Novltatesamericant MUSEUM PUBLISHED by the AMERICAN MUSEUM of NATURAL HISTORY CENTRAL PARK WEST at 79TH STREET, NEW YORK, N.Y
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NovltatesAMERICANt MUSEUM PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY CENTRAL PARK WEST AT 79TH STREET, NEW YORK, N.Y. 10024 Number 3000, 61 pp., 17 illustrations, 9 tables March 7, 1991 Morphological Transformation and Cladogenesis at the Base of the Adaptive Radiation of Miocene Hypsodont Horses RICHARD C. HULBERT, JR.1 AND BRUCE J. MACFADDEN2 CONTENTS Abstract ..................................................... 2 Introduction ................... .................................. 2 Acknowledgments ...................................................... 3 Abbreviations and Terminology ............................................... 4 Previous Investigations .................................................... 6 Materials and Methods .................................................... 14 Characters and Character States ............................................... 15 Description of Analyzed Taxa ................................................. 19 Analytical Methods ..................................................... 40 Results ..................................................... 40 Taxonomic Status of Merychippus insignis Leidy, Again ......... .................. 46 Chronological and Biogeographical Distribution of Merychippines ....... ........... 50 Conclusions ................. .................................... 56 References ................ ..................................... 57 'Postdoctoral Research Fellow, Department of Natural Sciences, Florida Museum of Natural History, University of Florida, Gainesville, Florida 32611; Present Address: Department of Geology and Geography, Georgia Southern University, L.B. 8149, Statesboro, Georgia 30460. 2 Curator, Department of Natural Sciences, Florida Museum of Natural History; Research Associate, Department of Vertebrate Paleontology, American Museum of Natural History. Copyright C) American Museum of Natural History 1991 ISSN 0003-0082 / Price $6.70 2 AMERICAN MUSEUM NOVITATES NO. 3000 ABSTRACT The genus Merychippus has traditionally com- gether form a monophyletic group within the prised a horizontal grade ofincipiently hypsodont, Equinae, and each includes species of merychip- middle Miocene, North American horses. Phy- pine-grade. "Merychippus" primus is the sister logenetic relationships both among merychippines taxon of the Equini + Hipparionini, and "M." and with younger, fully hypsodont clades were gunteri is the sister taxon to these three taxa. The poorly delineated at best, thus prohibiting detailed Equini consists oftwo monophyletic subtribes, the analysis of the radiation of grazing equids. A sin- Protohippina and the Equina. Known stratigraph- gle, most parsimonious, cladogram of 12 repre- ic ranges provide chronological constraints for the sentative merychippine-grade species based on 39 timing of the major cladogenetic events. The first cranial, dental, and postcranial characters was pro- occurrence ofboth the Hipparionini and the Equi- duced using the computer program PAUP. The ni is late Hemingfordian, ca. 17.5 to 17.0 Ma. early Hemingfordian species "Parahippus" leo- The status of Merychippus insignis is reconsid- nensis was used as an outgroup; its previously un- ered in light of the analysis by Evander (1986). In described cranial characters include a very shal- contrast to that study, we conclude that: (1) the low, unpocketed, poorly defined dorsal preorbital development of a protostyle on deciduous pre- fossa, no malar fossa, and a shallow nasal notch. molars displays high levels ofindividual variation In these and other cranial characters it closely re- in some quarry samples (e.g., Thomas Farm "P." sembles "Merychippus" primus. The resultant leonensis), and thus it is not a judicious character cladogram supports the following hypotheses: with which to make systematic decisions without Merychippus as traditionally used is a paraphyletic corroboration from others. (2) Specimens referred assemblage, and in a strictly phylogenetic classi- to M. insignis by Quinn (1955) and Evander (1986) fication should be limited to the type species and from the Point Blank Fauna of Texas represent the few others with which it forms a monophyletic Protohippus vetus. (3) The Lower Snake Creek group. Merychippus sensu stricto is a hipparionine, samples referred to M. insignis by Skinner and most closely related to Nannippus and Cormohip- Taylor (1967) probably do represent that species. parion. The tribes Hipparionini and Equini to- INTRODUCTION The radiation of the Equidae into the cur- fordian and much of the Barstovian, from sorial grazing ungulate adaptive zone in the about 18 to 14.5 Ma, equids of a less hyp- middle Miocene of North America is