Mesotherium

Mesotherium Temporal range: Early Pleistocene–Middle Pleistocene PreꞒ Ꞓ O S D C P T J K Pg N ↓
Skull of Mesotherium cristatum
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	†Notoungulata
Family:	†Mesotheriidae
Subfamily:	†Mesotheriinae
Genus:	†Mesotherium Serres, 1867
Type species
†Mesotherium cristatum Serres, 1857
Synonyms
Mesotherium: Typotherium Mesotherium cristatum M. hystatum Ameghino 1904 M. maendrum Ameghino 1887 M. pachygnathum Gervais & Ameghino 1880

Mesotherium ("Middle Beast")^[1] 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.^[2]

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.^[1] "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.^[3] "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;^[4] 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.^[1] 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^[4]) and Mesotheriidae the valid name of the family.^[1] 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.^[1]

Mesotherium was likely the size of a small sheep, and weighed around 55 kilograms (121 lb),^[1] or possibly up to 100 kilograms (220 lb), making it the largest known member of Typotheria.^[5] Like most rodents, it had superficially long upper incisors, which met at the tips,^[1] however, it had enamel on both the labial and lingual surfaces of the incisors, while rodents only have enamel on the labial surface.^[1] The lower incisors of Mesotherium were reminiscent of those of a rabbit's.^[1]

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.^[6] Because of this, Florentino Ameghino predicted in 1905, and confirmed in 1906 that Mesotherium would have a great toe.^[6]

Mesotherium was likely fossorial, in that it dug, possibly to find food,^[1][6] or to construct dens.^[5]

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.^[5]

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.^[7]

• The Origin and Evolution of Mammals (Oxford Biology) by T. S. Kemp
• Horns, Tusks, and Flippers: The Evolution of Hoofed Mammals By Donald R. Prothero, Robert M. Schoch Published 2003 JHU Press ISBN 0-8018-7135-2
• Forms of Animal Life: A Manual of Comparative Anatomy By George Rolleston, William Hatchett Jackson Published 1888 Clarendon Press
• The Century Dictionary: An Encyclopedic Lexicon of the English Language By William Dwight Whitney. Published 1890 The Century Company; original from Harvard University.
• Classification of Mammals Above the Species Level: Above the Species Level By Malcolm C. McKenna, Susan K. Bell, George Gaylord Simpson. Published 1997 Columbia University Press. ISBN 0-231-11013-8
• Neanderthals Revisited: New Approaches and Perspectives By Katerina Harvati, Terry Harrison. Publisher Springer ISBN 1-4020-5844-6

• Mesotherium in the Paleobiology Database
• Illustration of the head of a Mesotherium
• New Mesotheriidae (Mammalia, Notoungulata, Typotheria),geochronology and tectonics of the Caragua area, northernmost Chile^{[dead link]}