PALEOBIOLOGY OF ARCHAEOHIPPUS (MAMMALIA; EQUIDAE), A THREE-TOED HORSE FROM THE OLIGOCENE-MIOCENE OF NORTH AMERICA JAY ALFRED O’SULLIVAN A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2002 Copyright 2002 by Jay Alfred O’Sullivan This study is dedicated to my wife, Kym. She provided all of the love, strength, patience, and encouragement I needed to get this started and to see it through to completion. She also provided me with the incentive to make this investment of time and energy in the pursuit of my dream to become a scientist and teacher. That incentive comes with a variety of names - Sylvan, Joanna, Quinn. This effort is dedicated to them also. Additionally, I would like to recognize the people who planted the first seeds of a dream that has come to fruition - my parents, Joseph and Joan. Support (emotional, and financial!) came to my rescue also from my other parents—Dot O’Sullivan, Jim Jaffe and Leslie Sewell, Bill and Lois Grigsby, and Jerry Sewell. To all of these people, this work is dedicated, with love. ACKNOWLEDGMENTS I thank Dr. Bruce J. MacFadden for suggesting that I take a look at an interesting little fossil horse, for always having fresh ideas when mine were dry, and for keeping me moving ever forward. I thank also Drs. S. David Webb and Riehard C. Hulbert Jr. for completing the Triple Threat of Florida Museum vertebrate paleontology. In each his own way, these three men are an inspiration for their professionalism and their scholarly devotion to Florida paleontology. They are seeond to none. Dr. Douglas S. Jones emphasized to me the importance of evolutionary questions that influence the evolutionary histories of all taxa. Dr. John F. Eisenberg, with his eye ever on the big pieture, was able to remind me that the study of the evolutionary histories of partieular taxa is informative and fun, and, as did Dr. Brian K. McNab, taught me that ecology is just as influential as phylogeny in shaping those histories, and sometimes more so. (No hardening of the categories in these pages.) Over the years, additional guidance, assistance, and friendship were provided by many people, including Brian Ahem, Dr. L. Barry Albright, Brian Beatty, Jerry Bond, Dr. J. Daniel Bryant, Dr. Thure Ceding, Walter Cressler, Dr. Jason Curtis, Ruth Ann Czerenda, Phil D’Amo, Dr. David Dilcher, Dr. Peter Dodson, Dr. Jim Ehleringer, Dr. Robert Emry, Dr. Robert Evander, Robert Feranec, Marc Frank, Dr. David Froelich, Jeff Gage, Dr. Gina Gould, Diana Hallman, Peter Hallman, Dr. F. Glynn Hayes, Dr. George Hecht, C. Andrew Hemmings, Dr. Pennylynn Higgins, Dr. David Hodell, Casey Holiday, Dr. Robert Hunt, Steve Hutchens, Sue Hutchens, Dr. Pat Hylton, Dr. Carlos ludiea, Kenetha Johnson, Dr. Walter Judd, Dr. Donald Kennedy, IV Dr. John Krigbaum, Greg Labbe, Dr. W. David Lambert, Russ McCarty, Julie Meachan, Matthew C. Mihlbachler, Tangelyn Mitchell, Cyndi Moncrief, Gary Morgan, Irv Quitmeyer, Karen Pallone, Dr. David Philips, Jennifer Piascik, Roger Portell, Arthur Poyer, Dr. Donald R. Prothero, Dr. Jon Rieskind, Dennis Ruez, Dr. Eric Sargis, Dr. Bruce J. Shockey, Erika Simons, Dr. Josh B. Smith, Matt Smith, Dr. John Storer, Dr. Frederick S. Szalay, Dr. Carl Terranova, Dr. Kent Vliet, Laurie Walz, Skye White, Dr. Ron Wolff. I’d like to thank the Biological Sciences Program and Department of Zoology for various forms of support. Essential access to, and loans from, collections were provided by Charles Schaff and Dr. Parish A. Jenkins of the Museum of Comparative Zoology, Harvard University; John Alexander, Dr. Richard Tedford, and Dr. Jin Meng of the American Museum of Natural History, New York; Robert Purdy and Dr. Robert Emry of the National Museum of Natural History, Washington, D.C. Additional financial support was provided by a Lucy Dickinson Fellowship from the Department of Vertebrate Paleontology, Florida Museum of Natural History, a Grinter Fellowship from the Department of Zoology and the Graduate School, UF, an R. Jerry Britt Award from the FLMNH, a Gary Morgan Award from the Florida Paleontological Society, a Mitchell Hope Award from the Southwest Florida Fossil Club, a Theodore Roosevelt Award from the American Museum of Natural History, and travel grants provided by the Department of Zoology, College of Liberal Arts and Sciences, Graduate School, and Graduate Student Council of the University of Florida. V TABLE OF CONTENTS page ACKNOWLEDGMENTS iv ABSTRACT viii CHAPTERS 1 SMALL HORSE, BIG QUESTIONS I Background 1 Abbreviations and Conventions Used in This Study 8 2 A NEW SPECIES OF ARCHAEOHIPPUS (MAMMALIA; EQUIDAE) FROM THE ARIKAREEAN OF CENTRAL FLORIDA 9 Introduetion 9 Systematie Paleontology 