Arctotherium

Arctotherium was named by Hermann Burmeister in 1879. Arctotherium is part of the Tremarctinae subfamily of bears, otherwise known as the short faced bears, which also includes Arctodus (North American short faced bears), Plionarctos and Tremarctos (the Floridian and modern spectacled bear). along with other synonyms. Systematics Within Arctotherium, two clades are thought to exist- A. bonariense and A. tarijense have been described as the most derived species of the genus, whilst A. vetustum and A. wingei are regarded the most archaic, even more so than A. angustidens. Although A. wingei is only known from partial cranial and dental remains elsewhere, The Hoyo Negro specimens confirm that the A. wingei had a high degree of intraspecific morphological variation. The upper canine is very similar between species of Arctotherium, differing mainly in size. The canine of A. wingei is the smallest among the species. The lower canine of A. wingei presents two enamel ridges as in A. angustidens and A. tarijense, while in A. vetustum and A. bonariense there are three ridges. In A. vetustum, the distal ridge is very small and the mesial ridge is small, while in A. angustidens and A. tarijense both ridges are large. == Evolution ==

Tremarctinae Tremarctinae originate with their common ancestor, Plionarctos. Around the Miocene-Pliocene boundary (~5.3 Ma) Tremarctines, along with other ursids, experienced an explosive radiation in diversity, as C4 vegetation (grasses) and open habitats dominated, the world experienced a major temperature drop and increased seasonality, and a faunal turnover which extinguished 60–70% of all Eurasian faunal genera, and 70–80% of North American genera.Correspondingly, recent genetic studies suggest that the mean divergence dates for Arctotherium, Arctodus and Tremarctos was between 5.5Ma and 4.8 Ma,' and between Arctotherium and Tremarctos'' at 4.1 Ma. Recent research suggests that Arctotherium either emerged from the Tremarctos genus or was a sister lineage to Tremarctos, appears to be evolutionarily intermediate between P. harroldum and T. floridanus. South America The oldest dated confirmed remains of Arctotherium in South America are those of the gigantic A. angustidens from Buenos Aires, Argentina. What the evolutionary history of Arctotherium was beforehand, particularly regarding its sudden significant size, is unclear, though ursids are believed to have been part of the second phase of the Great Biotic Interchange, which is believed to have begun 1.8Ma. A. angustidens remains have been dated to between 1Ma to 0.4 Ma of the Pleistocene, which corresponds with the Ensenadan period. ==Description==

Skull Tremarctinae (and therefore Arctotherium) appeared to have disproportionately shorter snouts compared to most modern bears, hence the name "short-faced" was given to them. This apparent shortness is an illusion caused by the deep snouts and short nasal bones of tremarctine bears compared with ursine bears; Arctotherium had a deeper but not a shorter face than most living bears. Postcranial The shape of the elbow joint suggests the possibility of semi-arboreal locomotion for Arctodus sp., Arctotherium bonariense, and A. wingei, but the size of the elbow joint does not. As the medial epicondyle is particularly expanded in these species, it is likely that (as for the giant panda) the fossil Arctodus and Arctotherium retained this feature in relation to their higher degree of forelimb dexterity. such as for a scavenging lifestyle. Additionally, an analysis of the elbow joint of several Arctotherium species and an indeterminate Arctodus specimen suggested a mutual preference for mixed habitats. According to a 2009 study, the weight ranges for Arctotherium were calculated as follows- A. wingei at , A. vetustum at , A. tarijense at , A. bonariense between , and A. angustidens at . The study considered each end figure as the maximum hypothetical weight. Further studies calculated an A. tarijense specimen's weight (MACN 971) at , An extraordinarily large specimen of A. angustidens recovered in 2011 from Buenos Aires shows an individual estimated, using the humerus, to weigh between . However, the authors consider the upper limit as improbable, and say that is more likely. An estimated standing height for this A. angustidens individual is between . It would still make the species the largest bear ever found, and contender for the largest carnivorous land mammal known. ==Distribution and habitat==

