are below it Bases of
Tamiosoma are found both in place and on their sides. Those shells at the top of the
subunit protrude into subunit “2” and are generally vertically oriented, indicating that
the barnacles were preserved in life position. The subunit is well cemented with calcite
and weathers in relief.
Burrows are visible in subunit “2”, as discussed below. However, it is
convenient to note that because of the infilling of burrows, two distinct lithologic types
are present in subunit 2, in addition to the barnacles preserved there. Generally, the rock
is white and diatomaceous, with little color variation. The burrows are filled with a
more heterogeneous rock type, which contains black specks, interpreted to be pellets as
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47
Figure 11: The giant barnacle Tamiosoma gregaria, preserved exquisitely in situ. Rock
hammer for scale.
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49
Figure 12: Schematic drawing of Tamiosoma beds in measured section B. In situ
barnacles occur in each of the three subunits. Burrows are found within subunit 2, and
are designated with stippled pattern.
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Figure 13: Outcrop view o f Tamiosoma beds.
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53
Figure 14: Underside of subunit 3, as viewed from below. Subunit 2 has been eroded
away, leaving clusters of barnacles, still preserved upright, hanging from the well-
cemented shell bed.
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55
well as detrital minerals, and diatomaceous debris. This difference in lithology helps
distinguish the burrows from the surrounding sediment.
Bioturbation
Bioturbation is evident throughout much of the Wildhorse Canyon area. In most
areas, an ichnofabric index of 6, and complete homogenization of sediment provides the
only evidence of a once flourishing benthic biota. However, at two stratigraphic levels,
discrete burrows are present. The lowermost level, located 11m above the base of the
measured section, is directly above the cross-bedded sands, where numerous vertical,
unlined burrows are found in sand, as seen in Figure 15. These burrows measure 5-10
cm in length, and have an elliptical cross section, with the long axis measuring around
3 cm. The burrow fill is similar to the surrounding sand, but is well cemented, and
specimens weather in full relief. This is the only place in this area where these have
been found in situ, although similar casts have been found as float
Burrows are also present amongst the in situ specimens of T. gregaria in subunit
2, as seen in Figure 16. These burrows are filled with a darker, more heterogeneous
diatomaceous sediment than the surrounding material. This lithologic difference
between the burrow sediment in which they are formed allows the burrows to be easily
distinguished. These burrows were not filled biotically, but by sedimentation which
deposited sediment into the burrows. The differences in lithology between the burrow
and surrounding sediment are caused by a change in sediment supply to the area. The
result is a horizon which contains two distinct sediment types - one in the burrow, and
one outside of it, which is representative of an older depositional regime. The infilling
of burrows with sediment of a different type can completely replace a stratigraphic
horizon with rock of a different lithology. This modification of lithologic type by
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56
Figure 15: In situ burrows are found at one sandy stradgraphic interval, near the base of
measured section A. Pen for scale.
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58
Figure 16: Burrows within the diatomaceous subunit 2 contain sediment similar to the
matrix of the overlying shell bed (subunit 3).
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59
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60
infilling of a different type of sediment was noted by Curran (1994) in Bahamian
carbonate systems. In the case of the Bahamian carbonates, this type of change in
sediment composition was caused by transgression, which brought a different
depositional regime to the area. Savrda (1995) also notes that the presence of burrows
emplaced in a firmground and infilled with a distinctly different type of sediment is
common at transgressive surfaces and sequence boundaries.
These burrows are commonly indiscreet or motded, however when clearly
visible, they consist of one or more chambers which are connected to the surface
through narrow (2-3 cm) tunnels. These burrows were probably created by callianassid
shrimps, are unlined, and can be assigned to the ichnospecies Thalassanoides. Because
the burrow is unlined, the diatomaceous substrate must have been stiff, acting as a
firmground.
Thin Section Analysis
Thin-section analysis was used on layer “3” of the Tamiosoma beds, a shell bed
which contains numerous barnacles as well as bivalves, bryozoans, and a diatomaceous
matrix. The layer is well cemented, and bioclast supported. This shell bed is laterally
discontinuous, running approximately 100 m before thinning out and disappearing into
the hillside. One oversized thin section was made from the fragile shell bed. This was
taken perpendicular to the strike of the shell bed, which measures approximately 290
degrees (trending NW).
As is expected, the most com m on element of the shell bed is barnacle shells
(Fig. 17). While shells of Tamiosoma are present in the shell bed, there are also
numerous plates from the smaller barnacle, Menesiniella. These are generally covered,
and in some cases completely filled, by sparry calcite (Fig. 18). Matrix material is
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Figure 17: Microstructure of barnacle shell.
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63
Figure 18: Barnacle shell covered with sparry calcite crystals. Many voids within
barnacle shells are completely filled with this cement. Field of view=2.6 mm.
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65
quite diatomaceous, and contains very any clastic particles, including feldspar, quartz,
biotite, and lithics. The diatoms arc usually fragmented.
Not only does the presence of a firmground suggest a slowing or suppression of
sedimentation for a period of time, thin section analysis reveals the presence of calcium
fluorapatite pellets, indicating the beginning of pelletal phosphorite development. Some
of the dark specks visible in the infilling of the burrows may also be apatitic as well.
These pellets are gold to brown in color, rounded, and always contain some sort of
nucleus. The phosphate material is dark brown in plane light (Fig. 19) and is nearly
isotropic under crossed polars (Fig. 20). The nucleus is generally quartz, as seen in
Figure 21, however, biogenic materials have also been found with pelletal growth
around them. These include diatoms (Fig. 22) and an unidentified organic object, which
may be fish bone (Fig. 23, Fig. 24). This halo is well developed on the “fish bone” as
would be expected for an object that was originally phosphatic. This variation in pellet
morphology is similar to Graham’s (1980) illustrated pelletal phosphorite, from the
basal Monterey Formation, which also had a variety of nuclei, including foraminifera
and glauconite. Pelletal phosphates generally form during sediment starvation (Keller,
1992). This, combined with the presence of firmgrounds provides additional evidence
for sporadic deposition of sediments.
A third line of evidence for sediment starvation during the creation of the shell
bed is supported by bioerosion on the barnacle shells. As seen in Figures 25 and 26,
borings are common in barnacle shells, and can be identified as Entobia isp (Bromley
and D ’Alessandro, 1990). Entobia is created by boring sponges (Bromley and
D’Alessandro, 1990). In general it takes quite a while for these organisms to establish
themselves into the shell material, often more than 6 months (H. Lescinsky, pers.
communication 1998). Thus, the shell material must have been exposed at the seafloor
for a period of time before sedimentation resumed.
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66
Figure 19: Phosphate pellet, shown in plane light. Field of view=.65 mm.
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68
Figure 20: Phosphate pellet, shown under crossed polars. Field of view=.65 mm
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Figure 21: Phosphate pellet, shown in plane light. Note quartz nucleus. Field of
view=.65 mm.
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71
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72
Figure 22: A diatom serves as a nucleus for phosphate pellet creation. Field of
view=.65 mm.
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74
Figure 23: Unidentified object, possibly fish bone. Note phosphatic halo surrounding it.
Shown in plane light Field of view=1.3 mm.
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76
Figure 24: Unidentified object, possibly fish bone. Shown under crossed polars. Field
of view=1.3 mm.
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77
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Figure 25: Endobiont boring cross-cutting growth bands in barnacle shell. Shown
plane light Field of view=2.6 mm
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Figure 26: Endobiont boring cross-cutting growth bands in barnacle shell. Shown
plane light. Field of view=2.6 mm.
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82
In addition to stratigraphic understanding, thin sections provide valuable insight
into the paleoecology of the area. Diatoms are commonly found in the thin section,
although whole specimens are the exception, not the rule, as seen in Figure 27. Figure
28 shows a beautiful planktic specimen, while Figure 29 shows another which has been
damaged, possibly from the thin sectioning process, as well as a phosphate pellet
Other taxa seen in thin section are foraminifera, such as that seen in Figure 30. Sponge
spicules are uncommon, but present, as seen in Figure 31. Bryozoans, which are an
excellent indicator of normal marine salinity, can also be observed in thin section (Fig.
32). These branching bryozoa are also visible upon close inspection of the outcrop’s
matrix material. Echinoid plates are also present in the thin section, and are
characterized by linked, uniaxial calcite plates (Figs. 33 and 34).
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Figure 27: Diatom and microfossil debris. Field of view=. 31 mm.
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Figure 28: Centric diatom. Field of view=.31mm.
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Figure 29: Diatom and phosphate pellet. Field of view=.65 mm.
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88
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Figure 30: Foraminiferan test. Field of view=1.3 m m
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91
Figure 31: Sponge spicule and microfossil debris in matrix. Field of view=.65 mm
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92
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Figure 32: Cross section of bryozoan. Note pennate diatoms nestled in leftmost
chamber. Field of view= 2.6 mm.
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94
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Figure 33: Possible echinoid plates. Shown in plane light Field of view= ,65mm.
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97
Figure 34: Possible echinoid plates. Note uniaxial crystals. Shown under crossed
polars. Field of view=.65mm.
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X-Rav Diffraction
99
The widespread siliceous facies recorded by the Monterey and Pancho Rico
Formations record rapid deposition of diatomaceous ooze. This is indicative of climatic
cooling and increased upwelling along Coastal California in the Latest Miocene and
earliest Pliocene. This episode of high diatomaceous sedimentation lasted about 5 to 7
my. (Isaacs et al, 1983). The Monterey Formation records a variety of types of
preservation of this rock, ranging from altered diatomites to heavily altered cherts
(Isaacs et al., 1983). This diagenesis is common because of the large amounts of two
unstable components: biogenic silica, in the form of diatom and silicoflagellate tests,
and fine-grained organic matter.
Isaccs et al. (1983) recorded variable diagenesis in the Monterey Formation,
because of differences in burial temperature, depth, and original sediment composition.
The biogenic opal in diatom tests is opal-A, while much of the Monterey of opal-CT, or
opal-C. These changes can be described by the changes shown in Table 4.
X-ray diffraction patterns follow these changes. Unaltered diatomaceous rocks
are recognized by a characteristic low rise, in the patterns, while progressively altered
rocks show distinct peaks, indicating alteration into distinct mineral phases.
As seen in outcrop, the rocks of the Pancho Rico Formation show little signs of
alteration, and are quite soft, with microfossils visible in hand sample. To test the
amount of alteration, however, a sample from subunit “B” (not including burrow fill)
was analyzed using X-ray diffraction. A portion of this sample was also sent to Richard
Pollastro of the U.S. Geological Survey in Denver, Colorado, for comparison to similar
rocks of the Monterey Formation. This familiarity with the technique allowed for a
complimentary interpretation of the sample, including weight percent. Figure 35 shows
the X-ray diffraction pattern. A low rise is visible from approximately 20 to 32 degrees.
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100
Table 4: Diagenedc Changes in diatomaceous rock, as demonstrated by Isaccs et al.
(1983).
O r ig i n a l L i t h o l o g y : C h a n g e s t o : W it h m o r e d i a g e n e s i s , t o :
diatomite opal-CT chert quartz chert
diatomaceous shale (diatom-rich) opal-CT porcelainite quartz porcelainite
diatomaceous shale (diatom-
poor)
opal-CT mudrock quartz mudrock
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Figure 35: X-ray diffraction pattern for sediment found at stratigraphic level of
Tamiosoma buildups (Subunit 2).
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102
|_ _ (A
"o o a AljSUQlUl
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103
This is reminiscent of the opal-A rise as demonstrated by Isaacs et al. (1983). Peaks
from the presence of other crystalline minerals dominate the analysis, and serve to
override the subtle signal from the amorphous silica of opal. Pollastro (1998, pers.
comm.) noted that a large weight percent of clay minerals such as illite and smectite
serve to create non basal reflections which mask the curve which opal-a creates (which
should be visible from 20 to 26 degrees).
Peaks consistent with clay minerals are present Pollastro (1998, pers. comm.)
cites this as the majority of the sediment, with 50-60 weight percent from smectite or
randomly interstratified smectite and illite. This was probably derived as a primary
component of weathered ashes (Klein and Hurlbut, 1985). However, the large
component of clay minerals in comparison to other ash-derived minerals, such as
feldspar and quartz, indicates that the weathered ashes were redeposited into the Pancho
Rico Formation, rather than being a primary diagenetic feature of in situ ash beds
(Pollastro 1998, pers. comm.). Although no ash beds are readily observable in this
measured section, they are present elsewhere in the Pancho Rico Formation (J. Ingle,
pers. comm., 1997).
