''Rhyphodon'' 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 Notoungulate, who lived from the Middle to the Late

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

in what is today

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the southe ...

Description

This genus is only known from cranial remains, but by comparing it with some of its better known relatives, it is possible to reconstruct its appearance. ''Rhyphodon'' probably was a medium-sized herbivore, the size of a large

dog The dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated descendant of the wolf. Also called the domestic dog, it is derived from the extinct Pleistocene wolf, and the modern wolf is the dog's nearest living relative. Do ...

, with strong legs, each having five hooved toes. The skull, more than 20 centimeters long, had a relatively short muzzle and a primitive and complete set of teeth. The mandible had three incisors, a canine, four premolars and three molars. The teeth shared similarities with those of its relative ''

Periphragnis ''Periphragnis'' is an extinct genus of isotemnid notoungulates that lived from the Middle Eocene to the Early Oligocene in what is now Argentina and Chile. Description This animal was of robust build, with powerful, probably digitigrade legs ...

'', from which they were distinguished by a lack of cingulum and a greater coverage of wrinkled enamel on the molars. A fossil attributed to ''Rhyphodon'' preserves an

endocranial cast An endocast is the internal cast of a hollow object, often referring to the cranial vault in the study of brain development in humans and other organisms. Endocasts can be artificially made for examining the properties of a hollow, inaccessible sp ...

, allowing researchers to reconstruct the shape of various structures of the animal brain ; the endocranium was similar to other archaic

ungulate Ungulates ( ) are members of the diverse clade Ungulata which primarily consists of large mammals with hooves. These include odd-toed ungulates such as horses, rhinoceroses, and tapirs; and even-toed ungulates such as cattle, pigs, giraffes, cam ...

s 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 ''

Notostylops ''Notostylops'' ("south pillar face") is a genus of extinct South American ungulates from Eocene Argentina. Fossils of the genus have been found in the Sarmiento, Casamayor, Andesitas Huancache and Koluel Kaike Formations.

'', but with some observable differences. ''Rhyphodon'' had slightly smaller olfactory bulbs than the basal notoungulate ''Notostylops'', and those were folded downward. The cerebellum was larger and as broad as it was long, while the piriform lobes were laterally expanded. Compared to ''Notostylops'', the cerebral hemispheres were less convoluted. There was no separate optic canal and no posterolateral cerebral venous canal.

Classification

The genus ''Rhyphodon'' was first described in 1899 by Santiago Roth, based on fossil remains initially thought to be dating from the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...

and only later recognized as dating back to the Eocene. The type species, ''Rhyphodon lankesteri'' was discovered near Lago Musters in Patagonia in Late Eocene terrains. An earlier species, ''Rhyphodon angusticephalus'', is known from the Cañadón Colorado, in the

Chubut Province Chubut ( es, Provincia del Chubut, ; cy, Talaith Chubut) is a province in southern Argentina, situated between the 42nd parallel south (the border with Río Negro Province), the 46th parallel south (bordering Santa Cruz Province), the Andes ra ...

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 ...

. ''Rhyphodon'' has been historically placed within the family

Isotemnidae Isotemnidae is an extinct Family (biology), family of Notoungulata, notoungulate mammals known from the Paleocene (Las Flores Formation, Golfo San Jorge Basin, Las Flores Formation, Itaboraian) to Middle Miocene (Honda Group, Colombia, Honda Grou ...

, the earliest and least specialized known family of toxodonts. However, this group may have been paraphyletic, and constituted of various forms more or less specialized within Toxodonta ; according to a 2011 study, ''Rhyphodon'' is related to the genus '' Pampahippus''.Billet, Guillaume (2011). "Phylogeny of the Notoungulata (Mammalia) based on cranial and dental characters". Journal of Systematic Palaeontology. 9 (4): 481–97. doi:10.1080/14772019.2010.528456. OCLC 740994816.

References and Bibliography

*G. G. Simpson. 1948. The beginning of the age of mammals in South America. Part I. Bulletin of the American Museum of Natural History 91:1-232 *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 {{Taxonbar, from=Q60977375 Toxodonts Eocene mammals of South America Paleogene Argentina Fossils of Argentina Fossil taxa described in 1899 Prehistoric placental genera