NAPC 2001

Abstracts, Fe - Fre
(5/17/01)

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PRELIMINARY RESULTS OF FORAMINIFERAL STUDIES FROM THE LATE PLEISTOCENE AND HOLOCENE SEDIMENTS FROM THE CHUKCHI BORDERLAND, ARCTIC OCEAN

FEBO, Lawrence A., and Leonid Polyak, Byrd Polar Research Center, The Ohio State University, Columbus, OH, USA

Several piston and box cores from a shelf-to-basin transect are being investigated for paleoceanographic reconstructions as part of the Shelf-Basin Interaction Program. Reported here are results of foraminiferal studies from two sites on the Chukchi slope and adjacent Northwind basin.

Foraminifera were sampled every centimeter and counted in the 63-125 µm, 125-250 µm, and >250 µm size fractions. Piston core P-6 and box cores B4-B5 were taken on the Chukchi slope (400­600 mwd) and show a limited foraminiferal record. Foraminifera are low in numbers in the Holocene sediments of P-6, presumably due to strong CaCO₃ dissolution. However, sediments from the penultimate interglacial, tentatively correlated to Oxygen Isotope Stage 3, contain significant numbers of foraminifers. Planktic foraminifers are represented by Neogloboquadrina pachyderma (left coiling). Prevailing benthic species are Cassidulina teretis and Eponides tener. High contents of C. teretis in the Arctic Ocean are shown to be associated with the Intermediate Waters of Atlantic origin.

Box core B15 from the Northwind basin (2100 mwd) contains abundant foraminifers in Holocene sediments, which, in combination with dinocysts, allow us to better characterize the recent environments in the Chukchi Borderland. Three ¹⁴C ages were used to construct an age model. According to these ages and the lithostratigraphy, the core spans most of the Holocene and the last deglaciation. Holocene sedimentation rates are ~1.6 cm/ka—higher than most previously studied cores in the deep western Arctic Ocean. Foraminifers are nearly absent in the deglacial sediments and increase in abundance starting at ~11 ka. Two abundance peaks in planktic and benthic foraminifers occur between 6­7 ka and ~4 ka. Dominant benthic species include Stetsonia horvathi, E. tener, Epistominella tumidulus horvathi, and Cibicidoides wuellerstorfi. The combined changes in foraminiferal and dinocyst abundances and compositions provide a first record of biological productivity and sea-ice extent over the Chukchi Borderland during the Holocene.

FAUNAL SUCCESSION IN THE VENDIAN (TERMINAL PROTEROZOIC) DEPOSITS OF THE WHITE SEA REGION, NORTH OF THE RUSSIAN PLATFORM

FEDONKIN, Mikhail A., and Andrei Yu. Ivantsov, Paleontological Institute, Russian Academy of Sciences, Moscow, Russia

Siliciclastic deposits of the Vendian (Terminal Proterozoic System) exposed in the White Sea region seem to contain one of the most complete fossil record of the oldest metazoans. Faunal elements known separately from the Terminal Proterozoic of the Newfoundland, Great Britain, Namibia, South Australia, Urals, Arctic Siberia and other regions are discovered here in a succession. In addition, endemic invertebrates of complex morphology are discovered as well. Observations of fundamental importance concerning the time-space distribution of the Vendian fauna in the White Sea region are made: (a) growth of the taxonomic diversity of the body fossils and trace fossils upward the succession, (b) stage pattern in the biodiversity growth, (c) variety of the species' time ranges, (d) historical change in the paleofaunistic connection of the paleobasin, and (e) disappearance of the metazoan body fossils and bioturbations in the sediments of the brackish paleobasins. These facts may reflect the oldest known metazoan diversification in the marine habitats. At least six faunal assemblages named after the dominating fossil are identified in a sequence: Calyptrina-Beltanelloides, Ventogyrus, Inaria, Pteridinium, Charnia, Yorgia, Dickinsonia lissa. Wide geographic distribution of these fossil taxa opens the way to the globally correlatable biostratigraphic units (biozones and stages) to be established. A uranium-led zircon age 555.3 Ma for a volcanic ash by the top of the Charnia fossil assemblage in the sea cliffs of Zymnie Gory indicates a minimum age of the triploblastic metazoans because the bilaterian body fossils and trace fossils occur stratigraphically below. Biodiversity dynamics seem to have not any direct connection to the carbon isotope excursions in the Vendian ocean.

