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SESSION NO. 8 documented only on hardground hosts. Hardground sample occurrence and encrustation frequency fall within the expected range for shallow euphotic zone sclerobiont suites, but richness is significantly higher than expected. These data suggest that suites or sclerobiont taxa may be effective indicators of sedimentation in addition to paleodepth. 8-4 BTH 8 Green, Jeremy L. [218512] THE INFLUENCE OF BITE FORCE ON THE FORMATION OF DENTAL MICROWEAR IN XENARTHRANS (MAMMALIA) GREEN, Jeremy L., Geology, Kent State University at Tuscarawas, 330 University Dr NE, New Philadelphia, OH 446636, jgreen72@kent.edu and MCAFEE, Robert K., Ohio Northern University, Department of Biological and Allied Health Sciences, 525 South Main Street, Ada, OH 45810 Xenarthrans are a group of placental mammals (including tree sloths, armadillos, ground sloths, and glyptodonts) that lack enamel on their adult teeth, having instead an outer layer of softer orthodentine. Prior analyses of microscopic scars (termed “dental microwear”) on the chewing surface of xenarthran teeth reveal a correlation between orthodentine microwear patterns and feeding ecology, thereby providing a proxy for paleodiet in extinct xenarthrans. However, the specific formative mechanism of microwear features (e.g. scratches, pits) on xenarthran orthodentine remains poorly understood. It stands to reason that bite force (generated by mandibular closure during mastication) should influence the formation of microwear patterns, but no studies have specifically tested this hypothesis. We attempt to fill this gap in our knowledge by investigating the potential effects of variable bite force on microwear formation in tree sloths. Relative ratios of bite force were estimated from 16 skulls of Bradypus (three-toed sloth) and Choloepus (two-toed sloth) by applying a geometric model for calculating input forces from masticatory muscles (e.g., temporalis and masseter) relative to lever arm moments of the mandible. For each skull, microwear was examined on epoxy resin casts of the upper right tooth row. Using low-magnification (35×) light microscopy, we analyzed five microwear variables (i.e., number of scratches, presence of hypercoarse scratches, gouges, large pits, and crossscratches) on four tooth positions (M1–M4) from each skull. ANOVA tests were applied to compare each microwear variable with the estimated bite force at each tooth position per taxon. While there is some positive correlation of increasing microwear variables and strength of force as one moves posteriorly, the pattern is not consistent across the entire tooth row for either taxon. The lack of a significant correlation between microwear variables and bite force values suggests that tooth scars in sloths are not being generated by pure orthal closure of the mandible, but rather are more influenced from other jaw movements. Further analyses that incorporate all masticatory muscles to create a more realistic and three-dimensional assessment of the chewing cycle should help to clarify how microwear patterns are generated in theses taxa. 8-5 BTH 9 Guensburg, Thomas E. [218000] AGAINST HOMOLOGY OF CRINOID AND BLASTOZOAN ORAL PLATES GUENSBURG, Thomas E., Sciences Division, Rock Valley College, 3301 North Mulford Road, Rockford, IL 61114, t.guensburg@rockvalleycollege.edu, SPRINKLE, James, Department of Geological Sciences, Jackson School of Geosciences, University of Texas, 1 University Station C1100, Austin, TX 78712-0254, and MOOI, Rich, Dept. of Invertebrate Zoology and Geology, California Academy of Sciences, 55 Music Concourse Drive, San Francisco, CA 94118 Cambrian echinoderms include the earliest pentaradiate forms, blastozoans and edrioasteroids sensu lato, but no living classes. The earliest unequivocal crinoids appear early in the Ordovician. One side in the ongoing debate over crinoid origins recently promoted similar oral region morphology as evidence of blastozoan ancestry, the two historically assigned pelmatozoans (stemmed echinoderms). Instead we find evidence of ancestry within edrioasteroid-like taxa, recently found to also include stemmed forms. Does the oral region, and specifically oral plating surrounding the peristome (mouth) provide synapomorphies indicating blastozoan-crinoid monophyly? Proponents document morphologic similarities including: hydropore position, moveable peristomial and ambulacral cover plates, 2-1-2 ambulacral symmetry, arrangement of oral plates, and rigid attachment of the oral surface to the underlying calyx. These five traits are supposed to represent synapomorphies of some subset of blastozoans with crinoids. The early echinoderm record shows the first four of these traits actually comprise symplesiomorphies for all pentaradiate echinoderms, including edrioasteroids. Therefore they are uninformative for any blastozoan-crinoid linkage. The 2-1-2 symmetry and oral arrangement are closely related, the former constraining the latter. Lacking support from these similarities, blastozoan-crinoid oral homology becomes conjectural. Lastly, thecal rigidity represents an iterative theme in pentaradiate echinoderm evolution across the critical stratigraphic interval (Cambro-Ordovician), underscoring potential for homoplasy. Considering evidence from all other skeletal regions, we conclude that symplesiomorphy and homoplasy, not phylogenetic relationship, explain blastozoan and crinoid similarity, ruling against a pelmatozoan clade. 