Primitive boreosphenidan mammal (?Deltatheroida) from the Early Cretaceousof Oklahoma

We describe a new boreosphenidan mammal, Atokatheridium boreni gen. et sp. n., from the Early Cretaceous of Oklahoma, based on an upper molar and a te ntatively referred lower molar. The upper molar is characterized by a small protocone and unwinged conules, broad stylar shelf, paracone taller than m etacone, and lack of pre- and post-cingula. Comparisons with relevant Early and Late Cretaceous boreosphenidans suggest closest similarity to Deltathe roida, including one character (extreme development of the distal stylar sh elf, which projects labially and lacks cusps) interpreted as derived. The t entatively attributed lower molar shows similarity to Deltatheridium and th e ?aegialodontid genus Kielantherium in having the paraconid higher than th e metaconid, but differs from Kielantherium in having a differently shaped talonid. From Aegialodon it differs in having a vertically oriented (rather than semi-procumbent) paraconid and a larger talonid. We figure also two i solated trigonids, differing in size, which show some resemblance to that o f ?Atokatheridium. Deltatheroidans, despite their generally primitive denta l morphology, are otherwise surely known only from the Late Cretaceous, and are largely restricted to the Old World. If a deltatheroidan, the new taxo n implies a significant temporal range extension for the group, and provide s another biogeographic link between Cretaceous mammals of Asia and North A merica.