Mesotherium ("middle beast")[2] is an extinct genus of mesotheriid, a long-lasting family of superficially rodent-like, burrowing notoungulates from South America.

[1] 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.

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.

[4] Serres believed, based on its rodent-like incisors, ungulate-like body and the presence of a clavical (which most ungulates lack), that it represented an intermediate stage between Rodentia and "Pachydermata", an artificial assemblage of large mammals that was originally placed in Ungulata.

[2] According to a letter written by Scottish evolutionary biologist Hugh Falconer to English naturalist Charles Darwin, Serres believed that Mesotherium may have instead represented "a common centre towards which all mammalia got happily confounded".

[5] In 1867, as part of a series of monographs, Serres named Mesotherium cristatum, designating it the type species[6] and fully validating the genus.

[11] The muzzle, meanwhile, was fairly slender, a common trait of selective feeders,[2] contributing to its rodent-like appearance.

Like other mesotheriid typotheres, Mesotherium had cleft, nail-like ungual phalanges on its forelimbs,[2] while those of the hind limbs were more hoof-like.

Bruce J. Shockey, Darin A. Croft and Federico Anaya, in 2007, proposed that it, and most other mesotheriids, were scratch-diggers, using their chisel-like teeth to cut roots and loosen the substrate ahead of them, thereby supplementing the actions of their forelimbs.

Shockey, Croft and Anaya noted that this indicates a more selective diet, and lends some credence to the notion that mesotheriids were fossorial: high tooth crowns would protect against abrasive grit as much as they would grasses.

[2] However, Marcos D. Ercoli et al. suggested that Mesotherium's mouth was broad enough to function well while grazing, and that their cheek tooth morphology is very similar to that of rhinocerotids.

They did not, however, discard the hypothesis that Mesotherium could have fed on buried plant material, noting that both possibilities could have been true at the same time, and would have allowed it to exploit additional food sources when necessary.