EARLY CENOZOIC SURVIVORS: PUERCAN METATHERIA IN THE WESTERN INTERIOR

Survival across the Cretaceous-Paleogene (K-Pg) boundary was difficult, and mammalian extinction is estimated to be at least 75%. Whereas the Metatheria (marsupials and their closest fossil relatives) dominated the mammalian fauna during the latest Cretaceous with at least 20 species, exceedingly few crossed the K-Pg boundary. In earliest Paleocene (Puercan) time, only Thylacodon pusillus, T. montanensis, and Peradectes minor are currently known. Here we describe three fossils of metatherians from two Puercan localities in the Western Interior.

An isolated upper molar and a partial dentary were recovered from Puercan strata of the China Butte Member, Fort Union Formation in the Great Divide Basin (GDB), southern Wyoming. Based upon fossils described in prior studies, UCM locality 2011035 is considered early Puercan in age. We identify the isolated upper molar (UCM 103314) as T. pusillus due to similar morphology and size to the species. It bears stylar cusps A - E, with cusp B the widest mesiodistally. The stylar margin does not have a concave ectoflexus. There is no ridge connecting the stylar shelf to cusp C, as seen in T. montanensis, and the internal and anterior cristae are weakly developed. Additionally, the size of the upper molar as well as the molars in the dentary (UCM 103091) fall within the size range of T. pusillus, the largest of the three Puercan species.

Further south in the Denver Basin of Colorado, a left dentary (UMC 48598) from a middle Puercan locality in the Denver Formation falls outside the known size range of T. pusillus and likely represents a new species in the clade Herpetotheriidae. Although the teeth are morphologically similar to T. pusillus, the molars are approximately 30% larger than those of T. pusillus from the San Juan and Denver Basin.

The fossils of Thylacodon we report here from the GDB are the first to be described from that basin, despite many other eutherian and multituberculate jaws and teeth found at the same locality. Notably, the presence of a larger species of Metatheria from the Denver Formation increases the known diversity of metatherians in earliest Paleocene time. Further, it supports the hypothesis that mammalian body sizes increased within the first few hundred thousand years after the K-Pg extinction, as ecosystems recovered and new niches opened for adaptation.