''Tetramerorhinus'' is an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

of proterotheriid

litoptern Litopterna (from grc, λῑτή πτέρνα "smooth heel") is an extinct order of fossil hoofed mammals from the Cenozoic era. The order is one of the five great orders of South American ungulates that were endemic to the continent, until th ...

that lived during the Early and Middle

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

in what is now

Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the second-largest country in South America after Brazil, th ...

and

Peru , image_flag = Flag of Peru.svg , image_coat = Escudo nacional del Perú.svg , other_symbol = Great Seal of the State , other_symbol_type = Seal (emblem), National seal , national_motto = "Fi ...

Description

This animal resembled a small

horse The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 million y ...

, with slender, elongated legs. The body was approximately a meter long, and it is supposed to have weighed between 30 to 50 kilograms. The skull had a relatively elongated snout, with nasal bones longer than ''

Diadiaphorus ''Diadiaphorus'' is an extinct genus of litoptern mammal from the Miocene of Argentina ( Ituzaingó, Pinturas, Chiquimil and Santa Cruz Formations) and Bolivia ( Nazareno Formation), South America. Description ''Diadiaphorus'' closely rese ...

'' and with less developed upper

incisor Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, whe ...

s. The orbits were positioned near the middle of the skull. The molars had lower crowns than those of more derived proterotheriids, such as ''

Proterotherium ''Proterotherium'' (meaning "first beast") is an extinct genus of litoptern mammal of the family Proterotheriidae that lived during the Late Miocene of Argentina and Chile. Fossils of this genus have been found in the Ituzaingó Formation of Arge ...

''. 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 The neocortex, also called the neopallium, isocortex, or the six-layered cortex, is a set of layers of the mammalian cerebral cortex involved in higher-order brain functions such as sensory perception, cognition, generation of motor commands, sp ...

, and to the complexity of the

cerebellum The cerebellum (Latin for "little brain") is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or even larger. In humans, the cerebel ...

. 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 In animal anatomy, the rhinencephalon (from the Greek, ῥίς, ''rhis'' = "nose", and ἐγκέφαλος, ''enkephalos'' = "brain"), also called the smell-brain or olfactory brain, is a part of the brain involved with smell (i.e. olfaction). ...

was highly developed, and the piriform lobes were in a much less lateral position than those of archaic ungulates such as ''

Phenacodus ''Phenacodus'' (Greek: "deception" (phenax), "tooth' (odus)) is an extinct genus of mammals from the late Paleocene through middle Eocene, about 55 million years ago. It is one of the earliest and most primitive of the ungulates, typifying the fa ...

'', and presented a sort of circular depression of unknown significance. Numerous longitudinal grooves covered the surface of the neocortex. The

cranial nerves Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and f ...

were unusually arranged relatively to those of other ungulates, and the foramen ovale was posterior to the tentorium osseum.

Classification

The genus ''Tetramerorhinus'' was first described in 1894 by

Florentino Ameghino Florentino Ameghino (born Giovanni Battista Fiorino Giuseppe Ameghino September 19, 1853 – August 6, 1911) was an Argentine naturalist, paleontologist, anthropologist and zoologist, whose fossil discoveries on the Argentine Pampas, especially ...

, based on fossil remains found in

Early Miocene The Early Miocene (also known as Lower Miocene) is a sub-epoch of the Miocene Epoch made up of two stages: the Aquitanian and Burdigalian stages. The sub-epoch lasted from 23.03 ± 0.05 Ma to 15.97 ± 0.05 Ma (million years ago). It was prece ...

deposits in

Patagonia Patagonia () refers to a geographical region that encompasses the southern end of South America, governed by Argentina and Chile. The region comprises the southern section of the Andes Mountains with lakes, fjords, temperate rainforests, and gl ...

. Numerous species have been ascribed to this genus, such as ''Tetramerorhinus cingulatum'', ''T. fleaglei'', ''T. mixtum'', ''T. lucarius'', ''T. prosisttens''. ''Tetramerorhinus'' is a proterotheriid, a group of

litopterns Litopterna (from grc, λῑτή πτέρνα "smooth heel") is an extinct order of fossil hoofed mammals from the Cenozoic era. The order is one of the five great orders of South American ungulates that were endemic to the continent, until the G ...

with characteristics similar to those of

equids Equidae (sometimes known as the horse family) is the taxonomic family of horses and related animals, including the extant horses, asses, and zebras, and many other species known only from fossils. All extant species are in the genus '' Equus'', w ...

, although they were not closely related. ''Tetramerorhinus'' seems to have been a derived form, closely related to ''

Anisolophus ''Anisolophus'' is an extinct genus of proterotheriid from the Early to Middle Miocene of Argentina. The genus was named by Burmeister in 1885 to accommodate the species ''Anchitherium australe'', which they had named earlier in 1879. Soria then ...

'' and ''

''. Below is a phylogenetic tree of the Proterotheriidae, based on the work of McGrath ''et al''. 2020.

References

{{Taxonbar, from=Q3769142 Proterotheriids Miocene mammals of South America Neogene Argentina Fossils of Argentina Neogene Peru Fossils of Peru Laventan Colloncuran Friasian Santacrucian Colhuehuapian Fossil taxa described in 1894 Taxa named by Florentino Ameghino Prehistoric placental genera Golfo San Jorge Basin Sarmiento Formation Austral or Magallanes Basin Santa Cruz Formation