NAPC 2001

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(5/22/01)

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TAPHONOMIC BIAS AND INSECT DIVERSITY: A LESSON FROM THE BEETLES AND FLIES

SMITH, Dena M., CU Museum of Natural History, University of Colorado, Boulder, CO, USA

Labandeira and Sepkoski's 1993 work on the fossil record of insects was very important because it showed that insects have an extensive fossil record and that this record could be used to examine both paleontolgical and neontological hypotheses. However, exceptionally preserved, insect-rich assemblages may have had a great influence on this diversity curve. For example, large peaks in diversity in the Late Jurassic and in the Early Cenozoic may be attributable to the fossil rich Karatau assemblage and the Baltic Amber. How important is insect taphonomy in determining patterns of insect diversity over time? To answer this question, I reanalyzed the coleoptera (beetles) and the diptera (flies). Both of these insect orders are diverse and abundant today, they have a long fossil record (dating to the Permian), and they are also ecologically diverse. However, their distinct morphological features make them susceptible to different taphonomic biases, providing an opportunity to look at how differing preservational bias can affect within and among group diversity patterns.

Here, I present updated diversity data for the diptera and coleoptera and examine the type and number of depositional environments represented in each time interval, the condition of described taxa, and the change in species richness within deposits over time. Large increases in insect diversity during the early Cenozoic do appear to be artifactual as Labandeira and Sepkoski suggested. Not only are there more types of depositional environments during this time interval, but the quality of these deposits in terms of insect preservation are also exceptional. Differing preservational bias between the diptera and coleoptera seems to most strongly affect the condition of specimens and not long-term diversity patterns. Conducting future analyses that only examine equivalent depositional environments through time may help to give a less taphonomically biased picture of insect diversity trends over time.

FOSSIL PHYTOLITH CARBON ISOTOPE RECORDS OF GRASS PHOTOSYNTHETIC PATHWAYS

SMITH, Francesca A., Dept. of Geophysical Sciences, University of Chicago, Chicago, IL, USA

The development of grasslands ecosystems in the Tertiary provides a context for current and future responses of grasses and crops to anthropogenic climate and atmospheric changes. Although a variety of methods have been used to characterize past grasslands (i.e., paleobotanical records, vertebrate functional morphology, paleosol characteristics, herbivore tooth enamel d¹³C, and soil carbonate d¹³C), none of these proxies capture a record of the photosynthetic pathway (C₃ or C₄ of grasses specifically), as distinct from overall vegetation. Carbon isotope ratios of fossil grass phytoliths (silica bodies produced primarily by grasses), on the other hand, provide a direct record of grass photosynthetic pathway and therefore can detect proportions of C₃ and C₄ grasses. In order to examine changes in C₃/C₄ of grasses in the Great Plains during the Tertiary, phytoliths were extracted from paleosols from Kansas and Nebraska and the occluded carbon was combusted and analyzed isotopically. In contrast to the tooth enamel d¹³C record that shows no significant C₄ component prior to 7 Ma (Wang et al., 1994; Latorre et al., 1997), the d¹³C of phytoliths suggest a significant contribution from C₄ grasses prior to 7 Ma, potentially as far back as the Early Oligocene. The phytolith d¹³C reflects an expansion of C₄ grasses at about 7 Ma, similar to the shift observed in the tooth enamel d¹³C record. Therefore, the change from a C₃ to a C₄ diet observed in the tooth enamel could have been caused entirely by a shift in the photosynthetic pathway of the grasses, rather than the previously proposed shift from a C₃ woodland to a C₄ grassland (Wang et al., 1994). After the expansion of C₄ grasses seen at about 7 Ma, phytolith d¹³C shifts back toward C₃ values in the Early Pliocene. Potential drivers for these shifts are temperature, seasonality of precipitation and atmospheric CO₂/O₂ ratios. Given paleo-pCO₂ reconstructions indicating little change throughout the Neogene, the favored drivers are primarily climatic.