Quartz is represented by the tallest peak on the chart Pollastro (pers. comm.,
1998) estimated that quartz contributed 10-20 weight percent of the sample. Although
it is conceivable that this quartz peak is from altered diatoms, the presence of the
microfossils, often broken, but present, does not support this. However, there is
undoubtedly a component of detrital quartz, as the formation contains quartz-rich sand.
Such a small proportion of quartz could still give this pronounced peak, and is a more
probable solution.
Plagioclase feldspar is also present, and may contribute up to 10% by weight of
the sample (Pollastro, pers. comm., 1998). It is a detrital component, and is probably of
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104
similar origin as the feldspar seen in thin section (Fig. 36, on right, with two phosphatic
pellets).
Calcite exists in the sample. This could be from microfossils such as
foraminifera, megafossils, such as barnacles, or from carbonate cement All three are
present at this interval, and thus each probably contributed.
Although it is likely that other minerals exist in the rock, the numerous
reflections of minerals such as quartz and feldspar limits the detection of minerals
which contribute less then 2 percent of the weight of the sample. It is interesting to note
that no apatite is present in this rock, although pellets are visible in thin section taken
less than one half meter above this sample. This sample was taken from amongst the
barnacles, during an interval which is interpreted to be of relatively higher
sedimentation rate, not during a hiatal interval.
Carbon/Carbonate Analysis
Samples from twelve sites were collected for analysis. Because of the scattered
nature of the sample sites, stratigraphic sections A and B, which have sim ilar
stratigraphic thickness and lithologies were combined to create a composite
stratigraphic framework. Samples were dried, pulverized, and analyzed using Leco
carbon and carbonate analysis. The results of analysis for one sample, taken from the
top of measured section A were removed from the data analysis. This is due to the
anomalously high carbon and carbonate contents (over 6%), and the presence of visible
soil formation at the sample site. The remaining results are seen in Figure 37.
In general, carbon results show very low levels of organic carbon. Two peaks
are the exception to this rule; the peak at 39 m, with values over 2%, and the top of the
section, where values exceed this. The sample at 39 m was taken from the “event” bed
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105
Figure 36: Feldspar fragment and two phosphadc pellets filling in crack in barnacle
shell. Shown under crossed polars. Field of view=1.3 mm.
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107
Figure 37: Results for carbon and carbonate analysis. Closed dots for organic carbon,
open boxes for carbonate. Both values are in percent. Peak at 40 meters represents
distinct shelly bed, while peak at top of section is a result of weathering and soil
development
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108
G vfb o n a te
in
ro
in
C N
in
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G O
C N
m r o cn cn
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MetersUpsection
109
in measured section A. This interval contains a great deal of shell material, as well as
less diatomaceous rock. Those samples taken at the top of the section were near the top
of a hill. No lithologic changes can significandy account for the dramatic increase in the
carbon values. However, weathering can be quite significant. Thus, it is hypothesized
that recent soil formation contributed to the increasing carbon content
In general, carbonate patterns mirror those of total carbon. Most sites’ total
carbon is from the presence of carbonate material in the sample. The total organic
carbon content of the samples can be determined by subtracting carbonate from total
carbon. In some cases, TOC values were near 1.5%, however the majority of samples
had litde or no organic carbon. Some samples had carbonate values which were higher
than the experimentally determined carbon values. This could be due to analytical error
in either sample, or due to heterogeneity of sediment types in the samples. Due to the
sometimes shelly nature of the rock, it is possible for one sample to contain more shelly
material than others. The sample taken at 39 m was from a darker gray, shelly bed.
Although suggested above that TOC might be higher due to its darker color, there
appears to be litde or none, and carbonates contribute to the total carbon at that
stratigraphic level.
Thus, the surface sediments of the Pancho Rico Formation contain very litde
organic carbon. This could be due to the extensive surface weathering in the area, but it
may truly indicate a lack of organic carbon in the sediment. Subsurface portions of the
Pancho Rico Formation in the San Ardo oil field, located south of Wildhorse Canyon
are also muddy (Long 1957). Petroleum geologists from Texaco North American
Production (which owns the field) report that litde or no petroleum products exist in the
muds, which serve as caps to the sandy, petroleum rich strata below (J. Harris, pers.
comm., 1998).
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D i s c u s s i o n
110
The overall stratigraphy of the Pancho Rico Formation records deposition in a
shallow marine basin during the latest Miocene. In Wildhorse Canyon, the facies are
quite fine-grained. Yet the benthic megafauna are indicative of shallow marine depths.
Diatomaceous muds formed in upwelling systems are generally found on the inner
shelf.
The intense bioturbation found throughout the formation is indicative of a well-
oxygenated environment (Savrda, 1995). The presence of well-preserved burrows in
two portions of the field area records two very different, and unique depositional
settings which punctuated the history of the area. The stratigraphically lowest burrows
are interpreted to have been made by clams, and are found in very fine grained, subtly
cross-bedded sands. This stratigraphic interval likely represents the highest energy
environment preserved in the field area. The presence of cross-beds, yet lack of grading
within the weakly preserved channels is indicative of a relatively shallow, subtidal
system with a mature source area. Conversely, it could also represent a more distal
portion of a channel, with transport serving as an efficient sorting mechanism for the
transported sediment. These shifting sands may have provided an ideal habitat for
shallow infaunal clams, but a high rate of clastic input may have served to preserve the
burrows. Calcite cement, found throughout the formation, preferentially cemented the
sand-filled burrows, preserving them in full relief.
By contrast to the clam burrows found downsection, the presence of
Thalassanoides isp. provides one of many lines of evidence for firmground
development and suppression of sediment input at the same stratigraphic level at which
barnacles are found. As Savrda (1995) noted, Thalassanoides is often associated with
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I ll
transgressive sequence boundaries, in which fine grained sediments become dewatered
and firmgrounds develop. The matrix which infills the burrows contains phosphate
pellets as well, which are often associated with hiatal deposits.
It is hypothesized that three different depositional regimes are associated with
Tamiosoma’s unique paleoecology: These are the development of shell beds which
serve as attachment points for young barnacles, the rapid deposition of diatomaceous
sediment, which causes the barnacles to grow upward, and the development of hiatuses
without shell bed deposition, as demonstrated by firmground development and
Thalassanoides burrows. Calcium fluorapatite pellets were produced on the seafloor
during hiatuses, as indicated by their presence in the burrows, as well as amongst the
shells in the shell beds. Thus, for the interval in which T. gregaria flourished, it
appears that rapid deposition facilitated the growth of the extraordinary bases, while
hiatuses in deposition facilitated outward growth of the shells before upward growth,
creating a conical, rather than cylindrical, base. This dependence upon periods of
exposure may be the reason that these barnacles are found in limited stratigraphic
horizons and quantities - very specific conditions were required for them to flourish.
The unique multi-layer composition of the interval which contains abundant
Tamiosoma fossils is, overall, indicative of a transgressive interval. Despite this, the
complex substratigraphy is not entirely explained by transgression. All shell beds are
laterally discontinuous, some less than 1 m in length. This suggests heterogeneity of
the seafloor at any given time. Tamiosoma’s elongate morphology is clearly adapted to
a lifestyle of burial in the mud, termed mudsticking (Seilacher, 1985), and indicates a
fairly rapid depositional regime. Their presence, in situ, in the diatomaceous muds is
intriguing, considering the surrounding evidence of reduced sedimentation. It is likely
that the diatomaceous muds in which they flourished were deposited quickly and
sporadically. The presence of many broken microfossils, such as diatoms, as well as the
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112
lack of many benthic diatoms suggests that the diatomaceous sediment may have been
transported en masse, instead of settling through the water column, or that intense
bioturbadon broke up the mi - ifossils. Ingestion by organisms and dissolution in the
water column may also contribute to the broken nature of the sediment
The presence of Tamiosoma in such quantities is an example of an epibole: a
thin stratigraphic interval characterized by the anomalous abundance of species which
are normally rare or absent (Brett 1995). Three types of epibole exist: taphonomic,
ecologic, and incursion epiboles. Taphonomic epiboles are those which have
exceptional preservation. The presence of Tamiosoma, although well preserved, is not
an example per se of a taphonomic epibole. The shells’ construction of low-magnesium
calcite is quite stable, and the barnacles lived within the sediment with only a small
portion above the sediment-water interface, making burial not unusual.
Incursion epiboles represent an incursion of adults or larvae into an area for a
short period of geologic time, and may reflect unusual sedimentary conditions (Brett
1995). The development of shell beds may have facilitated a significantly higher
juvenile barnacle survival rate in the case of the Wildhorse canyon beds, however, the
presence of individual barnacles found in the sediments of the surrounding area does
not support this. Both incursion and ecologic epiboles are representative of local biotic
“events” and may be useful as marker beds in the stratigraphic record (Brett, 1995).
Brett (1995) recognizes ecological epiboles as a burst of abundance for
sometimes unapparent reasons, but hypothesizes that unusual environmental conditions
associated with transgression, and subsequent sediment starvation are responsible for
this. The presence of Tamiosoma in such great number is an example of an ecologic
epibole.
Thus, the Tamiosoma beds may represent a complex depositional response to
transgression. This transgression caused hiatal deposits within the bed, due to a change
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113
in the center of deposition (Brett, 1995). Abrupt pulses in this otherwise supressed
sedimentation may have helped create the ideal environment for Tamiosoma, creating
an ecologic epibole. However, local factors such as tectonic events or storms served to
sporadically transport sediment into the area, creating the subunits seen throughout the
deposits.
The up-section increase in small shell beds, containing small barnacles, is
indicative of a gradual shallowing of the basin, by both tectonic causes, and sediment
infilling. The lack of terrigenous sediment throughout the section indicates that this area
remained distal to sediment flow off the surrounding land.
At least one cobble whose provenance appears to be the Monterey Formation
has been found in Wildhorse Canyon. Although found out of place, it most likely came
from the Pancho Rico Formation, indicating erosion of the Monterey in the Late
Miocene. The presence of marine bivalve borings on this cobble indicates exposure on
the seafloor, which must have occurred during the deposition of the Pancho Rico
Formation, and excludes the nonmarine Paso Robles Formadon as the original source.
It is possible that rocks of the Monterey formadon served as the hardgrounds for the
smaller barnacle Menesiniella aquila to settle upon. As most of the specimens are
disarticulated, their transported origin is supported. No cobbles have been found in the
area with barnacles attached. It is probable that the environment which Tamiosoma was
adapted to was deeper and/or calmer than that which Menesiniella inhabited.
The gypsum in the Pancho Rico Formation was most likely formed at a much
later time than the other rocks. It cross-cuts bedding, and has no features of evaporidc
gypsum, such as stratigraphic evidence of shallowing, other evaporite minerals, and
horizontal orientation. Although the gypsum is not an evaporative feature in the Pancho
Rico Formation, its formation follows similar chemical pathways. Gypsum forms when
concentrations of Ca^+ and S0 4 ^‘ ions are high. Only then will solid CaS0 4 (gypsum)
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114
precipitate (Krauskopf and Beard, 1995). It is likely that pore waters which filtered
through subsurface cracks in the sediment contained high concentrations of the ions,
precipitating the gypsum. In the Pancho Rico Formation, the calcium ions were most
likely derived from calcite, while the sulfur was probably derived from pyrite. Pyrite is
a common authigenic mineral in the Monterey formation because of high organic
carbon content of the sediments (Isaacs et al., 1981). Though pyrite is not as common in
the Pancho Rico Formation, it s possible that organic sulfur was present in the sediment,
and served as a source for the gypsum.
Commercial-grade gypsum is also found in the overlying Paso Robles
Formation near the southern end of the Gabilan Range (Durham, 1974). Although none
is visible in the Paso Robles in this area, it is possible that the same setting which
deposited these large lenses could have also contributed to the emplacement of these
veins.
