''Oldfieldthomasia'' 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 ...

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

, probably related to the suborder

Typotheria Typotheria is a suborder of the extinct mammalian order Notoungulata Notoungulata is an extinct order of mammalian ungulates that inhabited South America from the early Paleocene to the Holocene, living from approximately 61 million to 11,0 ...

. It lived during the Middle

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 animal was approximately the size of a

marmot Marmots are large ground squirrels in the genus ''Marmota'', with 15 species living in Asia, Europe, and North America. These herbivores are active during the summer, when they can often be found in groups, but are not seen during the winter, w ...

, and probably superficially resembled a

rodent Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are na ...

. The 10 centimeters long skull was rather low and had an elongated muzzle ; the orbits, of large dimension, were posteriorly opened and located in the posterior area of the skull. The zygomatic apophysis was rather slender. The posterior part of ''Oldfieldthomasia debilitata'' was closely studied by

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 (20th century), modern ...

in 1936, through the realization of 55 very thin sections ; the study highlighted numerous cranial characteristics, such as the relationships between the various cavities. The tympanic and hypotympanic cavities were not separated and were devoid of spongy tissue ; the path of the pneumatic orifice can be followed between the epitympanic recess and the epitympanic sinus, filling free, and close to the size of tympanic cavity. The main sinuses of the veins and arteries were also visible : the carotid entered the tympanic cavity. As in all notoungulates, the cavity for the hyoid apophysis was quite large. The posterior jugular foramen and the median jugular foramen were notably external ; the external portion of the first constituted the posterior carotid foramen. The mandible was long and thin. The teeth formed a continuous and progressive series, with a complete series of 22 low-crowned (brachydont) teeth, both in the maxilla and the mandible. The canines were incisor-shaped. The protocone and the hypocone were almost identical, with an oblique and elongated central dimple and a posterior dimple stopped by a sort of hook fused to the ectoloph. An anterior and a posterior cingulum were present ; the molars had a mesostyle, while the premolars were triangular, with a fold on the metacone.

Classification

The genus ''Oldfieldthomasia'' was first described in 1901 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 terrains from the Middle Eocene of

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 ...

. Ameghino described several species, such as ''Oldfieldthomasia anfractuosa'', ''O. debilitata'', ''O. parvidens'' and ''O. traversa'', but it may be possible that some of these forms represents intraspecific variations within a single species. ''Oldfieldthomasia'' is the eponymous genus of the family

Oldfieldthomasiidae Oldfieldthomasiidae is an extinct family of notoungulate mammals known from the Late Paleocene to Late Eocene of South America. The family was classified by George Gaylord Simpson in 1945 and a synonym is Acoelodidae, defined by Florentino Ameghi ...

, a group of archaic notoungulates representative of the Eocene ; this family, however, is considered

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

by some researchers.G. Billet. 2011. Phylogeny of the notoungulata (mammalia) based on cranial and dental characters. Journal of Systematic Palaeontology, 9:481-497. ''Oldfieldthomasia'' may have been part of an early evolutionary radiation of the suborder

, a group of notoungulates who occupied during the

Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configura ...

various ecological niches comparable to those of rodents in other continents. Its closes relatives were '' Allalmeia'', ''

Colbertia ''Colbertia'' is an extinct genus of oldfieldthomasiid notoungulate. It lived from the Early to the Middle Eocene, and its fossilized remains were discovered in Argentina and Brazil. Description This animal was roughly the size of a Virginia ...

'' and ''

Dolichostylodon ''Dolichostylodon'' is an extinct species of mammal, belonging to the order Notoungulata. It lived during the Middle Eocene, and its fossilized remains were discovered in South America. Description About the size of a marmot, this animal was q ...

''.

References and Bibliography

*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. 1904. Nuevas especies de mamíferos, cretáceos y terciarios de la República Argentina ew species of mammals, Cretaceous and Tertiarty, from the Argentine Republic Anales de la Sociedad Cientifica Argentina 56–58:1-142 *G. G. Simpson. 1936. Structure of a primitive notoungulate cranium. Am Mus Novit;824:1–32. *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=Q51803739 Typotheres Eocene mammals of South America Paleogene Argentina Fossils of Argentina Taxa named by Florentino Ameghino Fossil taxa described in 1901 Prehistoric placental genera Golfo San Jorge Basin Sarmiento Formation