A SMALL ARCHAIC UNGULATE MAMMAL SKULL FROM THE EARLY PALEOCENE DENVER FORMATION OF COLORADO (CORRAL BLUFFS, EL PASO COUNTY)

The western North American fossil record documents the rise of mammals in the early aftermath of the Cretaceous–Paleogene (K–Pg) mass extinction, yet that record mostly consists of isolated teeth and jaw fragments. Recently, exceptionally preserved mammal skulls have been discovered in phosphatic concretions at Corral Bluffs—earliest Paleocene exposures of the Denver Formation just east of Colorado Springs, CO—which may help clarify the obscured phylogenetic relationships and ecologies of many early Paleocene mammals. Here we report the discovery of a well-preserved partial skull (DMNH EPV.136181) of a small-bodied archaic ungulate from ca. 600 ka after the K–Pg boundary at Corral Bluffs. The skull is mediolaterally compressed and missing much of the rostrum, but it includes partial maxillae and dentaries with posterior premolars and molars in occlusion on both sides and bones of the neurocranium and basicranium, including partial petrosals with the left side preserving aspects of the promontorium and tegmen tympani. Digital preparation of micro-CT scans has revealed dental features of DMNH EPV.136181 that resemble those of the conacodontine periptychid, Oxyacodon, such as (1) prominent hypocones on upper molars that project lingually beyond the protocones, (2) somewhat inflated P/p4 that are slightly smaller than M/m1, and (3) small paraconids on m1–3 situated on the midline. DMNH EPV.136181 also exhibits dental features that more closely resemble more basal archaic ungulates (e.g., Mimatuta, Maiorana), such as (1) a taller trigonid, (2) trigonid cusps that are not as closely appressed, and (3) a larger talonid basin. That combination of dental features suggests that DMNH EPV.136181 is a phylogenetically basal periptychid, and it therefore may help clarify aspects of early archaic ungulate systematics, such as the phylogenetic relationship between more basal taxa like Mimatuta and Maiorana and the Periptychidae. Further, given that crania of periptychids are primarily known only from derived and mostly large taxa, differences documented in the cranium of DMNH EPV.136181 (e.g., a petrosal with a prominent epitympanic wing, an anterodorsal process of its jugal that does not expand to occupy the lateral side of the anterior zygomatic root) may shed light on early cranial evolution within the Periptychidae.