DISCOVERY OF A CRANIUM OF STYLEMYS (REPTILIA: CHELONIA) FROM THE TURTLE COVE MEMBER OF THE JOHN DAY FORMATION, CENTRAL OREGON

SMITH, Matthew E., and Theodore J. Fremd, John Day Fossil Beds National Monument, Kimberly, OR, USA; and Roger C. Wood, Stockton State College, Pomona, NJ, USA

Although turtle remains are distributed throughout much of the 45 million year sequence represented by the deposits within the John Day Basin, almost all specimens in existing collections are represented only by shell material or limb elements. Particularly vexing is the lack of association of any diagnostic skull material with these typically low-arched carapaces. Previously, Stylemys from the John Day Formation has been described based solely on shell morphology due to a lack of cranial elements.

Here, we report the discovery of a nearly complete cranium of a tortoise referable to Stylemys (Leidy, 1851) in situ with several carapace fragments. This is the first reported occurrence of any chelonian skull material associated with postcranial elements from the John Day Formation. The stratigraphic position can be pinpointed with precision within the K1 unit of the Turtle Cove Member, approximately 8m above the Deep Creek Tuff recently dated to 27.5 ma. Little has been published on fossil turtles from the Northwest since the work of Hay and Gilmore's work in the early 1900s. The presence of additional material within a definable pedofacies suggests a unique opportunity to provide additional paleoecologic information, as well as population and taphonomic data, about the occurrences of these relatively abundant but rather poorly understood tortoises.

FURTHER INVESTIGATIONS OF AROID SEEDS FROM THE PRINCETON CHERT

SMITH, Selena Y., and Ruth A. Stockey, Dept. of Biological Sciences, University of Alberta, Edmonton, AB, Canada

Large numbers of fossil seeds of Keratosperma allenbyensis Cevallos-Ferriz & Stockey have been identified in the Middle Eocene (Allenby Formation) Princeton chert of British Columbia. These seeds have been previously assigned to the Araceae, Subfamily Lasioideae based on anatomical characters. Over 200 new specimens have been examined using the cellulose acetate peel technique modified for hydrofluoric acid. Seeds are campylotropous and have three rows of spiny ridges on the seed coat. Idioblasts that probably contained raphides are found scattered in the outer integument. The micropylar end of the seed has a thin-walled micropylar cover that was easily displaced at the time of germination. A prominent hypostase and podium are present at the chalazal end of the seed and an epistase has been observed at the micropylar end. Endosperm is present in these seeds and contains dark contents. Monocotyledonary embryos have been reported but most seed cavities show tissues that contain large numbers of fungal hyphae that replace embryo tissues. Anatomical comparisons with the extant lasioids Cyrtosperma ferox Linden et N. E. Br. and Urospatha sagittifolia (Rodsch.) Schott were made using paraffin and cryo embedding and sectioning techniques. A reconstruction of the fossil seeds was made from serial sections allowing the fossils to be compared to extant taxa based on external morphology. Keratosperma allenbyensis are the oldest known lasioid seeds in the fossil record and show closest similarities to Urospatha from Central and South America.

INTEGRATED RESEARCH ON FOSSIL AND EXTANT FLOWERING PLANTS VIA A DESCRIPTIVE DATABASE

SMITH, Una R., Los Alamos National Laboratory, Los Alamos, NM, USA

Although fossils are notoriously incomplete, what matters is not the completeness of fossils per se, but rather the completeness of their descriptions. Research on both fossil and extant taxa requires useful descriptions and, in most cases, more complete descriptions are more useful. Paleobotanists routinely describe fossil plants that are incomplete (e.g., dispersed pollen, leaves, fruits, seeds, and pieces of wood), but often the most difficult challenge is to obtain comparable descriptions of extant plants. To address this challenge, I will demonstrate effective ways to use a descriptive database of all extant flowering plants (Watson, L., and M.J. Dallwitz. 1992 onwards. The Families of Flowering Plants: Descriptions, Illustrations, Identification, and Information Retrieval. Version: 14th December 2000. http://biodiversity.uno.edu/delta/angio/). This enormous database of extant plant descriptions can be used to: (1) guide the painstaking work of identification and description; (2) evaluate the validity and completeness of published descriptions; and (3) test hypotheses of systematic affinity.

