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Hippopotamidae (hippopotami; order Artiodactyla, suborder Suiformes) A family of large, amphibious artiodactyls, perhaps descended from the Anthracotheriidae or (as recently proposed) from primitive Tayassuidae, which appear in the fossil record for the first time in sediments of Upper Pliocene age. Now confined to Africa, during the Pleistocene they were widespread in the warmer regions of the Old World. In modern forms the dentition is complete except for the outer incisors. The eyes are small and set high on the skull, and the nostrils can be closed. Each limb possesses four digits, all of which are functional. There are two monotypic genera, Hippopotamus amphibius (common hippopotamus) and Choeropsis liberiensis (pygmy hippopotamus).