New discoveries of early Paleocene (Torrejonian) primates from the Nacimiento Formation, San Juan Basin, New Mexico.

Primates underwent a period of diversification following the extinction of non-avian dinosaurs. Although the Order first appeared near the Cretaceous-Paleogene boundary, it is not until the Torrejonian (the second North American Land Mammal Age of the Paleocene) that a diversity of families began to emerge. One of the lithological units critical to understanding this first primate adaptive radiation is the early Paleocene Nacimiento Formation of the San Juan Basin (SJB; New Mexico). Primates previously described from this formation comprise six species of palaechthonid and paromomyid plesiadapiforms, all known from very limited material. Collecting has increased the sample of primate specimens more than fivefold. Included in the new sample is the first specimen of a picrodontid plesiadapiform from the Torrejonian of the SJB, referable to Picrodus calgariensis, and the first paromomyid specimen complete enough to allow for a species level taxonomic assignment, representing a new species of Paromomys. With respect to the 'Palaechthonidae', the current report describes large collections of Torrejonia wilsoni and Palaechthon woodi, and the first new specimens attributed to Plesiolestes nacimienti and Anasazia williamsoni since 1972 and 1994, respectively. These collections demonstrate previously unknown morphological variants, including the presence of a metaconid on the p4 of some specimens of T. wilsoni, a discovery that supports previous inferences about a close relationship between Torrejonia and Plesiolestes problematicus. This new sample considerably improves our knowledge of the poorly understood 'Palaechthonidae', and about the biostratigraphy, biogeography, and early evolution of North American primates. In particular, the rarity of paromomyids, the continuing absence of plesiadapid and carpolestid plesiadapiforms, and the presence of a number of endemic palaechthonid species in the SJB contrast with plesiadapiform samples from contemporaneous deposits to the north. Together, these data suggest that by the latter part of the early Paleocene primates had already developed not only an impressive diversity, but patterns of regional endemism.