Temporal range: Paleocene–Pleistocene
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 Pleistocene. 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 Pleistocene.
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.
A recent analysis based on collagen sequences obtained from Macrauchenia and Toxodon found that litopterns along with notoungulates form a sister group to perissodactyls, making them true ungulates. This contrasts with the results of some past morphological analyses which favoured them as afrotherians. It is consistent with some more recent morphological analyses which suggested they were basal euungulates.
- Order Litopterna — Litopterans (all members of the order extinct South American forms)
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- McKenna, Malcolm C; Bell, Susane K (1997). Classification of Mammals Above the Species Level. New York: Columbia University Press. ISBN 978-0-231-11013-6.
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