Almost all Arctotherium species appear to be largely restricted to the Southern Cone, particularly Argentina, with the richest records being in the Buenos Aires Province. a Blancan-age unassigned Arctotherium tooth from El Salvador, and A. wingei, which almost exclusively inhabited a more northern range. southern Bolivia, and Uruguay, although A. bonariense may have also been contemporary in Late Pleistocene Uruguay. A. tarijense has been described as having a very low density of fossil material in Patagonia. throughout the tropical savanna forests of Brazil to Bolivia, Venezuela, and into North America (Belize and Mexico, Yucatán Peninsula). A possible A. wingei specimen has also been found in northwest Argentina. == Diet ==

A. angustidens Using carbon isotopes, A. angustidens' diet has been posited to be omnivorous with a preference towards large quantities of meat. Beyond the scavenging of mega-herbivore carcasses, the type of tooth wear present amongst A. angustidens specimens, in addition to the frequency of broken teeth from most specimens (especially at older ages), suggests the active predation of large vertebrates, including but not limited to horses, tapirs, camelids, macraucheniids, glyptodonts, giant ground sloths, toxodontids, and gomphotheres by A. angustidens. Carbon isotope studies from southernmost Patagonia suggest that A. tarijense was a particularly active scavenger. Although carnivory increased the further south Arctotherium lived, carbon isotopes suggest that A. tarijense's prey weight limit peaked at 300 kg, leaving the (subadult and younger) mega-mammals, such as the gomphotheres, giant ground sloths, and toxodontids, to Smilodon populator and giant jaguars. However, a fragmented Arctotherium c.f. tarijense tooth from Baño Nuevo-1 cave in southern Chile preserves cavities, which could be interpreted as a consequence of consuming carbohydrate-rich foods such as fruit or honey. A further microwear analysis attempt of the tooth in 2015 was complicated by hard plant and bone consumption causing similar damage to teeth in omnivores. However, the diet of A. wingei was not necessarily orthodox, with carnivory likely peaking in times of resource instability. According to a 2021 study, the maximum prey for A. wingei would be around its own bodyweight (~). == Paleobiology ==

Hibernation Three A. angustidens individuals were discovered in a paleoburrow together (postulated to have been a mother with adolescent cubs, in association with Scelidodon and Scelidotherium skulls), which opens the possibility that A. angustidens used dens for hibernation. In contrast with the spectacled bear's tropical and temperate habitat, Pleistocene Argentina's seasonal and often harsh climate suggests quasi-hibernation would have been an effective strategy for survival, as ursine bears do today. As suitable paleoburrows are rare before the Great American Interchange, it has been suggested that predation and competition for dens by the newly arrived eutherian carnivores, especially by A. angustidens, increased the rate of xenarthran cave excavations. suggesting that the extant spectacled bear descends from an independent, later dispersal event from North America to that of Arctotherium, possibly after A. wingei became extinct in the Americas. Paleopathology Beyond the scavenging of mega-herbivore carcasses, the type of tooth wear present amongst A. angustidens specimens, in addition to the frequency of broken teeth from most specimens (especially at older ages), suggests the active predation of large vertebrates, including but not limited to horses, tapirs, camelids, macraucheniids, glyptodonts, giant ground sloths, toxodontids, and gomphotheres by A. angustidens. Of the dentition known from later Arctotherium species, only one specimen of A. bonariense exhibits the same cracked teeth which A. angustidens had, although extreme wear of the occlusal molar surface is common throughout the genus. A microwear analysis attempt of Arctotherium c.f. tarijense in 2015, suggests the consumption of hard plant and bone consumption caused similar damage to teeth in omnivores. The pathologies found on a huge specimen of A. angustidens, being multiple deep injuries which had long healed despite infection, demonstrate a lifestyle of conflict. == Paleoecology ==

