Prosotherium
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''Prosotherium'' 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 late
Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...
hegetotheriid notoungulate. It has been found in two
Argentinian 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 ...
fossil formations, ie, the
Sarmiento Formation The Sarmiento Formation (Spanish language, 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 y ...
in
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 ...
, and the
Agua de la Piedra Formation The Agua de la Piedra Formation (FAP, Spanish names include ''Estratos de Agua de la Piedra'' and ''Complejo Volcano-sedimentario del Terciario inferior'')Combina et al., 1994, p.418 is a Late Oligocene (Deseadan in the South American land mammal ...
in
Mendoza Province Mendoza (), officially the Province of Mendoza, is a province of Argentina, in the western central part of the country in the Cuyo region. It borders San Juan to the north, La Pampa and Neuquén to the south, San Luis to the east, and the r ...
.


Description

This animal was similar to
rabbit Rabbits are small mammals in the family Leporidae (which also includes the hares), which is in the order Lagomorpha (which also includes pikas). They are familiar throughout the world as a small herbivore, a prey animal, a domesticated ...
s, in aspect and in size. Compared to its relatives, it would have been about 20% larger than ''
Propachyrucos ''Propachyrucos'' is an extinct genus of late Oligocene Hegetotheriidae, hegetotheriid notoungulate. It is known from a few mandibular fragments from the Sarmiento Formation, Chubut Province, Argentina. Description ''Propachyrucos'' may have bee ...
'', '' Pachrukhos'', and '' Paedotherium'', but slightly smaller than '' Medistyllus''. Its hind legs were particularly long.


Crania

Its skull was light and thin, notably in the posterior part. The
tympanic part of the temporal bone The tympanic part of the temporal bone is a curved plate of bone lying below the squamous part of the temporal bone, in front of the mastoid process, and surrounding the external part of the ear canal. It originates as a separate bone (tympan ...
was particularly developed, even more than 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, Sarm ...
'', and it is probable that its auricle was quite large, similar to the ears of a
hare Hares and jackrabbits are mammals belonging to the genus ''Lepus''. They are herbivores and live Solitary animal, solitarily or in pairs. They nest in slight depressions called forms, and their young are precociality, able to fend for themselves ...
. The
maxilla In vertebrates, the maxilla (: maxillae ) is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The two maxil ...
and the
mandible In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...
were high and deep ; the mandible had a subtle coronoid process. The teeth were characterized by
incisor Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, wher ...
s pointing inward, molars and
premolar The premolars, also called premolar Tooth (human), teeth, or bicuspids, are transitional teeth located between the Canine tooth, canine and Molar (tooth), molar teeth. In humans, there are two premolars per dental terminology#Quadrant, quadrant in ...
s covered by a thin layer of 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. Structure The cells of cementum are ...
, generally on the outside part of the upper teeth and on the inside part of the lower teeth. 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 considerable development.


Postcranial skeleton

The
humerus The humerus (; : humeri) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius (bone), radius and ulna, and consists of three sections. The humeral upper extrem ...
was large and thin, like the
ulna The ulna or ulnar bone (: ulnae or ulnas) is a long bone in the forearm stretching from the elbow to the wrist. It is on the same side of the forearm as the little finger, running parallel to the Radius (bone), radius, the forearm's other long ...
and
radius In classical geometry, a radius (: radii or radiuses) of a circle or sphere is any of the line segments from its Centre (geometry), center to its perimeter, and in more modern usage, it is also their length. The radius of a regular polygon is th ...
.
Metacarpal In human anatomy, the metacarpal bones or metacarpus, also known as the "palm bones", are the appendicular bones that form the intermediate part of the hand between the phalanges (fingers) and the carpal bones ( wrist bones), which articulate ...
s and
phalanges The phalanges (: phalanx ) are digit (anatomy), digital bones in the hands and foot, feet of most vertebrates. In primates, the Thumb, thumbs and Hallux, big toes have two phalanges while the other Digit (anatomy), digits have three phalanges. ...
indicates that the hand was small and delicate. The
pelvis The pelvis (: pelves or pelvises) is the lower part of an Anatomy, anatomical Trunk (anatomy), trunk, between the human abdomen, abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also c ...
was elongated, slender and lightly build, while the
femur The femur (; : femurs or femora ), or thigh bone is the only long bone, bone in the thigh — the region of the lower limb between the hip and the knee. In many quadrupeds, four-legged animals the femur is the upper bone of the hindleg. The Femo ...
was equipped of a small head separated from the main bone. The femur was even longer than in ''Pachyrukhos'', and had a small trochanter curiously pushed back in the posterior part. Contrary to ''Pachyrukhos'', the
tibia The tibia (; : tibiae or tibias), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two Leg bones, bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outsi ...
and
fibula The fibula (: fibulae or fibulas) or calf bone is a leg bone on the lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long bones. ...
were completely separated. The
talus bone The talus (; Latin for ankle or ankle bone; : tali), talus bone, astragalus (), or ankle bone is one of the group of Foot#Structure, foot bones known as the tarsus (skeleton), tarsus. The tarsus forms the lower part of the ankle joint. It transmit ...
was likewise characteristic, with the trochlear nerve extant covering entirely the dorsal part of the bone, and a low and flat condylar crest. The
calcaneus In humans and many other primates, the calcaneus (; from the Latin ''calcaneus'' or ''calcaneum'', meaning heel; : calcanei or calcanea) or heel bone is a bone of the Tarsus (skeleton), tarsus of the foot which constitutes the heel. In some other ...
was relatively small, with an articulated facet for the
cuboid bone In the human body, the cuboid bone is one of the seven tarsal bones of the foot. Structure The cuboid bone is the most lateral of the bones in the distal row of the tarsus. It is roughly cubical in shape, and presents a prominence in its infer ...
slightly concave and occupying the entire distal part of the bone. The
metatarsal The metatarsal bones or metatarsus (: metatarsi) are a group of five long bones in the midfoot, located between the tarsal bones (which form the heel and the ankle) and the phalanges ( toes). Lacking individual names, the metatarsal bones are ...
was robust and shorter than in ''Pachyrukhos''. The distal phalanx was high and narrow, flattening and widening towards the terminal part. Its body was slender, with a very short tail and strong forelegs, although much shorter than the hind legs, equipped with long metatarsals. 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, Sarm ...
''. The hand seems more specialized than in ''Pachyrukhos'', and shares similarities with that of '' Paedotherium''. The metatarsals were elongated, especially the third.


