Plesiadapiform Postcrania, Primate Origins and Supraordinal Relationships

www.brooklyn.cuny.edu/web/academics/schools/naturalsciences/undergraduate/anthropology/faculty/stephenchester.php

Photo credit: Patrick Lynch

Dr. Stephen Chester (Brooklyn College / CUNY) describes his collaborative research on Paleocene and Eocene plesiadapiform skeletons.

Photo credit: Doug Boyer

The origin of Primates marks the first clear step in the divergence of primates, including humans, from other mammals. Phylogenetic and functional analyses of plesiadapiforms and other euarchontan mammals (primates, treeshrews, and colugos) are critical for testing hypotheses regarding aspects of the origin and early evolutionary history of primates. Plesiadapiforms are a diverse, likely paraphyletic or polyphyletic group of mammals known from the Paleogene of North America, Europe, and Asia. Members of this group of small, arboreal mammals have long been considered close fossil relatives of crown-clade primates (Euprimates) based on the similarity of morphological features such as molars with broad talonid basins and bunodont cusps (Simpson 1935, Szalay 1968). Recent comprehensive phylogenetic analyses have supported a monophyletic group that includes plesiadapiforms and euprimates (plesiadapiforms as stem-primates; Bloch et al. 2007, Silcox et al. 2010) or plesiadapiforms as stem-primatomorphs (stem members of a clade comprised of primates and colugos; Ni et al. 2013).

Purgatorius is the oldest and most primitive plesiadapiform and has been documented in the fossil record shortly after the extinction of the non-avian dinosaurs 66 million years ago. Purgatorius has long been viewed as a plausible ancestor for all other primates, yet had been known only from isolated teeth and jaw fragments since it was first discovered 50 years ago.

My colleagues (Jonathan Bloch, Doug Boyer, and William Clemens) and I recently discovered and described the first postcrania (ankle bones) of Purgatorius, which have diagnostic characteristics for mobility that can also be found in the ankles of arboreal primates and other euarchontan mammals today. Such postcranial specializations would have allowed Purgatorius to access angiosperm products such as fruit, flowers, and associated insect pollinators that were not directly available to many contemporary terrestrial mammals. Results of phylogenetic analyses that incorporate new data from these fossils support Purgatorius as the geologically oldest primate, and this discovery is the oldest fossil evidence that suggests arboreality played a key role in earliest euarchontan and primate evolution (Chester et al. 2015).

As was the case for Purgatorius, nearly all plesiadapiform species are known only from dental remains, and the several described partial skeletons of plesiadapiforms represent fairly evolutionarily derived and geologically younger members of their respective families. I am currently analyzing the postcranial morphology of several undescribed plesiadapiform skeletons with Eric Sargis and Jonathan Bloch. These include skeletons of micromomyid plesiadapiforms, which are quite primitive and uniquely similar to Purgatorius in several aspects of dental and tarsal morphology. I am also in the process of describing the oldest plesiadapiform skeleton with Thomas Williamson and colleagues. This dentally-associated skeleton includes the first postcrania known for the primitive plesiadapiform family Palaechthonidae. Analyses of these plesiadapiform skeletons and comparisons to basally divergent treeshrews, colugos, and fossil euprimates will allow for a more refined perspective of the primate and euarchontan morphotypes.

This research has received financial support from the National Science Foundation, Leakey Foundation, and the City University of New York.

 

References:

Bloch, J. I., Silcox, M. T., Boyer, D. M., Sargis, E. J. 2007. New Paleocene skeletons and the relationship of plesiadapiforms to crown-clade primates. Proceedings of the National Academy of Sciences 104:1159-64.

Chester, S. G. B., Bloch, J. I., Boyer, D. M., Clemens, W. A. 2015. Oldest known euarchontan postcrania and affinities of Paleocene Purgatorius to Primates. Proceedings of the National Academy of Sciences 112:1487-1492.

Ni, X., Gebo, D.L., Dagosto, M., Meng, J., Tafforeau, P., Flynn, J.J., Beard, K.C. 2013. The oldest known primate skeleton and early haplorhine evolution. Nature 498:60-64.

Silcox, M.T., Bloch, J. I., Boyer, D. M., Houde, P. 2010. Cranial anatomy of Paleocene and Eocene Labidolemur kayi (Mammalia: Apatotheria), and the relationships of the Apatemyidae to other mammals. Zoological Journal of the Linnean Society 160:773-825.

Simpson, G. G. 1935. The Tiffany fauna, upper Paleocene. II.-Structure and relationships of Plesiadapis. American Museum Novitates 816:1-30.

Szalay, F. S. 1968. Beginnings of Primates. Evolution 22:19-36.