The North American carnivorans that invaded South America probably quickly adopted the predatory niches formerly occupied by the native typical South American groups such as metatherian sparassodonts and phorusracids that had largely gone extinct shortly prior to their arrival. while Extinction Cave contains armadillo, Panthera atrox, jaguar, puma, collared peccary, Palaeolama mirifica, red brocket deer, Bison sp., Equus conversidens, and Smilodon fatalis from a mixed grassland / scrub savanna. A. wingei's association with Protocyon in the Hoyo Negro, another animal previously thought to be endemic to South America, suggests a complex relationship of faunal interchange long after the Great American Interchange. The first recorded Arctotherium specimens in South America occur alongside the earliest known South American records of other fauna of North American origin; deer, tapirs, felids (e.g. Felis, Homotherium, Panthera, Puma, Smilodon), and mephitids, in what is referred to as the Mesotherium cristatum biozone. Holoarctic origin ungulates and carnivorans experienced an uptick in diversity via speciation within South America; while native Xenarthrans also experienced a notable increase in diversity, there was a decrease in native Notoungulates. A. angustidens In the Ensenadan, A. angustidens was only rivalled in size by Smilodon populator, with Theriodictis platensis, Canis gezi, Protocyon scagliorum, Panthera onca and pumas rounding out the predator guild in the Early Pleistocene Argentina. The extinction of the scavenger-niche specialist procyonid Chapalmalania during this faunal turnover event has been hypothesized as being the gateway for A. angustidens' gigantism. A higher proportion of older A. angustidens individuals have been recovered than other Arctotherium species. A. bonariense, A. tarijense & A. vetustum , would have been favoured prey items and habitat for several southern species of Arctotherium''. A. bonariense, A. tarijense and A. vetustum appeared during the Bonaerian phase of the Lujanian faunal stage (400kya - 130kya), in what is referred to as the Megatherium americanum biozone. Although carnivory increased the further south Arctotherium lived, A. tarijense's prey weight limit peaked at 300 kg, leaving the (subadult and younger) mega-mammals, such as the gomphotheres, giant ground sloths, and toxodontids, to Smilodon populator and giant jaguars. Eastern South America In the low-density savanna forests of the Brazilian intertropical region, A. wingei, pumas and jaguars played a supporting role to the (also likely solitary) Smilodon populator's dominance of the regional predator guild, avoiding competition with Protocyon troglodytes in more open savanna. Being smaller and more herbivorous, A. wingei would have also likely competed with other smaller carnivorans present in the BIR, such as jaguarundi, Lycalopex, Chrysocyon, Cerdocyon, Theriodictis, Speothos, Nasua, Procyon, Eira, Conepatus, Galictis, and Leopardus. Paleo-ecological reconstructions Although mostly herbivorous, the modern spectacled bear is on occasion an active predator. The spectacled bear has several hunting techniques- principally, the bear surprises or overpowers its prey, mounts its back, and consumes the immobilised animal while still alive, pinning the prey with its weight, large paws and long claws. Alternatively, the bear pursues the prey into rough terrain, hillsides, or precipices, provoking its fall and/or death. After death, the prey is dragged to a safe place (usually a nest over a tree, or a forested area) and consumed, leaving only skeletal remains. These behaviours have been suggested as Arctotherium's hunting strategies as well. ==Extinction==

The last known records of Arctotherium are an ambiguous find of A. bonariense from Uruguay (cf./aff, either ~36,900 or ~14,485 BP of the Sopas Formation, A. tarijense at 10,345 BP in the Cueva Del Puma, Patagonia, Chile, and A. wingei at 12,850 BP in the Sistema Sac Actun (Yucatán), Mexico, with a possible record of 9,000 BP in Muaco, Venezuela. Globally, in the Quaternary Extinction Event, extinction favoured 'conservative morphologies' in ursid body plans, such as those found in the spectacled bear. For example, the more specialised teeth of Arctotherium could have limited its diet more than the contemporary spectacled bear, and thus have made Arctotherium more vulnerable to extinction. ==References==