ARTIFICIAL EVOLUTION FOR INTERACTIVE ART AND ENTERTAINMENT

SOMMERER, Christa, and Laurent Mignonneau, ATR Media Integration and Communications Research Lab, Kyoto, Japan

The true power of evolution lies in its ability to exploit emergent collective phenomena (Langton, 1996). One of the central concepts in Artificial Life research is to create artificial evolution via artificial selection and to test the possibilities of creating a self-sustaining system with open-ended evolution.

Much inspiration for creating digital life forms on a computer screen has been provided by Dawkin's "The Blind Watchmaker" Evolution Simulation Software published in 1986. Reynold's work in 1987 simulating the flocking behaviour of artificial birds was another milestone in establishing the idea of applying artificial life principles to computer graphics software. Ray's "Tierra" evolution simulator, created in 1991, finally brought the possibilities of software evolution to larger attention. But it was the emergence of advanced computer graphics technologies in the early 1990s that helped artists, like Sims (1991) to study the visual creation process itself.

Since 1994, we've applied artificial life principles such as artificial genetics, mutation, and evolution to interactive art works. Our systems include "A-Volve" (Sommerer et al., 1994), an interactive installation where users can create and interact with artificial life creatures, that live and evolve in a water-filled glass pool. Our "Life Spacies" system (Sommerer et al., 1997) consists of a web page where users type text messages to create Alife creatures. Here, written text is used as the genetic code to shape the creatures' design and determine their behaviours and interactions. Our latest system is called "IKI-IKI Phone" (Sommerer, 2001), a large-scale multi-user Alife art game for mobile phones. In this system networked users can create, evolve and exchange Alife creatures using their i-mode-enabled mobile phones.

More information is available at www.mic.atr.co.jp/~christa/.

LOWER CARBONIFEROUS (OSAGEAN) INVERTEBRATES FROM MEXICO: PALEOGEOGRAPHIC IMPLICATIONS

SOUR-TOVAR, Francisco, Sara A. Quiroz-Barroso, and Daniel Navarro-Santillán, Facultad de Ciencias, UNAM, Ciudad Universitaria, Coyoacan, México

Invertebrate fossil assemblages from the Lower Carboniferous of Tamaulipas and Oaxaca, states of East and Southeast Mexico, are typical of marine environments associated with continental margins. Among the diverse invertebrates found, it is noteworthy that a very high percentage of the species described have also been reported as typical of several localities of the Mid-Continent Paleoprovince. This similarity extends to previously described Pennsylvanian faunas from Oaxaca, indicating that during most of the Carboniferous this Paleoprovince extended from the central-east region of the United States to at least the region now occupied by Oaxaca State, Southeast Mexico. Among the Osagean species that allowed the establishment of the former relationship, and which have been described for Oaxaca, we find the rostroconch Pseudomulceodens cancellatus associated to the binomial of brachiopods Actinoconchus lamellosus-Torynifer pseudolineatus and several species of genus Syringothiris. This association is also found in Tamaulipas, where brachiopod diversity is greater, and includes the presence of Barroisella sp. Orbiculoidea sp., Rugosochonetes multicostatus, Rotaia subtrigona, Camarophorella sp., Cleiothyridina tenuilineata and Beecheria chouteauensis. Several ideas recently proposed relate to the origin of tectonic terranes that formed east and southeast Mexico, and it is believed that there are exotic terranes, possibly derived from Gondwana and of Proterozoic or Early Paleozoic age. Although the timing of this North American accretionary event is still under discussion, faunal similarities suggest that the coupling of North America and these terranes in the area studied, was a pre-Mississippian or Early Mississippian event.

