New omomyoids (Euprimates, Mammalia) from the late Uintan of southern California, USA, and the question of the extinction of the paromomyidae (plesiadapiformes, primates)

Abstract

Paromomyidae has been thought to represent the longest-lived group of stem primates (plesiadapiforms), extending from the early Paleocene to late Eocene. We analyzed primate material from the late-middle Eocene of southern California that had initially been ascribed to cf. Phenacolemur shifrae. This material falls at the lowest end of the size range for the family. The Californian specimens also exhibit several dental features that are atypical for paromomyids, such as a strong paraconid on the third lower molar, and differ from early Eocene species of Phenacolemur in lacking a distally expanded distolingual basin on upper molars. This combination of traits is more typical of earlier paromomyids with more plesiomorphic morphologies (e.g., Paromomys) and as such is inconsistent with the late age of these specimens. The purported paromomyids Ph. shifrae and Ignacius mcgrewi known in deposits of similar age are comparably tiny and share many of the characteristics found in the southern California material that distinguish them from typical early Eocene paromomyids. However, these traits are shared with some trogolemurin omomyoid euprimates that are of similar size. We argue that the material from southern California, along with Ph. shifrae and I. mcgrewi, should be transferred to a new genus of trogolemurin omomyoid. Purported European records of paromomyids later than the earliest middle Eocene are reconsidered and found to be non-diagnostic. After the early middle Eocene, only a single tooth of a paromomyid can be confirmed, indicating that the group suffered near-extinction, possibly correlated with the Early Eocene Climatic Optimum.