NAPC 2001

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(5/17/01)

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DEPOSITIONAL ENVIRONMENTS IN THE TYPE AREAS OF THE CHADRONIAN, ORELLAN, WHITNEYAN, AND ARIKAREEAN NALMAS, NEBRASKA, USA

LAGARRY, Hannan E., James B. Swinehart, and Leigh Anne LaGarry, Conservation and Survey Division, University of Nebraska, Lincoln, NE, USA

This paper summarizes recent 1:24,000-scale geologic mapping of the type areas of the Chadronian (Peanut Peak Mbr., Big Cottonwood Creek Mbr., Trunk Butte beds, Chadron Fm.), Orellan (Orella Mbr., Brule Fm.), Whitneyan (Whitney Mbr., Brule Fm.), and Arikareean (Sharps beds, Brule Fm.; Ash Creek beds; Monroe Creek Fm.; Harrison Fm.; Coffee Mill Butte beds; Harrison Fm.; Upper Harrison beds) North American land mammal ages (NALMAs). These NALMAs were originally defined as geochrons bounded by prominent marker beds (e.g., "upper purplish-white layer"). However, our map units are based on lithology and allow a more refined understanding of the sequence and distribution of depositional environments across the region during late Eocene through early Miocene time. The Chadronian NALMA is represented by deposition of eolian and fluvially reworked volcaniclastic silts over broad, relatively stable uplands and lowlands with incised valleys, intermittent lakes, and evaporitic basins. Orellan time was marked by a changed to fluvially dominated environments with repeated incision and backfilling of small valleys. During Whitneyan time eolian volcaniclastic silts were deposited over the entire region. Fine-grained eolian deposition continued into early Arikareean time. In early Arikareean time, three E-W drainages were incised, backfilled by coarse-grained sediment, and buried by eolian volcaniclastic sands. At least three ergs existed in the region during this time. These occasionally stabilized ergs were followed by the deposition of additional valley fills; burial by volcaniclastic eolian silt, and intermittent landscape stability. This work was supported by the 1996­2000 Conservation and Survey Division STATEMAP program.

REVISED PALEOGEOGRAPHY OF BASAL WHITE RIVER GROUP STRATA (EOCENE), NORTHERN GREAT PLAINS, USA

LAGARRY, Leigh Anne, Conservation and Survey Division, University of Nebraska, Lincoln, NE, USA; Mary Anne Holmes, Dept. of Geosciences, University of Nebraska, Lincoln, NE, USA; and Hannan LaGarry, Conservation and Survey Division, University of Nebraska, Lincoln, NE, USA

Based on geologic mapping and lithologic correlations of basal White River Group strata in NW Nebraska and adjacent areas, we expand the geographic extent of the Chamberlain Pass Formation (CPF). The CPF consists of pedogenically modified white channel sandstones and extensive red and green overbank silty claystones deposited on a broad, shallowly incised, well-drained, heavily vegetated stable landscape supporting deciduous hardwood open canopy and riparian forests in a humid, strongly seasonal climate. We have mapped or observed facies of the CPF west to the Sherrill Hills of eastern Wyoming, north to the Railroad Buttes and Big Badlands of South Dakota, and east to Keya Paha County in north-central Nebraska. Previous workers identified similar beds at Lance Creek, Wyoming, Slim Buttes, South Dakota, and in the Little Badlands of North Dakota. Reinterpretation of subsurface data suggests that the CPF filled a >61 m deep NW to SE trending paleovalley across the Nebraska panhandle and extends south to Morrill County, Nebraska. Vertebrate fossils from the areally extensive overbank facies of the CPF are rare due to the long duration of surface exposure and acidic environments of soil formation. Fossils recovered from the channel sandstones include a lower jaw, associated upper teeth, and postcranial elements of a brontothere more primitive than those in Chadronian faunas. Destruction of bone by severe pedogenic modification makes correlation of the CPF based on fossil mammals nearly impossible, but earlier workers assumed a Duchesnean age for the sandstone facies. Our reexamination of this unit suggests that the CPF represents a time of widespread landscape stability and is the earliest preserved regionally extensive Tertiary deposit of the northern Great Plains. This work was funded by The University of Nebraska-Lincoln Conservation and Survey Division STATEMAP project and the Nebraska National Forest. Fossils discussed here are housed in the University of Nebraska State Museum.

