Arctotherium Wingei

''Arctotherium'' ("''bear beast''") is an extinct genus of the Pleistocene short-faced bears endemic to Central and South America. ''Arctotherium'' migrated from North America to South America during the Great American Interchange, following the formation of the Isthmus of Panama during the late Pliocene. The genus consists of one early giant form, ''A. angustidens'', and several succeeding smaller species, which were within the size range of modern bears. ''Arctotherium'' was adapted to open and mixed habitat. They are genetically closer to the spectacled bear (''Tremarctos ornatus''), than to '' Arctodus'' of North America, implying the two extinct forms evolved large size in a convergent manner, perhaps to facilitate dominating other carnivores in the competition for the biggest carcasses.

Evolution

Tremarctinae

''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) and '' Tremartcos'' (the Floridian and modern spectacled bear). Tremarctinae originate with their common ancestor, '' Plionarctos,'' in the Middle Hemphillian (earliest Late Miocene, ~10 Ma) of North America; ''Plionarctos'' is last recorded in the early Blancan (Early Pliocene, ~3.3 Ma). Around the Miocene- Pliocene boundary (~5 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 4.8 Ma, and between ''Arctotherium'' and ''Tremarctos'' at 4.1 Ma. Interestingly, all three genera are first recorded from the Blancan (Late Pliocene) of North America, with the first possible record of ''Arctotherium sp.'' being a tooth found in the Cuscatlán Formation of El Salvador, along with a partial '' Borophagus'' skull, dated to the latest Pliocene (2.588 Ma).

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. ''A. angustidens'' remains have been dated to between 1Ma to 0.7 Ma of the Pleistocene, which corresponds with the Ensenadan period (although the younger dates are uncertain).

''A. angustidens'' was eventually replaced at the start of the Lujanian (~700,000 years ago) by medium-sized ''Arctotherium'' species- first by ''A. vetustum'' ( Middle Pleistocene), then shortly thereafter by the more robustly built ''A. bonariense'' (Middle / Late Pleistocene), along with ''A. tarijense'' ( Middle Pleistocene to the Early Holocene)''.'' While the smallest but most widespread species, ''A. wingei'', is only confirmed from the Late Pleistocene and Early Holocene, the species' more tropical disposition is thought to greatly limit fossilisation. That, along with ''A. wingeis more ancestral position in ''Arctotherium'', suggests an origin in the Middle Pleistocene. 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''. Of these successor species, ''A. tarijense'' and ''A. wingei'' are by far the most successful when taking into account temporal & geographic range, and the frequency of fossil finds. A separate Andean form of ''Arctotherium'' is also suggested to have existed at the end of the Pleistocene, if those finds aren't representative of the currently Holocene-restricted '' T.ornatus''.

Curiously, while ''Arctotherium'''s known species dramatically shrank in size after ''A. angustidens'', '' Arctodus'' underwent the opposite transformation, transitioning from the medium-sized ''A. pristinus'' to the gigantic ''A. simus''. by the end of the Pleistocene.

Description

Taxonomy

''Arctotherium'' was named by Hermann Burmeister in 1879. Tremarctinae (and therefore ''Arctotherium'') appear to have a disproportionately shorter snouts compared with most modern bears, giving them the name "short-faced." 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. Although size can help differentiate the species, only cranial and dental identifiers can be used to differentiate between species. 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.

With regard to locomotion, although the shape of the elbow joint suggests '' Arctodus sp''., ''Arctotherium bonariense'', and ''A. wingei'' the possibility of retaining semi-arboreal adaptations, 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 character in relation to their higher degree of forelimb dexterity. As these genera's convergent evolution towards large body size that could have been favoured by scavenging adaptations, then high degree of proximal dexterity could have been advantageous for these species and retained in the Tremarctinae lineage in spite of size evolution.

Size

''Arctotherium'' species ranged between a variety of sizes, both between species and individuals of the same species. The sole remaining Tremarctine bear, the spectacled bear, exhibits strong sexual dimorphism, with adult males being 30%–50% larger than females. Phylogenetic bracketing, along with ''Arctotherium'''s ability to exploit a variety of resource rich/poor environments and niches, can help explain ''Arctotherium'''s morphological diversity.

Various attempts to calculate each species' body mass have been made; for example, a 2006 study calculated the mean weight of two species, ''A. bonariense'' at ~110 kg (hypothetical typical weight range = 106 kg-122 kg), and ''A. tarijense'' at ~139 kg (102 kg-189 kg).

According to a 2009 study, the weight ranges for ''Arctotherium'' were calculated as follows- ''A. wingei'' at 51 kg-150 kg, ''A. vetustum'' at 102 kg-300 kg, ''A. tarijense'' at 135 kg-400 kg, ''A. bonariense'' between 171 kg-500 kg, and ''A. angustidens'' at 412 kg-1,200 kg. The study considered each end figure as the maximum hypothetical weight. Further studies calculated an ''A. tarijense'' specimen's weight (MACN 971) at 231 kg, and ''A. wingei'' specimens from the Brazilian intertropical region at ~83 kg.

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.

Paleobiology

Early Pleistocene

The first recorded ''Arctotherium'' specimens in South America occur alongside the earliest known South American records of several other carnivorans: the sabre-toothed cats '' Smilodon'' and '' Homotherium'', the puma (''Puma concolor''), the jaguar (''Panthera onca''), some large 25 kg–35 kg canids, and several smaller (<15 kg) mustelids, canids, felids and mephitids.

