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ABSTRACT GREEN, JEREMY LANE. Enamel ABSTRACT GREEN, JEREMY LANE. Enamel-Reduction and Orthodentine in Dicynodontia (Therapsida) and Xenarthra (Mammalia): An Evaluation of the Potential Ecological Signal Revealed by Dental Microwear. (Under the direction of Mary H. Schweitzer and Julia Clarke.) Mastication produces dental scar patterns (microwear) linked to diet in extant mammals, providing a baseline for reconstructing paleodiet in extinct taxa. Although microwear on enamel is a well-accepted proxy for dietary inference, microwear on orthodentine has received less attention. Because some synapsids lack enamel on their teeth in maturity (e.g., xenarthran teeth, proboscidean tusks), our understanding of paleoecology from microwear in these animals is limited. This dissertation addresses the central question: are orthodentine microwear patterns reliable indicators of ecology in synapsids? Chapter 1 evaluates the distribution of enamel-reduction across Synapsida to select appropriate taxa to target for this research. Enamel-reduction exhibits homoplasy and has independent origins in 13 synapsid taxa. Members of Dicynodontia, Proboscidea, and Xenarthra were chosen for orthodentine microwear analysis because these groups are species-rich with total enamel loss on teeth in maturity. Chapter 2 establishes the correlation between orthodentine microwear and diet in extant xenarthrans using 255 tooth specimens from 21 species. Tooth sampling loci were standardized across taxa. Mean scratch number was significantly higher for extant xenarthran folivores and frugivore-folivores than carnivore-omnivores or insectivores. This supports utility of microwear in distinguishing herbivorous from non-herbivorous taxa. Orthodentine microwear was examined in the Pleistocene ground sloth Nothrotheriops shastensis to extend this correlation to extinct taxa. Statistically, microwear in N. shastensis was most similar to extant folivores, corroborating independent evidence for herbivory in this taxon. Chapter 3 examines the necessity of standardized tooth sampling (used in chapter 2) by quantifying intertooth variation of microwear in each of the four extant xenarthran dietary groups. Distribution of microwear features between tooth loci is more conserved in folivores than other groups. Significant variation in the latter may stem from uneven distribution of bite-force in long-faced animals (two-toed sloths, armadillos) relative to short-faced folivores (three-toed sloths). These results support standardized tooth comparison in future analyses of paleodiet in extinct xenarthrans. Chapter 4 tests hypotheses of possible tusk function (sexual display, feeding) of extinct Permo-Triassic dicynodonts through phylogeny and orthodentine microwear analysis. Optimization of tusk function in living mammals supports evolution of tusks through sexual selection, with an associated feeding function arising second in some taxa. Tusk orthodentine microwear was compared between extant analogues (Loxodonta, Odobenus) and extinct dicynodonts to assess use of tusks for feeding in dicynodonts. Microwear on tusks of extant analogues was significantly different between taxa that use tusks for feeding (Loxodonta) versus those that do not (Odobenus), which supports a link between feeding behavior and tusk microwear. However, only three of 24 sampled dicynodont tusks had original, unaltered microwear. This low yield precluded statistical comparison with extant taxa, so the utility of orthodentine microwear in reconstructing tusk function in extinct taxa remains uncertain. This dissertation supports the utility of orthodentine microwear as a proxy for ecology in extant mammals. Analysis of fossil material supports this method as a proxy for paleodiet in Xenarthra, although more research is needed to test whether microwear can reconstruct tusk feeding function in extinct taxa. Life history of dicynodonts includes not only feeding behavior but also growth, among other traits. Chapter 5 uses bone histology to reconstruct growth patterns in a large late Triassic dicynodont, Placerias hesternus. Cortical microstructure of 21 dissociated long bones supports rapid growth in early ontogeny, with slowed osteogenesis in late ontogeny. Although microstructure of primary bone is similar between P. hesternus and geologically older and smaller dicynodonts, extent of secondary remodeling in long bones is significantly higher in the former, supporting differential growth between taxa. These data need to be analyzed using a more complete ontogenetic series to understand more about the evolution of growth strategy in Dicynodontia. Enamel-Reduction and Orthodentine in Dicynodontia (Therapsida) and Xenarthra (Mammalia): An Evaluation of the Potential Ecological Signal