FOSSIL DECAPOD CRUSTACEANS IN THE HIGH LATITUDES: PIONEERS AND SURVIVORS

FELDMANN, Rodney M., Dept. of Geology, Kent State University, Kent, OH, USA; and Carrie E. Schweitzer, Stark Regional Campus, Kent State University, Canton, OH, USA

Recent studies of paleobiogeographic patterns of decapod crustaceans in high latitudes strengthens earlier observations, based upon a limited number of taxa, that the high latitudes were important sites of origin of families and genera. Ongoing collection of decapods from both hemispheres reveals that the pattern is much more robust than previously thought. As many as 42 genera of lobsters, mud shrimp, and crabs have their earliest geologic occurrence in the high latitudes in the time interval from the Cretaceous to the Eocene. Several of these taxa are among the longest-ranging decapods and are represented by extant species. Geographic and ecologic patterns of most genera reflect diverse dispersal and adaptive patterns. High latitude, bipolar, amphitropical, and global distribution patterns are documented, suggesting that decapods are plastic with regard to latitudinal conditions; photoperiod and water temperature per se do not appear to limit their geographic distribution. However, general absence of decapods in post-Eocene Antarctic settings and their low diversity in high northern latitudes suggests that shallow water taxa may be excluded from areas characterized by low water temperature coupled with low seasonal temperature fluctuation. Among taxa known to have high latitude origins, water depth and substrate preference of many taxa have changed from shallow water, coarse to medium grained substrates to deeper water, fine grained substrates; the reverse pattern has not been observed. Nearly all are confined to normal marine salinities and none has given rise to fresh water or non-marine descendants. The geologic record of high-latitude fossil decapod crustaceans is now sufficiently robust to serve as a test against distributional patterns of more frequently studied megainvertebrates. Distributional patterns corroborate those illustrated by other groups but the ecological patterns suggest that decapod genera may be more versatile than those in groups such as the mollusks, echinoderms, and bryozoans.

HEMIAUCHENIA, A HIGH-CROWNED BROWSER FROM FLORIDA

FERANEC, Robert S., Dept. of Integrative Biology, University of California, Berkeley, CA, USA

Previous studies analyzing the hypsodonty of the genus Hemiauchenia suggest, due to its high-crowned teeth, that it was a grazer—a feeder of monocots, while other studies based on muzzle width suggest that it was a browser or mixed-feeder—a feeder of dicotyledons or both dicotyledons and monocotyledons. The analysis of stable carbon isotope values from the tooth enamel of Hemiauchenia provides another method to determine feeding strategy. Florida provides an ideal location to conduct a study on herbivore feeding strategy because unlike other regions of the country almost all the present-day dicotyledons use the C₃ photosynthetic pathway, while most of the monocots use the C₄ photosynthetic pathway. The modern flora would therefore suggest that if Hemiauchenia were a grazer the stable carbon isotope values of tooth enamel should reflect C₄ monocot feeding (>-2.0). If Hemiauchenia were a browser, the isotopic value should reflect ingestion of C₃ dicots (<-8.0). During glacial periods, climate and vegetation models suggest further dominance of C₄ monocots, which would result in similar stable isotope values as expected for the modern flora. The d¹³C values for Hemiauchenia from the Blancan through the Rancholabrean in Florida average more negative than -8.0 , a C³ dicotyledon diet. This study, which extends from the late Tertiary through the Quaternary, suggests that Hemiauchenia was predominantly a high-crowned browser over the past four million years. Because Hemiauchenia is high-crowned and not a grazer, hypsodonty may not be an appropriate measure of feeding strategy or habitat type in all circumstances.