LATE WHITNEYAN, ARIKAREEAN, AND EARLIEST HEMINGFORDIAN OREODONTS (MAMMALIA: ARTIODACTYLA: AGRIOCHOERIDAE AND OREODONTIDAE) FROM THE JOHN DAY FORMATION OF CENTRAL OREGON

LANDER, E. Bruce, Paleo Environmental Associates, Inc., Altadena, CA, USA; and Theodore J. Fremd, John Day Fossil Beds National Monument, Kimberly, OR, USA

Based on the shared occurrences of age-diagnostic agriochoerid and oreodontid species and subspecies, stratigraphically superposed land mammal assemblages in the John Day Formation (JDF) can be correlated with taxonomically similar land mammal assemblages in the upper White River, the Arikaree, and the lower Hemingford Groups (WRG, AG, HG, respectively) in the central Great Plains (CGP). Late Whitneyan assemblages in the lower Turtle Cove Member of the JDF are dominated by Agriochoerus antiquus guyotianus and Eporeodon occidentalis occidentalis, which also occur in Whitney B/C intervals and/or in the Leptauchenia beds of the Brule Formation of the WRG, and by the endemic Eucrotaphus trigonocephalus.

Earliest Arikareean assemblages of the JDF are dominated by E. occidentalis major and also contain Oreodontoides oregonensis, A. a. guyotianus, and E. trigonocephalus. With the exception of E. trigonocephalus, these taxa also occur in the Brown Siltstone member of the Brule Formation and/or in the Gering and Sharps Formations of the AG. Late early Arikareean assemblages, which occur above the Deep Creek Tuff in the JDF, are dominated by Merycochoerus superbus and also contain E. o. major, O. oregonensis, and Merycoides pariogonus, all of which also occur in the upper Gering and Sharps Formations and in the Monroe Creek Formation of the AG.

Assemblages of presumed early late Arikareean age in the JDF contain Merycochoerus chelydra chelydra, which also occurs in the Harrison Formation of the AG. Latest Arikareean assemblages of the JDF are dominated by the endemic Paroreodon parvus and contain Hypsiops breviceps, which also occurs in the upper Harrison Formation and probably the Upper Harrison Beds of the AG.

Earliest Hemingfordian assemblages in the Haystack Valley Member of the JDF contain Merycochoerus matthewi, Merychyus elegans arenarum, and possibly Merycoides longiceps, which also occur in the lower Runningwater Formation and the Upper Rosebud Beds of the HG.

HETERASTRIDIUM: A PLANKTONIC INVERTEBRATE IN THE UPPERMOST TRIASSIC

LARSEN, Martin C., Dept. of Geology, University of Montana, MT, USA

Restricted to the uppermost Triassic strata, Heterastridium are spherical fossils hypothesized to have increased in size during their evolution. Further, they are postulated, based on functional morphology and wide paleogeographic distributions, to have exploited a pelagic-planktonic mode of life. To test the first hypothesis, measurements of the diameters were made, along with the ages of each specimen correlate with age based on ammonoid and conodont biochronology. A clear trend of size increase through geological time follows Cope's rule and makes this taxon useful in biostratigraphic classification and dating. The specimens for the project consist of material in the Research Collection at the University of Montana, the U.S. Geological Survey's collections, and specimens from a parallel study at the University of British Columbia, Vancouver. To test the mode of life a suitable program was required in order to determine the original porosity of the aragonitic skeleton. Thin sections of the specimens were studied to assess the porous construction of the aragonitic coenosteum, which consist of numerous radial series, zooidal tubes and coenenchyme. These observations, Heterastridium's occurrence in nearly all marine facies, and its worldwide distribution, support the pelagic-planktonic hypothesis.

