''Theosodon'' is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

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 ...

mammal from the Early to Middle Miocene of South America.

Description

''Theosodon'' was long-legged with a long neck resembling modern

llama The llama (; ) (''Lama glama'') is a domesticated South American camelid, widely used as a meat and pack animal by Andean cultures since the Pre-Columbian era. Llamas are social animals and live with others as a herd. Their wool is soft ...

s or

guanaco The guanaco (; ''Lama guanicoe'') is a camelid native to South America, closely related to the llama. Guanacos are one of two wild South American camelids, the other being the vicuña, which lives at higher elevations. Etymology The guanaco ...

s. It was large for a litoptern, reaching up to in length and weighing up to . It had a long neck and

tapir Tapirs ( ) are large, herbivorous mammals belonging to the family Tapiridae. They are similar in shape to a pig, with a short, prehensile nose trunk. Tapirs inhabit jungle and forest regions of South and Central America, with one species inh ...

-like, three-toed feet, and like other litopterns and modern horses, tapirs and rhinos, it bore its weight on its middle toes. Extraordinarily, rather than having nostrils at the front of its head, ''Theosodon'' had its nostrils on the top of its snout, halfway between the forehead and the tip of the snout, and its nostrils pointed upwards rather than forwards, possibly as an adaptation for browsing on prickly vegetation.

History and species

''Theosodon'' has been known since the 19th century, and by 1910 seven species had been described within the genus, all from the early

Miocene The Miocene ( ) is the first epoch (geology), 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 mea ...

Santa Cruz Formation The Santa Cruz Formation is a geological formation in the Magallanes/Austral Basin in southern Patagonia in Argentina and in adjacent areas of Chile. It dates to the late Early Miocene epoch, and is contemporaneous with eponymous Santacrucian ...

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 ...

. The name ''Theosodon'' means "god tooth". The first fossils were only dental remains, but it is unknown why this name was given. Though seven species had originally been described, the exact number of discovered species remains uncertain as it varies between different authors. There are either ten or seven species, and in 2014 Schmidt and Ferrero put forward that the genus needed a full taxonomic revision due to this issue.

''"Theosodon" hystatus''

In 1931, fossils found in the late Miocene Arroyo Chasicó Formation were assigned to the genus under the name ''Theosodon hystatus''. This was questioned in 1995, and in 2014 it was moved to the genus ''

Paranauchenia ''Paranauchenia'' is an extinct genus of South American litopterns belonging to the family Macraucheniidae. It is known only from fossil finds in Argentina. It possessed three toes and long limbs. The species ''Paranauchenia denticulata'' lived i ...

'' due to similarities to the species ''Paranauchenia denticulata''. This move was supported by phylogenetic analysis that showed the two species formed monophyletic clade.

''"Theosodon" arozquetai''

In 2018 a new species of macraucheniid litoptern was described based on the partial remains of a skull and two

metapodials Metapodials are long bones of the hand (metacarpals) and feet (metatarsal The metatarsal bones, or metatarsus, are a group of five long bones in the foot, located between the tarsal bones of the hind- and mid-foot and the phalanges of the toes ...

. A phylogenetic analysis tentatively linked the species to ''Theosodon'' as a sister clade and it was named ''Theosodon arozquetai'' pending revision. Its body mass was estimated to be somewhere from . It dated to the middle Miocene, about 13 million years ago, somewhat later than most other ''Theosodon'' species.

Classification

''Theosodon'' is in the subfamily Cramaucheniinae within the family

Macraucheniidae Macraucheniidae is a family in the extinct South American ungulate order Litopterna, that resembled various camelids. The reduced nasal bones of their skulls was originally suggested to have housed a small proboscis, similar to that of the sai ...

. A phylogenetic analysis of the family in 2014 found that Cramaucheniinae is a paraphyletic group, and that ''Theosodon'' is a sister clade to the subfamily Macraucheniinae, the least primitive of the members of Cramaucheniinae.

Paleobiology

''Theosodon'' was a terrestrial and

cursorial A cursorial organism is one that is adapted specifically to run. An animal can be considered cursorial if it has the ability to run fast (e.g. cheetah) or if it can keep a constant speed for a long distance (high endurance). "Cursorial" is often us ...

animal that may have lived in both forests and more open environments. Due to its size and its long neck, ''Theosodon'' was likely a high browser, stripping leaves off of trees and shrubs high off the ground. ''Theosodon'' had a slender jaw compared to many other

s, indicating its food was softer, such as dicotyledons. As its nostrils are upward-facing and at the centre of its head rather than the front, ''Theosodon'' may have used this adaptation to feed on thorny plants. Some modern animals such as giraffes feed on thorny plants, and their nostrils are also further back and face more upwards than forwards. Other fossils were found in the Sarmiento Formation of Argentina, the

Yecua Formation The Yecua Formation is a geological Formation in what is now Bolivia. Studies suggest that the Yecua Formation preserves a coastal setting with humid to semiarid floodplains, shorelines and tidal as well as shallow marine environments including ...

of Bolivia, the Honda Group of Colombia, the Chucal and

Río Frías Formation Río Frías Formation ( es, Formación Río Frías) is a Middle Miocene geologic formation made up sedimentary rock located in Aysén Region, western Patagonia. The formation crops out along the upper couse of Cisnes River ( es, Río Cisnes).Mar ...

s of Chile and the Ipururo Formation of Peru.''Theosodon''
at Fossilworks.org

References

{{Taxonbar, from=Q2708140 Macraucheniids Miocene mammals of South America Laventan Colloncuran Friasian Santacrucian Colhuehuapian Neogene Argentina Fossils of Argentina Neogene Bolivia Fossils of Bolivia Neogene Chile Fossils of Chile Neogene Colombia Fossils of Colombia Honda Group, Colombia Neogene Peru Fossils of Peru Fossil taxa described in 1887 Taxa named by Florentino Ameghino Prehistoric placental genera Golfo San Jorge Basin Sarmiento Formation Austral or Magallanes Basin Santa Cruz Formation