NEW OCCURRENCES OF COPROLITES FROM THE ARIRIPE BASIN (UPPER JURASSIC AND LOWER CRETACEOUS, BRAZIL)

SOUTO, Paulo R.F., Departo. de Geologia, Inst. de Geologia, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil

The aim of this study is to provide new paloentological information on coprolites collected from different areas of the Araripe Plateau region, northeastern Brazil. The material consists of samples collected from three units: the Brejo Santo Formation (Upper Jurassic) and the Rio da Batateira and Santana Formations (Lower Cretaceous). The Araripe Basin sediments were deposited in shallow water of an epicontinental sea preserving a series of richly fossiliferous strata with abundant fishes, some quelonians, crocodiles and pterosaurs. The recent discovery of coprolites in these sediments reveals new morphotypes. Comparative treatment and analysis of the chemical components provides new information regarding the level of spatial distribution and density for these paleocommunities. The coprolites from the Brejo Santo Formation are large, cylindrical forms and smaller ovoid forms with anisopolar and polar ends. Those from the Rio da Batateira Formation are spherical to ovoid in shape and some are over two centimeters in diameter. The two coprolite assemblages from the Santana Formation include calcareous concretions (ichthioliths) collected from the Romualdo Member. These oval and cylindrical forms have isopolar and anisopolar ends. Internally, they show enterospires marks and the remains of shells and bones. Coprolites from the Ipubi Member are covered by thin layers of mudstone, are oval and cylindrical shaped and of different sizes.

POTENTIALLY FALSE INTERPRETATIONS OF DINOSAUR NESTS AND NESTING, WITH AN EXAMPLE FROM THE ALLEN FORMATION, PATAGONIA, ARGENTINA

SPENCER, Lee A., Earth History Research Center, Southwestern Adventist University, Keene, TX, USA; and Elaine G. Kennedy, Geoscience Research Institute, Loma Linda University, Loma Linda, CA, USA

The primary difficulty in accurately identifying dinosaur nest sites and behaviors has been the lack of defining criteria, both paleontological and sedimentological. If eggs are found, especially associated eggs or abundant eggshell fragments, they are almost always assumed to be nests. Sedimentological data has been minimized or ignored.

The Allen Formation in the Rio Negro Province, Argentina, preserves numerous localities with either whole eggs or abundant eggshell fragments, which have been interpreted as representing nesting areas. A 4 m² quarry was excavated through part of a layer containing abundant eggshell fragments utilizing both paleontological and sedimentological techniques. The eggshell fragments were randomly oriented throughout a 1.5 m thick mudstone; the mudstone lacked depositional structures;and the layer was exposed over 1.5 km. The fragments were taxonomically diverse and were mixed with other biota, including fish and turtle fragments as well as plant debris. We interpreted the depositional environment as a storm surge deposit.

Sedimentological structures were also noted at the site of whole eggs. The eggs were found at the toe of a cross-bedded sandstone. The cross-beds draped across the eggs and were not cut by any parental digging activity. Rip-ups, mud lenses and mud stringers were noted in the basal portion of the sandstone.

The sedimentological context of all sites examined involved transport. No nest structures were seen. Preliminary studies in North America and Europe have yielded data suggesting that nesting behavioral interpretations there could also benefit from additional sedimentological research.

CHANGES AMONG REEF ECOSYSTEMS DURING THE EARLY MESOZOIC

STANLEY, George D., Jr., Dept. of Geology, The University of Montana, Missoula, MT, USA

Tropical reef ecosystems, devastated after the end-Permian event, did not recover until 12­14 million years later—well into the Middle Triassic. There are unanswered questions about the lengthy lag time and the Triassic recovery did not follow a gradual logistical curve. The lag time and subsequent rapid return of reef ecosystems and carbonate sedimentation in the Middle Triassic, suggests that protracted marine environmental perturbations arrested recovery.

