PHYLOGENY OF A NEW EARLIEST PALEOCENE (PUERCAN) ARCTOCYONID FROM THE GREAT DIVIDE BASIN, WYOMING, AND IMPLICATIONS FOR UNDERSTANDING RELATIONSHIPS AMONG THE EARLIEST ‘CONDYLARTHS’

Studies suggest that the radiation of placental mammals after the Cretaceous-Paleogene (K-Pg) extinction was explosive and occurred within a few hundred thousand years of the boundary. Earliest Paleocene (Puercan) faunas are vital for testing this hypothesis. A new early Puercan (Pu1) fauna has been discovered from the China Butte Member of the Fort Union Formation in the Great Divide Basin, southern Wyoming. The fauna includes a new arctocyonid ‘condylarth’ genus and species, described here. Based on a sample of 18 specimens (including 14 partial dentaries) whose combined dentitions represent the p2, p4, and m1–3, the new taxon appears most similar in size and molar morphology to early Puercan arctocyonid Oxyprimus erikseni, but differs significantly in its p4 morphology. To examine the relationships between the new taxon and other Puercan arctocyonids and periptychids from the Western Interior of North America, a phylogenetic analysis utilizing 17 taxa (including 16 ‘condylarth’ species and outgroup species Cimolestes incisus) and 52 dental characters was performed. Characters were aggregated from a number of previous phylogenetic analyses of ‘condylarth’ taxa, and scored based on direct comparative study of specimens and casts from several museum collections, as well as descriptions of dental morphology in the literature. The resulting strict consensus tree of 177 steps shows that the new arctocyonid from the Great Divide Basin is closely related to Oxyprimus spp. and Protungulatum donnae. In addition, the Puercan Periptychidae appear to be paraphyletic; the early Puercan periptychids Mimatuta morgoth, Mimatuta minuial, and Maiorana noctiluca appear more closely allied with the Arctocyonidae, while the remaining Puercan periptychid taxa Conacodon spp., Ampliconus browni, Oxyacodon priscilla, and Mithrandir gillianus form a monophyletic clade. This analysis is the first to focus on a large set of Puercan arctocyonid and periptychid taxa across geographically-widespread localities, and, with the addition of the new species from the Great Divide Basin, suggests that diversity among early Puercan ‘condylarths’ is higher than previously recognized.