THE CENOZOIC EVOLUTION OF ANTARCTIC RADIOLARIAN FAUNAS

LAZARUS, David. B., Museum fuer Naturkunde, Berlin, Germany

Antarctic radiolarian faunas today are moderately diverse and highly endemic, and differ radically in composition from adjacent faunas of temperate waters. Fossil faunas are well preserved in deep sea sediments and provide a record of evolutionary change that can be compared to paleoenvironmental changes.

Mid-Eocene and older faunas from the Southern Ocean show some endemism but are largely dominated by temperate water cosmopolites. Beginning in the late Eocene endemism increases. This appears to be in response to the development of a distinct Antarctic water mass and polar circulation system, caused by the separation of southern Australia from Antarctica. The endemic fauna spreads from relatively near the continent in the late Eocene to a broad region comparable in area to the modern Southern Ocean by the early Oligocene. Later Oligocene and Neogene faunas appear to evolve largely in situ, although there are several shorter lived episodes of decreased endemism caused by the influx of "warmer" water species. Late Neogene to Recent faunas are dominated by extinction and the overdominance of just a few species, notably those of Antarctissa.

Although tectonics plays a major role in triggering water mass changes, most radiolarian evolution appears to be more directly driven by changes in the water masses and currents themselves. The late Neogene decrease in diversity appears to be linked to reduced Southern Ocean productivity, possibly the result of increases in sea-ice coverage. Lastly, many Cenozoic species in the Antarctic appear to have had closely related sister taxa in either low-latitude upwelling areas, or in north polar regions, suggesting that allopatric speciation between polar regions has been important as well, with upwelling regions serving as stepping stones for faunal transfer.

EVALUATING THE ACCURACY OF DRILLING FREQUENCY AS AN ESTIMATE OF PREY PREFERENCE AND PREDATION INTENSITY

LEIGHTON, Lindsey R., Dept. of Geology, University of California, Davis, CA, USA

Studies of drilling predation often use drilling frequency, the percentage of individuals of a given taxon or community that have been drilled, as an estimate of (a) the predator's prey preference, and (b) the intensity of drilling predation in a given community. Estimations of prey preference assume optimal foraging by the predator. Encounters between most drilling predators and their prey are sequential; upon encountering potential prey, the predator's choice is, either to attack, or continue searching. For optimal foragers, the decision whether to take a prey type is based on whether the available prey will provide more net energy per unit time than the average of other taken prey types in the system; the encounter rate with these other prey must be considered. The predator maximizes net energy over time when a given prey type is always ignored or always taken: the zero-one rule. This result implies that all taken prey types should have similar drilling frequencies; drilling frequency is not indicative of prey preference. Observed differences in drilling frequency may be due to differences in encounter rate, or to time-averaging.

Analysis of pooled drilling frequencies of communities, as a means of assessing relative predation intensity, holds promise, but may be questionable in systems in which the predator need not drill its prey. For example, naticid gastropods do not need to drill Ensis; the prey can be penetrated through a gape. As prey that do not require drilling probably cost less energy to kill, they usually will be among those prey taken. Optimal foraging models reveal that the abundance of such prey not only effect overall pooled drilling frequency, but also the pooled drilling frequency for drilled prey. Practical examples from the Devonian and the Neogene demonstrate this phenomenon.

PALEOBIOLOGY AND PALEOBIOGEOGRAPHY OF CORUMBELLA, A LATE NEOPROTEROZOIC EDIACARAN-GRADE ORGANISM

LESLIE, Stephen A., Dept. of Earth Sciences, University of Arkansas at Little Rock, Little Rock, AR, USA; Loren E. Babcock, Dept. of Geological Sciences and Byrd Polar Research Center, The Ohio State University, Columbus, OH, USA; Anne M. Grunow, Byrd Polar Research Center, The Ohio State University, Columbus, OH, USA; and Georg R. Sadowski, Instituto de Geosciencias, Universidade de São Paulo, São Paulo, Brazil