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115
PALEOECOLOGY OF THE PANCHO RICO FORMATION
Although the Pancho Rico Formation has received adequate, although not
excessive, documentation lithologically, no true paleoecological interpretation of faunas
has been made. Long (1957) wrote an entire masters thesis, entitled “The Stratigraphy
and Paleontology of the Type Locality of the Pancho Rico Formation, Salinas Valley,
California” with little mention of the megafauna. His paleontological interpretations
were based on foraminifera, and yielded a latest Miocene age of the fauna, which was
corroborated by the presence of Tamiosoma, regarded as an index fossil of the Hny-
period. Durham and Addicott (1965) made the most notable attempts at reconstructing
the paleoecology of the faunas of the Pancho Rico Formation, providing detailed lists of
taxa at each locality they documented (Durham and Addicott, 1965). These lists remain
the most comprehensive to date. Using the assemblages present at field sites, they
determined that the embayment in which these strata were deposited was shallow
marine, and contained a normal assemblage, including gastropods, salinity-sensitive
echinoids, barnacles, bivalves, and other taxa. No discussion was made of the
anomalous growth habits of Tamiosoma. Also surprising, the common bryozoans found
at multiple localities in the formation were not mentioned by them.
Long (1957, p. 51) cites that the “fauna is sparse... indicating] that conditions
were not suitable for a great many forms that were living during this period of time.”
While a great deal of the rock is bare of fossil life, or does contain sparse faunas in
many areas, others are quite rich. The presence of bryozoans and echinoids, both highly
intolerant of fluctuating water chemistry (Clarkson, 1986), indicates that conditions
during at least part of the basin’s history were those of a stable, normal marine system.
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116
The numerous diatoms are indicative of high nutrient levels, which indirectly may have
served to provide numerous food sources for the faunas of the basin.
The highly bioturbated nature of the formation also points to an active benthic
biota. Discrete burrows such as Thalassanoides isp. as well as unidentified “clam”
burrows provide evidence of crustacean and mollusc an burrowing; body fossils of
infaunal or semi-infaunal molluscs, such as Turritella are additional support. During
deposition of the Pancho Rico Formation, unlike the underlying Monterey Formation,
the basin was oxic, as evidenced by the massive rocks. This could be due to its shallow
depth, and to good circulation. This circulation supported the high nutrient levels which
were so necessary for flourishing diatoms.
Foraminifera
Numerous shallow water foraminifera are preserved in the rocks of the Pancho
Rico Formation. Previous research by Long (1957) recorded twelve species of
foraminifera in a Standard oil field core, near Pancho Rico canyon. He proposed two
foraminiferal zones of the Pancho Rico formation based on the foraminiferal
assemblages shown on Table 5.
Hughes (1963) recognized fewer taxa from the Pancho Rico formation, without
subdividing the formation. These are shown in Table 6.
For this study, foraminifera were collected by disaggregating the loosely
lithified rock in boiling water. This mixture was then sieved using a 200 micron screen
to remove the fine silt sized particles. The resulting sediment was then dried in the oven
and foraminifera were removed individually form the surrounding particles. Only
sample 12, from the Tamiosoma beds, contained a significant number of foraminifera.
In addition to inorganic particles, small fragments of bryozoans, barnacles, and
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117
Table 5: Foraminiferal zones of the Pancho Rico Formation, as defined by Long (1957)
at the type section of the formation.
Lower Zone (1250 feet) Upper Zone (max. thickness 400 feet)
Eponides exigua (H.B. Brady) Eponides exigua (H.B. Brady)
Nonionella miocena Cushman Nonionella miocena Cushman
Elphidiella hannai (Cushman and Grant) Elphidiella hannai (Cushman and Grant)
Elphidium hughesi Cushman and Grant Elphidium hughesi Cushman and Grant
Pullenia malkinae Coryell and Mossman Pullenia malkinae Coryell and Mossman
Bulimina ovata D’Orbigny Elphidium tumidum Natland
Buliminella elegantissima (D^Orbigny) Quineloculina sp.
Buliminella subfusiformis Cushman Virgulina sp.
Virgulina californiensis Cushman
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118
Table 6: Carbonate microfossils of the Pancho Rico Formation, as discussed by Hughes
(1963).
Foraminifera O ther
Elphidiella hannai ((Bushman and Grant) Ostracods
Elphidium hughesi Gushman and Grant Small Black Sporbo
Eponides usisquocensis"
Buliminella eleganxissima (D’Orbigny)
Eponides exigua (H.B. Brady)
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119
molluscs were also present in the sediment samples. The foraminifera found in this
study varied from those previously published by Long (1957) or Hughes (1963) - the
assemblage appears to be incomplete, most likely due to preservational biases. Of those
foraras found, some have not been previously identified from the formation. Species
found in this study were not keyed to species level, and include the following (R.
Douglas, pers. comm., 1998):
Epistominella- 2 sp. (identified as Eponides by Long (1957) and Hughes (1963)
Elphidium sp.
Elphidiella sp.
Rosalina sp.
Pulleina sp.
Nonion sp.
Although of interest paleoecologically, previous published reports of
foraminifera have been of little use biostratigraphically, and do not settle the question of
the age of the Pancho Rico Formation. However, the assemblage does point to a very
shallow water (<50 meters), low energy depositional environment, as is supported by
the lithologic characteristics of the rocks.
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Paleobiology and Paleoecology of Tamiosoma gregaria
Eralutton and Biology
120
Barnacles represent one of the most successful marine groups in their
exploitation of hard substrates. These include rocks, boats, pilings, driftwood, and
marine animals such as whales and turtles. Shells of other marine invertebrates are also
a common settling point However, their dominance is generally limited to hard
substrates. Thus Tamiosoma gregaria represents not only a strange morphology, but a
unique expansion into a previously underutilized habitat for the Cirripedia.
The barnacles are a unique group of crustaceans due to their planktonic larval
form, but sedentary adulthood (Coull and Bell, 1983). As such, they are useful in both
biologic and paleobiologic studies, as their habitat can be well established, and can even
be found in situ. (Doyle et al., 1996). The distribution of barnacles is generally divided
into zones, some reaching well above sea level. The upper limit of these zones is
generally limited by physical factors such as water level, while lower limits are usually
associated with biotic interactions such as predation (Coull and Bell, 1983).
Barnacle evolution can be traced through the excellent fossil record of the
Cirripedia, and by comparison of larval and adult forms to other Maxillopod
crustaceans (Schram, 1982). The parasitic Ascothoracican barnacles probably represent
the most primitive barnacle body form (Schram, 1982). Modem nonpedunculate
barnacles probably are derived from the pedunculate Scalpellidae. The Balanomorpha,
to which Tamiosoma gregaria belongs, may represent a polyphyletic group (Newman
and Ross, 1976).
The thoracican barnacles can be classified according to the structure discussed
by Newman (1996), as seen in Table 7.
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121
Balanomorph barnacles are characterized by one basal disk, six wall plates, and
four opercular plates, each of which have their own taphonomic properties. The basal
plate serves to attach the barnacle to the substrate, whether it is a rock, shell, wood, or
another barnacle, rhis attachment is facilitated by a powerful and permanent cement
which makes the base plate the most enduring pan of the barnacle in the fossil record.
These are relatively common in tertiary rocky-shore habitats, and are referred to in
Doyle et al., (1996) as “barnacle stubs”. Unfortunately, taxonomic determination
cannot be made from the base. However, disarticulated plates, which can be useful for
such determination, are commonly found in deposits associated with the eroded stubs
(Doyle et al., 1996).
Attached to the basal plate are six wall plates: the carina, two carinolaterals,
rostrum, and two rostrolaterals (Fig. 38)(Anderson, 1994). Depending on the species,
size, and environmental conditions, these plates can be fused to the base, or loosely
attached by tissue. Barnacles grow by addition of shell material at the edge of the base
plate, in conjunction with addition at the bottom of each wail plate (Anderson, 1994).
This is facilitated by a thin layer of tissue between the two. While active growth is still
occurring, the base and wall plates must remain unfused, thus limiting the durability of
younger barnacles when subjected to currents, storms, and other physical processes
(Whittlesey and Bottjer, 1996).
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122
Table 7: Classification of the barnacles, from Newman (1996).
Class Maxillopoda Dahl, 1956 Silurian-Recent
Subclass Cirripedia Burmeister, 1834 Cambrian-Recent
ORDER SESSELIA Lamarck, 1818
Suborder Brachylepadomorpha Withers, 1923 U. Juras.- Recent
Family Brachylepadidae Woodward, 1901
Family Neobrachylepadidae Newman and Yamaguchi, 1995
Suborder Verrucomoipha Pilsbry, 1916 U. CreL- Recent
Family Neoverrucidae Newman, 1989 Recent
Family Proverrucidae Newman, 1989 U. Cret.
Family Verrucidae Darwin, 1854 U. Cret- Recent
Suborder Balanomorpha Pilsbry, 1916 Paleo.-Recent
Superfamily Chionelasmatoidea Buckeridge, 1983b Eo.- Recent
Family Chionelasmatidae Buckeridge, 1983b
Superfamily Pachylasmatoidea Utinomi, 1968
Family Pachylasmaddae Utinomi, 1968
Subfamily Eolasmatidae Buckeridge, 1983a
Subfamily Pachylasmaddae Utinomi, 1968
Superfamily Chthamaloidea Darwin, 1854
Family Catop hragmidae Utinomi, 1968
Family Chthamalidae Darwin, 1854
Subfamily Euraphiinae Newman and Ross, 1976
Subfamily Notochthamalinae Foster and Newman, 1987 Eo.-
Recent
Subfamily Chthamalinae Darwin, 1854
Superfamily Coronuloidea Leach, 1817 U. Eo.- Rec.
Family Chelonibiidae Pilsbry, 1916
Family Platylepadidae Newman and Ross, 1976
Family Emersoniidae Ross, 1967
Family Coronulidae Leach, 1817 U. Mio.- Recent
Superfamily Tetraclitoidea Grovel, stat nov. (Newman 1993)
Paleo.- Recent
Family Bathylasmaddae Newman and Ross, 1971
Subfamily Bathylasmadnae Newman and Ross, 1976
Subfamily Hexelasmadnae Newman and Ross, 1976
Family Tetracliddae Grovel, 1903 Eo.- Recent
Subfamily Austrobalaninae Newman and Ross, 1976
Subfamily Tetraclitellinae Newman and Ross, 1976
Subfamily Tetraclitinae Grovel, 1903 Oligo.- Recent
Superfamily Balanoidea Leach, 1817 Eo.-Recent
Family Archaeobalanidae Newman and Ross, 1976
Subfamily Archaeobalaninae Newman and Ross, 1976
Subfamily Acastinae, Kolbasov, 1993
Subfamily Bryozobiinae Ross and Newman, 1996
Subfamily Elminiinae Foster, 1982 U. Oligo.- Rec.
Subfamily Semibalaninae Newman and Ross, 1976
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Table 7 (continued)
Family Pyrgomatidae Gray, 1825
Tribe Pyrgomatini Ross & Newman, 1995 Mio.-Rec.
Subfamily Hexacreusiinae Newman 1996 Recent
Subfamily Ceratoconchinae Newman and Ross, 1976
Subfamily Megatrematinae Holthuis, 1982
Subfamily Pyrgomatinae Gray, 1825
Tribe Pyrgopsellini Ross & Newman, 1995
Tribe Hoekiini Ross and Newman, 1995
Family Balanidae Leach, 1817 Oligo.- Rec.
Subfamily Balaninae Leach, 1817
Subfamily Concavinae Zullo, 1992 Oligo.-Rec.
Subfamily Megabalaninae Newman, 1979 Oligo.-Rec.
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124
Figure 38: Top view of a balanomorph barnacle, idealized. Abbreviations: C- Carina,
CL- Carinolateral, R- Rostrum, RL- Rostrolateral, T- Ter gum, S-Scutum.
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125
T
BASE
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126
Fusion of the wall and base plates provides advantages: the overall shell strength
is increased, allowing barnacles to populate areas of vigorous current, such as the
intertidal area, where multiple species have successfully populated marine environments
up to the splash zone (Stanley and Newman, 1980). Barnacles whose plates are fused
together have a greater likelihood of being preserved whole, biasing the fossil record
towards larger barnacles (Whittlesey and Bottjer, 1996). Furthermore, as taxa with
multielement skeletons, they can be found in a variety of preservational states, ranging
from intact to thoroughly disarticulated. This tendency to disarticulate make barnacles a
powerful tool for inference on the conditions of the depositional regime, as well as
taphonomic study (Donovan, 1996).