THE RELATIONSHIP BETWEEN DIVERSITY AND ABUNDANCE: AN EXAMPLE FROM THE LOWER-MIDDLE ORDOVICIAN BOUNDARY OF THE GREAT BASIN

FINNEGAN, Seth, and Mary L. Droser, Dept. of Earth Sciences, University of California, Riverside, CA, USA

The traditional focus of paleobiological and paleoecological research has been on issues of diversity. The study of diversity trends provides essential insights into patterns and processes of ecological and evolutionary change. However, diversity is only one aspect of ecological systems. An in-depth understanding of paleoecological change also requires data on depositional environments, biogeography, and abundance. Diversity and abundance may be decoupled at a variety of temporal, taxonomic, and geographic scales. Communities with low diversity but high abundance are well documented as characteristic colonizers of disturbed environments, and underline the importance of considering abundance and diversity separately. Analyses of abundance data may lead to recognition of patterns of ecosystem transformation not apparent from taxonomic diversity data alone. As the previous example illustrates, this may be particularly true during so-called "critical intervals." One such interval is the Lower-Middle Ordovician boundary. A growing body of evidence suggests that the sweeping evolutionary and ecological changes of the Ordovician radiations are disproportionately concentrated around this interval. Preliminary data from the boundary interval of the Great Basin indicate that there is a significant disconnect between diversity and abundance of brachiopods at this time. A dramatic increase in the abundance of brachiopods is seen at the Lower-Middle Ordovician boundary in a variety of environments. This increase in abundance does not correspond with Ordovician patterns of brachiopod diversification, and represents an important step towards the establishment of Paleozoic ecosystem types. There is also evidence, though controversial, of a converse pattern of decoupled diversity and abundance among trilobites. While they undergo a major radiation at the base of the Middle Ordovician, the abundance of trilobite material declines significantly in studied sections. More study is needed to confirm the geographic and temporal extent of this trend.

DINOSAURIA AND ASSOCIATED VERTEBRATE FAUNA OF THE LATE CRETACEOUS CHRONISTER SITE OF
MISSOURI

FIX, Michael F., Dept. of Geology, St. Louis Community College at Florissant Valley, and Dept. of Physics, University of Missouri St. Louis, St. Louis, MO, USA; and Guy Darrough, Missouri Ozark Dinosaur Project Inc., Arnold, MO, USA

The late Cretaceous (Campanian) Chronister site of Southeastern Missouri stands unique as the only known locality in the state containing Mesozoic terrestrial vertebrate remains. Fossils occur within a clay that shows considerable soft sediment deformation due to the close proximity of a normal fault, on whose downthrown block it has been preserved.

The environment of deposition as indicated by faunal and stratigraphic evidence, is a body of fresh to brackish water that was at least occasionally connected to the Gulf of Mexico, which was probably no more than 20 kilometers away. Allocthonous boulders of Paleozoic rocks in the clay are probably associated with faulting, but evidence suggests that parts of the deposit may also represent debris flows.

Abundance of bone is highly variable, but in some parts of the deposit is high enough to constitute a "bone bed." Many bones have been damaged by stresses that accompanied deposition, tectonic sediment deformation, and some are crushed, which could be due to compaction or to bioturbation. Some bones show evidence of predation or scavenging.

The fauna includes three types of dinosaur that have been positively identified: Hypsibema missouriensis, which is considered to be a hadrosaur of uncertain affinities; an undetermined genus of tyrannosaurid; and an undetermined genus of dromaeosaurid. More problematic is the assignment of a single phalanx to an undetermined genus of ornithomimid. The deposit also contains aquatic vertebrates: crocodilians, Leidyosuchus sp.; turtles (the most common faunal element), Adocus punctatus and Naomichelys speciosa; a sirenid amphibian, Habrosaurus sp.; and several fish, including Lepisosteus sp. (gar), an amiid, (bowfin), and a sparid (sheepshead/porgies). Current excavation by the Missouri Ozark Dinosaur Project Inc., is being conducted under an enclosure to keep out water, and utilizes a 60 square meter hanging grid to facilitate detailed mapping and taphonomic record keeping.

