Mesotheriidae

Mesotheriidae Temporal range: Late Eocene–Middle Pleistocene PreЄ Є O S D C P T J K Pg N
Mesotherium cristatum
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	†Notoungulata
Suborder:	†Typotheria
Family:	†Mesotheriidae Alston, 1876
Subfamilies and Genera
†Fiandraiinae †Fiandraia †Mesotheriinae †Altitypotherium †Caraguatypotherium †Eotypotherium †Eutypotherium †Hypsitherium †Mesotherium †Microtypotherium †Plesiotypotherium †Pseudotypotherium †Typotheriopsis †Trachytheriinae †Trachytherus

†Fiandraiinae

†Mesotheriinae

†Trachytheriinae

Mesotheriidae ("Middle Beasts") is an extinct family of notoungulate mammals known from the Eocene through the of South America. Mesotheriids were small to medium-sized herbivorous mammals adapted for digging.

Mesotheriids were small to medium sized notoungulates; larger forms were approximately the size of a sheep (Shockey et al., 2007). Additionally, the family is characterized by specializations of the teeth and skeleton. In the dentition, all mesotheriids have ever-growing incisors with enamel restricted to the anterior surface, a condition termed gliriform, as it also occurs in Glires (rodents and lagomorphs). The cheek teeth (premolars and molars) of mesotheriids are high-crowned (hypsodont) and in advanced members of the family, the cheek teeth are also ever-growing (Shockey et al., 2007). Mesotheriid skeletons are heavily built and show features associated with digging in living mammals. In particular, fossorial characteristics of mesotheriids include deeply fissured claws, presence of a sesamoid bone in the elbow and reinforcement of the pelvic girdle by addition of vertebrae to the sacrum and fusion of the sacrum and innominate (Shockey et al., 2007).

A biomechanical study of the skeleton of three mesotheriid genera (Trachytherus, Plesiotypotherium, and Mesotherium) spanning the temporal range of the family indicates that most or all mesotheriids were adapted for digging (Shockey et al., 2007). Shockey et al. concluded that mesotheriids likely dug for roots and tubers and were most similar in their diet and behavior to living wombats, although no living group is perfectly analogous. Extensive burrowing was considered possible but unlikely given the relatively large size of most mesotheriids.

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