Corumbella werneri Hahn et al., 1982, is the most abundant macroscopic body fossil in the Tamengo Formation (Corumbá Group; upper Neoproterozoic) of Mato Grosso do Sul, Brazil. Recent collecting in Mato Grosso do Sul has yielded new material that sheds considerable new light on the morphology of this organism and its phylogenetic affinities. Comparison with Corumbella from the western United States provides important support for a late Neoproterozoic age for the Tamengo Formation of Brazil, and potentially for a relatively close juxtaposition of the Amazon craton and the Laurentian craton during the latest Neoproterozoic. The morphology and phylogenetic affinities of Corumbella werneri, type species of the genus, is reinterpreted based on the new material from the Tamengo Formation. Corumbella secreted a narrow, elongate, tetraradially symmetrical tube. Reinterpretation of tube morphology and new evidence of reproduction by means of budding indicate close affinities with the present-day coronate scyphozoan cnidarian Stephanoscyphus, and possibly the Paleozoic-Mesozoic conulariids. Corumbella may have value for interpreting paleogeographic relationships between terranes rifted from the most recent late Neoproterozoic supercontinent (Rodinia or Pannotia). Corumbella species are known from only two Neoproterozoic localities: western Brazil and western United States. We recognize that paleogeographic interpretations based on such limited data must be viewed with caution. However, the shared presence of Corumbella on both the Amazonian and Laurentian cratonic areas is consistent with the hypothesis that these areas were in relatively close juxtaposition at the close of the Neoproterozoic. Likewise, the shared presence of Cloudina in western Brazil and the western United States further supports the hypothesis of a relatively close paleogeographic position between the Amazonian and Laurentian cratons.

THE CAMBRIAN EXPLOSION: MOLECULAR DIVERGENCE TIMES, PROBABILITIES, AND SIMULATIONS

LEVINTON, Jeffrey, Dept. of Ecology and Evolution, SUNY at Stony Brook, Stony Brook, NY, USA; Lindsey Dubb, Dept. of Genetics, University of Washington, Seattle, WA, USA; and Gregory Wray, Dept. of Biology, Duke University, Durham, NC, USA

The fossil record is currently consistent with a model approximating a near basal rapid Cambrian divergence of animal groups. In concordance with the estimate of Wray, Levinton, and Shapiro (1996) most molecular estimates of the Protostomata-Deuterostomata divergence time are considerably older than the Cambrian, centering around 800­900 million years ago. These estimates, however, are widely scattered and it is possible that a number of biases make molecular divergences unreliable. But if they are unreliable then a paradox arises: The order of molecular divergence estimates of protostome-deuterstome, echinoderm-chordate, and agathan-gnathostome fishes can be recovered with highly improbable accuracy relative to a null model (p = 10^(-5) to 0.03, depending upon assumptions). If the divergences occurred in a very short time, as indicated by fossil occurrences, and if the molecular clock is so inaccurate, then why are the divergence orders recovered so faithfully? We are conducting simulations of evolutionary radiations, varying the ratio of the time of divergence to subsequent evolution using various molecular evolution models. Parameters incorporating an extreme Cambrian explosion time (ca. 10 my) or a slightly more prolonged time (ca. 30 my) do not permit accurate phylogeny reconstruction. Success did not exceed 80 percent at the time of the Cambrian without an extended divergence time interval of at least 100 my. Using larger numbers of taxa (25) the proportion of clades that were correct was quite small unless very large divergence times were used. This seems inconsistent with our success in getting such estimates of order of divergence time from actual data. Our results militate against the likelihood of an Early Cambrian or slightly longer explosion of the animal phyla, as apparently recorded by the fossil record.

