New Mexico Geological Society Annual Spring Meeting — Abstracts

A new fossil vertebrate local fauna (NMMNH localities 4723, 4725) at the head of Willow Wash, northwestern San Juan Basin represents the first new Puercan site from New Mexico to be reported in almost a century. It is approximately midway between the classic late Puercan (Pu3) sites at the heads of the West Fork of Gallegos Canyon and Alamo Wash. Fossils are stratigraphically restricted to a narrow zone about 3.5 m thick at the base of a channel sandstone complex approximately 15 m above the base of the Nacimiento Formation. This zone correlates with the "Taeniolabis zone" of other localities and is therefore of late Puercan (Pu3 age). The fauna from this site is here termed the Split Lip Flats Local Fauna (LF).

The Split Lip Flats LF is unique among late Puercan faunas previously reported from the San Juan Basin in that it includes a rich microfauna. Initial surface collecting and wet screening and picking of approximately 100 kg of matrix from several sites have yielded a diverse fauna that includes gars, stingrays, turtles, glyptosaurine lizards, crocodilians, and mammals. Mammals, represented primarily by isolated teeth, are the most abundant and diverse component of the fauna.

A preliminary list of mammals from the site includes Taeniolabis taoensis, Periptychus sp., Mithrandir gillianus, Conacodon cophator, Choeroclaenus turgidunculus, numerous unidentified arctocyonids, numerous small multituberculates, several isolated teeth of small marsupials, and numerous small eutherians. Significant among the small eutherian specimens is an associated partial set of lower teeth referrable to the poorly known Cimolestid Cimolestes simpsoni, as well as teeth of several new or poorly known taxa. Additional collecting and a thorough description of the Split Lip Flats LF will result in a better characterization of late Puercan mammals and add to a better understanding of the evolutionary radiation of mammals at the beginning of the Paleogene of North America.