Protypotherium

''Protypotherium'' is an extinct genus of notoungulate mammals native to South America during the Oligocene and Miocene epochs. A number of closely related animals date back further, to the Eocene. Fossils of ''Protypotherium'' have been found in the Deseadan Fray Bentos Formation of Uruguay, Muyu Huasi and Nazareno Formations of Bolivia, Cura-Mallín and Río Frías Formations of Chile, and Santa Cruz, Salicas, Ituzaingó, Aisol, Cerro Azul, Cerro Bandera, Cerro Boleadoras, Chichinales, Sarmiento and Collón Curá Formations of Argentina.

Taxonomy

''Protypotherium'' was a typical representative of the Interatheriidae, a group of typotherian notoungulates with rodent-like appearances, usually with slender forms. The genus has a wide stratigraphic and geographic distribution, around 29 million years. Fossils assigned to ''Protypotherium'' have been found in numerous localities in Argentina, Bolivia, Chile, and Uruguay. Material was copied from this source, which is available under a Creative Commons Attribution 3.0 International License The oldest occurrence of ''Protypotherium'' dates back to the Late Oligocene (Deseadan) Fray Bentos Formation of Uruguay.

The genus ''Protypotherium'' was first described in 1882 by Florentino Ameghino, based on fossil remains found in the Ituzaingó Formation in Entre Ríos Province, Argentina, in soils dating from the Late Miocene. The type species is ''Protypotherium antiquum''. Another well-known species is ''P. australe,'' also from the Santa Cruz Formation, but several other species have been attributed to this genus, such as ''P. altum'', ''P. attenuatum'', ''P. claudum'', ''P. colloncurensis'', ''P. diastematum'', ''P. distinctum'', ''P. endiadys'', ''P. minutum'', ''P. praerutilum'', and ''P. sinclairi,'' all found in various localities in Argentina in Lower and Middle Miocene deposits. A species from Chile, ''P. concepcionensis'' was described in 2019.

''Protypotherium'' was a rather specialized member of the interatheriids, akin to the bizarre ''Miocochilius''; these two forms, according to a 2017 study, formed a monophyletic derived clade within the family Interatheriidae. In the same study, it is indicated that the species ''P. australe'' would be the most basal known species of the genus and may be ancestral to the other species of ''Protypotherium'' and the genus ''Miocochilius''. In spite of its name, ''Protypotherium'' was not an ancestor of ''"Typotherium"'', a genus that is now considered to be a synonym of '' Mesotherium'', another notoungulate belonging to another family, the Mesotheriidae.

The following cladogram of the Interatheriinae is based on Vera ''et al.'' 2017, showing the position of ''Protypotherium''. Material was copied from this source, which is available under a Creative Commons Attribution 4.0 International License

Species

The following species of ''Protypotherium'' have been described:''Protypotherium''
at Fossilworks.org

* ''P. altum''
* ''P. antiquum''
* ''P. attenuatum''
* ''P. australe''
* ''P. claudum''
* ''P. colloncurensis''
* ''P. diastematum''
* ''P. distinctum''
* ''P. endiadys''
* ''P. minutum''
* ''P. praerutilum''
* ''P. sinclairi''

* ''P. concepcionensis''

Description

''Protypotherium'' was slightly larger than a rabbit, measuring about in length. The body and legs as well as the tail of this animal were relatively long, while its neck was short. It probably resembled a rodent, possessing slender limbs with four, digitigrade clawed feet. Its rat-like skull contained a set of 44 unspecialized teeth.

From the shape of its claws, ''Protypotherium'' would have been adept at digging and likely took over the burrows of other animals.

Skull and dentition

The skull of ''Protypotherium'' was about halfway up the cranial vault, was slightly descending in the anterior part; the posterior part, on the other hand, lowered abruptly, in contrast to similar forms such as ''Cochilius'' (whose cranial vault was not inclined). The orbit was in a nearly median position, very open posteriorly, and the nasal bones were very advanced. The orbital arch was robust and was the direct extension of the lambdoid ridges. The maxillary bone alone formed the lower margin of the skull. At the back of the skull, the squamous bone and mastoid were highly developed. Seen from above, the skull had an oval outline but narrowed sharply at the beginning of the snout. The temporal fossa continued backward in a deep reinforcement between the lambdoid and sagittal ridges. The auditory region was characterized by a hypotympanic sinus much smaller than the tympanic cavity. In some species, the inner wall of the tympanic membrane was thick and filled with a fine spongy tissue. The epitympanic sinus was of medium size. The two branches of the mandible were firmly joined in the anterior part; the posterior part of the mandible was very elevated.

