Meridiungulata

Meridiungulata might have originated in South America from a North American condylarth ancestor,[5] and they may be members of the clade Laurasiatheria, related to other ungulates, including artiodactyls and perissodactyls.[6] It has, however, been suggested the Meridiungulata are part of a different macro-group of placental mammals called Atlantogenata.[7]

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.[8]

Results from the sequencing of collagen from Pleistocene fossils of the notoungulate Toxodon and the litoptern Macrauchenia have indicated that at least these two orders are indeed laurasiatheres, and form a sister group to odd-toed ungulates.[9][10] This result has been corroborated with mitochondrial DNA extracted from Macrauchenia, which points towards a branching date of 66 million years ago.[11] Panperissodactyla has been proposed as the name of an unranked clade to include perissodactyls and their extinct South American ungulate relatives.[9]

Astrapotherium magnum skull

The following classification is from Rose 2006:

• †Meridiungulata
  • †Didolodus (incertae sedis)
  • †Eoastrapostylops
  • †Amilnedwardsia (Notopterna?)
  • Order †Notoungulata
    • Suborder †Notioprogonia
      • †Henricosborniidae
      • †Notostylopidae
    • Suborder †Toxodontia
      • †Isotemnidae
      • †Notohippidae
      • †Leontiniidae
      • †Toxodontidae
      • †Homalodotheriidae
    • Suborder Typotheria
      • †Oldfieldthomasiidae
      • †Interatheriidae
      • †Archaeopithecidae
      • †Mesotheriidae
      • †Campanorcidae
    • Suborder Hegetotheria (Typotheria?)
      • †Archaeohyracidae
      • †Hegetotheriidae
  • Order †Litopterna (Panameriungulata)
    • †Protolipternidae
    • †Notonychopidae (Notopterna?)
    • †Macraucheniidae
    • †Adianthidae
    • †Proterotheriidae
  • Order †Astrapotheria
    • †Eoastrapostylopidae
    • †Trigonostylopidae (Trigonostylopoidae?)
    • †Astrapotheriidae
  • Order †Pyrotheria
    • †Colombitheriidae
    • †Pyrotheriidae
  • Order †Xenungulata
    • †Carodniidae