Early Eocene Paromomyidae (Mammalia, Primates) from the southern Bighorn Basin, Wyoming: Systematics and evolution

Abstract

We present a critical review of the alpha taxonomy and evolution of Eocene North American paromomyid primates, based on analysis of more than 570 stratigraphically controlled dental and gnathic specimens from the early Eocene of the southern Bighorn Basin, Wyoming (Wasatchian, Willwood Formation). In addition to documenting numerous previously unpublished specimens of known taxa (including deciduous teeth), we also describe a new species,Phenacolemur willwoodensisn. sp., from the upper part of the Willwood Formation (Wa 5 and 6). The new species is intermediate in size betweenPhenacolemur simonsiandPhenacolemur citatusand has both primitive features (e.g., retention of m2–3 paraconids, relatively long molar trigonids) and derived traits (e.g., relatively reduced paraconid on m1 and no p4 paracristid, unlikeParomomys). Overall patterns of dental evolution in southern Bighorn Basin paromomyids provide some support for previously hypothesized periods of faunal change (Biohorizons). In particular,Phenacolemur praecoxevolves into the similarly sized but morphologically distinctPhenacolemur fortiorat Biohorizon A, andP. fortioris replaced byP. citatusjust below Biohorizon B. Two taxa previously believed to have become extinct at Biohorizon A(Ignacius graybullianus, P. simonsi)are shown to have persisted about a million years longer than previously thought. The Bighorn Basin paromomyids are of general interest in comprising a very dense sample that allows for the study of patterns of evolution against the backdrop of well-understood patterns of change in other mammalian lineages, and in climatic variables.