The dentition is complete, with the mandible having 44 teeth and no diastema. The upper incisors were rooted and provided with enamel on the outer surface. The canines were incisor-shaped, while the premolars had weak roots. The molars had two ribs on the outer wall. An internal groove divided the tooth into two almost equal lobes. An unworn molar showed an ectoloph, two convex inner crescents, and a ridge departing from the ectoloph. The anterior margin of each crescent joined the ectoloph. The lower incisors were divided longitudinally by a deep groove and resembled those of present-day hyraxes. The teeth were partially overlapping: one molar covered the posterior-external margin of the previous molar. An opposite outer and inner groove divided the molars into two lobes; the posterior lobe was shorter. A slight anterior-internal groove disappeared over time as tooth wear progressed.

The upper incisors of ''Protypotherium'' are characterized by a three-layered schmelzmuster represented by outer radial enamel. The prism diameter is ~6 μm. The Hunter-Schreger bands are thin (20–50 μm thick) and oblique. The interprismatic matrix forms closed coats near the outer enamel service and interrow sheets near the enamel dentine junction, and is intermediate to modified in the Hunter-Schreger bands. Lower incisors of ''Protypotherium'' are characterized by a one-layered schmelzmuster with Hunter-Schreger bands. Prism diameter is ~6 μm. Hunter-Schreger Bands are generally oblique and steady, even if they present a low decussation on both sections and are less discernible on some transverse sections. The interprismatic matrix forms closed coats in the entire thickness, but it is also slightly anastomosing near the enamel dentine junction. Material was copied from this source, which is available under a Creative Commons Attribution 4.0 International License

''Protypotherium'' has euhypsodont premolars and molars, and premolariform premolars. The upper molars have a lingual sulcus that separates the protolophs from the metalophs. The third and fourth premolars have a sub-triangular shape, with the mesio-distal length shorter than the labio-lingual breadth and a smaller size than the first and second molars. The first and third incisors are compact (i.e. lacking spaces between the teeth) and are imbricated another mesiodistally. The second premolar has a shorter talonid than the trigonid. The third and fourth premolars are relatively smaller than the first and second molars. The first and third molars have sub-triangular trigonids, while the first and second molars have longer talonids than the trigonids. Material was copied from this source, which is available under a Creative Commons Attribution 3.0 International License

Compared with the related '' Interatherium'' and ''Cochilius'', ''Protypotherium'' had well-differentiated third and fourth premolars compared to molars, and numerous other dental features. In contrast to ''Miocochilius'', moreover, ''Protypotherium'' possessed the canine-shaped lower first premolar and shorter lower third molar.

Regarding the species ''Protypotherium sinclairi'', it can be distinguished from other species of ''Protypotherium'' by its dentition, with all teeth having a thick cementum covering. The third and fourth premolar both have a shorter anteroposterior diameter of the talonid than the trigonid, while the first and second premolars are short and non-molariform. The teeth of this species are smaller than those of ''P. australe'', but larger than those of'' P. praerutilum'' and ''P. attenuatum'', all of which are Santacrucian in age. The posterior lower premolars show proportionally larger buccolingual diameter of the talonid than those in the Santacrucian species.

Postcranial skeleton

The skeleton of ''Protypotherium'' is well known, especially regarding the species ''Protypotherium australe''. Fifteen dorsal vertebrae, seven lumbar vertebrae and five sacral vertebrae were probably present. The tail was long, with at least eighteen vertebrae.

The scapula possessed a slightly convex coracoid margin. The scapular spine was tall and narrow, with a small apophysis of the acromion and a large metacromion. The humerus was very stout in the proximal region, with two low tuberosities; the distal end was enlarged, with the entepicondyle developed and provided with a large foramen. The radius was strongly curved antero-posteriorly and rather gracile; the ulna, on the other hand, was curved laterally. The hand was tetradactylous, as opposed to the related ''Miocochilius,'' which had only three fingers, two of which were functional, with an alternating structure of the carpus and relationships between the metacarpals. The weight was discharged between the second and third metacarpals, which were almost equal in length, while the fourth was shorter and the fifth was much reduced. The scaphoid bone had a strong articulation for the radius and rested on the greater trochanter and the trapezoid. In the outer part, the scaphoid articulated with the lunar bone by means of a small facet of the apophysis related to the great bone. The lunate bone was in contact distally with the great bone and the hamate, and laterally it was in contact with the cuneiform bone via a large surface. The joints of the first phalanges were limited to the plantar and distal surfaces. The phalanges nail joints were laterally compressed and provided with a small incision at the end.