8-6 BTH 10 Aucoin, Christopher D. [218100] A PRELIMINARY COMPARISON OF THE LATE ORDOVICIAN BUTTER SHALES OF THE CINCINNATI ARCH AUCOIN, Christopher D., Department of Geology, University of Cincinnati, Cincinnati, OH 45211-0013, aucoincd@mail.uc.edu, BRETT, Carlton E., Department of Geology, University of Cincinnati, Cincinnati, OH 45221-0013, MALGIERI, Thomas J., Department of Geology, University of Cincinnati, 500 Geology/Physics Building, Cincinnati, OH 45221-0013, and THOMKA, James R., Department of Geology, University of Cincinnati, 500 Geology-Physics Building, University of Cincinnati, Cincinnati, OH 45220 The Upper Ordovician Cincinnatian strata of Ohio, Kentucky and Indiana contain several claystone units, colloquially referred to as “butter shales” or “trilobite shales”. These units are widely known for their relative abundance of well preserved trilobites Isotelus and Flexicalymene. Previous studies have focused on trilobite taphonomy and bed characteristics of individual butter shales; however, there has been little comparison between butter shales to assess lateral variation, or broader stratigraphic/facies context. This study takes a comparative approach by examining the geographic extent of each claystone unit, as well as lateral variations in bed thickness, paleoecology, taphonomy and clay sedimentology within and between various butter shales. Field study and review of literature has identified three major butter shale units in the Waynesville Formation and several minor ones in the older Grant Lake and Arnheim formations. The Treptoceras duseri shale, the most extensively studied of the claystones, has produced a mollusk-dominated fauna adapted for environments characterized by muddy substrates, high turbidity and rapid sedimentation in contrast to the brachiopod dominated fauna of the surrounding units. Surprisingly, however, at least three of the butter shales also contain zones with corals (Tetradium) and small stromatoporoids, unusual fauna for clay-dominated environments. 10 2013 GSA Abstracts with Programs These coral/sponges are frequently overturned and heavily bored and encrusted, indicating reworking in relatively shallow water conditions during pauses in sedimentation. In terms of sequence stratigraphy, butter shales appear to be consistently situated within highstand portions of third-order cycles, apparently amplified by analogous phases of higher-order cycles. We suggest that this common position within a 3 rd and 4 th order stacking reflects a sedimentational “sweet spot”, in which progradation during shallowing, regressive conditions permitted episodic pulses of mud deposition. 8-7 BTH 11 Zambito, James J. [218267] NEW INSIGHTS INTO THE TRILOBITE AND CONODONT BIOSTRATIGRAPHY OF THE MIDDLE-UPPER DEVONIAN GENESEE GROUP IN EASTERN NEW YORK STATE ZAMBITO, James J. IV, Department of Geology and Geography, West Virginia University, 330 Brooks Hall, 98 Beechurst Street, Morgantown, WV 26506-6300, james.zambito@ mail.wvu.edu and DAY, Jed, Geography & Geology, Illinois State Univ, Normal, IL 61790-4400 During the Middle Devonian (Givetian) Global Taghanic Biocrisis, numerous trilobite families underwent extinction including the Homalonotidae. In northeastern North America, the last occurrence of homalonotid trilobites (Dipleura dekayi) has previously been reported in strata deposited during the final stages of the Taghanic Biocrisis (semialternans Zone –latest Middle Givetian) including the Tully Formation of the northern Appalachian Basin and the Petoskey and Thunder Bay formations of the Michigan Basin. Globally observed low-oxygen conditions associated with the Taghanic Biocrisis, represented by the Geneseo and Antrim black shales in the Appalachian and Michigan basins, respectively, have been interpreted to be at least in part responsible for the local extinction of the Homalonotidae and other trilobite families. Recent geologic mapping in eastern New York State (northern Appalachian Basin) has documented the occurrence of Dipleura dekayi in siliciclastic-dominated, nearshore post-Taghanic strata that were deposited