Scleractinian corals appeared suddenly in the mid-Triassic without ancestors. They likely evolved from soft-bodied, anemone-like animals that survived the perturbations and later acquired calcification. After their debut, these corals waited over 20 million years before assuming major constructional roles on reefs. The first Mesozoic reefs in the Middle Triassic were dominated by microbes, calcareous sponges, bryozoans, calcareous algae and microproblematica which, at higher taxonomic levels, were similar to Permian reefs. Changes took place during the Late Ladinian-Early Carnian interval, with increases among sphinctozoid sponges, "Tubiphytes," and other microproblematica. Late Carnian-early Norian reefs diversified with corals and sponges, and a smaller mass extinction separates this interval from the maximum reef development during the latest Triassic (mid-Norian to Rhaetian), when new higher taxa appeared, sphinctozoid sponges expanded and corals came to dominate. The appearance of more Permian Lazarus taxa during this interval, than in the Middle Triassic is perplexing.

The end-Triassic mass extinction triggered a "sudden" reef collapse like the Permo-Triassic pattern, but the Early Jurassic lag was shorter, lasting 6­8 ma. Panthalassan refuges are evoked to explain the survival of some taxa. It was not until the mid-Jurassic that a new stable ecosystem re-emerged, which included corals, stromatoporoids and algae. Jurassic reef changes relate to survival of key taxa, climatic amelioration, and the opening of the proto-Atlantic.

WEAK COMPETITION THROUGHOUT THE MARINE REALM: CONSEQUENCES FOR POPULATION GROWTH, RATE OF SPECIATION, AND GLOBAL DIVERSIFICATION

STANLEY, Steven M., and Morton K. Blaustein, Dept. of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA

Paleontologists have too often invoked competitive interactions to explain patterns of evolution and extinction for marine taxa. Bob Paine's caging experiments on rocky shores in the 1960s showed that, by preventing potentially dominant species from excluding others, predation and physical disturbance can allow numerous species to coexist. Similar caging experiments have shown that intense predation prevents food or space from limiting population sizes of most soft-bottom species. Even on organic reefs, where space is at a premium, the presence of competitive networks suppresses competitive exclusion. Ironically, weak competition in the marine realm should reduce rates of speciation. Incipent species must arise frequently as small, reproductively isolated populations, but in order to succeed they must expand numerically. For terrestrial taxa in which niche partitioning occurs, an incipient species can expand immediately by exploiting a previously unutilized resource. In the marine realm, however, where thousands of species utilize the same kinds of food and space, a typical incipient species joins what might be termed a meganiche—a communal niche shared by numerous species separated by trivial differences in resource requirements. Lacking unique access to a limiting resource, the population of the incipient species will not easily expand beyond its small initial percentage of the total population occupying its meganiche; extinction is therefore likely. This condition has probably contributed to low overall rates of speciation in the marine realm and to accelerated speciation following mass extinctions, when populations of incipient species have exploded because predation has weakened. Weak competition in the marine realm is also incompatible with the idea of logistic increase and saturation at a diversity plateau for Paleozoic taxa. In fact, weak competition accounts for the unbridled diversification of marine life since the Permian mass extinctions, which decimated taxa normally characterized by high rates of extinction (the Paleozoic Fauna).