11 Discussion 24 Conclusions 30 3 POPULATION DYNAMICS OF ARCHAEOHIPPUS BLACKBERGI (MAMMALIA; EQUIDAE) FROM THE MIOCENE THOMAS FARM FOSSIL SITE OF FLORIDA 31 Introduction 31 Materials and Methods 33 First Reproduction in Modem Females 37 First Reproduction in Modem Males 39 Results 40 Discussion 44 Estimates of First Reproduction in Fossils 44 Age Dependent Mortality 46 Neonate Mortality Estimates 49 Estimates of Average and Potential Longevity 50 Paleoenvironmental Reconstmction of the Thomas Farm Fossil Site Based on Interpretations of Social Structure 51 Paleoenvironmental Reconstruction of the Thomas Farm Fossil Site Based on Interpretations of Dietary Preferences 52 Conclusions 53 VI 4 STABLE ISOTOPIC ANALYSIS OF EVOLUTIONARY HETEROCHRONY, BODY SIZE REDUCTION, AND DIETARY SPECIALIZATION IN ARCHAEOHIPPUS BLACKBERGIYROU THE MIOCENE THOMAS FARM FOSSIL SITE 55 Introduction 55 Heterochrony Affects Morphology, Life History, and Behavior 58 Stable Isotopes in Tooth Enamel Arehive Environmental and Behavioral Data... 61 Materials and Methods 63 Results 65 Diseussion 69 Patterns of variation in S'*0 69 Evaluating Dietary Differentiation in yl. blackbergi and P. leonensis 76 Body Size and Resouree Partitioning 76 Patterns of variation in 5*^C 78 Conclusions 82 5 CLADISTIC ANALYSIS OF ANCHITHERE HORSES 86 Introduction 86 Notes on Two Important Taxa 93 Materials and Methods 96 Characters Used in this Analysis 98 Taxa Used in this Analysis 102 Results 103 Discussion 1 12 Conclusions 119 6 SUPPOSITIONS AND CONCLUSIONS 121 Dwarfing and the Gulf Coastal Plain 122 Conclusions 127 APPENDIX - COMPARISONS OF KEY TAXA 131 Observations on the Postcranial Skeleton 138 Observations on Some Very Early Florida Equids 140 Observations on Material at the AMNH 143 Comments on Materials at USNM 148 LIST OF REFERENCES 149 BIOGRAPHICAL SKETCH 162 vii . Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy PALEOBIOLOGY OF ARCHAEOHIPPUS (MAMMALIA; EQUIDAE), A THREE-TOED HORSE FROM THE OLIGOCENE-MIOCENE OF NORTH AMERICA By Jay Alfred O’Sullivan May 2002 Chairman: Dr. Bruce J. MacFadden Major Department: Zoology The Oligocene-Miocene three-toed horse Archaeohippus (Mammalia; Equidae) is a brachydont browser that exhibits phyletic body size reduction and parahippine pedal adaptations. This study provides a summary of previous work regarding Archaeohippus, documents a new species of Archaeohippus, includes a life table analysis of Archaeohippus blackbergi from Thomas Farm, presents an analysis of stable isotopes from tooth enamel of three fossil horse taxa, and demonstrates phylogenetic relationships of Archaeohippus in a cladistic analysis. Previous students of middle Cenozoic equids have been polarized in the phylogenetic placement of Archaeohippus Some, emphasizing the importance of the primitive low-crowned teeth, have aligned this genus with the anchitheres sensu stricto such as Anchitherium. Those who focused on the shared derived condition of the manus and pes linked Archaeohippus with Parahippus. A new species from the middle Arikareean is the oldest and smallest known member of the genus. Its cheek teeth have viii relatively advanced characters such as crochets, additional enamel plications, and triangular hypostyles enclosing postfossettes, and its manus has reduced side digits and elongated medial phalanges; these characters link it with primitive parahippines. The life table analysis of the population of Archaeohippus blackbergi from the early Hemingfordian Thomas Farm fossil site displays an age-dependent mortality spike in the young adult cohort that may be due to male combat, and which differs from the mortality profile of sympatric Parahippus leonensis and a population of Miohippus obliquidens, a close outgroup from the Oligocene of Wyoming. Analysis of stable isotopes of carbon from tooth enamel failed to elucidate dietary differentiation of the Thomas Farm equids. Oxygen isotopes revealed reduced ontogeny in A. blackbergi relative to P. leonensis but not M. obliquidens. A cladistic analysis of anchithere grade equids supported the monophyly of the genus Archaeohippus and placed it within the primitive parahippines. This study provides insight into the paleobiology of Archaeohippus, the small browsing ecomorph of the first great equid adaptive radiation. This radiation produced a variety of equid forms in response to intensive global climate change that resulted in the fragmentation of forests and the appearance of extensive savanna ecosystems across North America. IX CHAPTER 1 SMALL HORSE, BIG QUESTIONS Background Although the fossil remains are relatively fragmentary, the late Oligocene and early Miocene of