Arctotherium
Arctotherium is an extinct genus of short-faced bears endemic to Central and South America from the Late Pliocene to the end of the Late Pleistocene. 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 Early Pleistocene species Arctotherium angustidens is one of the largest known bears and possibly the largest terrestrial carnivorous mammal ever, with some individuals suggested to exceed one tonne in body mass and reach in standing height, while later species such as A. bonariense, A. tarijense and A. wingei were smaller and comparable in size to living bears. Like living bears, species of the genus were omnivorous, with the degree of meat consumption varying between species, with Arctotherium angustidens suggested to have been highly carnivorous, while A. wingei was largely herbivorous.
The last species of the genus went extinct around 13,000-10,000 years ago as part of the Late [Pleistocene extinctions|end-Pleistocene extinction event], along with most other large mammals across the Americas.
Taxonomy
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, Plionarctos and Tremarctos. In the 19th and early 20th centuries, specimens of Arctotherium were occasionally referred to Arctodus, and vice versa, 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. 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 highland form of Arctotherium is also suggested to have existed at the end of the Pleistocene, consisting of the type A. wingei specimen from Tarija and an Arctotherium sp. individual from El Rodeo, in contrast with the larger and more robust Brazilian A. wingei specimens.Although A. wingei is only known from partial cranial and dental remains elsewhere, the A. wingei skeletons identified from Hoyo Negro in Mexico are the most complete known for its species, the Arctotherium genus and all extinct Tremarctine bears. The Hoyo Negro specimens confirm that the A. wingei had a high degree of intraspecific morphological variation.
Cladogram
Below is a cladogram showing proposed relationships between Arctotherium species, based on morphological data.Diagnostics
Size can be a useful indicator in differentiating between species of Arctotherium, but cranial and dental features need to be examined for a definite identification.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 Tremarctines, along with other ursids, experienced an explosive radiation in diversity, as C4 vegetation 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. Notably, all three genera are first recorded from the Late Blancan of North America, with the first possible record of Arctotherium sp. being a tooth found in the Cuscatlán Formation of El Salvador, dated to the latest Pliocene. Arctotherium may have existed in North America for several million years without being present in the fossil record, or possibly island-hopped to mainland South America prior to formation of the Isthmus of Panama. However, genetic research suggests that Arctotherium originated in the Late Pliocene, coinciding with the establishment of a direct land connection between North and South America approximately 3 Ma.Genetic research on the mitochondrial DNA of tremarctine bears indicates Arctotherium was more closely related to Tremarctos than Arctodus. However, a preliminary investigation of tremarctine bear's nuclear DNA suggests an extensive history of hybridization between Tremarctos and Arctodus in North America, although hybridization with Arctotherium is possible, either in Central America throughout the Pleistocene or as Tremarctos migrated southwards into South America at the end of the Pleistocene.
Recent research suggests that Arctotherium either emerged from the Tremarctos genus or was a sister lineage to Tremarctos, ''Plionarctos harroldum may have been the ancestor of both genera as a transitional P. c.f. harroldum specimen from Washington state 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.A. angustidens went extinct at the start of the Lujanian, replaced by medium-sized Arctotherium species. The first recorded successor species was A. vetustum, then shortly thereafter by the more robustly built A. bonariense, along with A. tarijense ''. 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. wingei's more ancestral position in Arctotherium'', suggests an origin in the Middle Pleistocene.
Description
Skull
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.In comparison with other studied Tremarctine bears Arctotherium tarijense exhibited major morphological differences in the anatomy of its auditory bulla, showing some similarities with ursine bears. Researchers suggest that this morphology gave A. tarijense a relative high vestibular sensitivity and possibly greater agility, and therefore a better ability to explore diverse habitats. The canalis semicircularis lateral also suggests A. tarijense had a 40° head posture, which being higher than A. angustidens and A. bonariense could infer a greater capacity for long distance vision in A. tarijense.
Baby (deciduous) teeth have also been recovered from a 4-5 month old A. tarijense specimen, and Arctotherium angustidens. Although the baby teeth of A. tarijense resemble those of ursine bears, the adult teeth are divergent.
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 the fossil Arctodus and Arctotherium retained this feature in relation to their higher degree of forelimb dexterity. As these genera convergently evolved towards increased body size, this high degree of proximal dexterity may have been advantageous and therefore retained in the Tremarctinae, such as for a scavenging lifestyle.A. bonariense is believed to have convergently evolved several adaptations with Arctodus simus and Agriotherium / Huracan, such as proportionally longer limbs, very large body size, short broad rostrums, premasseteric fossa on the mandible, and possible carnassial shears. Additionally, an analysis of the elbow joint of several Arctotherium species and an indeterminate Arctodus specimen suggested a mutual preference for mixed habitats. The postcranial skeleton of A. tarijense is unknown.File:Rare Spectacled or Andean Bear.jpg|thumb|The last short-faced bear, and the ecological successor of A. wingei, is the spectacled bear.|256x256px
Size
Arctotherium consists of one early giant form, A. angustidens, and several succeeding smaller species, which were within the size range of modern bears. 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.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 ~, and A. tarijense at ~.
