Litopterna
{{Taxobox
color = pink | name = Litopterna | regnum = Animalia | phylum = Chordata | classis = Mammalia | ordo = LitopternaThe Litopterna, also known as the pseudo-horse, is an order of fossil mammals from the Tertiary Period that displays toe reduction. Three-toed, and even a one-toed horselike form were developed.
This order, known only from South America, 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 seem to have originated in South America from the South American condylarths, and therefore to have the same source as the latter. However, there is a growing number of scientists who believes the Litopterna (together with other South-American ungulates) originated completely independent from the other Ungulates, thus are unrelated to the condylarths. They proposed a new clade to contain these groups: the Meridiungulata.
The Litopterna, like the notoungulates and toxodonts, 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 played a role in the ecosystem similar to horses in Laurasia.*Order Litopterna - Litopterans (all members of the order extinct South American forms)
**Family Protolipternidae - incertae sedis
**Superfamily Macrauchenioidea
***Family Macraucheniidae
***Family Notonychopidae
***Family Adianthidae
**Superfamily Proterothrrioidea
***Family Prototheriidae*McKenna, M. C, and S. K. Bell. 1997. Classification of Mammals Above the Species Level. Columbia University Press, New York, 631 pp.
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