The ischium was broad posteriorly, and the pubical area was small. The femur was straight and somewhat flattened anteroposteriorly. The greater trochanter slightly exceeded the articular head, while the third trochanter was well developed and in a fairly proximal position. The condyles were large. The tibia and fibula were usually separated, but sometimes distal co-ossification was present. The distal joint of the tibia was divided by a prominent ridge into two equal cavities. The fibula was thin. The trochlea of the astragalus was long and medium deep; the ridges of the talus were equal to each other, the neck long and the head globular. The calcaneus did not articulate with the navicular bone and had a large facet for the fibula. The phalanges possessed the same structure as those of the hand, but were larger. It is likely that at least the feet of ''Protypotherium'' were digitigrade.

Paleobiology

''Protypotherium'' was mainly a herbivore, but it is possible that ''Protypotherium'' fed occasionally on carrion as well. The legs clearly show robust nail phalanges, thanks to which the animal could dig burrows or modify those abandoned by other animals.

A 2021 study concerning numerous fossils of the teeth of various species of ''Protypotherium'' showed that there is a trend in the preservation of tooth pattern, increase in size and decrease in number of species over time. This could be correlated with a global trend of cooling temperatures, indicating a deterioration of paleoenvironmental conditions during the Miocene. There also appears to have been a latitude shift in the distributional range of these animals: from Lower Miocene Patagonia to northern areas of South America towards the end of the Miocene.

Paleoenvironment

Fossils of ''Protypotherium'' have been found in various fossiliferous stratigraphic units in South America. Several specimens come from the Santa Cruz Formation in the Austral Basin in southern Patagonia, Argentina, with other finds from the Cerro Azul, Cerro Boleadoras, Ituzaingó, Cerro Bandera, Chichinales, Collón Curá Formations, and the Sarmiento Formations of the Colorado, Austral, Paraná, Neuquén, Cañadón Asfalto, and Golfo San Jorge Basins, as well as the Aisol and Salicas Formations of the same country. Furthermore, fossil finds of ''Protypotherium'' have been found in other countries, such as the Fray Bentos Formation of Uruguay, also in the Paraná Basin, the Muyu Huasi Formation of the Muyu Huasi Basin in Bolivia, the Nazareno Formation in the same country, of the Tupiza Basin, and the Cura-Mallín Formation of the Cura Mallín Basin of Argentina and Chile and the Río Frías Formation of the Magallanes Basin in Chile.

In the Chichinales Formation, which is known for its local mammal fauna, ''Protypotherium'' would have coexisted with astrapotheres, the notoungulates '' Cochilius volvens'', ''Colpodon'', '' Hegetotheriopsis sulcatus'' and '' Hegetotherium'', the litoptern ''Cramauchenia'', the rodents '' Australoprocta'', '' Caviocricetus'', ''Eoviscaccia'', and '' Willidewu esteparius'', the armadillos ''Proeutatus'' and ''Stenotatus'', and the sparassodont '' Cladosictis''. Bird remains from the formation are comparatively poor. A part of a tibiotarsus has previously been classified as an undetermined species of psilopterine phorusrhacid. Other birds include an undetermined wading bird, '' Opisthodactylus horacioperezi'', a species of rhea, and '' Patagorhacos'', a phorusrhacid. During the Miocene the area likely consisted of open but wooded environment with temperate climate and a proximity to freshwater.

The Sarmiento Formation has provided a wide assemblage of mammals, consisting of pyroclastic deposits in an arid desert environment. Among these mammals were the astrapotheres ''Astrapotherium'' and ''Parastrapotherium'', the fellow notoungulates '' Argyrohippus'', ''Cochilius'', ''Colpodon'', '' Interatherium'' and ''Pachyrukhos'', the litopterns ''Cramauchenia'', ''Lambdaconus'', ''Paramacrauchenia'', '' Proheptaconus'', ''Prolicaphrium'', ''Pternoconius'', '' Tetramerorhinus''
and '' Theosodon'', the xenarthrans '' Hapaloides'', '' Holomegalonyx'', '' Nematherium'', '' Peltephilus'', ''Proeutatus'', '' Proschismotherium'', ''Prozaedyus'', '' Stegotherium'', and ''Stenotatus'', the metatherians '' Acyon'', '' Acrocyon'', '' Arctodictis'', ''Borhyaena