DIATOM EVIDENCE FOR FLUCTUATIONS IN THE FRESHWATER BUDGET AT RUSH RANCH, NORTHERN SAN FRANCISCO BAY, CALIFORNIA: A 3,000 YEAR RECORD

STARRATT, Scott W., U.S. Geological Survey, Menlo Park, CA, USA

Diatoms from tidal marsh sediment cores collected at Rush Ranch, north of Suisun Bay were used to reconstruct the history of salinity variability in the northern part of San Francisco Bay over the past 3000 years. This site was chosen because it is located midway between the normal marine environment of San Francisco Bay and the freshwater environment of the Sacramento and San Joaquin Rivers. The composition of modern diatom assemblages in fresh-water, brackish water, and saltwater marshes are utilized to calibrate the late Holocene assemblages. Variations in both individual taxa and a simple ratio (which summarizes the relationship between salinity and individual taxa) serve as proxies for salinity. The ratio summarizes proportions of taxa commonly found in freshwater, brackish water, and intertidal environments. While this ratio is useful in determining large-scale salinity variations, it does not account for changes in the abundance of individual taxa. Dominant taxa in each assemblage vary downcore, indicating that variation in salinity is only one parameter in a complex set of factors that control the temporal and spatial distribution of diatoms in San Francisco Bay marshes. At the Rush Ranch site there appears to be evidence of broad-scale salinity cycles. Prior to European contact about 150 years ago, there are two intervals dominated by freshwater taxa and two intervals dominated by intertidal taxa. Although these cycles do not appear to be directly related to well-established global climate patterns, minor fluctuations during the last 1800 years resemble the lake level record for Mono Lake, California, implying contemporaneous increases in precipitation on both sides of the Sierra Nevada. Other proxies (¹³C and pollen) at Rush Ranch show similar broad-scale cycles in salinity.

DIATOM STUDIES IN THE WESTERN UNITED STATES: FROM ECONOMIC DEPOSITS TO INDICATORS OF PAST CLIMATE CHANGE

STARRATT, Scott W., U.S. Geological Survey, Menlo Park, CA, USA

Diatoms were first reported from rocks of the western United States more than one hundred years ago. While the economic importance of those deposits was recognized, it was almost seventy years before their potential use in biochronological and paleolimnological studies was recognized. The oldest known freshwater diatom flora is at least middle Eocene in age. While there is limited evidence of Paleogene diatomaceous deposits, there is a marked increase in both the number of deposits and the diversity of the floras beginning in the early Miocene. The association of these deposits with numerous layers of volcanic ash, as well as diverse mammalian faunas, has led to a well-constrained chronology for the evolution of freshwater diatoms in the western US. It is possible to broadly divide the floras into four evolutionary assemblages, based primarily on centric planktonic genera. The earliest assemblage (middle Eocene through early Miocene) includes members of the Aulacoseiraceae and various families of pennate diatoms. The Neogene and Quaternary assemblages include members of the Hemidiscaceae and Thalassiosiraceae. The evolution of early Miocene assemblages coincides with major radiations of marine taxa, high stands of sea level and the creation of tectonic basins throughout the Great Basin. Floral evolution in the middle and late Miocene may have been related to major oceanographic and climatic changes which resulted in increased rates of diatom evolution. While the morphological similarity of many of the late Neogene and Quaternary taxa to modern forms simplifies paleoenvironmental interpretation, this taxonomic longevity makes them difficult to use in high-resolution stratigraphic studies. Taxonomic and stratigraphic resolution will likely be improved through increased use of the scanning electron microscope and comparison with well-dated floras from other regions of the world.

THE IMPACT OF REPRODUCTIVE STRATEGY ON CEPHALOPOD EVOLUTION

STEPHEN, Daniel A., Dept. of Geology & Geophysics, Texas A&M University, College Station, TX, USA