THE FATE AND FUTURE OF THE COLORADO DELTA CLAM: PALEOECOLOGICAL EVIDENCE FOR ENDANGERMENT, PALEOECOLOGICAL GUIDELINES FOR RECOVERY

FLESSA, Karl W., Carlie R. Rodriguez, and David L. Dettman, Dept. of Geosciences, University of Arizona, Tucson, AZ, USA; Miguel A. Tellez-Duarte and Guillermo A. Avila-Serrano, Facultad de Ciencias Marinas, Universidad Autonoma de Baja California, Ensenada, BC, México; and Sarah Noggle, Dept. of Geosciences, University of Arizona, Tucson, AZ, USA

Paleoecological evidence can be used to identify the cause of species endangerment. Before dams and diversions on the Colorado River, the mactrid bivalve Mulinia coloradoensis was the most abundant mollusk inhabiting the tidal flats of the Colorado Delta. More than 90% of the ~2 trillion shells that make up the islands of the delta are from this species. Recent surveys of the delta's living fauna encountered only twelve specimens of M. coloradoensis—less than 1% of the shelly individuals found.

The d¹⁸O values in shells of pre-dam M. coloradoensisare significantly more negative than d¹⁸O values in live-collected shells of Chione cortezi, the most common bivalve living on the delta today. This indicates that M. coloradoensis lived in waters much less saline than found today and strongly implicates the diversion of Colorado River flow as the cause of the population crash of the species.

Paleoecological evidence can also be used to prescribe restoration efforts. Although complete resumption of the Colorado River's flow to its delta would likely restore the population of this species, this simple prescription is politically unrealistic: all of the Colorado River's water is now diverted for agricultural or domestic uses in the US and Mexico. Partial restoration might be feasible if upstream water users could agree on an "allocation for nature." What percentage of the river's flow would be needed to restore only a part of the population of Mulinia coloradoensis? We examined d¹⁸O values in shells from Las Isletas, 50 km south of the river's mouth, where a viable population existed despite near-complete mixing of river water and normal Gulf water. Assuming that this population was a source rather than a sink, the d¹⁸O values can be used to calculate the necessary proportion of river water and the required flow in acre-feet per year.

TESTING CLOCKS: ASSESSING MORPHOLOGICAL, MOLECULAR, AND PALEONTOLOGICAL DATA ON THE TIMING AND RATE OF EVOLUTION IN MAMMALS

FLYNN, John J., and Sarah M. Zehr, Dept. of Geology, The Field Museum, Chicago, IL, USA

Dramatic differences of opinion exist regarding the effects of rates of character evolution on both phylogeny reconstruction and estimating clade ages/divergence times, and there remain significant conflicts between molecular and morphological/fossil record inferences. Several lines of evidence contradict recent molecular clock suggestions of 50­100% greater ages (than derived from the fossil record) for the mammalian orders, although it is clear that much remains to be done in assessing the cause of the conflict between these data.

Within Mammalia, the Carnivora are an excellent group for establishing pattern, tempo and mode of evolution. Our analyses indicate that multiple lines of evidence (nucleotide sequences, morphology, fossil record) generally yield congruent phylogenetic results, and combining them can greatly increase robustness and resolution. But phylogeny-based analyses of change in molecules indicate wide variance within genes and across taxa. To test for rate heterogeneity we applied several "relative" and "absolute" (calibrated by multiple fossil-constrained divergence ages within the Carnivora) rate tests. Likelihood ratio tests on individual molecular data sets (mitochondrial cyt b, ND2, ND3; nuclear c-myc,TBG and transthyretin intron) showed significant rate heterogeneity (p <0.001) in every one. To focus on which taxa may be responsible for the rate heterogeneity, Tajima's (1993) 1D relative rate test was performed for all pairwise comparisons of carnivoran taxa using several outgroups—these preliminary tests, and those of Flynn (1996), suggest there is rate heterogeneity among many carnivoran lineages for all of these molecular data sets. Fossil-constrained "absolute" rate calculations indicate up to 1.5 orders of magnitude differences in nucleotide changes/time across the carnivoran radiation. This may be a general pattern in many groups, but it has been documented for only a few, and may be of significance for interpreting molecular clock "inverse calculations" of divergence ages from nucleotide differences among taxa.

