The Teeth of Being on the Order of 500 Miles Or 800 Kilometers
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Papers on Paleontology - Recent Numbers 7. Devonian Strata of Emmet and Charlevoix Counties, Michigan by R. V. Kesling, R. T. Segall, and H.O. Sorensen 8. Strata and Megafossils of the Middle Devonian Silica Formation by Robert V. Kesling and Ruth B. Chilman 9. Revision of Upper Ordovician and Silurian Rocks of the Northern Peninsula of Michigan by Robert V. Kesliizg 10. Index and Bibliography of Late Cenozoic Freshwater Mollusca of Western North America by Dwight W. Taylor 1 1. The Fossil Catfishes of North America by John G. Lundberg 12. Studies on Cenozoic Paleontology and Stratigraphy (Papers Presented at Hibbard Memorial Symposium, May, 1974, in Ann Arbor) edited by Gerald R. Smith and Nancy E. Friedland 13. Geology and Paleontology of the Early Pleistocene (Late Blancan) White Rock Fauna from North-Central Kansas by Ralph E. Eshelman 14. Fishes of the Pliocene Glenns Ferry Formation, Southwest Idaho by Gerald R. Smith Fishes of the Miocene-Pliocene Deer Butte Formation, Southeast Oregon by Peter G. Kinznzel Museum of Paleontology The University of Michigan Ann Arbor, Michigan 48 109 CRANIAL ANATOMY AND EVOLUTION OF EARLY TERTIARY PLESIADAPIDAE (MAMMALIA, PRIMATES) Reconstructed Clarkforkian landscape in northwestern Wyoming, with Plesiadapis dubius and several Plesiadapis cookei in the foreground, and Chironzyoides rnajor in the background. Cranial Anatomy and Evolution of Early Tertiary Plesiadapidae (Mammalia, Primates) Philip D. Gingerich University of Michigan Museum of Paleontology PAPERS ON PALEONTOLOGY NO. 15 Papers on Paleontology No. 15 The Museum of Paleontology University of Michigan Ann Arbor, Michigan 48 109 Gerald R. Srnitlz. Director Edited by Nancy E. Friedland June 1 , 1976 I . Introduction ..........................1 VI . Dental Function and Evolution ............63 Geographic Distribution of Fossil Localities . .2 Dental Function .....................63 Dental Terminology ..................2 Incisor Function ...................63 Dental Measurements .................5 Premolar and Molar Function ..........64 Institutional Abbreviations .............6 Dental Evolution .....................66 Acknowledgments ...................6 Evolution of Size ...................66 Reduction of Dental Formula ..........68 Incisor Evolution ...................69 Evolution of the Fourth Premolar ........70 I1 . Variation. Species. and Phylogeny Reconstruction .9 Evolution of the Third Molar ...........71 Biological Species in the Fossil Record .......9 Discussion .........................71 Variation in Fossil Populations ............10 Tooth Size and Species Diagnosis ..........12 VII . Cranial Anatomy ......................75 Phylogeny Reconstruction ..............15 Dentition .......................... 76 Dental Formula ....................76 Tooth Eruption ....................77 Nasal Region ....................... 78 Orbital Region ......................79 111 . Systematic Revision ....................17 Basicranial Region ....................79 Genus Pronothodectes .................18 Middle Ear .......................79 Genus Nannodectes ...................19 Brain ............................. 80 Genus Plesiadapis ....................24 North American Species ..............25 VIII . Notes on the Paleobiology of Plesiadapidae ....83 European Species ..................34 Postcranial Anatomy and Evolution ........83 Genus Chiromyoides ..................40 Paleobiology ........................85 North American Species ..............40 Origin of Early Eocene Mammals of Modern European Species ..................43 Aspect ...........................86 Genus Platychoerops ..................44 Noilzina Addenda ....................47 IX . Phylogeny and Classification of Primates ......89 Relationships within Plesiadapiformes .......89 Plesiadapiformes and other Primates ........90 Phylogeny and Classification of Primates .....93 IV . North American Biostratigra~h~and Phylogeny . 49 x . conclusions ........................-99 Biostratigraphy ......................49 Phylogeny Reconstruction from the Phylogeny .........................56 Fossil Record ......................99 Clarkforkian Mammal Age ...............56 North American and European Biostratigraphy and Correlation ....................100 Plesiadapis and the Phylogeny of Primates ...100 V . European Biostratigraphy. Phylogeny. and Literature Cited ..................... 103 Correlation ..........................59 Biostratigraphy ......................59 Appendix ..........................109 Phylogeny .........................59 Correlation .........................60 Plates ............................ 