THERE'S NOT MUCH MEAT IN A BRYOZOAN ZOOID: SMALL PREDATORS AND BOREHOLES VERSUS BIG INDUCTIVE REASONING IN CHEILOSTOME BRYOZOANS

LIDGARD, Scott, Dept. of Geology, Field Museum of Natural History, Chicago, IL, USA

One major evolutionary trend in cheilostome bryozoans is the broad secular increase of calcified armament of the frontal wall of the zooid, in all likelihood to protect the soft tissues inside from predators or mechanical stress. An inductive explanation uses the observation that variation in zooid calcification is greatest in the exposed frontal wall, not in the lateral, transverse or basal walls. Fossil drilling predation is evidenced by small, centrally located and uniformly positioned boreholes. Yet, this fossil evidence is actually quite sparse. Reports of modern predators are dominated by omnivorous fishes and macroinvertebrate grazers, and from nudibranchs at the shallow extreme of bryozoan depth ranges. A broad survey of reported bryozoan predators reveals that zooid-scale boring and demineralizing predators do exist, including turbellarians, nematodes, errant polychaetes, and both juvenile and adult prosobranch and opisthobranch gastropods. Some of these predators' feeding behaviors also circumvents skeletal armament by attacking fleshy everted polypides or boring through uncalcified zooid operculae without leaving skeletal boreholes. Skeletal/non-skeletal weight ratios and mechanical resistence to puncture suggest that for some drilling or demineralizing predators, dining on thickly calcified zooids may be overpriced energetically relative to uncalcified zooids. Species with uncalcified frontal walls make up a disproportionately large percentage of all cheilostomes reported in these predators' diets. Lastly, fossil skeletal evidence of frequent reparative zooid budding in taxa with uncalcified frontal walls supports the probable existence of zooid-scale predators from the earliest stages of cheilostome diversification. Small predators have probably been grossly underrepresented by collecting and observation biases.

MARINE ANGIOSPERM PATELLOGASTROPODS FROM THE DOMINICAN REPUBLIC NEOGENE: A SPATIAL-TEMPORAL WAY POINT BETWEEN EOCENE EUROPE AND RECENT NORTH AMERICA

LINDBERG, David R., W. Brian Simison, and Emina Begovic, Dept. of Integrative Biology and Museum of Paleontology, University of California, Berkeley, CA, USA

Specialist herbivores are rare in marine systems, and often specialist species thought to be more likely to become extinct. Marine sea grass limpets first appearance in the Eocene of the Paris Basin with a high morphological diversity that is correlated with fossil sea grass leaf widths. Today there are four species in North Pacific, one extinct species in Northwestern Atlantic, and one species in Australia. In the North Pacific the distribution of plants and limpets are not congruent. There are two patterns: In the North—Lottia alveus and Lottia angusta track the northern range of Zostera marina, while in the south—Lottia depicta occurs only on southern populations of Zostera marina and Lottia paleacea occurs on southern populations of Phyllospadix spp. A molecular phylogeny of these taxa was necessary to resolve relationships because of possible homoplasies in sea grass morphology. Both COI and 16s mDNA genes produce a phylogeny that suggests two distinct radiations of sea grass limpets in North Pacific—Lottia alveus in the North and Lottia depicta and Lottia paleacea in the south. The lineage of Lottia depicta and Lottia paleacea was tropical and present in the continuous Panamic-Caribbean region. The ancestral clade is likely represented in the Miocene­Pliocene fossils of the Dominican Republic and shell structure data connects these taxa back to the Eocene Paris Basin fauna. Without phylogeny the ecological scenario of adaptive radiation associated with habitat diversification would have remained plausible, but untested.

EOCENE ANGIOSPERM REMAINS FROM VANCOUVER
ISLAND

LITTLE, Stefan A., and Ruth A. Stockey, Dept. of Biological Sciences, University of Alberta, Edmonton, AB, Canada

Large numbers of permineralized angiosperm remains have been recovered from the Late Eocene Appian Way locality from Vancouver Island, British Columbia. Fossils are represented by water washed fruits, seeds, leaves and toredo-bored wood remains in calcareous concretions containing numerous gastropods that help to date the sediments. Fossils are studied using the cellulose acetate peel technique. The angiosperm remains are found in close association with those of cupressaceous/taxodiaceous conifers and at least three types of ferns. Some of the most prominent recognizable fruits and seeds include those of Juglandaceae, Annonaceae, Cornaceae (Mastixioideae) and Magnoliaceae. Fruits and seeds are closely comparable to the London Clay Flora of southern England and the Clarno Nut Beds Flora of western North America. At least 14 kinds of undescribed fruits and seeds have been preliminarily identified from Appian Way. Due to the large number of specimens, there is much promise for future study despite the apparent transport of these plant parts prior to deposition and preservation in a shallow marine environment.