a well sodont, phenetically more homogeneous known and familar example to most students grade, Merychippus s.l., predominated. Al- of paleontology and evolution. This resulted though tentative suggestions of phylogenetic in the presence in North America of at least relationships between particular species of 11 late Miocene, hypsodont, monophyletic Merychippus s.l. and certain advanced genera clades (usually recognized as the genera date back to Matthew (in Osbom, 1918: 98), Pseudhipparion, Neohipparion, Hipparion, the predominant 20th century classifications Nannippus, Cormohipparion, Protohippus, of the Equidae usually retained these taxa in Calippus, Pliohippus, Astrohippus, Onohip- a single, horizontal genus, Merychippus. This pidium, and "Dinohippus"). As they are rel- is hardly surprising, as it was the then ac- atively distinct in morphology, recognition cepted practice for groups at the base ofadap- and study ofthese taxa from the later part of tive radiations to be assigned to horizontal, the radiation (the late Barstovian, Claren- paraphyletic groups on the basis of shared donian, and Hemphillian, or ca. 14.5 to 4.5 primitive (symplesiomorphic) character states Ma) is relatively straightforward, and work and overall phenetic similarity (e.g., the tra- is nearing completion of an alpha-level tax- ditional composition of the Reptilia, Insec- onomic survey of these horses (MacFadden, tivora, Condylarthra, and Miacidae). The 1980, 1984a, 1984b, 1986; Webb and Hul- reasons behind the classic subfamilial and ge- bert, 1986; Hulbert, 1987, 1988a, 1988b, neric classifications embodying the "evolu- 1988c; MacFadden and Hulbert, work in tion of the horse" are identical (but on a progress). However, during the late Heming- smaller scale) to those described by Kemp 1991 HULBERT AND MACFADDEN: MIOCENE HYPSODONT HORSES 3 (1988b: 2) for the Synapsida: a relatively justify this study. (1) In addition to exam- complete fossil record, a large morphological ining well-known, previously studied speci- difference between end members spanned by mens, included are important discoveries that numerous structural intermediaries, and eas- fill many of the morphological, chronologi- ily observed transformation sequences for cal, and geographical gaps in the fossil record some characters. that hindered previous investigations. (2) New For horses, this remained the case until the geochronological and biochronological pre- mid- 1970s, until the widespread application cision now allows calibration ofcritical Neo- of phylogenetic systematics, and the realiza- gene faunas to within 1 m.y. or less (Flynn tion that characters other than those of the et al., 1984; Tedford et al., 1988). And, (3) cheekteeth were equally important in assess- the advent of computer programs for phy- ing relationships. Since then several species logenetic analysis allows simultaneous anal- previously included in Merychippus, or new ysis of many more characters and taxa than species ofmerychippine grade, have been ex- could be reasonably expected in previous plicitly allied to late Miocene genera using a studies. philosophy of phylogenetic, or vertical clas- The alpha-level systematics of merychip- sification (Skinner and MacFadden, 1977; pine equids are by no means completely un- MacFadden, 1984a; Webb and Hulbert, 1986; derstood, and is the subject of on-going re- Hulbert, 1987, 1988a, 1989). Thelatterstudy search by a number of investigators, notably was one of the first to present a cladogram Evander (1985, 1986, work in progress). demonstrating the relationships of a broad These efforts will undoubtedly contribute to suite of merychippine species with those of the reconstruction of equid phylogeny, and younger genera. The present study is in part quite possibly contradict some of the results a refinement of that analysis, and attempts presented here. Nevertheless, since Leidy's to answer the following questions regarding initial description of Merychippus insignis the adaptive radiation of hypsodont horses: over 130 years ago, significant progress has 1. What are the phylogenetic interrelation- been made in the study of merychippine ships among the oldest-known merychip- horses (summarized below), and many spe- pines, and how are they related to more cialists have repeatedly demonstrated the va- "advanced" equids? lidity of a number of species. These taxa are 2. What was the precise timing, pattern, and the focus of this study, whose primary goal geographical distribution of cladogenetic is to answer the fifth question posed above. events during the early phases of the evo- lution of hypsodont horses? ACKNOWLEDGMENTS 3. What minimum age can be placed on the nearest common ancestor ofall advanced, Earlier drafts of the manuscript were crit- hypsodont equids? ically reviewed by S. D. Webb, R. M. Hunt, 4. How does the fossil record compare with M.