''Xesmodon'' is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of mammal. It lived from the Middle to the Late

Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...

, in what is today

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...

Taxonomy

''Xesmodon'' was named by Carlos Berg in 1899. Its type species is ''Xesmodon langi'', and a second species, ''Xesmodon prolixus'', may also belong to the genus. Both species were named by

Santiago Roth Santiago Roth (14 June 1850 – 4 August 1924) was a Swiss Argentine paleontologist and academic known for his fossil collections and Patagonian expeditions. Life Kaspar Jakob (Spanish: Santiago) was born and raised in Herisau, Canton Appenzell ...

in 1899. The type and only known specimen of ''Xesmodon langi'' is a poorly preserved skull found at the locality Cañadón Colorado in

Argentina Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...

. The type and only known specimen of ''Xesmodon prolixus'' is a fragmentary mandible with two teeth found near Lago Musters in the

Chubut Province Chubut ( from Tehuelche language, Tehuelche 'transparent'; ) is a provinces of Argentina, province in southern Argentina, situated between the 42nd parallel south (the border with Río Negro Province), the 46th parallel south (bordering Santa ...

. Both species are from the

Mustersan The Mustersan age is a period of geologic time (48.0–42.0 Mya (unit), Ma) within the Eocene epoch of the Paleogene, used more specifically within the South American land mammal age (SALMA) classification. It follows the Casamayoran and precedes th ...

South American land mammal age The South American land mammal ages (SALMA) establish a geologic timescale for prehistoric South American fauna beginning 64.5 Ma during the Paleocene and continuing through to the Late Pleistocene (0.011 Ma). These periods are referred to as age ...

. Some researchers classify ''Xesmodon'' in

Didolodontidae Didolodontidae is a possibly paraphyletic family of "condylarth" mammals known from the Paleogene of South America, with most specimens known from Argentina. They were generally small-medium in body size, and had a bunodont dentition. A close rel ...

, whereas others classify it in Anisolambdinae, a subfamily of

Proterotheriidae Proterotheriidae is an extinction, extinct family of Litopterna, litoptern ungulates known from the Eocene-Late Pleistocene of South America. Members of the group were small-medium sized cursorial Herbivore, herbivores with brachydont Tooth, teet ...

. Proterotheriidae is part of the order

Litopterna Litopterna (from "smooth heel") is an extinction, extinct order of South American native ungulates that lived from the Paleocene to the Pleistocene-Holocene around 62.5 million to 12,000 years ago (or possibly as late as 3,500 years ago), and we ...

, whereas didolodontids are considered

condylarths Condylarthra is an informal group – previously considered an order – of extinct placental mammals, known primarily from the Paleocene and Eocene epochs. They are considered early, primitive ungulates and is now largely considered to be a wast ...

, albeit likely to be closely related to litopterns.

Description

This genus is known from partial cranial remains, permitting to partially reconstruct its appearance. The skull was rather long and low, and had particularly large orbites positioned near the middle of the skull. The nasal bones were elongated, while the frontal region was flat and table-like; long and narrow postorbital apophyses separated from this region, to which jugal apophyses were not opposed. The zygomatic arch was rather thin. The upper molars of ''Xesmodon'' had a very strong independent hypocone.

History of study

In 1899,

named a new genus and species of mammal, ''Glyphodon langi'', on the basis of a poorly preserved skull from Patagonia. He identified it as having

litoptern Litopterna (from "smooth heel") is an extinct order of South American native ungulates that lived from the Paleocene to the Pleistocene-Holocene around 62.5 million to 12,000 years ago (or possibly as late as 3,500 years ago), and were also pres ...

affinities. In the same paper, he named the species ''Megacrodon prolixus''. However, the name ''Glyphodon'' had already been given to a snake by

Albert Günther Albert Karl Ludwig Gotthilf Günther , also Albert Charles Lewis Gotthilf Günther (3October 18301February 1914), was a German-born British zoologist, ichthyologist, and herpetologist. Günther is ranked the second-most productive reptile tax ...

in 1858, so later the same year, Carlos Berg proposed the replacement name ''Xesmodon'' for Roth's genus. In 1948,

George Gaylord Simpson George Gaylord Simpson (June 16, 1902 – October 6, 1984) was an American paleontologist. Simpson was perhaps the most influential paleontologist of the twentieth century, and a major participant in the modern synthesis, contributing '' Tempo ...

tentatively reclassified ''Megacrodon prolixus'' as a second species of ''Xesmodon''. Simpson was unsure whether ''Xesmodon'' was a didolodontid or a

. In 1983, Richard Cifelli assigned ''Xesmodon'' to Anisolambdinae. In research by Miguel Soria published in 2001,Soria died in 1989 and his thesis was published posthumously in 2001. he classified ''Xesmodon'' in Didolodontidae.

References

Works cited

* * * * * * {{Taxonbar, from=Q60977295 Proterotheriids Eocene mammals of South America Paleogene Argentina Fossils of Argentina Fossil taxa described in 1899 Prehistoric placental genera