Meridiungulata is an extinct clade with the rank of cohort or superorder, containing the South American ungulates Pyrotheria (possibly including Xenungulata), Astrapotheria, Notoungulata and Litopterna. It is not known if it is a natural group; 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 Merdiungulata died out following the invasion of South America by North American ungulates and predators during the Great American Interchange; a few species from both orders survived until the end-Pleistocene extinctions.
Meridiungulata may 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 is, however, entirely possible the Meridiungulata are actually 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 will remain tenuous unless DNA is found to confirm this relationship. Some paleontologists have also challenged the monophyly of Meridiungulata by suggesting that the pyrotheres may be more closely related to other mammals, such as Embrithopoda (an African order that may be related to elephants), than to other South American ungulates.
Results from the sequencing of protein remains from Pleistocene fossils of the taxa Toxodon (Notoungulata) and Macrauchenia (Litopterna) have indicated that at least these two orders are indeed laurasiatheres, and more specific closely related to odd-toed ungulates.
The following classification is from Rose 2006:
- Suborder †Notioprogonia
- Suborder †Toxodontia
- Suborder Typotheria
- Order †Astrapotheria
- Order †Pyrotheria
- Order †Xenungulata
- Muizon & Cifelli 2000
- Hunter & Janis 2006
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