117 ABSTRACT The Plesiadapidae are an important family of early fossil mammals presently known from middle Paleocene through lower Eocene strata in North America and Europe. Specimens of Plesiadapis are among the most common Paleocene fossil mammals, they are known from stratigraphic intervals spanning some 12 million years, and they changed and diversified greatly through this period of geological time. Thus the Plesiadapidae are an ideal group for detailed biostratigraphic and evolutionary study. Virtually all known specimens of Plesiadapidae were studied to: (1) determine the extent to which it is possible to study the evolution of a group of early Tertiary mammals at the species level, (2) improve the correlation of middle Paleocene through early Eocene continental sediments within and between North America and Europe, and (3) clarify the relationship of Plesiadapis to living and fossil primates. A stratophenetic method of phylogeny reconstruction based on phenetic linking of fossils in adjacent stratigraphic intervals is proposed. Species in the fossil record are viewed as arbitrarily divided segments of biologically distinct evolutionary lineages that differ morphologically from other such segments, and a complete systematic revision is included. Tile family Plesiadapidae includes five valid genera and 26 species (one of which, Plesiadapis russelli, is newly described). Four distinct evolutionary lineages were present in North America during the late Paleocene. Three lineages were present in Europe, two of which paralleled very closely the evolution of their counterparts in North America. These pairs of North American and European lineages are so similar that they may have been parts of the same evolving pangeographic distribution, supporting the idea that there was a continuous or nearly continuous continental connection between North America and Europe during the late Paleocene. The relatively complete stratigraphic record of Plesiadapidae in North America documents gradual phyletic evolu- tion in both discrete and continuous morphological characters. Strong character divergence in tooth size (and by inference body size) is documented following the initiation of sympatry in recently separated sister lineages. Other niorphological characters do not show this pattern of character divergence but undergo considerable parallel evolution presumed to indicate parallel adaptations. Similarly, adaptation is inferred to be the cause of two documented evolu- tionary reversals in the evolution of cusp patterns on incisors and premolars. Continued study of mammalian groups with detailed fossil records promises to considerably improve our understanding of speciation in natural populations, and of trends in the evolution of adaptations. The Torrejonian, Tiffmian, and Clarkforkian in North America are subdivided into eight biostratigraphic range zones based on consecutive species in a single evolving lineage of Pronothodectes and Plesiadapis. Six biostratigraphic range zones are recognized in the evolution of Plesiadapis and Platychoerops during the Thanetian and Ypresian in Europe, but mammalian faunas fro111 additional intermediate intervals remain to be sampled in Europe. The Plesiadapis cookei zone in North America is correlated with the Sparnacian Plesiadapis russelli zone in Europe on the basis of Plesiadapis and other contained mamlnalian faunas, and it is concluded that the Clarkforkian mammal age in North America is earliest Eocene, rather than latest Paleocene as it has conventionally been regarded. The Plesiadapidae appear to have been herbivorous, basically terrestrial animals, most of which thrived at high population densities in warm temperate climates of the late Paleocene. Their closest living analogues in the mammal- ian faunas of North America and Europe today would appear to be rodents of the family Sciuridae (marmots, ground squirrels, etc.). It is possible that rodents evolved from a generalized early or middle Paleocene primate, but substan- tive fossil evidence is lacking. Rodents, like many other groups of modern mammals, appeared abruptly in the fossil record at the beginning of the Eocene when a subtropical climate again returned to Europe and North America, replacing the warm temperate climate of the late Paleocene. Being adapted to a temperate climatic regime, Plesiadapis became extinct in North America when it was replaced by new subtropical forms. In Europe Plesiadapis evolved rapidly into Platychoerops, wl~ichsubsequently became extinct several million years later. The presence of a temper- ate climatic minimum during the late Paleocene in the areas of known fossil deposits helps to explain the gap in the fossil record between generalized primates of the subtropical early and middle Paleocene, and their abruptly appearing relatives of modern aspect in the subtropical early Eocene. The dental and cranial morphology of primitive Plesiadapidae