Skull of Palaechthon nacimienti

Abstract

SPECIES in or near the ancestry of living primates first appear in the late Cretaceous and early Palaeocene of North America. Subsequent adaptive radiation of the Purgatorius-like ancestral stock produced the plesiadapoid families (Plesiadapidae, Carpolestidae, Paromomyidae) of the middle and late Palaeocene. Specialised members of all three families survived into the early Eocene, the paromomyid genus Phenacolemur persisting into the late Eocene. Most of the plesiadapoid species are known only from incomplete dentitions. In 1948, a crushed but nearly complete skull of a paromomyid was recovered from strata of middle Palaeocene age in the Kutz Canyon area of the San Juan Basin, New Mexico. The specimen has been described by Wilson and Szalay¹, who assign it to a new species (P. nacimienti) of the genus Palaechthon, known also from the mid-Palaeocene of Montana and Wyoming. The loss of the upper and lower first premolars excludes P. nacimienti from the ancestry of some of the Eocene prosimian lineages. Nevertheless, its persistently primitive molar morphology suggests that it may more closely resemble the last common ancestor of the plesiadapoids and the Eocene primates of modern aspect than do other plesiadapoids for which cranial remains are known. Skulls or partial skulls are known for dentally more specialised genera of each plesiadapoid family: Plesiadapis (Plesiadapidae), Carpolestes (Carpolestidae), and Phenacolemur (Paromomyidae).) We present here a reconstruction of the skull of P. nacimienti, together with some preliminary functional interpretations of its cranial and dental anatomy.