below fair-weather wave base. Although these strata have been mapped as Genesee Group, they have yet to be assigned a formation-level designation. An integrated stratigraphic approach, including litho-, sequence-, and conodont biostratigraphy has provided new insight into the stratigraphic succession in the eastern Genesee Group, representing marine shelf through non-marine settings. Lithostratigraphic correlation places the occurrence of Dipleura dekayi above the level of the Fir Tree Limestone of western, offshore sections. Application of a sequence stratigraphic model has enabled the identification of time-rich intervals such as flooding surfaces and sequence boundaries that have yielded conodonts. Preliminary conodont biostratigraphic data suggests that Dipleura dekayi persisted into at least the latest Middle Devonian Lower subterminus Zone (=Lower disparilis Zone). Preliminary mapping further suggests that Dipleura dekayi may have even survived into the Late Devonian. Similar to other northern Appalachian Basin taxa that persisted through the Taghanic Biocrisis, Dipleura dekayi apparently found intrabasinal refuge in oxygenated, nearshore shelfal settings. 8-8 BTH 12 Wulf, Shane [218741] TESTING SPECIES-ABUNDANCE MODELS OF THE HUGHES CREEK SHALE (CARBONIFEROUS) OF SOUTHEASTERN NEBRASKA WULF, Shane1 , JOHNSON, Daryl1 , and HANGER, Rex A. 2 , (1) Geography & Geology, University of Wisconsin-Whitewater, Whitewater, WI 53190, WulfSA11@uww.edu, (2) Geography & Geology, University of Wisconsin-Whitewater, 800 West Main Street, Whitewater, WI 53190 The Hughes Creek Shale Member of the Foraker Formation (Carboniferous) is exposed in road and stream cuts in Richardson County of southeastern Nebraska. Two coeval exposures separated by approximately 16 km were sampled extensively for (mostly) invertebrate fossil specimens, yielding over 5,000 individual specimens. Brachiopods dominate, but Bryozoans, Echinoderms, Molluscs , Cnidarians Arthropods and Chordates are also represented among the 36 species recovered. All taxa were identified to the lowest taxonomic level possible, then counted using (mostly) MNI (minimum number of individuals) methods. Counts of the fauna were then compared with the geometric, log-series and log-normal species abundance models. For a null hypotheses of no difference between actual data and the models, goodness of fit tests of all samples for both exposures were not significant for the geometric and log series models, but highly significant (P< 0.005)for the log-normal model. The log-normal model of species abundance fits many large, mature communities today, and that assumption is extended to the Hughes Creek Shale fauna. At both exposures, maximal faunal diversity occurs less than one meter above presumably anoxic, black shales, suggesting that reassembly of these mature paleocommunities occurred quickly once oxic conditions returned. 8-9 BTH 13 Rivera, Alexei A. [218247] ECOLOGY OF LATE MESOZOIC HETEROMORPHIC AMMONITES: A CASE FOR ALGAL SYMBIOSIS? RIVERA, Alexei A., 20404 Peridot Lane, Germantown, MD 20876, alexei.a.rivera@gmail.com Once touted as inadaptive products of “racial senility”, the heteromorphs curiously depart from the typical planispiral form of ammonites and occupy a radically divergent range of shell morphologies. Some resembled hairpins, others snails or even worms. Late Mesozoic heteromorphs, which include the ancylocones and hamiticones, probably arose suddenly through a single mutation and have been subject to a number of ecological interpretations. The recognition that algal symbiosis is widespread among bivalves and gastropods suggests that perhaps other molluscan stocks, for instance these aberrant ammonites, also served as hosts for photosynthetic unicellular algae. To benefit from this mutualistic relationship, such hosts obviously require tissues that are exposed to sunlight. Although ontogeny controls life position, functional morphology strongly indicates that the terminal aperture of adult ancyloconic and hamiticonic shells were oriented upward towards the ocean surface. These openings conceivably sported a radial fan of delicate filtering tentacles, which may have been adapted both for ensnaring plankton and providing the extensive surface area necessary for efficient algal photosynthesis. Several species possess structural features characteristic of an internal or semi-internal shell, allowing for increased mantle exposure. Indeed, most reconstructions of these heteromorphs argue that they were shallow-water vertical migrants living in the epipelagic zone, well within the bathymetric depths at which light penetrates. While ancylocones and hamiticones were not exclusively restricted to tropical latitudes, they were apparently more abundant and competitive in oligotrophic habitats. And though massive, robust skeletons and rapid calcification rates are usually associated with benthic hosts, such as reef-building corals, giant heteromorphs are known from the fossil record. Lastly, geochemical evidence from the Cretaceous hamiticone Polyptychoceras reveals a significant inverse correlation between δ18O and δ13C stable isotope values (‰ VPDB), which is consistent with the hypothesis of photosymbiosis.