The North American carnivorans that invaded South America, including short-faced bears and '' Smilodon'', probably quickly adopted the predatory niches formerly occupied by the native typical South American groups such as metatherian sparassodont carnivores that had largely gone extinct shortly prior to their arrival.

''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 is hypothesized as being the gateway for ''A. angustidens''' gigantism. 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 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. Moreover, pathologies found on a huge specimen of ''A. angustidens'', being multiple deep injuries which had long healed despite infection, demonstrate a lifestyle of conflict.

Three ''A. angustidens'' individuals were discovered in a paleoburrow together (postulated to have been a mother with adolescent cubs), which opens the possibility that ''A. angustidens'' lived in family groups. ''A. tarijense'' and ''A. wingei'' are also hypothesized to have utilised dens. 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. ''A. angustidens'' is thought to have reoccupied caves excavated by Xenarthra, such as the mylodonts '' Glossotherium'' and '' Scelidotherium'', and the pampatheriid ''Pampatherium''. 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 increased the rate of xenarthran cave excavations.

Middle Pleistocene onwards

It has been suggested that as a diverse carnivore guild became established in South America, ''Arctotherium'' began to revert to a more classically ursid omnivorous diet as the ecosystem matured, with ''A. tarijense'' being an adaptable, cosmopolitan omnivore, and ''A. wingei'' being largely herbivorous. However, the diet of ''A. wingei'' was not necessarily orthodox, as several bite marks on recovered fossils of herbivores, such as '' Glossotherium'' and ''Equus'', are suggested to have been inflicted by scavenging short-faced bears across Lujanian South America. Carnivory would have likely peaked in times of resource instability.

''A. bonariense'' & ''A. tarijense''

''A. bonariense'' and ''A. tarijense'' had a typical prey weight of 100 kg, with a maximum of 300 kg. ''A. tarijense'' competed against '' Smilodon populator'', giant jaguars (''Panthera onca mesembrina''), pumas, ''Lycalopex, Cerdocyon, Conepatus, Didelphis,'' and ''Dusicyon avus'' in Late Pleistocene Argentina, occasionally hunting camelids and horses as a supplement to scavenging, smaller prey and herbivory. 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, macraucheniids, giant ground sloths, glyptodonts, and toxodontids, to ''Smilodon populator'' and giant jaguars. '' Smilodon fatalis'', ''Arctotherium bonariense'', ''Canis nehringi,'' maned wolves, and humans would have also joined this predator guild at various stages of the Lujanian. 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.

''A. vetustum'' & ''A. wingei''

Along with clues from various teeth of ''A. wingei'', carbon isotope studies suggest that ''A. wingei'', at least in the Brazilian intertropical region, were highly herbivorous, specialising in C3 vegetative matter such as fruits and leaves. This is not to diminish potential carnivory in ''A. wingei'', as the same study pointed to isotope spikes indicating the consumption of the ground sloth ''Nothrotherium'' ''maquinense'' (hypothesized as a preference for younger individuals and opportunistic scavenging), and ''A. wingei'' itself, which could represent cannibalism for juveniles or cubs, as observed in American black bears and polar bears. According to a 2021 study, the maximum prey for ''A. wingei'' would be around its own bodyweight (~83 kg).

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.'' Additionally, as dire wolves (''Aenocyon dirus'') and '' Smilodon fatalis'' inhabited north-western South America, and were joined by American lions, grey wolves and coyotes in Central America, ''A. wingei'' would have been a member of various predator guilds across the species' range.

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. However, although the spectacled bear is capable of climbing trees, ''Arctotherium'' is thought to be non-arboreal.

Distribution and habitat

Other than ''A. wingei,'' the Blancan ''Arctotherium'' sp. tooth from El Salvador, and a possible record of ''A. vetustum'' in Brazil, all ''Arctotherium'' species appear to be largely restricted to the Southern Cone, particularly Argentina, with the richest records being in the Buenos Aires Province.

By the Late Pleistocene, ''A. tarijense'' held domain over the open and semi-arid Pampas and Patagonian habitats east of the Andes, inhabiting Argentina, Patagonian Chile, 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.

On the other hand, ''A. wingei'' spanned across the northern, more mixed/forested and tropical parts of the continent, throughout the tropical savanna forests of Brazil to Bolivia, Venezuela, and into North America ( Belize and the Yucatán Peninsula, Mexico). ''A. wingei'''s association with '' Protocyon'' in the terminal Pleistocene of the Yucatán, another animal previously thought to be endemic to South America, suggests a complex relationship of faunal interchange long after the Great American Interchange.

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. Interestingly, the remains of ''A. wingei'' in the Hoyo Negro of the Yucatán appear to be in association with human remains.

'' Tremarctos'' does not appear in the South American fossil record until the Holocene, 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. The modern spectacled bear may have hybridised with ''Arctotherium'' as they migrated southwards into South America. Globally, in the Quaternary Extinction Event, extinction favoured 'conservative morphologies' in ursid body plans, such as those found in the '' T. ornatus''.

References

{{Taxonbar, from=Q2076664

Pleistocene bears
Pleistocene carnivorans
Prehistoric carnivoran genera
Pleistocene mammals of North America
Blancan
Pleistocene El Salvador
Fossils of El Salvador
Pleistocene mammals of South America
Uquian
Ensenadan
Lujanian
Pleistocene Argentina
Pleistocene Bolivia
Pleistocene Brazil
Pleistocene Chile
Pleistocene Venezuela
Fossils of Argentina
Fossils of Bolivia
Fossils of Brazil
Fossils of Chile
Fossils of Venezuela
Fossil taxa described in 1857