USING TAPHONOMY TO DETERMINE PALEOLOGICAL PARAMETERS OF THE CHRONISTER LOCAL FAUNA, BOLLINGER COUNTY, MISSOURI

FORIR, Matthew, Dept. of Geology, St. Louis Community College, St. Louis, MO, USA

The Chronister site yields a localized concentration of Late Mesozoic (continental Upper Cretaceous) vertebrate fossils in the southeastern Ozarks of Missouri. The site is isolated from marine Upper Cretaceous strata of the Gulf Embayment 20 km to the south. It is considered to be a localized pocket of continental Late Cretaceous clay preserved in a down dropped block associated with a structurally complex area near Glen Allen, Missouri. Believed to be Campanian in age from the similarity of its hadrosaurian material with that of New Jersey, the locality is the original site of Missouri's first dinosaur, Neosaurus missourensis Gilmore and Stewart, 1945. Taphonomic observations on the site, interpret fracturing of concentrated dinosaur bone as green bone fracturing taking place at a Cretaceous watering hole. Dinosaur bone and chelonian shell showing evidence of predation in the form of puncture marks also reinforces the watering hole model. Extensive cherty layers associated with the bone bearing clays, also exhibit what is interpreted as compression fracturing of many of the chert cobbles, possibly as a consequence of the migration of a large number of herbivores. The Chronister site has been difficult to work not only because of the presence of extensive overburden but also because associated bone material is highly disarticulated and fragmented, again suggestive of activity associated with the watering hole model. Incorporated in with the bone bearing clays are anomalous boulders and residual chert derived from mid Paleozoic formations that are otherwise absent from the area. Such boulders, some of which are over one meter in diameter, are believed to have fallen or slid from cliffs which flanked the watering hole during the Cretaceous but which today have been removed by erosion.

BIOSTRATIGRAPHY OF THE ENTELODONTIDAE (MAMMALIA: ARTIODACTYLA) FROM THE JOHN DAY BASIN, OREGON

FOSS, Scott E., and Theodore J. Fremd, John Day Fossil Beds National Monument, Kimberly, OR, USA

The temporally continuous units of the John Day Formation form an ideal basis for biostratigraphic investigations. Entelodont fossils have been discovered in all four currently recognized members of the John Day Formation (Big Basin, Turtle Cove, Kimberly, and Haystack Valley). Datable tuffs are present in all four members. The Turtle Cove and Kimberly members are further subdivided into thirteen recognizable subunits (units A­M), each of which contain at least one, but usually numerous, datable primary ash-fall tuff layers.

At least three valid species of entelodonts (Archaeotherium caninus, A. calkinsi, Daeodon shoshonensis) have been recovered from the John Day Formation that can be correlated to these subunits, and which, therefore, may be dated with accuracy. Unlike other depositional regions of North America, the temporal record of entelodonts is relatively complete in the John Day. It is possible that there were multiple faunal exchange events between Asia and North America, and that the John Day species represent a continuous record of entelodont habitation in North America from at least the early Oligocene until the earliest Hemingfordian. The biostratigraphic framework established in the John Day allows the direct correlation of entelodont taxa across North America and allows a more complete calibration of entelodont evolution and biogeography world-wide.

CARBON ISOTOPES AND HIGH C₄ BIOMASS DURING THE MIOCENE IN THE GREAT PLAINS, USA

FOX, David L., and Paul L. Koch, Dept. of Earth Sciences, University of California, Santa Cruz, CA, USA

