Meridiungulata is an extinct clade with the rank of cohort or superorder, containing the South American ungulates Pyrotheria, Astrapotheria, Notoungulata and Litopterna. It is not known if it is a natural group; it is known that both Litopterna and Notoungulata form a clade based on collagen evidence, but the placement of the other members is uncertain. it was erected to distinguish the ungulates of South America from other ungulates. Relationships between the orders inside Meridiungulata remain unresolved and it could well be a "wastebasket taxon". Most Meridiungulata died out following the invasion of South America by North American ungulates and predators during the Great American Interchange, but a few of the largest species of notoungulates and litopterns survived until the end-Pleistocene extinctions. The notoungulate Mixotoxodon was able to invade North America to as far as present-day Texas.
Taxonomy
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, than to other South American ungulates. Results from the sequencing of collagen from Pleistocene fossils of the notoungulate Toxodon and the litopternMacrauchenia have indicated that at least these two orders are indeed laurasiatheres, and form a sister group to odd-toed ungulates. This result has been corroborated with mitochondrial DNAextracted from Macrauchenia, which points towards a branching date of 66 million years ago. Panperissodactyla has been proposed as the name of an unranked clade to include perissodactyls and their extinct South American ungulate relatives.
