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Tetramerorhinus is an extinct genus of Proterotheriidae Litopterna that lived during the Early and Middle Miocene in what is now Argentina and Peru.
The endocranial cast of a specimen of Tetramerorhinus lucarius has been preserved, which allowed to reconstruct certain structures of the animal's brain. The brain was quite large relatively to the size of the skull, due to the development, especially in the anterior part, of the neocortex, and to the complexity of the cerebellum. The olfactory lobes were small and separated by a deep notch, while the cerebellum was elongated and separated from the two cerebral hemispheres by a deep and narrow depression. The rhinencephalon was highly developed, and the piriform lobes were in a much less lateral position than those of archaic ungulates such as Phenacodus, and presented a sort of circular depression of unknown significance. Numerous longitudinal grooves covered the surface of the neocortex. The cranial nerves were unusually arranged relatively to those of other ungulates, and the foramen ovale was posterior to the tentorium osseum.
Tetramerorhinus is a proterotheriidae, a group of litopterna with characteristics similar to those of equidae, although they were not closely related. Tetramerorhinus seems to have been a derived form, closely related to Anisolophus and Proterotherium.
Below is a phylogenetic tree of the Proterotheriidae, based on the work of McGrath et al. 2020.
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