Revealed by Dental Microwear by Jeremy L. Green A dissertation submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Marine, Earth, and Atmospheric Sciences Raleigh, North Carolina April 1, 2009 APPROVED BY: _______________________________ ______________________________ Mary H. Schweitzer Julia Clarke Committee Chair Committee Co-Chair ________________________________ ______________________________ Nicholas Fraser Daniel Kspeka Committee Member Committee Member ________________________________ ______________________________ Lonnie Leithold Vince Schneider Committee Member Technical Consultant ii DEDICATION This work is dedicated to my parents Lane and Susan, my sister Kelly, and my love Kathryn, who have encouraged and supported me in the pursuit of my dreams, and also to the memory of my grandparents, David and Margaret Green and Fred and Estella Crane. I also dedicate this work to Todd Engstrom, Sharon Herman, Roger Portell, Richard Hulbert, Matthew Wedel, Gina Semprebon, Mary Schweitzer, Julia Clarke, Nick Fraser, Daniel Ksepka, Lonnie Leithold, and Vince Schneider, professionals who have pushed me to be a better scientist. iii BIOGRAPHY I was born and raised in the pine woods north of Tallahassee, Florida and grew up with a passion for nature and natural history. In my early enthusiasm, I discovered the wonder and magic of prehistoric animals and have aspired to be a paleontologist ever since. This inspiration led me to the University of Florida, where as an undergraduate student I discovered my love for teeth in the jaws of a mastodon. My early research on mastodon teeth eventually led me to North Carolina State University, and paved the way to this dissertation. I plan to continue my quest for understanding the connection between dental microwear and paleoecology in future research, and always to remember that ‘correlation is not causation.’ iv ACKNOWLEDGMENTS I would like to thank my advisor, Mary H. Schweitzer and my committee members, Julia Clarke, Nicholas Fraser, Daniel Kspeka, Lonnie Leithold, and Vince Schneider, for their advisement and encouragement of this research. Michael Sutherland and Laurie Godfrey provided statistical consultation. Brian Beatty, Eric Dewar, Daniela Kalthoff, Elizabeth Townsend provided constructive feedback on microwear methodology. I also thank Jack Horner, Robert McAfee, and Kevin Padian for analytical feedback. Thanks to Gina Semprebon and Nikos Solounias for providing training in the low-magnification microwear method, and to the NC State Paleo Reading Group (Clint Boyd, Tim Cleland, Drew Eddy, Elizabeth Johnson, Adam Smith, AJ DeBee, Brett Viar) and anonymous peer-reviewers for critical comments on the manuscripts herein. The following institutions generously allowed access and sampling of their extant mammal and fossil vertebrate collections: American Museum of Natural History, New York, NY; Florida Museum of Natural History, Gainesville, FL; Museum of Comparative Zoology, Harvard, Cambridge, MA; National Museum of Natural History, Smithsonian Institution, Washington D.C.; North Carolina Museum of Natural Sciences, Raleigh, NC; North Carolina State University Research Collection, Raleigh, NC; and University of California Museum of Paleontology, Berkeley, CA. Partial funding of this research was provided by the following grants (awarded to JG except where noted): Doris O. and Samuel P. Welles Research Award, Gary S. Morgan v Student Research Award, Geological Society of America Student Research Award, National Science Foundation (EAR-0541744 and EAR-0548847, awarded to MS), R. Jerry Britt Paleobiology Research Award, and Sigma-Xi Grant-In-Aid of Research Award. vi TABLE OF CONTENTS LIST OF TABLES ................................................................................................................. x LIST OF FIGURES ............................................................................................................... xii CHAPTER 1: ENAMEL-REDUCTION IN VERTEBRATES AND THE POTENTIAL OF ORTHODENTINE MICROWEAR AS A PROXY FOR PALEOECOLOGY .............. 1 Dental Microwear Analysis and Dentine ................................................................... 2 The Problem ............................................................................................................... 2 The Broad Question ................................................................................................... 4 The Biology and Evolution of Dentine ...................................................................... 5 The Ontogeny of Vertebrate Teeth ............................................................................ 5 Dentinogenesis and Amelogenesis ...........................................................................
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