Functional morphology of plesiadapiform distal phalanges and implications for the evolution of arboreality in Paleogene euarchontans.

With a few exceptions, crown-clade Primates differ from other arboreal mammalian clades by having nails instead of claws on most post-axial digits. Distal phalanx morphology of close extant and fossil relatives of crown-clade Primates provides a context in which to study the evolution of this characteristic feature. Plesiadapiforms are a diverse group of extinct arboreal mammals closely related to crown-clade Primates (together classified as total clade Pan-Primates) that have distal phalanges that are indicative of having supported keratinous claws, with the only documented exception being that of Carpolestes simpsoni which may have had a nail on its hallucal phalanx. To contextualize morphological variation among plesiadapiform distal phalanges, we investigated the influence of behavior and phylogeny on post-axial distal phalanx morphology using a broad sample of extant mammalian taxa (273 distal phalanges from 67 species) compared to those known for plesiadapiforms (26 specimens from 9 species). Results show that plesiadapiform distal phalanges share characteristics with those of both extant generalized arborealists/scansorialists and antipronograde specialists, consistent with previous behavioral reconstructions of frequent vertical clinging and climbing based on other regions of the skeleton. Distal phalanges of Plesiadapis cookei, Plesiadapis tricuspidens, and the "Le Quesnoy plesiadapid" are most similar to those of extant species that clasp branches between claws and the rest of the autopod when climbing. In contrast, known distal phalanges of micromomyids and the plesiadapoid C. simpsoni share some similarities with those of extant taxa that rely more heavily on grasping with digital pads when climbing. [ABSTRACT FROM AUTHOR]