Mesotherium

Mesotherium ("middle beast") is an extinct genus of mesotheriid, a long-lasting family of superficially rodent-like, burrowing notoungulates from South America. It is one of the youngest notoungulates, spanning the Early-Middle Pleistocene, and is the last known member of Typotheria. It was first named by Étienne Serres in 1857, though initially lacked a type species. Another genus name Typotherium, was put forward by Auguste Bravard, and three species were included: however, the name Typotherium was not applied to any particular specimens, and is thus invalid. A type species for Mesotherium, M. cristatum, was named in 1867, securing its status as a valid genus. M. cristatum spanned the Early-Middle Pleistocene.

In many regards, Mesotherium is convergent with rodents, to the point where Serres suggested that it was either a missing link between them and "pachyderms" or close to the ancestry of all mammals. Though fairly small compared to some notoungulates, it was the largest typothere, possibly weighing up to 100 kg (220 lb). The teeth of Mesotherium grew continuously, and its cheek teeth were high-crowned, indicating a diet heavy in abrasive materials. It was probably a grazer for the most part, though, judging by the fact it shared some attributes with burrowing animals, it may also have dug for food underground.