Classification

The genus ''Prosotherium'' was first described in
1897 Events January * January 2 – The International Alpha Omicron Pi sorority is founded, in New York City. * January 4 – A British force is ambushed by Chief Ologbosere, son-in-law of the ruler. This leads to a punitive expedit ...
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 ...
, over fossilized remains discovered in
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 ...
in Late Oligocene terrains. The type species is ''Prosotherium garzoni'', and Ameghino described in this same paper ''P. triangulidens'', which was a bit larger than the type species, as well as ''P. robustum''. ''P. quartum'', a fourth species, was described in 1901 by Ameghino.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 Only ''Prosotherium garzoni'' is still considered valid.F. D. Seoane, S. R. Juñent, and E. Cerdeño. 2017. Phylogeny and paleobiogeography of Hegetotheriidae (Mammalia, Notoungulata). Journal of Vertebrate Paleontology 37(1):e1278547:1-13 ''Prosotherium'' was a member of the
Hegetotheriidae Hegetotheriidae is an extinct family of notoungulate mammals known from the Oligocene through the Pliocene of South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a c ...
, a clade of light,
rodent Rodents (from Latin , 'to gnaw') are mammals of the Order (biology), order Rodentia ( ), which are characterized by a single pair of continuously growing incisors in each of the upper and Mandible, lower jaws. About 40% of all mammal specie ...
-like or hare-like
Notoungulata 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 ...
. ''Prosotherium'' was a member of the Pachyrukhinae subfamily comprising several hare-like genera. It is possible that ''Prosotherium'' was an ancestral form or a close relative of '' Paedotherium'', more specialized. In a 1999 unpublished Ph.D. dissertation, the ''
Propachyrucos ''Propachyrucos'' is an extinct genus of late Oligocene Hegetotheriidae, hegetotheriid notoungulate. It is known from a few mandibular fragments from the Sarmiento Formation, Chubut Province, Argentina. Description ''Propachyrucos'' may have bee ...
'' species ''P. ameghinorum'' and ''P. simpsoni'' were synonymized with ''Prosotherium garzoni.'' This was supported by a 2019 reassessment as well as a number of other studies. This means ''Prosotherium garzoni'' is known from an almost complete specimen, ie, the holotype of ''Pr. ameghinorum.'' The 2019 study recovers the features of ''P. ameghinorum'' as consistent with those of juvenile ''Prosotherium garzoni'' (although the holotype of ''P. ameghinorum'' was not directly examined'').'' Despite this, until 2019, phylogenetic analyses regularly scored ''Propachyrucos'' for cranial and post-cranial characters, which are known only from the synonymized ''P. ameghinorum'' and ''P. simpson''i.


Paleobiology

The Late Oligocene terrains of Patagonia present at least three genera of hegetotheres with hypsodont teeth : ''Prosotherium'', ''
Propachyrucos ''Propachyrucos'' is an extinct genus of late Oligocene Hegetotheriidae, hegetotheriid notoungulate. It is known from a few mandibular fragments from the Sarmiento Formation, Chubut Province, Argentina. Description ''Propachyrucos'' may have bee ...
'' and '' Medistylus'', the latter of which is sometimes considered an interatheriid. This implies a strict repartition of
ecological niche In ecology, a niche is the match of a species to a specific environmental condition. Three variants of ecological niche are described by It describes how an organism or population responds to the distribution of Resource (biology), resources an ...
s among the Oligocene hegetotheres, but also reflects the remarkable evolutionary radiation of rodent-like ungulates during the
Cenozoic The Cenozoic Era ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterized by the dominance of mammals, insects, birds and angiosperms (flowering plants). It is the latest of three g ...
of
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 ...
, and suggest environmental differences between the fauna of Patagonia and the fauna of
Uruguay Uruguay, officially the Oriental Republic of Uruguay, is a country in South America. It shares borders with Argentina to its west and southwest and Brazil to its north and northeast, while bordering the Río de la Plata to the south and the A ...
and
Bolivia Bolivia, officially the Plurinational State of Bolivia, is a landlocked country located in central South America. The country features diverse geography, including vast Amazonian plains, tropical lowlands, mountains, the Gran Chaco Province, w ...
, where those animals were absent.


References

{{Taxonbar, from=Q60978547 Typotheres Oligocene mammals of South America Deseadan Paleogene Argentina Fossils of Argentina Fossil taxa described in 1897 Taxa named by Florentino Ameghino Prehistoric placental genera Golfo San Jorge Basin Sarmiento Formation