A new species of Carpolestes (Mammalia, Plesiadapoidea) from the late Paleocene of southern Wyoming: assessing changes in size and shape during the evolution of a key anatomical feature

Abstract

A new and phylogenetically basal species of Carpolestes, the youngest and most derived genus of the plesiadapoid family Carpolestidae in North America, is described from a late Tiffanian (Ti-5) site in Sweetwater County, Wyoming, USA. Carpolestids differ from closely related plesiadapoid clades in having an enlarged, multicuspidate, blade-like P₄ that is partly convergent on that of multituberculates and other mammals showing plagiaulacoid dental adaptations. With some notable exceptions, the evolutionary history of North American carpolestids is characterized by the progressive development of larger and more elaborate P₄ blades through time. In particular, species of the monophyletic genus Carpolestes differ from species assigned to the earlier and apparently paraphyletic genus Carpodaptes in terms of both the size and shape of their P₄. A geometric morphometric analysis reveals that, with respect to P₄ shape, the closest approximation to the highly derived morphology of Carpolestes is made by Carpodaptes hobackensis, which is one of the smallest known species of Carpodaptes. In contrast, the largest known species of Carpodaptes, Carpodaptes jepseni, has a P₄ that falls within the metric range of variation for species of Carpolestes, yet Carpodaptes jepseni shows a uniquely derived P₄ shape that seems to exclude it from any special phylogenetic relationship with Carpolestes. A phylogenetic analysis based on dental characters reconstructs Carpodaptes hobackensis as the sister group of the Carpolestes clade. Shape seems to have been a more important factor than size during the final transformation of the blade-like P₄ of North American carpolestids.

http://zoobank.org/urn:lsid:zoobank.org:pub:212F9ECC-DA9A-44F8-BE7E-43F3EBAD636A

Keywords:

• Tiffanian
• geometric morphometrics
• phylogenetics
• Primatomorpha

Acknowledgements

Our fieldwork in the Paleocene Fort Union Formation in southern Wyoming is facilitated by personnel from the Wyoming State Office of the Bureau of Land Management (Paleontological Resources Use Permit PA15-WY-234). We thank Christopher Norris, Marilyn Fox and Daniel Brinkman of the Peabody Museum of Natural History at Yale University for making the hypodigm of Carpolestes dubius from the Bighorn Basin available for comparative study. Access to specimens permanently deposited in the collections of the Carnegie Museum of Natural History was facilitated by Amy C. Henrici. We thank all of the colleagues, students, and volunteers who have helped collect Paleocene mammals in southern Wyoming through the years. Thanks also to Gareth Dyke and an anonymous reviewer, whose comments improved the final manuscript. We dedicate this paper to the memory of our friends and former colleagues Gustav F. Winterfeld and Alan R. Tabrum, each of whom collaborated on multiple field expeditions to southern Wyoming.