DIVERSITY PATTERNS OF CRETACEOUS CORALS

LOESER, Hannes, Estación Regional del Noroeste, Instituto de Geología, Universidad Nacional Autónoma de México, Hermosillo, Sonora, México

Variation in specific and generic richness of corals through the Cretaceous is presented using an extensive computer database of worldwide Cretaceous coral indications. After an increasing number of taxa in the Early Cretaceous, which ceased in the Early Aptian, a phase of extinction took place that lasted until the Early Cenomanian. During the Cenomanian many new genera and species appeared, but became rapidly extinct and were replaced by Turonian taxa. A second phase of evolution started in the Coniacian and continued into the Santonian, but was overlapped by an extinction phase from the Late Santonian that ceased at the K/T boundary. The phases are represented by different taxonomic groups. Whereas the Early Cretaceous is dominated by genera that originated in the Middle to Late Jurassic, the Late Cretaceous is characterized by genera that became established during the Early and Late Cretaceous. Generally, genera and species were long-lived. The endemism of species is extremely high; faunas of the European Boreal, Tethys, the Asian Tethys and Caribbean provinces have a high number of joint genera but also show numerous endemic genera and many endemic species.

Species richness and evolutionary changes are compared to the sea level changes and the abundance of continental-margin anoxic events. The richness of taxa correlates with a high sea level and the absence of anoxic events, but there is no clear evidence that these conditions have also influenced the evolution of the organism group. The coral diversity patterns are finally compared with these of rudist bivalves and foraminifers.

TIME-DELAYED PLANT EXTINCTION AFTER THE END-PERMIAN ECOLOGIC CRISIS

LOOY, Cindy V., Johanna H.A. Van Konijnenburg-Van Cittert, and Henk Visscher, Laboratory of Palaeobotany and Palynology, Utrecht University, The Netherlands; David L. Dilcher, Paleobotany Laboratory, Florida Museum of Natural History, University of Florida, Gainesville, FL, USA; and Richard J. Twitchett, Dept. of Earth Sciences, University of Southern California, Los Angeles, CA, USA

Irrespective of climatic zonation or floral provinciality, terrestrial vegetation was dramatically affected by the end-Permian ecological crisis. The expanded nature of the Permian-Triassic siliciclastic sedimentary record in East Greenland provides a unique opportunity for a high-resolution palynological analysis of the vegetation dynamics associated with ecosystem collapse and initial recovery. Six successional steps are recognized: (a) rapid decline of closed gymnosperm woodland; (b) proliferation of opportunistic herbaceous lycopsids; (c) establishment of high-diversity open shrubland vegetation; (d) renewed lycopsid proliferation; (e) extinction of typical late Permian gymnosperms; and (f) establishment of cosmopolitan low-diversity open shrubland vegetation, characteristic for the beginning of the Mesozoic. This succession reveals some unanticipated patterns. There exists a significant time lag, possibly with a duration of 0.5­0.6 million years, between terrestrial ecosystem collapse and selective extinction among late Permian plants. Furthermore, ecological crisis resulted in an initial increase in regional plant diversity instead of a decrease. These patterns resemble modeled "extinction debt" responses of multi-species metapopulations to progressive habitat destruction.

DEEP SEA BENTHIC FORAMINIFERA AND SURFACE OCEAN BIOPRODUCTIVITY: WHAT MORE DO WE NEED TO KNOW?

