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Abstract The Periptychidae, an extinct group of archaic ungulates (‘condylarths’), were the most speciose eutherian mammals in the earliest Paleocene of North America, epitomizing mammalian ascendency after the Cretaceous–Paleogene (K–Pg) mass extinction. Although periptychids are mostly known from fragmentary gnathic remains, the Corral Bluffs area within the Denver Basin, Colorado, has yielded numerous exceptionally well-preserved mammalian fossils, including periptychids, from the earliest Paleocene. Here we describe a partial cranium and articulated dentaries plus an additional unassociated dentary fragment of a small-bodied (~273–455 g) periptychid from ca. 610 thousand years after the K–Pg mass extinction (Puercan 2 North American Land Mammal ‘age’) at Corral Bluffs. Based on these new fossils we erectMilitocodon lydaegen. et sp. nov. The dentition ofM. lydaeexhibits synapomorphies that diagnose the Conacodontinae, but it is plesiomorphic relative toOxyacodon, resembling putatively basal periptychids likeMimatutaandMaioranain several dental traits. As such, we interpretM. lydaeas a basal conacodontine. Its skull anatomy does not reveal clear periptychid synapomorphies and instead resembles that of arctocyonids and other primitive eutherians.M. lydaefalls along a dental morphocline from basal periptychids to derived conacodontines, which we hypothesize reflects a progressive, novel modification of the hypocone to enhance orthal shearing and crushing rather than grinding mastication. The discovery and thorough descriptions and comparisons of the partialM. lydaeskull represent an important step toward unraveling the complex evolutionary history of periptychid mammals.