Mesotherium

Mesotherium ("Middle Beast") is an extinct genus of mesotheriid, a long-lasting family of superficially rodent-like, burrowing notoungulates from South America. It one of the youngest notoungulates, and the last known member of Typotheria. It was first named by Étienne Serres in 1867, and only contains a single species, Mesotherium cristatum, spanning the Early-Middle Pleistocene.

Etymology
Serres named Mesotherium so due to his belief that it was an intermediate between rodents and pachyderms (or ungulates), due to its large upper incisors, and its size and proportions. "Serres—by a happy inspiration proposed calling it Mesotherium—as being a common centre towards which all mammalia got happily confounded," Hugh Falconer wrote Darwin in April 1863. "Bravard sent it home under the name of Typotherium as being the central type from which all mammals diverged." It was Serres' view that there was only one underlying animal type.

Despite Serres' having officially named the genus Mesotherium, it was known from the late 19th century to the early 20th century under the name "Typotherium", given it by the French palaeontologist living in Argentina, Auguste Bravard; under this name, Bravard sent the skull he found to Paris, which led to the family being named "Typotheriidae", and served as the basis for the order Typotheria. As the name Mesotherium had been published earlier in the same year as Typotherium, Mesotherium was declared the valid name of the genus, (Simpson, 1980 ) and Mesotheriidae the valid name of the family. Nevertheless, as the rules do not apply to anything above the family, the name of the order Typotheria is still in use, but refers to a wider range of rodent-like notoungulates.

Description
Mesotherium was likely the size of a small sheep, and weighed around 55 kg, or possibly up to 100 kg, making it the largest known member of Typotheria. Like most rodents, it had superficially long upper incisors, which met at the tips, however, it had enamel on both the labial and lingual surfaces of the incisors, while rodents only have enamel on the labial surface. The lower incisors of Mesotherium were reminiscent of those of a rabbit's.

The ankle joint of Mesotherium was made up of a "ball-and-socket" arrangement between the astragalus and the navicular, as well as a sliding articulation of the calcaneocuboid joint, which would cause extension-flexion in the ankle, as well as supination-pronation of the foot. Because of this, Florentino Ameghino predicted in 1905, and confirmed in 1906 that Mesotherium would have a great toe.

Mesotherium was likely fossorial, in that it dug, possibly to find food, or to construct dens.

Ecology
Mesotherium is suggested to have been a mixed feeder that likely consumed a large quantity of grasses, with its wide snout allowing for bulk feeding. In comparison to earlier mesotheriids, it probably consumed less hard food items.

Chronology
The oldest known specimens of the genus date to the Early Pleistocene. The youngest known specimen of the genus dates to around 220,000 years ago during the late Middle Pleistocene.