The opercular plates, the terga and scuta (2 each) serve to protect the barnacle
against many things: predation, infilling by sediment, and desiccation when in the
intertidal area (Anderson, 1994). These are attached by muscles and are by necessity
not fused to the rest of the barnacle. Upon the death of the barnacle, the wall plates
often fall back into the shell. However, if the barnacle shell is transported after death,
the opercula are often lost (Doyle et al., 1997). Barnacles which are found in situ have
a relatively good chance of having the opercular plates present, as sediment filling the
barnacles serves as a plug. In many instances, because of the upright nature of barnacle
growth, and the low energy environment inside the shell, the opercular plates remain in
life position relative to each other (K. Whittlesey, pers. obs.). While live barnacles
could well be transported to a site of deposition before death, the in situ nature of
opercular plates provides evidence that little (if any) transport occurred after death.
Unfortunately, the likelihood of finding these plates in the shell in older museum
collections is often highly reduced. The Los Angeles County Museum of Natural
History has a significant number of fossil balanomorph barnacles in its invertebrate
paleontology collection, but preparators have separated them. For clusters of barnacles,
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127
a vial of opercular plates contributes little to our understanding of the individual, as the
opercular plates arc permanently disassociated from their respective base. Biological
collections are more likely to preserve the opercular plates with the individual.
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Taxonomy
128
It should be borne in mind, that the recognition of the Fossil Pedunculated
Cirripedes by the whole of their valve and peduncle, is identical with recognising a
Crustacean by a single definite portion of the carapace, without the great advantage
of having its received the impress of the viscera of the included animal’s body:
knowing this, and yet often having the power to identify with ease and certainty a
Cimpede by one of its valves, or even by a fragment of a valve, adds one more to
the many known proofs of the exhaustless fertility of nature in the production of
diversified yet constant forms.
-Charles Darwin, 1851, p. 5
The giant sessile barnacle, Tamiosoma gregaria, is found in Mio-Pliocene
deposits of Central California. The odd morphology of Tamiosoma posed a unique
taxonomic question, and while originally identified as a barnacle (Conrad 1857a), it was
classified as a rudist bivalve (Conrad, 1864 &1865, Gabb, 1866 & 1869) for over a
decade. Tamiosoma regained its cimpede status as Balanus estrellanus with Conrad
(1876), after considerable bickering among prominent geologists. Dali (1902) clarified
Tamiosoma’s arthropod affinity with his discussion in Science. Since then, its
taxonomic position relative to other barnacles has been questioned numerous times.
Most recently, Zullo (1964 &1992) suggested that T. gregaria was synonymous with an
extant species, Balanus (Menesiniella) aquila.
Recent collected diatomaceous sediments of the Pancho Rico Formation,
Wildhorse Canyon, King City, CA, have revealed in situ barnacles which still contain
their associated opercular plates. Other, distinctly different, opercular plates are also
found at the same locality. Although size generally distinguishes the two types of
barnacle plates, those of comparable size have a distinct morphology indicating two
separate taxa. The smaller barnacles lack the grossly elongate bases which are
characteristic of Tamiosoma gregaria, and should be classified under the genus
Menesiniella (Newman, 1982 as M. aquila (Pilsbry)).
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129
Recent specimens of Menesiniella have two distinct shell forms - a robust form,
with transparietal radii, and a thinner-shelled form, with disparietal radii. In forms with
transparietal radii, wall plates completely overlap, creating a conical-shaped barnacle.
Those barnacles which have disparietal radii have wall plates that do not completely
overlap near the aperture, creating a more cylindrical barnacle whose aperture is
surrounded by the pointed ends of the wall plates like a crown. It is the latter which is
found in the Pancho Rico Formation, as illustrated by Figure 39. Although
ecophenotypy remains a strong possibility, these two forms will be designated as
subspecies.
Although Tamiosoma gregaria has been described numerous times, based on
the large, elongate bases (Fig. 40), but without opercular plates or with the plates of
other barnacles found in the same strata, no one has ever described the distinct, and
remarkably different opercular plates found inside the barnacles at Wildhorse Canyon,
which is believed to be the type locality. Woodring et al. (1940) described and
illustrated similar barnacle bases from the Kettleman Hills, San Joaquin Valley, of
California, and noted the presence of two types of opercular plates in the Pancho Rico
Formation. However, the Kettleman Hills opercular plates illustrated were not those
presumed to be of T. gregaria from in the Pancho Rico, but rather a concavinine
barnacle similar to Menesiniella aquila. Zullo (1979) mentioned the “reef-building”
habit of T. gregaria, but pictured opercular plates similar to the smaller barnacles which
do not possess the elongate bases. These opercular plates are similar to those of
Menesiniella aquila, and are most likely the reason why he synonymized T. gregaria
and M. aquila. Thus, over 100 years after first description, the complete Tamiosoma
gregaria, as found at the type locality in Wildhorse Canyon, has yet to be described
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Figure 39: Photograph of Menesiniella aquila, with opercular plates.
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132
Figure 40: Cluster of bases of the giant barnacle, Tamiosoma gregaria. No wall plates
are present. Preservation of bases without wall plates has led to many incomplete
descriptions of Tamiosoma.
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134
Systematic Description
Family Balanidae Leach 1817
Subfamily Balaninae Leach 1817
Tamiosoma Conrad 1857 (1856)
Type: T. gregaria Conrad 1857, p. 315
Diganosis: as for the species.
Tamiosoma gregaria Conrad 1857:315
Synonymy: Balanus estrellanus (Conrad 1857b, p. 72 (in Gabb (1869)), 1876, p. 273,
1877, p. 156) Radiolites gregaria (Conrad, 1864, p. 214,1865, p. 363, Gabb, 1866,
p.92, 1869, p. 61-63, plate XVII), Balanus sp. Pilsbry 1916, Balanus (Tamiosoma)
gregarius of Woodring, Stewart and Richards (of Wildhorse Canyon, not of Ketdeman
Hills). Tamiosoma gregaria Conrad 1877
NOT Balanus gregarius of Zullo (1979, 1992), or Addicott (1978).
Type Locality (Neotype): LACMEP 12264, North side of Wild Horse Road,
Near King City, Salinas Valley, Monterey County, CA.
Diagnosis: The basis has been described by Pilsbry (1916) as well as numous others; it
is the opercular plates that distinguish Tamiosoma as a balanine rather than a concavine,
as follows.
Scutum (Fig. 41), with normal growth ridges, no longitudinal striations; higher
than wide; relatively thick, biconvex, concave inward; wide strip of tergal margin
inflected 45*; occludent margin curved, smooth; exterior heavily worn apically;
exposing single radial line; pronounced articular ridge short, less than 1/2 length of
tergal margin; adductor ridge sharply defined, running from basal margin to at least
articular furrow, in some cases to apex, shallow bifurcation about halfway down; lateral
depressor pit continuous with cavity on tergal side of adductor ridge; rostral depressor
muscle pit continuous with slight cavity beneath internal ridge of occludent margin.
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135
Tergum biconvex (Fig. 41); weak to very weak radial striae; length excluding
tergal spur greater than width; occludent margin straight or slightly convex; basal
margin straight to slightly sinuous; spur furrow closed throughout its length and spur
depressed below surface of the valve; articular margin nearly straight, but upper 1/3 is
completely abraded away (by abrasion against sheath) to depth of spur furrow;
depressor muscle crests conspicuous but short; apex sharp and elongate.
Supplimentary description covering wall and basis: Wall plates with
transparietal radii; generally not fused to basis, but may be fused to each other; heavy
erosion or bioerosion on exposed distal internal margins.
In adults, basis wide, often tall, generally with cancellate infilling of the space
(Fig. 42). Externally, irregular longitudinal grooves and growth bands present Wall
thick; growth band present in thin section. Significant phenotypic plasticity exists in
basis shape, as well as variation throughout development
Subfamily Concavinae Zuilo, 1992
Genus Menesiniella Newman, 1982
Menesiniella aquila zulloi subsp. nov.
Synonymy: Balanus gregarius of Woodring et a l (1940, in part); Durham and Addicott
(1965, plates 1 &2); Addicott (1978); Zullo(1964, 1979)
Opercular plates as described and illustrated by Zullo (1979). Whole individual
from Wildhorse Canyon (about 10 m strati graphically above Tamiosoma beds)
illustrated in Figure 39. Subspecies definition based on wall plate morphology, with M.
aquila zulloi possessing thin, disparietal radii. This is the only morphology seen in the
Pancho Rico Formation. However recent individuals collected off the coast of
California can be divided into two distinct morphologies- the disparietal form of M.
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136
Figure 41: Opercular plates of Tamiosoma gregaria. Although each is definitely from
Tamiosoma, they are not from one individual. Scuta on top row, terga on bottom.
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Figure 42: Bases often show longitudinal lines and concentric banding.
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aquila zulloi, and the transparietal form of M. aquila aquila. Both contain similar
opercular plates.
Discussion
Balanid taxonomy relies heavily on the characteristics of the opercular plates of
the taxa, because they often show the most marked differences in construction. In
addition, because the barnacle grows through accretion of shell material, rather than
molting, the juvenile form is often preserved in the older parts of the adult opercular
plate. The opercular plates of Tamiosoma gregaria’s show little similarity to M. aquila,
even in the juvenile form. Most noticeable is the lack of longitudinal striae and
ornamentation on the scutum of Tamiosoma gregaria, unlike that of M. aquila. The
shapes of the scuta are also different, with T. gregaria being much wider. The presence
of the striae is a positive trait of all barnacles of the Concavinae (Zullo, 1992). Thus,
without redefining the Concavinae as a group, Tamiosoma is transferred from the
Concavinae, to the Balaninae.
The history of taxonomy of Tamiosoma gregaria is based not on
misidentification of barnacles skeletal components, but the lack of complete specimens,
combined with at least one other barnacle population preserved in the Pancho Rico
Formation. Tamiosoma gregaria is rarely preserved as well as specimens in Wildhorse
Canyon. The quiet water and rapid sedimentation associated with deposition of the
Tamiosoma beds provides the ideal preservational system for the fragile giant barnacle
in its entirety. Although large, most components are not fused, and only excellent
preservation will keep a specimen intact Thus, it is not surprising that other localities
have not yielded complete specimens. However, it is curious that Zullo (1979) did not
observe these same opercular plates when he visited the type section.
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The history of description of Tamiosoma has been hampered a great deal by this
incomplete nature. Conrad (1857a) first identified Tamiosoma as a barnacle, but
wavered through his years, identifying it as Balanus estrellanus (1857b, in Gabb
(1869)) and later (1865) as a bivalve. Its durable bases bears a marked similarity to the
rudist bivalve Radiolites, as believed by Conrad (1864, 1865) and Gabb (1866, 1869).
While Conrad (1864, 1865) regarded this “rudist” as being characteristic of the
California Cretaceous, Gabb (1866) noted that he had never seen one, and that Conrad’s
localities may have been misidentified. Gabb (1869), when finally receiving a
specimen, described it as a rudist, but noted that the fossil was from the “bituminous
shale”- a characteristically Miocene unit Conrad, however regained his original belief
that Tamiosoma was indeed a barnacle though he again identified it as Balanus
estrellanus (Conrad, 1876). Dali (1902) later discussed its taxonomic affinites,
identifying the giant barnacle as Tamiosoma. Pilsbry (1916) described and illustrated
specimens from Wildhorse Canyon which are now housed in the National Museum of
Natural History, however no opercular plates were observed.
Woodring (1940) noted the presence of large barnacles in the Etchegoin
Formation of the Kettleman Hills, California. These barnacles are of a sim ilar ecology
as those in Wildhorse Canyon, found in life position in isolated stratigraphic horizons.
The bases are of similar construction. Each possesses the ubiquitous, elongate bases
found on Tamiosoma. They were found in association with opercular plates, though no
mention was made as to whether they were found inside the barnacle. These opercular
plates, as figured in Woodring et al. (1940) are different than those found in the Pancho
Rico Formation. They mention, however, opercular plates which are “exceptionally
thick” and “not as strongly sculptured”, from the Pancho Rico Formation in Wildhorse
Canyon, and note that the barnacles are distinguishable from those of Kettleman Hills.
Due to the uneven distribution of scuta and terga, 8:1 and 77:1, respectively, in the
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142
Kettleman Hills deposits, it is probable that the opercular plates were found in
association in the surrounding sediment, not in the barnacle itself, as more terga would
likely have been preserved in situ.
