Tetramerorhinus is an extinct genus of proterotheriid litoptern that lived during the Early and Middle Miocene in what is now Argentina and Peru.^[2]

Quick Facts Scientific classification, Type species ...

Tetramerorhinus Temporal range: Early Miocene-Middle Miocene (Colhuehuapian-Laventan) ~21–11.8 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Left lower jaw of Tetramerorhinus mixtum
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	†Litopterna
Family:	†Proterotheriidae
Subfamily:	†Proterotheriinae
Genus:	†Tetramerorhinus Ameghino 1894
Type species
†Tetramerorhinus fortis Ameghino, 1894
Species
T. cingulatum Ameghino 1891 T. fleaglei Kramarz & Bond 2005 T. mixtum Ameghino 1894 T. lucarius Ameghino 1894 T. prosistens Ameghino 1899
Synonyms[1]
Genus synonymy Tichodon Ameghino, 1894 T. cingulatum Proterotherium cingulatum Ameghino, 1891 Tetramerorhinus fortis Ameghino, 1891 Licaphrium cavum Zittel, 1893 Proterotherium principale Ameghino, 1894 Tichodon quadrilobus Ameghino, 1894 T. lucarius Diadiaphorus santaecrucis Lydekker, 1894 Licaphrium proclivum Ameghino, 1894 Proterotherium brachygnathum Ameghino, 1894 Proterotherium nitens Ameghino, 1894 Proterotherium perpolitum Ameghino, 1894 Proterotherium karaikense Ameghino, 1904 Proterotherium politum Ameghino, 1904 Proterotherium brachygnathus Scott, 1910 T. prosistens Proterotherium prosistens Ameghino, 1899 Diadiaphorus zamius Ameghino, 1899 Heptaconus obcallatus Ameghino, 1899 T. mixtum Proterotherium mixtum Ameghino, 1894 Proterotherium pyramidatum Ameghino, 1894 Proterotherium browni Scott, 1910 Proterotherium dodgei Scott, 1910

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This animal resembled a small horse, with slender, elongated legs. The body was approximately a meter long, and it is supposed to have weighed between 30 and 50 kilograms. The skull had a relatively elongated snout, with nasal bones longer than Diadiaphorus and with less developed upper incisors. The orbits were positioned near the middle of the skull. The molars had lower crowns than those of more derived proterotheriids, such as Proterotherium.^[3][4]

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.^[5]

The genus Tetramerorhinus was first described in 1894 by Florentino Ameghino, based on fossil remains found in Early Miocene deposits in Patagonia. Numerous species have been ascribed to this genus, such as Tetramerorhinus cingulatum, T. fleaglei, T. mixtum, T. lucarius, T. prosisttens.^[6][7]

Tetramerorhinus is a proterotheriid, a group of litopterns with characteristics similar to those of equids, 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.^[8]