''Isotemnus'' 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 ...

notoungulate Notoungulata is an extinct order of ungulates that inhabited South America from the early Paleocene to the end of the Pleistocene, living from approximately 61 million to 11,000 years ago. Notoungulates were morphologically diverse, with forms re ...

belonging to the family Isotemnidae. It lived from the Late Paleocene to the Middle

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 ...

of what is now

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 ...

Description

This genus was smaller than '' Thomashuxleya'' and '' Periphragnis'', and did not exceed 50 kilograms in weight. Its build was comparable to a modern

peccary Peccaries (also javelinas or skunk pigs) are pig-like ungulates of the family Tayassuidae (New World pigs). They are found throughout Central and South America, Trinidad in the Caribbean, and in the southwestern area of North America. Peccari ...

, with a body relatively massive and strong and sturdy legs. Compared to other Eocene notoungulates, like basal

Notohippidae Notohippidae is a paraphyly, paraphyletic extinct Family (biology), family of Notoungulata, notoungulate mammals from South America. Notohippids are known from the Eocene and Oligocene Epoch (reference date), epochs.McKenna, Malcolm C., and Bell ...

and

Notostylopidae Notostylopidae is an extinct family comprising five genera of notoungulate mammals known from the Late Paleocene (Riochican) to Early Oligocene (Tinguirirican) of Argentina, Brazil and Chile in South America South America is a continent ...

, ''Isotemnus'' had an humerus whose distal part had a high medial trochlear crest, while the bicipital radial tuberosity was almost unexistant. The astragalus had a broad and low trochlea with a short neck. The calcaneus had rectangular fibular facets, and an unusually thick sustentaculum. Several of the distinctive anatomical leg characteristics of ''Isotemnus'' could be due to its smaller size ; ''Periphragnis'' and ''Thomashuxleya'', while very similar, had different characteristics.

Classification

''Isotemnus'' is the eponymous genus of the family Isotemnidae, a possibly paraphyletic group of notoungulates including the most basal forms of

toxodonts Toxodontia. Retrieved April 2013. is a suborder of the meridiungulate order Notoungulata. Most of the members of the five included families, including the largest notoungulates, share several dental, auditory and tarsal specializations. The gr ...

. ''Isotemnus'' was one of the most archaic and basal of the isotemnids. The type species is ''Isotemnus primitivus'', first described in 1897 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, especial ...

, based on fossil remains found in terrains dating back from the Early Eocene of

Argentine Argentines, Argentinians or Argentineans are people from Argentina. This connection may be residential, legal, historical, or cultural. For most Argentines, several (or all) of these connections exist and are collectively the source of their ...

Patagonia Patagonia () is a geographical region that includes parts of Argentina and Chile at the southern end of South America. The region includes the southern section of the Andes mountain chain with lakes, fjords, temperate rainforests, and glaciers ...

. Other species were later attributed to the genus, such as ''I. ctalego'' (Early Eocene), ''I. haugi'' (Early Eocene, initially described as ''Leifunia haugi''), ''I. latidens'' (Middle Eocene). Fragmentary remains attributed to ''Isotemnus'' were discovered in Late Paleocene formations in Argentina.

Bibliography

*F. Ameghino. 1897. Mammiféres crétacés de l’Argentine (Deuxième contribution à la connaissance de la fauna mammalogique de couches à Pyrotherium) retaceous mammals of Argentina (second contribution to the knowledge of the mammalian fauna of the Pyrotherium Beds) Boletin Instituto Geografico Argentino 18(4–9):406-521 *F. Ameghino. 1901. Notices préliminaires sur des ongulés nouveaux des terrains crétacés de Patagonie reliminary notes on new ungulates from the Cretaceous terrains of Patagonia Boletin de la Academia Nacional de Ciencias de Córdoba 16:349-429 *F. Ameghino. 1902. Notices préliminaires sur des mammifères nouveaux des terrains Crétacé de Patagonie {preliminary notes on new mammals from the Cretaceous terrains of Patagonia]. Boletin de la Academia Nacional de Ciencias de Córdoba 17:5-70 *G. G. Simpson. 1935a. Occurrence and relationships of the Rio Chico fauna of Patagonia. American Museum Novitates 818:1-21 *G. G. Simpson. 1935b. Descriptions of the oldest known South American mammals, from the Rio Chico Formation. American Museum Novitates 793:1-25 *G. G. Simpson. 1967. The beginning of the age of mammals in South America. Part II. Bulletin of the American Museum of Natural History 137:1-260 *Vucetich, M. G., & Bond, M. (1982). Los primeros Isotemnidae (Mammalia, Notoungulata) registrados en la Formación Lumbrera (Grupo Salta), del noroeste argentino. Ameghiniana, 19(1–2), 7–18. *M. O. Woodburne, F. J. Goin, M. S. Raigemborn, M. Heizler, J. N. Gelfo and E. V. Oliveira. 2014. Revised timing of the South American early Paleogene land mammal ages. Journal of South American Earth Sciences 54:109-119 *Malena Lorente 2020. "The Limb Anatomy of Isotemnus, One of the Most Basal Toxodontid Notoungulates (Mammalia, Paso Del Sapo Fauna)," Ameghiniana, 57(2), 80–89. {{Taxonbar, from=Q97387785 Toxodonts Prehistoric placental genera Paleocene mammals of South America Eocene mammals of South America Paleogene Argentina Fossils of Argentina Taxa named by Florentino Ameghino Fossil taxa described in 1897 Golfo San Jorge Basin Sarmiento Formation