Reconstructing Equid Mobility in Miocene Florida Using Strontium
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Reconstructing Equid Mobility in Miocene Florida Using Strontium Isotopes A thesis submitted to the Graduate School of the University of Cincinnati in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Geology McMicken College of Arts and Sciences By: Jenelle Wallace B.S. State University of New York Geneseo Advisory Committee: Brooke Crowley, Ph.D. - Committee Chair Josh Miller, Ph.D. Aaron Diefendorf, Ph.D. i ABSTRACT Despite extensive research on the evolutionary history and ecology of horses, we know surprisingly little about the mobility of now-extinct species. We used strontium isotope ratios (87Sr/86Sr) in tooth enamel to reconstruct mobility patterns of horses from two Miocene fossil sites in northern Florida, USA: Thomas Farm (ca. 18 Ma) and Love Bone Bed (ca. 9.5 to 9 Ma). Gomphotheres and tapirs from Love Bone Bed were also included to represent dedicated browsers. Based on modern mobility patterns of extant taxa, we expected that small-bodied or browsing taxa would have 87Sr/86Sr similar to that of local limestone, indicating a relatively sedentary lifestyle. Large-bodied and grazing taxa should have higher and more variable 87Sr/86Sr, reflecting a higher degree of mobility across differing geologies. Surprisingly, the majority of taxa at both sites have higher 87Sr/86Sr than expected for underlying Eocene bedrock, and instead are similar to contemporary Late Oligocene to mid-Miocene seawater. There are several potential scenarios to explain this, one of which seems most plausible. Most individuals may have foraged close to the coast, where vegetation would have been heavily influenced by marine-derived strontium via sea spray. Tapirs have considerably higher and more variable 87Sr/86Sr than horses or gomphotheres at Love Bone Bed which may reflect their aquatic foraging preference. It is likely tapirs consumed aquatic vegetation growing in the river channel that is preserved at Love Bone Bed. Although insignificant, slightly higher 87Sr/86Sr among gomphotheres suggests a small contribution of river water to vegetation growing near the riverbanks. Although our interpretations must be considered provisional, our results suggest that even during their peak radiation, horses were relatively sedentary in Florida. ii iii ACKNOWLEDGMENTS First and foremost, I would like to thank my advisor, Dr. Brooke Crowley, for her immeasurable guidance and support. With her help, I have been able to study what I truly love and will be forever grateful. Additional thanks to my committee members, Dr. Joshua Miller and Dr. Aaron Diefendorf, for their manuscript feedback and advocation of critical thinking. I would also like to thank Marian Hamilton for her insight in my data interpretations and Tim Phillips for his assistance with graphics. This project wouldn’t have been made possible without the generosity of the Florida Museum of Natural History for loaning me sample materials. Special thanks to Dr. Richard Hulbert and Dr. Bruce MacFadden for helping me with specimen acquisition as well as Dr. Robert Feranec for allowing me to utilize some of his samples. I am also very appreciative of Dr. Gideon Bartov for allowing me to participate in sample analysis, which proved to be an invaluable experience. Office mates and fellow graduate students, specifically, Shannon Neale, Kristen Hannon, Jeff Hannon, Abbey Padgett, Lizzie Orr, Stephanie Heath, Abby Kelly, and Zoey Dodson have made my time at UC an unforgettable experience who have shown me that even strenuous days can be full of laughter. I would like to acknowledge and express my gratitude to the American Society of Mammologists, Association of Women Geoscientists, Geological Society of America, Sigma Xi, and the University of Cincinnati Geology Department for funding this research. Finally, I would like to dedicate this thesis to my parents and Brandon Gaylord, for selflessly providing guidance and endless encouragement in times when I needed it the most. iv TABLE OF CONTENTS Abstract ....................................................................................................................................... ii Acknowledgements ................................................................................................................... iv List of Figures ........................................................................................................................... vii List of Tables ........................................................................................................................... viii List of Appendices ..................................................................................................................... ix 1. Introduction .............................................................................................................................1 1.1 Strontium Isotope Background ...............................................................................1 1.2 Background on Equids .............................................................................................3 1.3 Patterns of Mobility in Herbivores ..........................................................................4 1.3.1 Equid Mobility ...........................................................................................5 1.3.2 Using Strontium Isotopes to Track Mobility ...........................................6 2. Methods ....................................................................................................................................7 2.1 Regional Setting ........................................................................................................7 2.2 Site Descriptions ........................................................................................................9 2.3 Species Descriptions ................................................................................................10 2.4 Sample Selection, Preparation, and Analysis .......................................................13 2.4.1 Data Analysis ............................................................................................14 3. Results ....................................................................................................................................15 4. Discussion...............................................................................................................................17 4.1 Why Are Tapir Strontium Isotope Ratios Distinct? ............................................21 5. Conclusions ............................................................................................................................22 6. References ..............................................................................................................................24 v Figures ........................................................................................................................................38 Tables .........................................................................................................................................46 Appendix ....................................................................................................................................50 vi LIST OF FIGURES Figure 1: Paleogeographical and modern bedrock map of northern Florida with corresponding strontium isotope ratios .............................................................................................................. 38 Figure 2: General illustration of potential strontium sources within an ecosystem .................. 39 Figure 3: Box plot of Thomas Farm and Love Bone Bed taxa with references to global seawater strontium .................................................................................................................................... 40 Figure 4: Regressions between ln body mass and median 87Sr/86Sr and ln body mass and variance for Love Bone Bed species with and without tapirs .................................................... 41 Figure 5: Box plots comparing Love Bone Bed taxa with differing diets and habitats ............ 42 Figure 6: Regression between ln body mass and variance for all equids ................................. 43 Figure 7: Box plots comparing equids with differing diets and habitats .................................. 44 Figure 8: Possible scenarios to explain why taxa have similar strontium isotope ratios to contemporary seawater .............................................................................................................. 45 vii LIST OF TABLES Table 1: Morphological and ecological details of Thomas Farm and Love Bone Bed taxa ..... 46 Table 2: Mean strontium isotope ratios for all taxa .................................................................. 48 Table 3: Statistical comparison of diet and environment between Love Bone Bed taxa .......... 49 viii LIST OF APPENDICES Appendix I: Sample details....................................................................................................... 50 ix 1. Introduction The evolution of horses (Equidae) is one of the most notable radiations in the history of mammals. During the Miocene Epoch, ca. 23-5 million years ago (Ma), closed forests slowly shifted into open grasslands (Latorre et al., 1997; Passey et al., 2002; Retallack, 1997), which provided a novel habitat for grazing mammals, and the diversity of ungulate species exploded. Between 23 and 8 Ma, the number of equid genera more than quadrupled (MacFadden, 1992; Maguire