8-10 BTH 14 Rivera, Alexei A. [218060] A NEW TEST OF THE PUNCTUATIONAL MODEL USING PRESENTLY RADIATING CLADES OF BIVALVE MOLLUSKS AND MAMMALS RIVERA, Alexei A., 20404 Peridot Lane, Germantown, MD 20876, alexei.a.rivera@ gmail.com Critical tests using higher taxa with remarkably disparate evolutionary properties, such as bivalve mollusks and mammals, may aid in distinguishing between phyletic gradualism and the punctuational model of evolution in the fossil record. Bivariate regression analysis reveals that median bivalve lifespan (a more widely measured proxy for generation time) and the fractional increase in the number of species per million years, R, are significantly positively correlated for thirteen radiating bivalve families and genera, which is precisely the opposite of the relationship predicted by gradualism. These molluscan stocks generally represent a single adaptive radiation made possible by mantle fusion and siphon formation, which allowed for an astonishing expansion of infaunal life habits during the Mesozoic and Cenozoic Eras. Because the elevated turnover at the end of the Cretaceous Period about 65 MYA essentially interrupted the taxonomic diversification of five siphonate families (Veneridae, Mactridae, Tellinidae, Donacidae, and Teredinidae), their times of first appearance were adjusted via a simple rarefaction technique. In spite of this correction, no inverse correlation is detected within the Bivalvia. Nor is any identified linking median generation time and R among seven radiating mammalian families, again contrary to the gradualistic premise. Inasmuch as these results clearly strengthen the empirical validity of the punctuational model, there are, in fact, multiple variables that determine rates of evolution. Ample abundance data from hundreds of living bivalve and mammalian species can theoretically control for the plausible “overriding” effects of population size and stability, but these estimates could themselves be influenced by several factors, notably sampling intensity of existing populations as well as human-induced activity. 8-11 BTH 15 Blahnik, Caitlin [218747] MOUTH-SIZE ESTIMATION OF THE SHARK, PETALODUS OHIOENSIS, FROM THE HUGHES CREEK SHALE (CARBONIFEROUS) OF SOUTHEASTERN NEBRASKA BLAHNIK, Caitlin and HANGER, Rex, Geography & Geology, University of Wisconsin- Whitewater, Whitewater, WI 53190, BlahnikCE10@uww.edu The Chondrichthyan shark, Petalodus ohioensis, is the only carnivorous species commonly recovered in the Hughes Creek Shale Member of the Foraker Formation (Carboniferous) of the US midcontinent. Teeth are the only body parts known for the species, and their morphology suggest that their diet consisted of the shelly invertebrates of the diverse benthic fauna. Despite numerous reconstructions actual body size and mouth size remain unknown. Tomita (2011) developed a regression method for estimating mouth size (and subsequently body size) from isolated tooth elements. Using an extension of the Tomita method on teeth collected from two different exposures of the Hughes Creek Shale in Richardson County, Nebraska, supplemented by measurements taken from the literature, allow for preliminary estimates to be made for the first time on a Paleozoic shark species (upper jaw lengths up to 10cm). Such jaw sizes could easily accommodate any potential prey species from the documented fauna of the Member, although no actual Petalodus-bite marks have ever been recorded. Tomita (2011) created the method for extant and younger fossil species of the Lamniformes, and our uniformitarian extension of the method to extinct Petalodontiformes remains tentative. 8-12 BTH 16 Fontana, Thomas M. [218780] EOCENE TURTLES FROM THE DISTAL DEPOSITS OF THE CATHEDRAL BLUFFS TONGUE (WASATCH FORMATION), RED DESERT, WYOMING FONTANA, Thomas M. and BARTELS, William S., Department of Geological Sciences, Albion College, Albion, MI 49224, TMF10@albion.edu This study describes an unusual assemblage of fossil turtles from Eocene deposits of the Cathedral Bluffs Tongue of the Wasatch Formation in the Green River Basin of Wyoming. Cathedral Bluffs deposits range from conglomerates, coarse sandstones, and conglomeratic to sandy mudstones deposited by alluvial fans close to the Wind River mountain source (basin margin environment), through braided and meandering stream sandstones and mudstones, to lake-margin mudflat