It is well known that species richness is far greater for ammonoids than for nautiloids, and that in general ammonoid taxa have much shorter durations than nautiloid taxa, but the cause of this disparity has puzzled paleontologists for years. Why did the Ammonoidea become so taxonomically diverse and rapidly evolving, while the Nautiloidea did not? One possible solution to the mystery of cephalopod phylogenetic patterns is that taxa in clades characterized by short stratigraphic ranges and high total diversity had a semelparous reproductive strategy (parental mortality follows mass spawning event), whereas taxa in clades with long stratigraphic ranges and low total diversity had an iteroparous reproductive strategy (repeated, isolated breeding events). Previous workers have noted that "r-selected" species tend to be semelparous with many, relatively small offspring, and that "K-selected" species tend to be iteroparous with few, relatively large offspring. Although the correlation between reproductive strategy and r-versus-K population dynamics is not without exception, it has been suggested that cephalopod embryonic size correlates with reproductive strategy, small embryos indicating semelparity and large embryos indicating iteroparity. To examine these questions, I analyzed original data from the Carboniferous of the southern mid-continent as well as data from the literature. For ammonoid genera in the data set, mean embryonic shell size is 0.91 mm, and mean taxonomic longevity is 13.5 m.y. For nautiloid genera in the data set, mean embryonic shell size is 8.46 mm, and mean taxonomic longevity is 47.7 m.y. Differences between ammonoids and nautiloids are statistically significant, and these preliminary results support the hypothesis that reproductive strategy has played a major role in shaping the evolutionary history of the Cephalopoda.

GRASSHOPPER EVOLUTION AND THE SPREAD OF GRASSES

STIDHAM, Thomas A., and Caroline A.E. Strömberg, Dept. of Integrative Biology and Museum of Paleontology, University of California, Berkeley, CA, USA

Grasshoppers (Orthoptera: Caelifera) are a diverse clade of herbivores especially common in grassland and savanna habitats, where they can consume more grass than sympatric mammalian ungulates. Analysis of grasshopper feces demonstrates that they do ingest phytoliths, and therefore phytoliths likely cause observed mandibular wear. Despite their great diversity, grass consumption appears to be restricted to the derived acridoid grasshoppers. At present, the oldest acridoids are Eocene, contemporaneous with early fossil grasses. Several subfamily-level clades in the crown-group of the grass-eating Acrididae first appear in Miocene prior to the proposed spread of grasslands in the middle Miocene. In addition, many of the fossil members of these grass-eating clades have been collected in deposits that contain grass pollen and grass macrofossils. Given the apparent origin of derived acridoids in the Eocene and their Miocene radiation, these grasshoppers appear to have tracked the evolution of grasses and expansion of grasslands over geologic time. Additionally, grass consumption may have been a key innovation in grasshopper evolution that allowed them to take advantage of a new food resource and allow them to diversify.

UPPER CAMBRIAN/LOWER ORDOVICIAN PLATED MOLLUSKS OF THE OZARK UPLIFT

STINCHCOMB, Bruce L., Dept. of Geology, Florissant Valley Community College, St. Louis, MO, USA

A variety of problematic fossils of undoubted molluscan affinity occurs near the Cambrian-Ordovician boundary in the Ozark Uplift of Missouri and Arkansas. Controversy exists as to whether these fossils represent early and primitive amphinurans or whether they are representatives of an extinct molluscan class or classes. Support of the latter position is suggested by the following: (1) Some plated mollusks exhibit horseshoe shaped pattern of muscle scars identical to those of undoubted monoplacphorans. Such musculature is totally unlike that of the amphinurans. (2) Valves of many plated mollusks, such as Matthevia, are morphologically quite different from the valves or plates of amphinurans. (3) With extensive collecting, a seven or eight valved specimen should have been found, as other multiplated organisms such as trilobites, have been found in an articulated condition in the same horizons associated with plated mollusks. Rare, articulated, plated mollusks have been found, but only with two to four valves. Support for plated mollusks being amphinurans include the presence of ornamentation in the form of small shell perforations, which are similar to the perforations occupied by aesthetes of modern amphinurans, and the similarity in morphology of some plated mollusk valves to those of undoubted amphinurans.

Various models have been suggested for these plated mollusks. These include a sixteen-valved "monster" based upon large numbers of valve assemblages, the "stegosaur" model of Yochelson for Matthevia and a single valve (tapeworm) model in which each valve covered only a single animal which broke away from a "string" of such animals which could reproduce by asexually "budding." This model explains the occasional two- to four-valved specimens, which are sometimes found.