Protypotherium

Extinct genus of notoungulates

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. The taxonomy of the genus and the species within has a long and complicated history. Other genera of interatheriids such as Epipatriarchus, Eudiastatus, and Toxdontophanus, have been named, but no complete specimens exist, making comparison and classification difficult. Most modern scientists consider these genera to be junior synonyms of Protypotherium, and it is thought to contain the following species; P. australe, P. praerutilum, P. antiquum, P. altum, P. attenuatum, P. claudum, P. colloncurensis, P. diastematum, P. distinctum, P. minutum, P. endiadys, P. sinclairi, and P. concepcionensis. The most completely known species is P. australe, so most reconstructions of the genus are based on it.

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. 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.

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. The thickness of its dentition suggest that it was a grazer, eating mainly grass. Protypotherium has euhypsodont (well high crowned) 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. 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. with other finds from the Cerro Azul, Cerro Boleadoras, Cerro Bandera, 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. Among these mammals were the astrapotheres Astrapotherium and Parastrapotherium, the fellow notoungulates Argyrohippus, Cochilius, Colpodon, Interatherium and Pachyrukhos, Lambdaconus, Paramacrauchenia, Proheptaconus, Prolicaphrium, Pternoconius, Tetramerorhinus the xenarthrans Hapaloides, Holomegalonyx, Nematherium, Peltephilus, Proeutatus, Proschismotherium, Prozaedyus, Stegotherium, and Stenotatus, the metatherians Acyon, Acrocyon, Arctodictis, Borhyaena, Cladosictis, Palaeothentes, and Sipalocyon, Caviocricetus, Hypsosteiromys, Neoreomys, Paradelphomys, Parasteiromys, Perimys, Prospaniomys, Prostichomys, Protacaremys, Protadelphomys, Sarremys and Soriamys, and the primates Homunculus, Mazzonicebus and Tremacebus. The late surviving meridiolestidan Necrolestes was also present. Multiple species of Protypotherium lived during the Early Miocene in the Santa Cruz Formation of Argentina, which preserves mostly a coastal environment, but also forested and grassland regions. The area had little rainfall, so forests developed around lakes and rivers, giving Santa Cruz a diverse environment. During the Miocene, the climate was similar to those of the coasts of Chile with semi temperate forests and oceanic winds. Grasslands began spreading into Argentina during the Miocene, though much of inner Patagonia was still arid with small rainforests in between. Large, herbivorous, South American ungulates such as the astrapothere Astrapotherium, the toxodont notoungulates Adinotherium, Homalodotherium and Nesodon shared the niche of low browsers, along with the litopterns Adianthus, with the rabbit like interatheres such as Interatherium and the hegetotheres Hegetotherium and Pachyrukhos being frugivorous. Both mammalian and avian carnivores inhabited the area, the largest being the phorusrhacid Phorusrhacos. Marsupials also lived in the region, including the large carnivorous sparassodonts Borhyaena and the smaller sparassodonts Acyon, Cladosictis, and Sipalocyon. Xenarthrans in the Santa Cruz Formation were fairly common, such as the ground sloths Analcimorphus, Analcitherium, Eucholoeops, Hapalops, Hyperleptus, Nematherium, Megalonychotherium, Planops, Prepotherium, Schismotherium, Trematherium, and Xyophorus, and the armadillos Cochlops, Eucinepeltus, Proeutatus, Propalaehoplophorus, Prozaedyus, Stegotherium, and Stenotatus. Schistomys, Scleromys, Spaniomys, and Stichomys are also known. There were also primates found in the formation, such as Carlocebus and Homunculus. The Collón Curá Formation and the Colloncuran age of South America represent a time when more open environments with reduced plant covering predominated, similar to semiarid and temperate to warm, dry woodlands or bushlands. The open environment allowed more cursorial (adapted for running) and large animals to occur, contrasting with the earlier conditions during the late Early Miocene, with its well developed forests with tree dwelling animals. Forests would then have been restricted to valleys of the cordillera mountain ranges, with few tree dwelling species. This change happened progressively during the earlier Friasian stage. The transition towards more arid landscapes would have happened simultaneously with climate changes that corresponded to the Middle Miocene Climate Transition, a global cooling event which had a drying effect on continents. In addition to the mammals that characterize sediments of this age, there are also a few fossils of birds, reptiles, amphibians, and fish.