and meandering stream fine sandstones and mudstones farthest from the mountains (basin center environment). The fossils are recovered from the most distal deposits of the Cathedral Bluffs Tongue along Bush Rim where it thins, interfingers with, and pinches-out into the lacustrine deposits of the Laney Shale Member of the Green River Formation. Typical basin center assemblages contain a diverse array of trionychid (soft-shelled), emydid (slider, pond, box, and painted), baenid (extinct snapper-like), and dermatemydid (river) turtles. The upland deposits of the basin margin contain only an undescribed emydid (informally referred to as “Southpassemys”) that was adapted to the faster flowing alluvial fan streams. At Bush Rim, however, “Southpassemys” occurs alongside the common basin center (lowland) turtles. Several other unusual co-occurrences of mixed upland and lowland reptile and mammal groups have also been noted in the Bush Rim assemblage. It appears that during the earliest Bridgerian (Br1a), rapid movement on the Wind River Thrust may have caused the alluvial fan (upland) environments to rapidly expand into the basin center lake-margin environment, bringing together what had been geographically distinct upland and lowland vertebrate faunas (“a forced fauna”). In other vertebrate groups, where upland and lowland forms have been forced together, it has been noted that the most similar animals tend to evolve away from one another morphologically. A morphometric analysis of the large sample of Bush Rim “Southpassemys” specimens indicates little morphological difference from basin margin populations other than a reduction in size. The smaller size of this turtle in the Bush Rim sample, however, may be an evolutionary response to competition with the larger bodied basin center emydid species Echmatemys wyomingensis and E. septaria. 8-13 BTH 17 Claes, Christopher [218769] GEOGRAPHIC INFORMATION SYSTEM ANALYSIS OF THE DISTRIBUTION OF LIVING REPTILIANS WITH RESPECT TO CLIMATE AND ITS POTENTIAL FOR GENERATING QUANTITATIVE PALEOCLIMATIC ESTIMATES CLAES, Christopher, BARTELS, William S., and MCRIVETTE, Michael W., Geological Sciences, Albion College, 611 E. Porter St, Albion, MI 49224, cac13@albion.edu Through the use of Geographic Information System (GIS) technology (ArcMap), biogeographic ranges of North American turtles, lizards, and crocodilians were plotted and overlain to generate biodiversity maps for reptilian groups (guilds) which were then analyzed against nine climate maps representing measurements of temperature, seasonality, environmental moisture, and solar radiation. SESSION NO. 8 When diversity was regressed against climatic parameters, minimum and maximum climatic values were linked with the biodiversity of each reptile guild. The X-Y plots produced by linear regression analyses provide minimum and maximum climatic values associated with guild diversities and the overlap of the ranges of different estimates can then be used to establish criteria for assessing paleoclimatic conditions based on reptilian diversity in well sampled fossil assemblages. Crocodylids are restricted by cool mean annual and cold month temperatures and high seasonal temperature change. Lizard diversities have strong positive correlations with high mean annual temperature, solar radiation, and low rainfall. Aquatic and semiaquatic turtle diversities have strong positive correlations with high mean annual temperature, warm summers, and high humidity but are restricted by high seasonal temperature change and low cold month temperature. Terrestrial turtles thrive with low seasonality and warm summers but unlike aquatic turtles, are less controlled by cold month temperatures and environmental moisture. These climatic limits were then applied to fossil assemblages (approximately 60,000 University of Michigan Museum of Paleontology specimens) from the Paleocene and Eocene of western Wyoming to create paleoclimate estimates. Analysis of these faunas indicate mean annual and cold month temperatures, seasonal temperature changes, and mean annual precipitations that were similar to conditions that exist today along the Gulf Coast of North America. The current data indicates relatively minor climatic change through most of the early Paleogene, but supports a warming and wetting of the area into the middle Eocene about 50Ma. This research hopes to contribute both to the paleoclimate record and to the understanding of biodiversity as it changes in response to a dynamic earth and atmosphere. 