Southernmost occurrence of Purgatorius sheds light on the biogeographic history and diversification of the earliest primate relatives

Fossils of the earliest known euarchontan mammals, purgatoriid plesiadapiforms, first appear during the aftermath of the Cretaceous/Paleogene mass extinction. The oldest and most basally branching plesiadapiform, Purgatorius, has been documented in Montana and Saskatchewan during the earliest Paleocene Puercan North American Land Mammal Age, but no plesiadapiforms have been recovered from southern basins of the Western Interior until later in the Paleocene. This temporal and geographic gap has made the early biogeographic history and diversification patterns of plesiadapiforms unclear. Here we report Purgatorius fossils recovered from extensive screen-washing of sediments from the Corral Bluffs study area, Denver Basin, Colorado, which represent the first Puercan occurrence of a plesiadapiform south of Montana. Isolated teeth of Purgatorius described here exhibit a unique combination of features, and may represent a new, early diverging species. This discovery bridges the temporal and geographic gap among the earliest plesiadapiforms and suggests a northern origin followed by a southerly diversification in the earliest Paleocene. The presumed absence of Puercan plesiadapiforms in southern regions of the Western Interior of North America was at least in part due to sampling biases. Additional screen-washing efforts in these regions will further clarify biogeographic patterns of plesiadapiforms and other small-bodied earliest Paleocene mammals.