''Propachyrucos'' 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 hegetotheriid notoungulate. It lived from the Late

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...

to the Early

Miocene The Miocene ( ) is the first 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 means "less recen ...

, 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 the size of a large

hare Hares and jackrabbits are mammals belonging to the genus ''Lepus''. They are herbivores, and live solitarily or in pairs. They nest in slight depressions called forms, and their young are able to fend for themselves shortly after birth. The ge ...

, with a 10 centimeter long skull. Its body was slender, with a very short tail and strong forelegs, although much shorter than the hind legs, equipped with long metatarsals. The skull had a complete dentition, but the second and third incisors, as well as the canines and the first lower premolars, were vestigial. The first upper incisors were very large, and showed a considerable development. The molars had two internal folds separated by a further deep fold, like the

Interatheriidae Interatheriidae is an extinct family of notoungulate mammals from South America. Interatheriids are known from the Middle Eocene (Mustersan) to the Early Pliocene (Montehermosan).McKenna & Bell, 1997Linares, 2004 These animals were principally sm ...

; dental

cementum Cementum is a specialized calcified substance covering the root of a tooth. The cementum is the part of the periodontium that attaches the teeth to the alveolar bone by anchoring the periodontal ligament.Illustrated Dental Embryology, Histology, a ...

was present, but to a lesser extent than in ''

Interatherium ''Interatherium'' is an extinct genus of interatheriid mammal from the Early to Middle Miocene (Colhuehuapian-Mayoan). Fossils have been found in the Santa Cruz, Collón Curá and Sarmiento Formations in Argentina.Fernández, M. (2015). Rev ...

''. The lower teeth were flat on the outside and bilobed on the inside. The tibia and fibula were not fused, and the forearm and lower leg bones were respectively shorter than the humerus and femur, but the hind legs were much longer than the forelegs. The second and third fingers of the hand were of similar length, and the fourth was smaller ; the fifth finger was small, even smaller than in its relative ''

Pachyrukhos ''Pachyrukhos'' is an extinct genus of hegetotheriid notoungulate from the Early to Middle Miocene (Colhuehuapian-Friasian in the SALMA classification) of Argentina and Chile. Fossils of this genus have been found in the Collón Curá, Sarmien ...

''. The hand seems more specialized than in ''Pachyrukhos'', and shares similarities with that of ''

Paedotherium ''Paedotherium'' is an extinct, potentially paraphyletic genus of Notoungulate, belonging to the family Hegetotheriidae, composed of small-sized, rodent or lagomorph-like Meridiungulata, South American ungulates. Four species are unambiguously re ...

''. The metatarsals were elongated, especially the third.

Classification

''Propachyrucos'' was 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, especially ...

, based on a right mandible fragment found in

terrains of the

Sarmiento Formation The Sarmiento Formation (Spanish: ''Formación Sarmiento''), in older literature described as the Casamayor Formation, is a geological formation in Chubut Province, Argentina, in central Patagonia, which spans around 30 million years from the mid- ...

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 ...

, initially believed by Ameghino to have been

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 ...

. The type species is ''Propachyrucos smithwoodwardi'', but other species have been attributed to the genus, such as ''P. aequilatus'', ''P. robustus'', and ''P. ameghinorum'', the latter species, described in 1945 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 ...

from fossils found 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 ...

, is known from a remarkably complete skeleton, permitting to reconstruct the animal appearance with high precision. However, recent researches tend to indicate that all the other species of ''Propachyrucos'' are in fact synonymous with the type species. ''Propachyrucos'' was a hegetothere, a group of notoungulates that evolved in isolation on the South American continent, giving rise to forms resembling

s and

rabbit Rabbits, also known as bunnies or bunny rabbits, are small mammals in the family Leporidae (which also contains the hares) of the order Lagomorpha (which also contains the pikas). ''Oryctolagus cuniculus'' includes the European rabbit speci ...

s. ''Propachyrucos'' was a member of Pachyrukhinae, a subfamily of hegetotheres particularly similar to modern

Lagomorph The lagomorphs are the members of the taxonomic order Lagomorpha, of which there are two living families: the Leporidae (hares and rabbits) and the Ochotonidae (pikas). The name of the order is derived from the Ancient Greek ''lagos'' (λαγ� ...

Paleoecology

''Propachyrucos'' and its relatives were animals well suited to running and jumping. Body proportions similar to those of ''Propachyrucos'' can also be found in the

Cainotheriidae Cainotheriidae is an extinct family of artiodactyls known from the Late Eocene to Middle Miocene of Europe. They are mostly found preserved in karstic deposits. These animals were small in size, and generally did not exceed in height at the s ...

, a group of

Artiodactyl The even-toed ungulates (Artiodactyla , ) are ungulates—hoofed animals—which bear weight equally on two (an even number) of their five toes: the third and fourth. The other three toes are either present, absent, vestigial, or pointing poster ...

s from the

Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...

References and bibliography

*F. Ameghino. 1897. Mamiferos Cretaceos de la Argentina. Segunda contribucion al conocimiento de la fauna mastologica de las capas con restos de Pyrotherium. Boletin Instituto Geografico Argentino 18:406-521 *S. Roth. 1898. Reconocimiento de la Region Andina de la República Argentina. Apuntes sobre la Geología y la Paleontología de los Territorios del Río Negro y Neuquén (Diciembre de 1895 á Junio de 1896) econnaissance of the Andean Region of the Argentine Republic. Notes on the Geology and Paleontology of the Territories of the Río Negro and Neuquén (December 1895 to June 1896)1-56 *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 *G. G. Simpson. 1945. A Deseado Hegetothere from Patagonia. American Journal of Science 243(10):550-564 *E. Cerdeño and M. Reguero. 2015. The Hegetotheriidae (Mammalia, Notoungulata) assemblage from the late Oligocene of Mendoza, central-western Argentina. Journal of Vertebrate Paleontology 35(2):e907173:1-14 {{Taxonbar, from=Q56216946 Typotheres Oligocene mammals of South America Miocene mammals of South America Paleogene Argentina Neogene Argentina Fossils of Argentina Fossil taxa described in 1897 Taxa named by Florentino Ameghino Prehistoric placental genera Golfo San Jorge Basin Sarmiento Formation