†Fiandraiinae †Mesotheriinae †Trachytheriinae Mesotheriidae ("Middle Beasts") is an extinct family of notoungulate mammals known from the Oligocene through the Pleistocene of South America.

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).

[6] The family reached its greatest diversity in the Miocene,[5] and mesotheriids persisted into the middle Pleistocene, in the form of the type genus, Mesotherium.

[3] In addition to Mesotheriidae, Typotheria traditionally includes other small bodied notoungulates in the families Oldfieldthomasiidae, Interatheriidae, and Archaeopithecidae.

[9] These families have traditionally been placed in a separate suborder, Hegetotheria, but phylogenetic studies indicate that their exclusion would render Typotheria paraphyletic.