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Arctocyonidae (superorder Protoungulata, order Condylarthra) An extinct family of unspecialized primitive mammals (Eutheria), with more than 20 genera, which lived from the Cretaceous to the Eocene but were most abundant in the Palaeocene. Some were the size of modern bears and were probably omnivorous; others were smaller and herbivorous. The body was slim, the limbs slender, the feet clawed. The brain case was small, and the teeth primitive, with moderately sharp cusps. Protoungulatum of the N. American Upper Cretaceous is the oldest known ungulate.