Thus, the question arises whether the Kettleman Hills barnacles represent the
same species, without the proper opercular plates preserved. Such a mistake was made
by Zullo (1979) when he synonymized T. gregaria with Menesiniella aquila based on
the presence of ubiquitous M. aquila opercular plates in the Pancho Rico Formation.
Another likely scenario is that the elongate, mudsticking habit represents a specific
physiological response to the ubiquitous soft-sediment environments of the latest
Miocene, and not a species specific trait. Thus, multiple species of barnacles might fill
that ecological niche. This is the most likely scenario, and reduces the importance of T.
gregaria as being the only mudsticking barnacle, though historically, the Wildhorse
Canyon barnacles represent the type specimen locality, and as such should retain proper
taxonomic distinction.
Zullo’s (1964) synonymy of T. gregaria with Menesiniella (then Balanus)
aquila was based on a specimen of M. aquila collected by the Alan Hancock
Foundation, in sediments off the shore of Southern California. This specimen showed
modification of its basis similar to that of T. gregaria. However, this specimen is
missing from the Hancock Foundation’s collection, now curated by the Los Angeles
County Museum of Natural History. This modification of the base can be found in a
wide variety of barnacles, and is present in other recent specimens of M. aquila from
the Los Angeles County Museum of Natural History. Zullo (1964) noted the
modification of bases in fossil barnacles from California and Baja California which
possessed the elongate bases, and opercular plates similar to that of Menesiniella
aquila. This prompted him to synonymize the two. However, Zullo’s (1964) synonymy
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143
was probably based on the presence of Menesiniella opercular plates in the Pancho Rico
Formation, as opposed to their presence inside the elongate bases.
Specimens of recent Menesiniella aquila in the Los Angeles County Museum of
Natural History are remarkably similar to those found in the Pancho Rico Formation,
such as those discussed by Addicott (1978) from USGS locality 903, the shell bed near
San Lucas, California. Zullo (1979) recognized figured specimens of Menesiniella
aquila as Balanus gregarius (Tamiosoma gregaria). In doing so, he recognized the
fossil as worthy of distinction from other Concavinine barnacles at the genus level, by
synonymizing it with a taxon which was distinguishable at the genus level. Thus,
although Newman proposed Menesiniella as a subgenus in 1982, and Zullo (1992)
recognized it as a subgenus as well, Zullo’s (1979) genus-level recognition of
Menesiniella as Balanus gregarius elevates Menesiniella to the genus level.
The current classification of the Balaninae thus appears in Table 8, with
Tamiosoma and Menesiniella placed in their respective subfamilies.
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Table 8: Classification of the Balaninae (Modified from Newman and Ross, 1979)
Family Balanidae Leach 1817 Oligocene-Recent
Subfamily Balaninae
Balanus DaCosta 1778 Oligocene-Recent
Tamiosoma Conrad 1857 ?L. Oligocene, Middle Miocene-Pliocene
Zulloa Ross & Newman 1996
Fistulobalanus Zullo, 1984
Tetrabalanus Cornwall, 1941
Subfamily Concavinae Zullo 1992 Oligocene- Recent
Concavus Newman, 1982 L. Oligocene-Recent
Arossia Newman 1982 M. Miocene-Recent
Menesiniella Newman 1982 Miocene-Pliocene
Chesaconcavus Zullo, 1992 L. Oligocene-Pliocene
Paraconcavus Zullo, 1992 L. Miocene-Recent
Subfamily Megabalaninae Newman 1979 Oligocene-Recent
Fosterella Buckeridge 1983 L. Pliocene-Recent
Notomegabalanus Newman 1979 L. Miocene-Recent
Austromegabalanus Newman 1979 M. Miocene-Recent
Megabalanus Hoek, 1913 Oligocene-Recent
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F u n c t i o n a l M o r p h o l o g y
Tamiosoma gregaria occupied a morphological niche which is currently
underutilized by modem barnacles. This morphology has brought comparison with
some rudist bivalves. This shell form is convergent with not only the Bivalvia, but also
with richthofenid brachiopods and solitary corals, and has brought upon terminology
used by them as well, such as “reef’ (Woodring et al., 1940) or “bioherm” (Tinsley and
Dohrenwend, 1979). Morphologically, the basis of Tamiosoma is constructed similarly
to that of some rudists, with a cancellate structure filling the lower portion of the basis,
which was easily deposited and contributed to the structural integrity of the shell.
Individual specimens of T. gregaria attached themselves to any hard, suitable
substrate. Most specimens found show little or no evidence of this original anchor,
although in some cases the substrate is large, such as the scallop seen in Figure 43. In
other cases, the material first used as an anchor may have been small, and could quickly
be engulfed by rapid shell growth. Barnacles which clustered could settle on each
other, though generally in clusters the individuals radiate from a single point, rather
than overlapping, indicating settlement at approximately the same time. Specimens
which grew on each other are seen in Figure 44. Although the small anchor would
provide limited support for such a large barnacle, the close packing exhibited by some
individuals, in addition to sediment around and between individuals, would have held
the adult barnacle securely. Additionally, the calm waters of the embayment
(characterized by fine-grained sedimentary deposition) would have generally precluded
currents strong enough to dislodge such a creature.
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Figure 43: Scallop which served as anchor for juvenile barnacles to settle upon.
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Figure 44: Some barnacles served as anchors for other individuals. Lens cap for scale.
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E c o l o g y
“Reef-building” barnacles have received little attention in terms of their
ecology, except with the exception of Tinsley and Dohrenwend’s (1979) brief mention
in their field guide. However, it is clear that Tamiosoma’% ecology deserves close
attention. The closest ecological analog which has been closely studied are rudist
bivalves. Although rudists inhabited numerous ecologies (Gili et al., 1995), many filled
a similar depositional setting to that of Tamiosoma. This convergent morphology
caused numerous turn of the century scientists to identify Tamiosoma as Radiolites
(Conrad, 1864), and proceeded to confuse the stratigraphy of the area, as Miocene rocks
were labeled Cretaceous.
Gili et al. (1995) are strong proponents of some rudists as mudstickers, not reef-
builders, in the Cretaceous. Many inferences into ecology of an organism can be made
by understanding how its morphology was influenced by the environment This method
of interpreting paleoecology using functional morphology has been championed by
Adolf Seilacher. A mudsticking ecology as defined by Seilacher (1985) is well-suited
to muddy areas in which sedimentation is rapid, serving as support for the elongate
morphology.
The term “reef’ itself has been plagued with controversy. Many buildups of
shell material in the fossil record have been classified as reefs, despite few unifying
features. Gili et aL (1995) classified reefs as “robust biogenic frameworks” with
“topographical relief".
As with many elevator rudists (in which the commissure exhibited upward
growth) these barnacle buildups are not “reefs” by any terminology. Instead, they can
best be characterized as laterally tabulate or lenticular units which show little to no
topographic relief over the seafloor. These structures’ shapes were probably determined
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151
by the sedimentation rate and bottom morphology. At over 100 m long, with little
internal dip, this barnacle “bioherm” can best be described as a lithosome (Gili et al.,
1995).
Individually, the relationship of individual barnacles can be described based on
their density and relation to other individuals. Kauffman and Sohl (1974) created a
classification scheme for rudist relationships which is illustrated in Figure 45.
Individuals are the fundamental building blocks of these groupings, which grade into
each other through space and time. Associations are defined as loose accumulations of
individuals in a specific area. Clusters are “relatively small lenticular or pod-like
frameworks comprising one or a few generations of closely spaced rudists” (Kauffman
and Sohl, 1974; p. 433). These may form a loose network, and individuals are in close
contact Thickets are defined as “a close aggregation of clusters or individuals into
basically a tabular framework with its lateral extent greater than vertical extent”
(Kauffman and Sohl, 1974; p. 437). With this definition, a structure with shape, based
on specific relationships between individuals is thus defined. They (Kauffman and Sohl;
1974) also note that this structure may be monospecific. Coppices are topographically
low frameworks in which the previous generation of rudists serves as a base for the
following generations. This shell debris is thus in situ. Biostromes, as defined by
Kauffman and Sohl (1974), are a “complex framework or assemblage of different types
of frameworks characterized by broad lateral extent, thin vertical extent, high collective
diversity, and a matrix ...of coarse bioclastic debris.” (Kauffman and Sohl, 1974; p.
441). Banks are a continuation of the coppice structure, becoming larger, and having a
significant topographic relief. Theoretically, banks can grow to become patch reefs and
barrier reefs, as seen in corals today.
When Tamiosoma is compared to these rudist buildups, it is apparent that reef is
not the appropriate term to describe the relationship between individuals. In many
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Figure 45: Kauffman and Sohl’s (1974) classification of Cretaceous Carribean rudist
frameworks. If this classification is applied to T. gregaria buildups, most would fall
into “associations”, though development up to “thicket” stage was observed in isolated
areas. As noted by Gili et al. (1995), “framework” may not be the appropriate term for
these growth fabrics, because the structure did not confront wave action.
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TH ICK ET TH ICK ET
CLU STER CLUSTER CLUSTER
ASSOCIATION INOIVIDUAL ASSOCIATION
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places, Tamiosoma forms clusters. Tamiosoma also forms thickets, with the lateral
extent greater than vertical extent The definition of coppice implies that multiple
generations contribute to the vertical extent of the buildup. This is indeed true for the
Tamiosoma beds, as seen by overlapping barnacles, and the multiple subunits which
contain in situ barnacles. However, these layers cannot be described as a biostrome, for
the arrangement of shell material never becomes complex, and the unit is generally
monospecific in terms of creating the framework. Thus, by Kauffman and Sohl’s (1974)
classification, Tamiosoma comes nowhere near true “reefbuilder” status, and thus, its
classification as such was ill-founded.
Like T. gregaria, rudists are dependent on a hard substrate, such as a shell
fragment, for larval colonization, yet are never found on a hard substratum. Their
lifestyle is dependent on muddy bottoms. Elevator congregations of rudists are also
commonly found in life position, as they were already embedded in the sediment, and
are generally distributed in fine grained, muddy deposits - indicative of calmer waters.
Likewise, these barnacles are in situ, due to similar deposidonal circumstances. In situ
deposits of rudists are common because of these quiet deposidonal environments. This
growth within the sediment, not above it is termed “constratal growth” (Fig. 46), and
can be tested by observing bioerosion patterns on the exterior of the individual, as was
done by Kauffman and Sohl (1974). Those whose growth is constratal should show
little or no bioerosion. As discussed below, bioerosion was also analyzed on
Tamiosoma, and the general lack of bioerosion on the exterior supports this hypothesis.
Some have suggested that some rudists had symbiotic zooxanthellate algae
(Kauffman and Johnson 1988). Yet rudists (specifically Hippuritids) are often found in
muddy sediments (Gili et al., 1995). This muddy environment may have been turbid
and nutrient rich, as shown by associated organic-rich marls (Gili et al., 1995). As
corals and photosymbiotic bivalves usually prosper in oligotrophic areas, this type of
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Figure 46: Illustration of “constratal” growth fabric, from Gili et al. (1995). With
exception of the ammonite, this could well be a picture of Tamiosoma gregaria living in
diatomaceous muds which are characteristic of the Pancho Rico Formation.
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turbid environment would not be conducive to photosynthesis (Gili et al., 1995). This
environment is similar to that in Wildhorse Canyon - turbid and nutrient enriched water,
which supported a diversity of life, but may not have been well-lit. Gili et al. (1995)
also notes that rudists are commonly found oriented in a particular direction, which is
related to current flow. Although the Wildhorse Canyon lithosome has a distinct strike
of 290 degrees, it is necessary to note that this strike of the high density outcrop, is
formed by the presence of shell beds in the area, although barnacles are found
throughout the area in the same stratigraphic interval. Because this strike represents
shell beds and not the distribution of barnacles themselves, and because
paleogeographic reconstructions are poorly constrained, and are on a much larger scale
than this mapping, any inferences into paleocurrent would be unfounded, or weak at
best
Both Kauffman and Sohl (1974) and Gili et al. (1995) note the lack of binding
and encrusting organisms on rudists. With the exception of one tubeworm observed by
the author (Fig. 47), no encrusters have been observed on Tamiosoma gregaria.
Although these barnacles commonly grew in clusters, they never created a framework
capable of creating much relief above the seafloor.
These ecological similarities are striking, and warrant broader examination of
environmental circumstances which allow creatures such as Tamiosoma to develop,
thrive, and just as suddenly, disappear.
