Notoungulata

Description and ecology

Notoungulates are united by a number of morphological characters of the skull, including a large epitympanic sinus (part of the inner ear) within the squamosal bone, as well as a "crochet" on the metaloph (a raised crest present on mammal teeth) of the upper molars. Notoungulates varied widely in body size, with early diverging notoungulates like Simpsonotus, and some hegetotheriid and interatheriid typotherians having a body mass of approximately 1–2 kilograms (2.2–4.4 lb), while the toxodontid Toxodon is suggested to have had a body mass exceeding 1,000 kilograms (2,200 lb). Typotheres generally occupied small-medium body size niches, while toxodontians were generally medium-large sized animals.

The families Interatheriidae, Hegetotheriidae, Mesotheriidae and Toxodontidae separately evolved high crowned (hypsodont) ever-growing (hypeselodont) cheek teeth, with high crowned species constituting the majority of notoungulates from the Late Oligocene onward. This adaptation was historically suggested to be the result of a diet increasingly incorporating grass, but this has been questioned, and other authors suggesting that it may have been due to the increasing intake of abrasive particles from volcanic sources.

Many typotheres have bodyforms convergent on rodents, hyraxes and rabbits, with some rabbit-like hegetotheriids suggested to have developed a rabbit-like bounding locomotion. The basal notungulate Notostylops and the mesotheriids are suggested to have engaged in digging, with mesotheriids suggested to have had an ecology similar to wombats. Toxodontids have sometimes been compared to rhinoceroses and hippopotamuses in overall bodyform and tooth morphology. The Miocene toxodontian Homalodotherium had claws on its forelimbs and is thought to have had an ecology similar to the extinct chalicotheres, rearing on its hindlegs to feed. Like perissodactyls, notoungulates were likely primitively hindgut fermenters, but it has also been proposed that some of them may have had fermentation more similar to ruminants based on their skeletal anatomy, though this is uncertain.

Evolutionary history

The oldest notoungulates appeared during the Paleocene, probably originating from "condylarth" ancestors that had migrated from North America. Notoungulates and other South American native ungulates reached their apex of diversity during the Eocene and Oligocene. Notoungulate species diversity was stable during the Miocene, though 45% of the family diversity of the group became extinct during the interval, including Homalodotheriidae, Leontiniidae, and Interatheriidae. The diversity of the group declined during the Pliocene and Pleistocene, which is coeval in time with the Great American Interchange, which allowed ungulates and other mammals from North America to enter South America. This decline has historically been attributed to competition with the new North American arrivals, though earlier views had probably overstated the importance of this, with climatic change also likely being an important factor. As part of the Great American interchange, the toxodontid Mixotoxodon migrated into Central and North America, with its furthest northern record being in Texas. The last hegetotheriids are known from the Early Pleistocene (with a supposed Middle Pleistocene record being considered questionable). The youngest known member of Typotheria, the mesotheriid Mesotherium, has its last records in the late Middle Pleistocene, around 220,000 years ago. The last notoungulates, the toxodontids Toxodon, Mixotoxodon and Piauhytherium became extinct at the end of the Late Pleistocene around 12,000 years ago as part of the Late Pleistocene megafauna extinctions, along with most other large mammals in the Americas. The extinctions followed the arrival of the first humans to the Americas and they have been suggested to have been a potential causal factor in the extinctions.