8-14 BTH 18 Baumann, Eric [218204] INVESTIGATING THE ECOLOGY OF EXTINCT PROBOSCIDEANS FROM THE CINCINNATI REGION USING STABLE ISOTOPES BAUMANN, Eric Jr, Geology, University of Cincinnati, 5359 Little Turtle Dr, South Lebanon, OH 45065, baumanei@mail.uc.edu and CROWLEY, Brooke, Geology and Anthropology, University of Cincinnati, 500 Geology Physics Building, 345 Clifton Court, Cincinnati, OH 45221 Like their modern African relatives, the extinct proboscideans of North America likely played important ecological roles, including maintaining open grasslands. Yet, the degree to which these animals utilized different plant resources and moved across the landscape has not been comprehensively investigated in North America. We used stable carbon (δ13C), oxygen (δ18O) and strontium ( 87Sr/ 86Sr) isotopes from tooth enamel to investigate the ecology of four mastodons (Mammut americanum) and eight mammoths (Mammuthus spp.) from southwestern Ohio and northwestern Kentucky. We also examined 87Sr/ 86Sr ratios in regional water bodies. We aimed to answer the following questions: Did these mammoths and mastodons have different dietary regimes? Were these proboscideans local residents or simply passing through the region when they died? If passing through, from where did they come? Based on tooth morphology and previous isotopic work, we expected that mammoths and mastodons would have differing δ13C values, indicating C grazing and C browsing niches, 4 3 respectively. We compared 87Sr/ 86Sr ratios in waters and proboscideans tooth enamel to identify local residents and potential migrants. We then used δ18O values to pinpoint possible origins for migrant individuals. As anticipated, mammoths have significantly higher δ13C values than mastodons. This suggests that mammoths may have consumed more C grasses, although 4 one mammoth has δ13C values suggesting a C -based diet. Overall, strontium isotope ratios 3 for proboscideans and local waters are indistinguishable. However, one Mammut molar has significantly higher 87Sr/ 86Sr ratios, suggesting this animal immigrated into the area from somewhere outside of the Midwest. Combined 87Sr/ 86Sr and δ18O values suggest this animal may have migrated from the southern Appalachians. These results will supply a foundation for future work on proboscideans and other extinct megafauna from the Midwestern United States. 8-15 BTH 19 Thomka, James R. [218106] SUBSTRATE-CONTROLLED VARIABILITY WITHIN ATTACHMENT STRUCTURES OF CARYOCRINITES (ECHINODERMATA: RHOMBIFERA) FROM THE MIDDLE SILURIAN OF SOUTHEASTERN INDIANA THOMKA, James R., Department of Geology, University of Cincinnati, 500 Geology-Physics Building, University of Cincinnati, Cincinnati, OH 45220, thomkajr@mail.uc.edu and BRETT, Carlton E., Department of Geology, University of Cincinnati, Cincinnati, OH 45221-0013 The hemicosmitid rhombiferan Caryocrinites is a common and conspicuous faunal element in a variety of Silurian marine environments. Where preserved, the dendritic radicular attachment structures of Caryocrinites can reliably be identified by the presence of a distinctive trilobate lumen and solid pseudocirrate radicles. A hardground surface within the Wenlock-age (Sheinwoodian) Massie Formation, exposed at the New Point Stone quarry near Napoleon, southeastern Indiana, is encrusted by a diverse assemblage of crinoid and blastozoan attachment structures, including structures that can be confidently attributed to Caryocrinites. This hardground is irregular, with slightly elevated, well-sorted, well-winnowed crests and more poorly sorted, coarser troughs. The surface is also host to fistuliporoid bryozoan-dominated microbioherms. The morphology of Caryocrinites attachments structures reflects local substrate conditions on this microbiohermal hardground: holdfasts on hardground crests are simple, approaching conical, and often cemented to other macrofossils, commonly diploporite thecal attachments; holdfasts on hardground troughs are more “typical,” comprising laterally branching, but thin, dendritic radix structures; holdfasts on microbioherms are extremely thickened by secondary stereom secretion—this undifferentiated stereom envelops the initial attachment site and proximal radicles. This segregation of attachment structure morphology is related to substrate properties. The stable and winnowed hardground crests require few radicles for stabilization, in contrast to the shifting, unstable bioclastic rubble of hardground troughs, which require greater surface area (i.e., lateral branching). The extreme thickening of structures on microbioherms is more enigmatic. Secretion of secondary stereom to prevent interaction with anoxic mud is unlikely given the diverse benthic fauna, but the swelling may be a response to interactions with microbes or bryozoans on the microbioherms. An interesting alternative involves purposeful growth of secondary stereom in order to prevent dislodgement from advantageous positions atop elevated microbioherms. 2013 GSA North-Central Section Meeting 11
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