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Figure 47: Tubeworm shell, found inside shell of T. gregaria. Cluster is found upright
(in life position), but barnacle iust have been dead before tubeworm settled in the
living chamber.
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Birerosion
Previous field research, conducted during the 1996 field season, documented the
presence of boring sponges, recognized by the ichnofossil Entobia, on both gastropod
and barnacle shells. It is the purpose of this portion of the study to determine how
abundant Entobia is on Tamiosoma, whether the distribution of borings is dependent on
size of the barnacle, and whether a spatial relationship exists between the sponge
borings and the barnacle shell. It is hypothesized that these borings can indeed provide
valuable information into the life habits of these barnacles.
These findings suggest that, in large barnacles, Entobia is rare, if not absent, on
the outside of the shells, but can be found on the inside of the shell, above where the
opercular plates would be placed. Thus, a distinct preference in boring sites is exhibited
by the sponges, which bored and lived near the bamacle’s aperture while both were
alive. The outside of Tamiosoma is generally pristine, due most likely to a high
sedimentation rate which prevented the successful settlement and boring of clionid
sponges. In Menesiniella, borings are rare, and, in most cases cannot be conclusively
identified as sponge borings.
Past Research
Paleoecological work using borings has been underutilized by the
paleontological community, but can provide valuable clues as to the life habits of fossil
organisms, as well as elucidating the presence of cryptic faunas present in many
environments. Lawrence (1968) attempted to quantify the use of clionid sponges in
paleoenvironmental analyses, and concluded that the technique is “hampered by
complex sponge-spreading patterns and by the fact that time relations between the two
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161
groups cannot be established in all occurrences” (Lawrence, 1968, p. 542). However,
clionid borings in coral-dominated environments have more recently been analyzed
(Edinger and Risk, 1994, Pleydell and Jones, 1988), and researchers have identified the
by-products of clionid borings as an important sediment source (Cobb, 1969). The
borings have important paleoecological implications for coral structure and strength as
well.
Others have seen borings as a very powerful paleoecological tool. For example,
Seilacher (1969) cited boring barnacles as an ideal tool for paleoecological studies, as
they have specific optimal orientations during their life spans. Similarly, Botq'er (1982)
used borings and epizoans in studies of the Cretaceous oyster, Pycnodonte mutabilis.
Lescinsky (1996) also believes that epibionts are an ideal tool for paleoecological
investigation, due to readily identifiable cross-cutting relationships between boring and
encrusting taxa, providing a time frame more precise than ordinarily feasible in
geologic studies.
Perhaps the most applicable study was done by Kauffman and Sohl (1974) on
Antillean Cretaceous rudist bivalves. Though taxonomically very different than
barnacles, rudists may be the most promising ecological analogs. By mapping the
distribution of epibionts on the rudists, they were able to provide interpretations of a
sediment line, below which the shell stayed buried, on the genus Barrettia. On the
rudist genus Chiapsella, sponge borings were found on the upper half of the attached
valve, and on the top of the free valve, in areas which were most likely exposed to light,
and most likely, free water flow (Kauffman and Sohl, 1974). Thus, it appears that the
distribution of clionid sponges is a fairly sensitive tool in inferring life orientation of
individuals, whether preserved in situ or not
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M ethods o f Investigation
Two outcrops of Tamiosoma buildups appear in the Wildhorse Canyon area - a
large, elongate exposure (#1), and a smaller, more western outcrop (#11). Both are
assumed to be at the same stradgraphic level, and may represent either separate
outcrops of a continuous reef-like buildup, or, more likely, remnants of a distinctly
patchy distribution of barnacles. Within this fossiliferous subunit of the upper Pancho
Rico Formation, three horizons have been identified (Fig. 12). These consist of “1”, a
shell bed consisting of densely packed barnacles, “2”, a diatomaceous layer which
contains in situ barnacles, including the fragile wall and opercular plates, and “3”, a
shell bed similar to “ 1” which facilitates the preservation of the sequence by protecting
the underlying strata from erosion.
Because of the fragile beauty of these deposits, the sampling area was chosen
carefully. All bulk samples were removed from a less-exposed edge of the smaller,
western outcrop (#11). Sampling was done in the “2” and “3” horizons, which are not
well defined in this area. Samples for this study were then collected using bulk
sampling techniques. Ten bags were labeled and shell debris and matrix were collected
from this site. Combined, these bags of diatomaceous mud and shell weighed in excess
of 20 kg.
Once collected, the samples were returned to the laboratory for analysis. Matrix
and shell fragments were separated and classified into three size categories, and six
shell types. Barnacle size was quantified by estimating the longest dimension of the
aperture. Those barnacles with apertures less than 2.5 cm were classified as small.
Those over 4 cm were classified as large. Those between the two were classified as
miscellaneous, and constitute a small percentage of the total sample. Shell fragments
were then sorted according to plates. Wall plates were defined as the entire plate, or the
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163
portion above the adductor ridge, provided that the proximal and distal position of the
fragment could be ascertained. Opercular plates were sorted individually. Finally,
broken wall plates (which did not fit the above category), bases, or unidentifiable
fragments were classified as "unidentified". Each fragment, provided that it was the
majority of a wall plate, was identified as one plate. Those found fused together were
counted based on the number of plates. Thus a bored portion of barnacle shell which
was comprised of 3 wall plates carried 3 times the weight of a single wall plate.
Because of the bulk sampling techniques, whole collected barnacles should receive
equal weighting (six plates apiece), while individual barnacle plates transported in are
less significant This should minimize the effect transported fragments receive, since
they may have resided at the surface longer, while m axim izing the attention placed on
those found in situ.
Collected barnacle plates were then analyzed for bioerosion. No encrusting
organisms, with the exception of other barnacles, were found on any barnacle. For each
piece exhibiting bioerosion, the orientation of the bioerosion was noted, as well as what
type. This method yielded:
A) Percentage of bored individuals.
B) Preferred orientation of borer, if any.
C) Relative timing of boring (i.e., while barnacle was alive, or after death.)
Unfortunately, little focus has been paid to borings on barnacles, nor of the
Pancho Rico Formation. Ascertaining the exact ichnospecies may also be tricky to those
well trained in the subject (Bromley and D’ Alessandro, 1990). Thus, it is difficult to
ascertain the exact ichnospecies of the borings. However, the following characterization
of boring types is safe to assume:
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A) Those borings which were small, round, and evenly distributed can be
classified as Entobia isp.
B) Small, isolated holes may be due to breakage, or may be borings. Assuming
that they are indeed borings, they may not safely be classified as Entobia.
C) In general, all borings and bioerosion require considerable time to develop.
Thus, all were considered proof positive of boring activity, though not
necessarily by boring clionid sponges.
Results
The results of this study are summarized in Table 9. While individual bags of
sample yielded varying numbers of barnacle plates, each had the same general patterns.
There was a large number of small wall plates (from Menesiniella), but only a few of
them (2%) had borings. Almost half (47%) of the large barnacles were bored. This may
be considered a minimum number, since most were missing the upper (oldest, most
exposed) edge of the wall plate.
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Table 9: Results of bioerosion study. Each bulk sample was taken from the same
location, at the same time.
Sample Size Wall-Clean Wall- Bored ? Clean ? Bored Terga Scuta
1 Large 8 2 8 0 4 2
Small 58 1 1 0 0 4
Misc. 0 0 45 0 1 4
2 Large 0 3 0 I 0
*
Small 37 1 4 0 0 3
Misc. 1 0 28 I 0 0
3 Large 0 1 1 1 2 1
Small 24 2 0 0 0 8
Misc. 4 1 4 I 0 0
4 Large 3 1 1 1 0 1 1
Small 31 2 2 0 1 5
Misc. 1 0 4 6 0 0
5 Large 2 0 5 0 2 2
Small 30 1 8 1 1 4
Misc. 0 0 0 0 0 0
6 Large 2 5 2 0 5 6
Small 19 2 0 0 0 3
Misc. 2 1 4 0 0 0
7 Large 0 0 2 0 0 3
Small 22 1 0 0 0 0
Misc. 2 0 1 0 0 0
8 Large 1 4 8 0 0 2
Small 43 0 1 0 1 7
Misc. 6 5 7 0 1 1
9 Large 0 4 0 1 2 1
Small 39 0 0 0 1 5
Misc. 3 1 2 0 0 0
10 Large 6 0 3 1 1 5
Small 35 2 3 0 0 4
Misc. 0 0 37 2 0 0
All Large 20 18 37 4 12 20
Small 334 8 19 1 5 37
Misc. 16 7 126 9 2 5
’ercent
Lg. 0.473
Sm. 0.023
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Of those small barnacles that were bored, only two incidences of boring could
be identified as Entobia isp. with any degree of confidence. However, some
unidentified, solitary borings showed evidence of thickening of the inner wall of the
shell by the barnacle, in response to boring. These borings may be from predatory
gastropods, which are a primary predator of barnacles (W. Newman, pers. comm.
1997). This indicates that, when the barnacle was alive, borings which penetrate a
surface of the barnacle which is not fully exposed should produce a modification of the
shell. One of the small barnacle shells was fully covered in Entobia isp. and was
fragmented, probably due to weakening of the shell. This is most likely due to
postmortem boring, and tells us little of the ecology of the barnacle itself, but indicates
that the shell fragment remained exposed at the surface for a period of time after the
barnacle’s death.
Large specimens showed a well-developed boring pattern around the top of the
wall plates, when borings were present. In addition to Entobia isp., branching borings,
and other miscellaneous forms of bioerosion were also present. In large specimens,
opercular plates tended to be wom down, but exhibited no specific borings, despite
being in close proximity to bored wall plates.
Discussion
The spatial distribution of clionid sponge borings provides an additional test of
the hypothesis of upward growth in response to high rates of sedimentation. If
Tamiosoma’s upward growth corresponded to growth high above the sediment-water
interface, as some have suggested that rudists did (Kauffman and Sohl, 1974), one
might expect to see borings all over the barnacle’s outward surface (Fig. 48). One
recent cluster of Megabalanus psittacus, archived at the Los Angeles County Museum
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Figure 48: Predicted bioerosion for barnacles which are exposed to water (and boring
organisms) throughout their lives. Borings cover the exterior, as well as portions of the
interior of the shell.
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Clionid sponge borings, External
| | ^ ^ l l l Clionid sponge borings, Internal
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169
of Natural History is covered by borings on all external surfaces. As this particular
cluster was collected for a restaurant in Chile, it is known that the barnacle was alive at
the time of collection. Thus, none of the bioerosion is postmortem. Cobb (1969)
observed penetration of crystalline calcite about three weeks after settlement of Clioncc,
organic materials in the shell of barnacles may have proven even tougher to penetrate
than inorganic calcite. Thus, a minimum of three weeks’ exposure is required for minor
evidence of boring; most likely the development of extensive borings and chambers
requires many months.
Conversely, if the barnacle lived with the majority of its shell in the sediment,
little opportunity would be provided for boring sponges to colonize the substrate. The
exception, however, is the inside of the wall, above the opercular plates. This area must
be sediment free for all of the barnacle’s life, and the weak current created by the
barnacle cirri could provide water flow to the sponge. Thus, it would not be surprising
to find Entobia on the upper inside edge of the wall plates, even in a barnacle
completely submerged in sediment This distribution has been observed in this study
(Fig. 49), and supports other indications that Tamiosoma lived with the basis below the
sediment-water interface. An identical bioerosion pattern was found on the inside of
recent Megabalanus psittacus. However, the exterior of the shell showed extensive
signs of bioerosion. This specimen was collected off of rocks, and was of similar size to
specimens of Tamiosoma. The lack of this type of bioerosion on Tamiosoma is
supportive of its subsurface life habit
Small borings have been observed on the outside of the large bases. These cup
shaped holes are probably the result of boring bivalves, and suggest that some creature
did have time to settle and remove shell material. When closely examined, it appear
that these borings coirespond to concentric growth bands found on the outside of the
barnacles. These bands are well developed, as opposed to the zones between them,
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Figure 49: Observed bioerosion on Tamiosoma gregaria.
