The phylogeny and evolution of Cretaceous–Palaeogene metatherians: cladistic analysis and description of new early Palaeocene specimens from the Nacimiento Formation, New Mexico

Metatherian mammals were the most diverse mammalian clade in North America through the Late Cretaceous, but they underwent a severe extinction at the Cretaceous–Palaeogene (K-Pg) boundary. In order to clarify the origin of Palaeogene metatherians and the pattern of metatherian survivorship across the K-Pg boundary we conducted an inclusive species-level phylogenetic analysis of Cretaceous and early Palaeogene metatherian taxa. This analysis includes information from new Palaeocene specimens from south-western North America. Both the phylogenetic topology and information from new specimens support the validity of the genus Thylacodon and justify the recognition of a new species, T. montanensis. Thylacodon is closely related to Swaindelphys and Herpetotheriidae, which must have diverged by the latest Cretaceous due to its close relationship with late Campanian Ectocentrocristus. Pediomyidae and ‘Peradectidae sensu lato’ together comprise a major metatherian clade. Maastrichtidelphys, from the Late Cretaceous of the Netherlands, is the oldest member of ‘Peradectidae sensu lato’, indicating a Cretaceous origination for this group. Therefore, the major groups Herpetotheriidae and ‘Peradectidae sensu lato’, represented almost completely by Palaeocene taxa, must have originated in the Late Cretaceous. The lineages leading to these clades include at least four lineages that must have crossed the K-Pg boundary and therefore confirm that the K-Pg boundary marked a profound extinction event for metatherians and suggests that Palaeogene taxa originated from only a few clades of Cretaceous species, all of which were relatively minor or very rare components of known Cretaceous mammalian faunas.