Litopterna (from "smooth heel") is an extinct order of fossil hoofed mammals from the Cenozoic era. The order is one of the five great orders of South American ungulates that were endemic to the continent, until the Great American Biotic Interchange brought new ungulate species. Like other endemic South American mammals, their relationship to other mammal groups had long been unclear, but recent 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. There were two major groups of litopterns: Proterotheriidae and Macraucheniidae. Proterotheriids were medium to large animals that evolved adaptations for fast running, and occupied a variety of niches that elsewhere were filled by animals such as goats and antelopes, mouse deer, and horses. Macraucheniids were large to very large animals with long necks; they evolved retracted nasal openings, indicating that a number of their species likely had a muscular upper lip or short trunk. They likely filled roles in the environment similar to camels, giraffes, sivatheres, and browsing rhinoceroses on other continents. Many types of litopterns were abundant in South American faunas, almost all ate plants, and the group reached its maximum diversity in the late Miocene. All litopterns displayed toe reduction – three-toed forms developed, and some proterotheriids had a single hoof on each foot. Together with Macraucheniopsis, Neolicaphrium, and Xenorhinotherium, Macrauchenia was among the youngest genera of litopterns, and these two appear to have been the only members of the group to survive the Great American Biotic Interchange. All four became extinct at the end of the Pleistocene. The genera that died out during this faunal exchange are presumed to have been driven to extinction at least in part by competition with invading North American ungulates. Evolutionary background This order is known only from fossils in South America and Antarctica, however, possible remains belonging to a litoptern, likely a macraucheniid, have been found in Mexico. Litopterns, like the notoungulates and pyrotheres, are examples of ungulate mammals that evolved independently in "splendid isolation" on the island continent of South America. Like Australia, South America was isolated from all other continents following the breakup of Gondwana. During this period of isolation, unique mammals evolved to fill ecological niches similar to other mammals elsewhere. The Litopterna occupied ecological roles as browsers and grazers similar