Litopterna (from "smooth heel") is an extinct order of fossil hoofed mammals from the Cenozoic era. The order is one of the five great clades of South American ungulates that gave the continent a unique fauna until the Great American Biotic Interchange. Like other endemic South American mammals, their relationship to other mammal groups had long been unclear, but genetic and proteomic evidence indicates that their closest living relatives are Perissodactyls (odd-toed ungulates) including horses, rhinoceros, and tapirs, and that litopterns are closely related to notoungulates, another widespread group of South American ungulates.