LOUBERE, Paul, Dept. of Geology and Environmental Geosciences, Northern Illinois University, DeKalb, IL, USA

Relative abundances of benthic foraminiferal taxa in deep-sea assemblages have long been generally associated with bioproductivity of the overlying surface waters. Over the past 5 years, specifically developed statistical tests have quantified this association on a global basis for surface sediments. The result was transfer functions for estimating past surface-ocean bioproductivity from fossil assemblages. These transfer functions have now been tested in the eastern equatorial Pacific. Tests include comparisons to benthic foraminiferal d¹³C, changing patterns of carbonate preservation, surface ocean nutrient-tracers and geochemical tracers of productivity such as Ba/Al and Al/Ti ratios. In all cases, the tests support the transfer function estimates. Our ability to estimate surface ocean bioproductivity with benthic foraminiferal assemblages is surprising because variations in the flux of organic carbon to the seabed are relatively small at water depths greater than 2000 m. We have the empirical evidence that this small difference has an important effect on assemblage formation; but the reasons for this are unclear. There is benthic community theory, which explains general ecologic stucture at the seabed, but a specific model for foraminiferal community response to changing organic carbon flux in the deep sea is lacking. Confident use of the benthic foraminiferal bioproductivity proxy, and its application to time periods preceding the late Pleistocene, requires the development of such a model.

RELATIONSHIPS OF MAJOR MESOZOIC MAMMALIAN CLADES

LUO, Zhe-Xi, Section of Vertebrate Paleontology, Carnegie Museum of Natural History, Pittsburgh, PA, USA; Zofia Kielan-Jaworowska, Instytut Paleobiologii PAN, Warszawa, Poland; Richard L. Cifelli, Oklahoma Museum of Natural History, Norman, OK, USA

We propose a phylogeny of 40 lineages of Mesozoic mammals, plus three lineages of non-mammalian cynodonts, based on a parsimony analysis of 255 osteological and dental characters. Mammals (mammaliaforms of some authors) are monophyletic. Within mammals, Sinoconodon is the most primitive taxon; and Sinoconodon, morganucodontids, docodonts, and a new early Jurassic taxon are successively closer to the mammalian crown group. Within the mammalian crown group, there is a division of an australosphenidan clade of Gondwana and a boreosphenidan clade of Laurasia. Within our phylogeny, allotherians (haramiyids and multituberculates), eutriconodonts, australosphenidans, and therians (including boreosphenidans) form a very robust clade. However, there can be two alternative placements for allotherians: either allotherians are sister-taxon to the clade of the stem and living therians by strict consensus of the parsimony analysis; or alternatively and with several additional steps, allotherians are placed outside the mammalian crown group by a topological constraint that is consistent with the traditional emphasis on the uniqueness of allotherian dentition. The difference between the two alternatives in tree statistics is small, given our dataset. There are also two alternative positions of the "eutriconodonts" in the Mesozoic mammal phylogeny, contingent on the placements of other clades such as allotherians, although we favor a monophyletic eutriconodont group nested within the mammalian crown group. Our phylogeny indicates that the "obtuse-triangle" symmetrodonts are paraphyletic, and they lack any unambiguous synapomorphies.

A NEW, PRIMITIVE GENUS OF SEA TURTLE FROM THE MIOCENE OF THE CENTRAL VALLEY OF CALIFORNIA

LYNCH, Shannon C., and James Ford Parham, Dept. of Integrative Biology, University of California, Berkeley, CA, USA

Although the history of California's sea turtles extends into the Late Cretaceous, specimens are extremely fragmentary and rare. An important exception is material from the middle Miocene (Barstovian NALMA) Sharktooth Hill locality, Round Mountain Silt Formation. in Kern County, California. In addition to bearing numerous enormous leatherback (Dermochelyidae) fossils, Sharktooth Hill has yielded a handful of hard-shelled sea turtle (Cheloniidae) specimens.

Here we present a new member of the Cheloniidae that is unlike all other Neogene cheloniids, but bears a striking resemblance to Erquelinnesia from the Eocene of Europe. The material of this middle Miocene turtle includes a curiously primitive femur, humerus and cranium, and several lower jaws. In order to understand its phylogenetic relationship with other members of Cheloniidae, cranial and postcranial characters were cladistically examined. Based on our analysis, this new turtle is the last surviving member of a clade of durophagous cheloniids that originated in the Late Cretaceous and, until now, was thought to have gone extinct in the early Eocene.