South American native ungulates

South American native ungulates, commonly abbreviated as SANUs, are extinct ungulate-like mammals of controversial affinities that were indigenous to South America prior to the Great American Biotic Interchange. They comprise five major groups conventionally ranked as orders—Astrapotheria, Litopterna, Notoungulata, Pyrotheria, and Xenungulata—as well as the primitive "condylarth" groups Didolodontidae and Kollpaniinae. It has been proposed that some or all of the members of this group form a clade, named Meridiungulata, though the relationships of South American ungulates remain largely unresolved. The two largest groups of South American ungulates, the notoungulates and the litopterns, were the only groups to persist beyond the mid Miocene. Only a few of the largest species of notoungulates and litopterns survived until the end-Pleistocene extinctions. Though most SANUs lived in South America, astrapotheres and litopterns are known from Eocene aged deposits in the Antarctic Peninsula and the notoungulate Mixotoxodon spread as far north as what is now Texas during the Great American Biotic Interchange. Meridiungulata might have originated in South America from a North American condylarth ancestor, and they may be members of the clade Laurasiatheria, related to other ungulates, including artiodactyls and perissodactyls. It has, however, been suggested the Meridiungulata are part of a different macro-group of placental mammals called Atlantogenata. Much of the evolution of meridiungulates occurred in isolation from other ungulates, a great example of convergent evolution. However, the argument that meridiungulates are related to artiodactyls and perissodactyls needs support from molecular sequencing. Some paleontologists have also challenged the monophyly of Meridiungulata by suggesting that the pyrotheres are more closely related to other mammals, such as Embrithopoda (an African order possibly related to elephants), than to other South American ungulates.