Litopterna

Litopterna Temporal range: Paleocene–Pleistocene PreЄ Є O S D C P T J K Pg N
Macrauchenia
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Infraclass:	Eutheria
Superorder:	†Meridiungulata
Order:	†Litopterna Ameghino, 1889

Litopterna (from Ancient Greek: λῑτή πτέρνα "smooth heel") is an extinct order of fossil hoofed mammals (ungulates) from the Cenozoic period that displayed toe reduction – three-toed forms developed; there was even a one-toed horselike form.

This order, known only from South America and Antarctica, was common and varied in early faunas and persisted, in decreasing variety, into the . Early forms are near the condylarths, to such an extent that the litopterns might be considered merely as surviving and diversely specialized condylarths. They are suspected of being descended from South American condylarths, and therefore to have the same source as the latter. However, an opposing view has been that litopterns (together with other South-American ungulates) originated independently from other ungulates and thus are unrelated to condylarths. A proposed clade containing these groups is the Meridiungulata. Macrauchenia was the youngest genus of litopterns, and was the only litoptern group to survive the Great American Interchange; it died out at the end of the .

The Litopterna, like the notoungulates and pyrotheres, are examples of ungulate mammals that arose relatively 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 to horses and camels in Laurasia.