Notoungulata is an extinct order of mammalian ungulates that inhabited South America during the Paleocene to the Holocene, living from approximately 57 Ma to 11,000 years ago. Notoungulates were morphologically diverse, with forms resembling animals as disparate as rabbits and rhinoceroses. Notoungula were the dominant group of ungulates in South America during the Paleogene and early Neogene. Their diversity declined during the Late Neogene, with only the large toxodontids persisting until the end of the Pleistocene. Several groups of Notoungulates separately evolved ever growing teeth like rodents and lagomorphs, a distinction among ungulates only shared with Elasmotherium.
Taxonomy
Notoungulata is divided into two major suborders, Typotheria and Toxodontia, alongside some basal groups which are potentially paraphyletic. Due to the isolated nature of South America, many notoungulates evolved along convergent lines into forms that resembled mammals on other continents. Examples of this are Pachyrukhos, a notoungulate that filled an ecological niche similar to those of rabbits and hares, and Homalodotherium, which resembled chalicotheres. The families Interatheriidae, Hegetotheriidae, Mesotheriidae and Toxodontidae separately evolved high crowned ever-growing teeth. During the Pleistocene, Toxodon was the largest common notoungulate. Most of the group became extinct after the landbridge between North and South America formed and allowed North American ungulates to enter South America in the Great American Interchange, and then to out-compete the native fauna. Mixotoxodon was the only member of the group to be successful in invading Central America and southern North America, reaching as far north as Texas.
This order is united with other South American ungulates in the super-order Meridiungulata. The notoungulate and litoptern native ungulates of South America have been shown by studies of collagen and mitochondrial DNA sequences to be a sister group to the perissodactyls, making them true ungulates. The estimated divergence date is 66 million years ago. This conflicts with the results of some morphological analyses which favoured them as afrotherians. It is in line with some more recent morphological analyses which suggested they were basal euungulates. Panperissodactyla has been proposed as the name of an unranked clade to include perissodactyls and their extinct South American ungulate relatives. Cifelli has argued that Notioprogonia is paraphyletic, as it would include the ancestors of the remaining suborders. Similarly, Cifelli indicated that Typotheria would be paraphyletic if it excluded Hegetotheria and he advocated inclusion of Archaeohyracidae and Hegetotheriidae in Typotheria. Notoungulata were for many years taken to include the order Arctostylopida, whose fossils are found mainly in China. Recent studies, however, have concluded that Arctostylopida are more properly classified as gliriforms, and that the notoungulates were therefore never found outside South and Central America. Based on an analysis of 133 morphological characters in 50 notoungulate genera, Billet in 2011 concluded that Homalodotheriidae, Leontiniidae, Toxodontidae, Interatheriidae, Mesotheriidae, and Hegetotheriidae are the only monophyletic families of notoungulates.