Changes in mammalian faunas and stable isotope records during the Miocene suggest global reorganization of terrestrial ecosystems, with increased abundance of C₄ plants (tropical grasses) relative to C₃ plants (trees, shrubs, cool climate grasses) after 8 Ma. In North America, middle Miocene faunas dominated by browsing herbivores were replaced by late Miocene faunas dominated by grazers, and species diversity decreased. Because C₃ and C₄ plants have distinct carbon isotope signatures, the isotopic composition of tooth enamel carbonate of mammalian herbivores indicates the proportion of these plants in the diet. The oldest unequivocal isotopic evidence from enamel for C₄ consumption in North America is at about 7 Ma; older mammalian herbivores had C₃-dominated diets and no species appear to have had specialized C₃ or C₄ diets. To investigate further the changes in North America during the Miocene, we measured the carbon isotope composition of pedogenic carbonate from the Great Plains. The carbon isotope composition of pedogenic carbonate reflects the mix of C₃ and C₄ plants that grew in the soil, providing an integrated signature of the entire local flora. To date we have analyzed 179 samples from seven formations in Nebraska, northwestern New Mexico, and western Texas that range in age from about 25 to 6 Ma. Mean composition of these nodules is -6.7 ± 0.70 permil. Assuming modern atmospheric composition and a +15 permil fractionation between plant biomass and pedogenic carbonate, C₃ and C₄ carbonate end members would be -11 and +3 permil, respectively. Thus, our data imply the persistent presence of up to 40% C₄ biomass throughout the study interval. This pattern contrasts with paleosol records from Pakistan and South America, ruling out secular changes in atmospheric composition or C₃ fractionation as explanations. Given that no mammals had specialized diets until after 7 Ma despite the apparent abundance of C₄ plants, our results suggest the possibility that ecological changes in North American faunas were not driven exclusively by changes in habitat.

EARLY TRIASSIC PROLIFERATION OF MICROGASTROPODS: AN UNUSUAL GLOBAL PHENOMENON

FRAISER, Margaret L., Richard J. Twitchett, and David J. Bottjer, Dept. of Earth Sciences, University of Southern California, Los Angeles, CA, USA

Estimated at nearly 5­10 million years long, the Early Triassic (Scythian) represents the biotic recovery interval following the most devastating biotic crisis in the history of life, the end-Permian mass extinction. This biotic recovery interval is the longest following any mass extinction and was a period of low-diversity, simply structured communities. Early Triassic invertebrate marine faunas have been characterized as morphologically simple, opportunistic, unspecialized, and cosmopolitan. Previously, many primarily qualitative studies have indicated that Lower Triassic strata are typically dominated by only a few species of bivalves and tiny snails. In particular, Lower Triassic shallow-water carbonates composed nearly entirely of microgastropods are a prominent component of Tethyan strata. This research is the first quantitative study to disclose how truly unique microgastropod-bearing rocks are to the Early Triassic. Field work in the western US, Japan and Europe has revealed that microgastropods are commonly the primary rock-forming allochems in tropical paleolatitude Lower Triassic outcrops. Similarly, analysis of the literature reveals references to Lower Triassic carbonates "rich in microgastropods" or containing "abundant small snails" from every ocean and for every stage of the Early Triassic. The rarity of microgastropod-bearing limestones in pre-extinction Permian strata and Middle Triassic strata demonstrates that the global occurrence of microgastropod-dominated carbonates in the Early Triassic is indeed an unusual phenomenon. One possible explanation for this phenomenon is that microgastropods behaved as "biotic recovery opportunists" during the Early Triassic and thrived in high-stress, low resource shallow-water carbonate environments during the aftermath of the end-Permian mass extinction. The dwarfed size of Early Triassic gastropods could largely be due to the effects of environmental stress such as decreased primary productivity during the Early Triassic.

THE BEGINNING OF MESOZOIC BIVALVE REEFS: EARLY JURASSIC "LITHIOTIS" FACIES BIOHERMS

FRASER, Nicole M., and David J. Bottjer, Dept. of Earth Sciences, University of Southern California, Los Angeles, CA, USA