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Clionid sponge borings, Internal
Clionid sponge borings, External
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172
which are generally wider, less well developed, and have longitudinal bands. Kauffman
and Sohl (1974, p. 407) observed a similar phenomenon in the rudist Antillocaprinia,
noting that “Clionid borings...are concentrated...in circular zones under concentric
lamellae.” Perhaps the well developed concentric bands correspond to a period of slow
or absent sedimentation, in which well developed shell structure was allowed to form,
while the larger zones of less developed banding correspond to fast rates of sediment
influx. The presence of the borings on the small bands supports this hypothesis, for the
period of time in which the concentric rings were allowed to develop would be a rime of
relatively low sedimentation.
However, this may not necessarily be the case. These zones may have a skeletal
ultrastructure which makes them more susceptible to boring. No studies have been
done to assess the role of tissue in preventing the growth and setdement of borers in
barnacles, but other taxa such as some molluscs have been shown to have inhibitors to
the growth of epi- and endobionts (Kauffman and Sohl, 1974). In gastropods, the
periostracum serves as the inhibitor to encrusters and borers (Bottjer, 1981). Bivalves
also possess a protective periostracum, but is limited to specific clades. Although each
phylogenetic group probably evolved these structures independently, they each serve to
protect against epi- and endobionts, as well as to reduce predation and achieve stability
in the substratum (Bottjer and Carter, 1980).
When compared to rudists, the morphological differences between cirripeds and
bivalves are of fundamental importance. Hippuritid rudists, which have a cup and cone
morphology, had an upper valve which was available for colonization, while barnacles
have only a hollow cone at the top. Some bivalves which are filter feeders concentrate
their water flow using siphons. Borings in some species show concentration around
what is interpreted to be the siphonal areas; others show fewer borings (Bromley and
D ’Alessandro, 1990, Kauffman and Sohl, 1974). Tamiosoma's suspension feeding
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173
lifestyle created a more even water flow across the upper wall plates. While this must
have helped the growth of sponges, the largest advantage of settling there was probably
a continuously sediment-free substrate in contact with the water column. The
ubiquitous bioerosion in this zone suggests that this area provides excellent habitat for
borers.
Edinger and Risk (1994) suggest that bioerosion is generally correlated with
nutrient levels. Their research demonstrated increased bioerosion in the Caribbean in
the Early Miocene, and showed independent evidence of increased nutrients in the form
of phosphorites, which are indicative of upwelling. In the case of the Pancho Rico
Formation, bioerosion is certainly present on T. gregaria, and independent evidence for
upwelling is available in the form of extensive diatomaceous sediments. Thus, one may
speculate that nutrient levels may have promoted bioerosion.
Tamiosoma gregaria is clearly unique in terms of morphology, but, in addition,
is preserved so well that many additional inferences in terms of geology can be made
from its presence. Borings created by clionid sponges provide an opportunity to test
hypotheses regarding the deposition of these beds, and support the theory that these
barnacles lived deep within the sediment, not elevated above it. Borings are most
widespread on the inside of the wall plates, above where the opercular plates would be
located. Small barnacles of similar, if not identical taxonomic identity have no well
developed Entobia isp., suggesting that they were not exposed for long enough periods
of time to develop the borings.
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174
Taphonomv of T. gregaria
The taphonomic aspects of Tamiosoma gregaria are quite different than the
other barnacles in the Pancho Rico formation. Unlike Menesiniella, and other barnacles
found scattered throughout the formation, T. gregaria's unique morphology confers
upon it radically different taphonomic properties.
Even in the largest individuals, wall plates are not fused to the bases, and are
thus readily susceptible to disarticulation. This may be an indicator of multiple things:
1) that T. gregaria was actively growing throughout its life, and 2) that the presence of
wall plates and in situ opercular plates is indicative of little or (more likely) no transport
after death.
The wall plates of Tamiosoma gregaria are robust and commonly at least
partially fused to each other. The thickness of these plates, yet relative weakness of the
articulation point of base and wall may suggest that the entire base, plus part of the wall
plates was buried, and that currents near the bottom were not strong. In this way, the
weakest point of the barnacle’s shell would have been supported by the tightly packed
sediment around it
T a p h o n o m y o f P a n c h o R i c o F m . D e p o s i t s
Despite relatively easy identification and general availability, opercular plates
have served little paleoecological utility. In this portion of the study, the distribution of
sizes in whole barnacles and their opercular plates is compared. In addition, the
taphonomic characteristics of Wildhorse Canyon deposits are compared to those of the
San Lucas shell bed.
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175
The bamacle-dominated San Lucas shell bed (USGS Locality 903) contains
some whole barnacles, but is generally composed of disarticulated plates. Scallops,
echinoids, gastropods, and bryozoans are also found but are a proportionally sm all
component of the fauna. Barnacle opercular plates, especially scuta, are common in this
bed, and these scuta vary a great deal in length, from 3 to 30 mm in length. Most of the
scuta are small, however, with half falling below 11 mm in length. Whole barnacles are
much less common.
Measurements on the extant Menesiniella aquila reveal that scutum length is
directly proportional to orifice length (Fig. 50). Thus, the distribution of scuta can be
used to predict the size distribution of barnacles. In both the shell bed and bioherm,
intact barnacles are less common.
By using scutum length to predict orifice length, it is evident that smaller
barnacles are proportionally less common than would be expected. It is likely that
smaller specimens of Menesiniella disarticulated during or before the high energy
deposition of the shell bed, while comparatively larger barnacles with fused wall plates
were more likely to survive transport Thus, the apparently narrow range of barnacle
sizes present in the shell bed is an artifact of preservation.
The barnacle deposit at Wildhorse Canyon contains vertically oriented, in situ
clusters of Tamiosoma, most over 10 cm high, as well as a small proportion of coarse
small barnacle (Menesiniella) debris. Bulk samples collected from the site reveal that
intact scuta are the more common opercular plate, and that many are incredibly large
(up to 30 mm long). Although in situ, whole barnacles are fragile, and quickly
disarticulate when exposed.
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176
Figure 50: Cross section of a barnacle (hypothetical). Plates are labeled as follows:
C- Carina, CL- Carino-lateral, RL- Rostro-lateral, R- Rostrum, T- Tergum, S- Scutum.
Muscles: TD- Tergal Depressor, LSD- Lateral Scutal Depressor, RDS- Rostral Scutal
Depressor.
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Aperture Length
C L
RL
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Site 1: USGS Locality 903: San Lucas Shell Bed
178
USGS Locality 903, described by Durham and Addicott (1964), is located near
San Lucas, CA, and is an isolated outcrop of coarse, bioclast supported shell material
and occasional pebbles in a fine to medium-grained sandstone matrix. The matrix is
well sorted, and contains mostly quartz grains, with a high percentage of biotite. Figure
51 shows this area.
The unit is dominated by disarticulated barnacle plates and proportionally few
whole barnacles. Other tax a include epifaunal scallops, multiple species of gastropods,
echinoids, and branching and encrusting bryozoans. The shell material is concentrated,
and shows only faint evidence of grading or sorting, although portions of the outcrop
contain significantly smaller fragments, and less complete fossils.
The most common barnacle debris is wall plates. Bases are also common, and
are generally conical, due to clustering. Clusters of bases can be found, and some show
the beginning of cancellate infilling at the tip of the cone. Whole barnacles, when
found, are filled with sand and shell debris, and do not contain terga or scuta.
The presence of taxa suited to both rocks (such as barnacles) and sand substrates
(such as scallops and irregular echinoids) suggests that there may have been multiple
source areas for the transported material, or one large, diverse source area containing
both hard and soft substrates. Time averaging, and fossil breakage may contribute to a
fossil assemblage which is different from the source paleoco mm unity.
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179
Figure 51: Shell bed near San Lucas. Boulders are all composed of shelly material.
Arrow points to shell bed.
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Site 2; Wildhorse Canvon
181
The Wildhorse Canyon barnacle deposit is an in situ deposit containing both
Tamiosoma gregaria, and Menesiniella aquila. Small outcrops are found at
approximately the same strarigraphic level in a 2.5 km square area.
Two species of barnacle have been idenitified from the deposit Menesiniella
aquila was not observed whole at this stratigraphic level, however wall plates and scuta
are common. Specimens of Tamiosoma gregaria are found vertically oriented within
the matrix. This suggests that Menesiniella was transported to the site, while
Tamiosoma is in situ.
In addition to the aberrant morphology of these large specimens of T. gregaria,
they inhabited soft sediment, an uncommon feature for barnacles. Shell debris
apparently served as substrate for the first barnacles to colonize the areas; later arrivals
grew on other individuals, or on the increased shell debris from these colonists. This
taphonomic feedback (Kidwell and Jablonski, 1983) eventually created a thriving
community of barnacles in an unusual environment.
Methods
Bulk sampling was used at each locality in order to prevent collecting biases.
Approximately 800 cubic centimeters of shell material was collected at USGS 903. At
the Wildhorse Canyon bioherm, an approximately equal amount were filled from the
outcrop, all at a single stratigraphic level.
Samples from USGS 903 were disaggregated by hand, or by placing them in a
sonifier. The material was then washed to remove the sandy matrix, leaving shell
material. This barnacle-dominated material was then carefully sorted, and any whole
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182
barnacles and opercular plates were removed. Because of the soft, fine-grained matrix
at the Wildhorse Canyon locality, opercular plates were removed without processing the
samples beforehand. Although fine-grained matrix was present at the time of sorting,
this most likely did not mask smaller plates, due to the color and textural differences
between shell material and matrix. The presence of small opercular plates from these
samples, of sizes comparable to that of the shell bed, supports this.
Once separated, for each sample, scuta were separated into right and left sides,
and measured with digital calipers. Left and right scuta were equally distributed, and
further analyses were conducted using the combined data set. Whole barnacles (all
without opercular plates) were put aside. Due to the large size and fragile nature of T.
gregaria, it is likely that the bulk sampling process of placing sediment in the sample
bags disarticulated any whole barnacles during collection.
Samples of extant Menesiniella aquila were measured from the invertebrate
zoology collections at the Los Angeles County Museum of Natural History. Each of the
barnacle specimens measured contained at least one scutum. Ten different
measurements of the shell, as well as scutum measurements, were taken from each
specimen. Using Statistica (StatSoft, 1994), a correlation matrix between variables was
created. It was ascertained that orifice length is most closely correlated with the length
of the scutum, because scuta fit within the orifice, and the lengths are thus constrained
by the orifice shape. Its high correlation value (R=.9941) supports this (Fig. 52).
Orifice lengths were then measured for each of the whole barnacles found in the bulk
samples.
For each scutum collected, the corresponding predicted orifice length was then
calculated using the regression-derived equation from M. aquila. This was then plotted
against the observed orifice measurements.
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183
Figure 52: Orifice length is linearly correlated with scutum length. Measurements were
made on extant specimens of Menesiniella aquila. This linear relationship was used to
predict the corresponding orifice length for each fossil scutum.
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184
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Scutum Length
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Analysis Of Data: San Lucas Shell Bed
The distribution of scuta lengths (Fig. 53) from the San Lucas shell bed shows a
left-skewed size-ffequency distribution (N=211), with the greatest percentage of scuta
falling into the 5-8 mm range. Small scuta (smaller than 5 mm) were a proportionally
small percentage of the scuta. This is due to the fragile nature of these scuta, which are
thin, and break easily, as opposed to a true lack of small barnacles. The relatively low
abundance of large scuta (characterized by large scuta) may represent a true upper limit
for this group of barnacles, as representative samples were large, thick, and less prone
to breakage than smaller barnacles.
Measurements on the extant barnacle, M. aquila, revealed a high correlation
between orifice length and scutum length. This linear relationship was then used to
predict the orifice length of the barnacle for all scuta collected.
The predicted orifice length distribution (Fig. 54) shows a similar trend for
whole barnacles. The increases and decreases in predicted abundance, such as the
increase in the 5-8 mm range, are due to predicted orifice lengths which fall into
different size categories than their corresponding scuta.
When whole barnacles’ orifice lengths are compared to the number of scuta, or
the predicted orifice lengths, it becomes apparent that scuta are significantly more
abundant (almost 10X more common than whole barnacles).
Statistical analysis was performed using Stastica (StatSoft, 1994). Basic
statistics were performed, as well as a correlated T-TesL The T-test showed that these
two data sets are statistically significant at the 100% level, with p = 0.00. Results are
seen in Table 10. Because of the small number of whole barnacles, it is better to
visually compare the predicted distribution of orifice lengths and observed orifice
lengths (Fig. 55) based on percent abundance (Figure 56). Both predicted and observed
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186
Figure 53: Barnacle scuta from San Lucas Shell Bed (USGS 903)(N=240) show a left-
skewed size-firequency distribution.
