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 . 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.