Prior to the Cretaceous reef-building rudists, the Early Jurassic is notable for bioherms constructed by a unique group of large aberrant bivalves that collectively form the "Lithiotis" facies: Lithiotis, Gervelleioperna sp., Lithioperna sp., Cochlearites and Mytiloperna. These bivalves dominated nearshore tropical ecosystems of the Early Jurassic (Pliensbachian) and abruptly went extinct at the end of the Early Jurassic (Toarcian). The dearth of reefs constructed by colonial organisms and the preponderance of filter-feeding bivalves in nearshore tropical environments is a common feature of later Mesozoic environments, beginning in the Early Jurassic and culminating in the large Late Cretaceous rudist reefs. "Lithiotis" facies bioherms in Oregon, California, Italy and Morocco have been used in this study. Line-intercept transects, bulk sample collection and species identification have been completed for each site. Original aragonite was preserved in some Italian specimens; d¹⁸O, d¹³C and Mg/Ca analyses are currently being performed on these specimens to quantitatively assess paleoenvironment. While most of the "Lithiotis" bioherms in Oregon and California are relatively low-diversity with minor constituents of nereinid gastropods and red algae, the Moroccan and some Italian sites exhibit higher diversity with corals, sponges and brachiopods. The increased presence of bivalve-constructed reefs in the Early Jurassic could have resulted from increased nutrient delivery to the shelves. The consequent increased rates of bioerosion and algal growth allowed filter-feeding bivalves to outcompete stenohaline groups such as scleractinians. An alternate hypothesis for the occurrence of bivalve constructed bioherms is that these organisms are adapted to euryhaline environments. While this explanation may be appropriate for the Oregon and some of the Italian sites, it is inadequate to explain the occurrence of "Lithiotis" facies bivalves with stenohaline organisms such as corals and sponges in Morocco and other Italian sites.

SPECIES-LEVEL EVOLUTIONARY PATTERNS IN NEOGENE STROMBIDS FROM THE DOMINICAN REPUBLIC

FREIHEIT, Jim, and Dana H. Geary, Dept. of Geology and Geophysics, University of Wisconsin, Madison, WI, USA

Due to their determinate growth patterns, gastropods of the family Strombidae are particularly suitable for evolutionary and ecological studies involving size-related trends of morphological change. Geary (1991) made an intriguing observation regarding strombids of the West Atlantic; starting in the Pliocene, they seem to have gotten bigger. This observation applies both within species (larger average adult size) and to the genus Strombus as a whole (smaller species go extinct; new species are larger than the former average). This implies widespread selective pressure against small size but the details of such size-related selection remain elusive.

We have examined a cluster of morphologically intergrading strombid species from the Mio-Pliocene of the Cibao Valley in the Dominican Republic in order to establish both the nature of the relationships between these species and what patterns of morphologic change they exhibit through time. End members of this group are the species Strombus bifrons and Strombus proximus. Morphometric analysis reveals a pattern of increasing size and robustness through time for individuals corresponding most closely to S. proximus. Preliminary results suggest a similar trend for S. bifrons. Future research will focus on unambiguously establishing this trend and on clarifying the evolutionary relationships between these groups.

APPRAISING THE SIGNIFICANCE OF COMPLEX ASSEMBLAGES AND DATA DEFRAGMENTATION: AN EXAMPLE FROM THE VOLCANICLASTICS OF EASTERN OREGON

FREMD, Theodore J., John Day Fossil Beds National Monument, Kimberly, OR, USA

Many stratigraphically complex basins are globally significant in terms of local preservation of long-term processes and events. These are qualitatively characterized by sequences of well-preserved biotas found in association with isochronous index strata that can be correlated with deposits elsewhere. Comparative assessment of the significant features of data-rich basins demonstrates that among such classic Tertiary settings as the Siwaliks and the Bighorn Basin is the lesser-known John Day Basin.

The remarkable temporal continuity of the Tertiary deposits in eastern Oregon was first recognized one hundred years ago by J.C. Merriam of the University of California, Berkeley. However, it was only after Merriam's development of an interdisciplinary group known as, "The John Day Associates," that it was realized that no region in the world shows more complete sequences of Tertiary land populations, both plant and animal, than the John Day Formations. We now realize that these series of strata are exceptional not only in terms of temporal continuity, but in that they also contain multiple, diverse localities formed in laterally variable intrabasin depositional environments, which allows comparison of time-equivalent paleobiomes.

Basins like the John Day are warehouses of stratigraphic information, indispensable for accurate phylogenetic and ecologic studies. Many of the data sets needed for complete analyses and comparative study of such areas are fragmented, however, as a result of non-integration of varied disciplines into a cohesive research framework, unsystematic institutional data storage, and overlooked inter-basin relationships. As a result, there are misconstructions of the quality of the fossil record appearing in overall tabulations, or a failure to recognize correlative units proximal to basin margins. Defragmentation can be achieved for long-term, interdisciplinary analysis of these basins and their margins when paleontological studies, data collection, and storage are performed with the big picture in mind.