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120
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188
Figure 54: Predicted barnacle orifice lengths calculated from scutum lengths, using the
model constructed using extant Menesiniella aquila.
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189
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190
Table 10: Statistical comparison of predicted orifice lengths (a proxy for overall
barnacle size) and observed lengths. These are statistically significant.
Predicted Orifice Lengths Observed Orifice Lengths
N 239 27
Minimum 3.97 2.4
Maximum 28.35 22.5
Mean 10.00 12.18
Standard Deviation 4.47 4.85
Skewness 1.62 0.15
Kurtosis 2.73 -0.41
T -Test Results: p=.0000 100% Significant
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191
Figure 55: Comparison of measured and predicted scutal distributions, as well as
distribution of observed aperture lengths. Abundance of complete barnacles is much
lower (N=27).
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192
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193
Figure 56: When viewed as a percentage, aperture lengths of collected barnacles shows
a normal distribution, while scutal distribution is skewed to smaller barnacles.
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194
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Length (mm)
195
aperture lengths have a similar spread, with similar standard deviations, indicating that
barnacles of all sizes (and their opercular plates) do become preserved in the fossil
record. The mean orifice length was higher for observed orifice lengths (12.18) than its
predicted counterpart (10.00). The kurtosis of predicted orifice lengths is high due to the
large number of small barnacles which form a steep curve (kurtosis = 2.73), as opposed
to the even distribution of a small number of barnacles, which forms a flat curve
(kurtosis = -0.41). Compared to the predicted left-skewed (skewness = 1.62)
distribution of orifice lengths, the normal (skewness = 0.15) distribution of barnacle
sizes shows that the larger barnacles have a better chance of being preserved. Thus
taphonotnic processes which affected deposition of the shell bed favored the
preservation of larger whole barnacles, although small barnacles should have been more
abundant
Analysis Of Data-Wildhorse Canvon
The distribution of scuta for the Wildhorse Canyon outcrop (Fig. 57) shows a
weakly bimodal distribution. This is, in part due to the small sample size (N=33), and
partly due to the two different types of barnacle scuta present - M. aquila, and the larger
ones of T. gregaria.
Using the regression-derived equation based on Menesiniella aquila, predicted
orifice lengths were calculated from observed scuta lengths (Fig. 58). Because no
whole barnacles were recovered from the bioherm, there were no observed orifice
lengths.
The predicted orifice lengths (Fig. 59) from the Tamiosoma beds were then
compared to that of the shell bed, as well as those observed at the shell bed. For all but
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196
Figure 57: Scutum length distribution from Wildhorse Canyon bulk samples (N=34).
Bimodal distribution is a result of size differences between Menesiniella and
Tamiosoma.
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197
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Scutuntengtftmm)
198
Figure 58: Predicted orifice lengths of barnacles from Tamiosoma beds, as compared to
observed scutal lengths (N=34).
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199
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9-12 15-18 21-24 27-30
Length (mm)
200
Figure 59: Predicted orifice lengths for Wildhorse Canyon differs from those in San
Lucas Shell Bed. In most size classes, the predicted scuta distribution from Wildhorse
Canyon is closer to the predicted value from the shell beds, suggesting that, if these
barnacles were under similar growth regimes, the in situ deposit is less biased.
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0 Predicted Orifice Lenths (San Lucas)
® Observed Orifice Lengths (San Lucas)
E3 Predicted Orifice Lengths (Tamiosoma beds)
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Length (mm)
202
two of the size ranges in which all three variables are represented (6-9 mm, 18-21 mm),
the predicted orifice ;engths from Wildhorse Canyon were closer to the predicted orifice
lengths of the shell bed (those opercular plates belonging to M. aquila). The second
peak of orifice lengths predicted from Wildhorse Canyon is indicative of Tamiosoma.
Discussion
Perhaps the primary factor governing scutum distribution is the structure of the
original paleocommunities. Environmental influences were very different between the
two, creating communities with different species composition, and hence very different
size class distributions. However, neither outcrop has been preserved without loss of
valuable paleoecological data due to preservational processes. Each species of barnacles
has taphonomic effects brought on by shell properties, and environmental preferences.
Barnacles found in the San Lucas shell bed (Af. aquila) are small and generally
show evidence of competition for substrate with other barnacles, often resulting in
clusters of barnacles with conical bases. The other taxa found, and the sandy matrix
associated with these barnacles, are indicative of a shallow sublittoral environment
(Stanton and Dodd, 1970). Fusing the wall plates to the basis may have been
advantageous for the barnacles of this environment, in which waves and currents were a
factor. Larger barnacles than predicted by scutum distribution are preferentially
preserved in the deposit due to this bond between basis and parietes, which only occurs
in fully grown individuals.
The fine-grained sediment and thin-shelled bivalve taxa suggest that the
environment in the Wildhorse Canyon area was subtidal, in relatively still water, below
wave base (Prothero, 1990). Barnacles from this area also show evidence of upward
growth, possibly as a response to the rapid accumulation of sediment around the basis,
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203
as opposed to crowding by other barnacles. The bimodal distribution of scuta cannot
be compared to whole barnacles, as they are difficult to collect in bulk samples.
However, qualitative comparison of parietal debris indicates that this distribution is
close to the true size ranges of barnacles present. If growth in the muddy environment
was dominated by sediment supply to the area, small barnacles may have been a
proportionally smaller component of the fauna. Larger individuals, once established,
would need only to direct growth upward, while smaller barnacles would need to both
expand as well as keep up with sediment influx.
Although whole T. gregaria specimens are common at certain stratigraphic
levels, Menesiniella is not generally found whole in the same deposits. Nor are their
bases found on any of the larger barnacles, which would have made an excellent
substrate for growth. It is likely that the two species are found in mutually exclusive yet
adjacent habitats, with the small barnacle debris transported into the deeper, calmer
waters where T. gregaria flourished.
While barnacles are made of stable low-magnesium calcite, and thus are well
preserved in the bed, very few aragonitic fossils are preserved, except as molds in the
fine-grained sediment. In deposits associated with high organic productivity, low pH
can develop in the sediment as a result of oxidation of monosulfides and pyrite, and the
development of sulfuric acid in the seawater (Brett, 1995). This can lead to early
dissolution of the aragonitic portion of the benthic biota. However, it is possible that
this dissolution was caused later through leaching as a result of weathering and soil
development The uncompacted nature of fragile molds support the latter hypothesis.
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204
FUTURE RESEARCH
Isotope analysis is a promising outlet for future study. The morphology of T.
gregaria , its age, and its relationship to sedimentation rate can be identified through
oxygen isotope values in the base of the shell. Barnacles incorporate oxygen isotopes in
proportion to the ambient oxygen 18 values (Killingley and Newman, 1983). Killingley
and Newman (1983) demonstrated that variations in the 0*8 value could be observed in
extant Menesiniella aquila, and are correlated with observed seasonal temperature
fluctuations. In an embayment such as that which filled the Salinas Valley, the 0
value of the water would vary as a function of both the water temperature and the
seasonal influx of fresh water draining off the land.
Barnacles often show bands of growth in their shells, as seen in Figure 60.
Although intertidal barnacles often show this type of banding in response to exposure,
subtidal barnacle would provide a regular source of calcite deposition. Because the
barnacles create their shells from stable low-magnesium calcite, the record of oxygen
isotope fluctuations should still be recorded in the shell.
Because the 0*8 value would vary according to both temperature and
freshwater influx, it may be difficult to decouple the two effects. However, a seasonal
cycle of change would record the ages of the barnacles, and thus their growth rates, and
by inference, the net rate of sedimentation.
Although the taxonomic nature of T. gregaria has been resolved, the nature of
other giant barnacles throughout the Pacific Coast has been far from resolved. Careful
analysis of the opercular plates of each specimen of barnacle attributed to “ Balanus
gregarius” must be done to understand the true taxonomic nature of Miocene barnacles.
This study may reveal one of two results: that T. gregaria is ubiquitous throughout the
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205
Figure 60: Thin section showing microscopic growth lines in barnacle shells. Such
layers may preserve a distinct isotopic composition relating to ambient temperature and
salinity.
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207
Miocene of California or that many barnacles developed large, elongate bases in
response to increased local sedimentation and productivity.
Furthermore, a rigorous renewal of sampling localities throughout California
would be invaluable in recovering complete specimens, as well as accurately recording
the stratigraphic context in which they were found. The interesting stratigraphic
occurrence of Tamiosoma in strata which display evidence of hiatuses suggests that this
morphology may be linked with very special depositional regimes, and other outcrops
should be examined to test whether or not this is unique.
Tectonically, precise reconstructions of motion along the San Andreas and faults
throughout the Salinas Valley would aid a great deal in accurate reconstructions of the
Salinas Basin during the latest Miocene through Pliocene.
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208
CONCLUSIONS
1. The fossil record of barnacles is excellent, especially in the Cenozoic. The in
situ nature of Tamiosoma gregaria in Wildhorse Canyon is exceptional, and allows a
rigorous analysis of paleoenvironmental factors which influenced the ecology and
morphology of this unique organism.
2. Stratigraphically, the Pancho Rico Formation is a diverse unit, characterized
by sandy and diatomaceous lithologies. The diatomaceous mudstone of the Wildhorse
Canyon area provides valuable evidence of this quieter deposition, removed from
terrigenous input Analysis of diatoms yields a Latest Miocene age, similar to that of the
Sisquoc Formation.
3. The Pancho Rico Formation was deposited in shallow water, in a fully
marine basin. Amounts of terrigenous input determined the lithology of the strata being
deposited.
4. Stratigraphic analysis indicates that Tamiosoma is most abundant at one
stratigraphic horizon which can be traced for hundreds of meters. In the most dense
beds, this horizon can be subdivided into three units: 1) a lower shell bed, 2) a
diatomaceous layer which contains in situ barnacles and Thalassanoides isp., and 3) an
upper shell bed.
5. Thin section analysis and the presence of Thalassanoides in a frrmground
suggest that sedimenation rates were reduced at that time period, while the elongate
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209
nature of Tamiosoma indicates that sedimentation rates were high. Combined, these
opposing results indicates that sedimentation rates fluctuated, possibly due to sea level
transgression.
6. Carbon/carbonate analysis indicates that the strata contain little organic
carbon, and variable amounts of carbonate. Much of this carbonat is due to soil
development
7. Although many have called Tamiosoma gregaria a “reef building” barnacle,
there is little evidence for bioherm development in the Pancho Rico Formation, where
Tamiosoma is preserved spectacularly in situ. While evidence of widespread barnacle
beds is present, there is not evidence that these formed any topographic relief. Rather,
most specimens are preserved in one horizon (at three outcrops), and probably represent
an ecological epibole - a chance flourishing of the barnacle due to favorable conditions
for growth and survival.
8. Taxonomically, these giant barnacles should be calledTamiosoma gregaria,
and not Balanus gregarius. Because of its features, Tamiosoma belongs in the
subfamily Balaninae. Many of the barnacles previously referred to as Balanus gregarius
are M enesiniella aquila zulloi. Menesiniella is a member of the Concavinae.
9. In the fossil record, populations are often difficult to distinguish due to the
many taphonomic factors which influence fossilization. The Pancho Rico Formation
contains both ends of the spectrum- an in situ barnacle bed, and a time-averaged
barnacle shell bed. It is clear that in situ deposits are better for studying paleoecology,
but many of the taxonomic errors which were committed in the study of Tamiosoma
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210
were from incomplete specimens. Thus in situ specimens are also necessary for solid
taxonomic work to be done. While it is still difficult to ascertain the degree to which
the assemblages may have been altered since they were living communities, using
opercular plates as a proxy for the entire barnacle may provide valuable insight into the
taphonomy and paleoecology of fossil barnacle assemblages.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
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Creator Whittlesey, Karen Elizabeth (author)

Core Title Barnacles as mudstickers? The paleobiology, paleoecology, and stratigraphic significance of Tamiosoma gregaria in the Pancho Rico Formation, Salinas Valley, California

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Degree Master of Science

Degree Program Geological Sciences

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag OAI-PMH Harvest,paleoecology

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Advisor Bottjer, David (committee chair), Douglas, Robert (committee member), Gorsline, Donn (committee member)

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