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 at, and A. wingei specimens from the Brazilian intertropical region 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.
While Arctotherium
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. The exceptions are the Tarija formation in southern Bolivia where three species have been recovered, a possible record of A. vetustum in Brazil, unassigned Arctotherium sp. postcranial remains from Rio Grande do Sul, a Blancan-age unassigned Arctotherium tooth from El Salvador, and A. wingei, which almost exclusively inhabited a more northern range.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. 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, 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.''A. bonariense'' & ''A. tarijense''
A. bonariense and A. tarijense had a typical prey weight of, with a maximum of. A. tarijense occasionally hunted camelids and horses as a supplement to scavenging, smaller prey and herbivory. 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''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. However, the diet of A. wingei was not necessarily orthodox, with carnivory likely peaking in times of resource instability. Isotope spikes from the Brazilian Intertropical Region also indicate the occasional consumption of the ground sloth Nothrotherium ''maquinense, and A. wingei itself, which could represent cannibalism for juveniles or cubs, as observed in American black bears and polar bears.Additionally, 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. 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, 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. 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, especially by A. angustidens, increased the rate of xenarthran cave excavations. Other Arctotherium species such as A. tarijense and A. wingei have been recovered from caves and are also hypothesized to have utilised dens.Relationship with other bears
Traditionally based on morphological data, Arctodus and Arctotherium were thought to be more closely related to each other than to Tremarctos. A 2016 mitochondrial DNA study found that the mitochondrial genome of Arctotherium is closer to the spectacled bear, than to Arctodus of North America. The authors of the study suggested that this implied Arctotherium and Arctodus evolved large size in a convergent manner. However, as Arctotherium and Arctodus had divergent evolutionary trends towards body size as the Pleistocene progressed, they are believed to have had separate selective pressures. As Arctotherium shrank in size as the Pleistocene cooled, Bergmann's rule is not believed to be the cause of the initial gigantism of A. angustidens. Instead, the initial lack of large predators and abundance of herbivores in South America when Arctotherium migrated into the continent likely fueled increased size. This reversed as the predator guild matured in the Middle Pleistocene, leading to the disappearance of A. angustidens and more herbivorous diets in Arctotherium.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.
However, the ranges of the spectacled bear, a specialist of highland Andean forests, and the more lowland & open-environ adapted Arctotherium, could have co-existed in South America. Researchers suggest an altitude of 1,800 meters represented a buffer zone between the genera.
Trackways
Footprints from Pehuén-Có under the ichnotaxon Ursichnus sudamericanus were assigned to Arctotherium tarijense due to their Late Pleistocene age. Similar in dimensions & body mass to a male spectacled bear, the tracks show an inward rotation of 10°, with the strides being 65cm in length. Long claws are visible, and digit III is longer than the others; the heel impression is absent in the manus.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, 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. Arctotherium angustidens appears first and was endemic to the Ensenadan faunal stage, while all subsequent species are only found in the proceeding Lujanian faunal stage. Arctotherium is believed to have been adapted to the open areas of arid and semi-arid habitats.After A. angustidens became extinct at the end of the Ensenadan, two forms begin to appear in the fossil record. The A. bonariense / A. tarijense species complex was composed of adaptable, cosmopolitan omnivores, whereas A. vetustum & A. wingei were largely herbivorous. However, as A. vetustum and A. wingei are the most archaic species of Arctotherium, their lineage must have existed before the emergence of A. angustidens in the Enseadan period of the Early Pleistocene. Although A. angustidens was highly carnivorous, it has been suggested that as a diverse carnivore guild became established in South America, the Arctotherium genus began to revert to more classic ursid diets as the ecosystem matured in the Middle Pleistocene.
Central America
The oldest known specimen of Arctotherium consists of a baby tooth found in the Río Tomayate locality of Cuscatlán Formation of El Salvador, along with a partial Borophagus skull, dated to the latest Pliocene. The archaic form and size of the tooth is closest to A. angustidens, however as only few other baby teeth has been recovered from Arctotherium, the tooth was only confidently assigned at the genus level as Arctotherium sp.File:Parque Estadual do Lajeado - TO.jpg|thumb|226x226px|Tropical savanna forests and grasslands, such as the modern Cerrado, would have supplanted the Amazon as the dominant biome of Pleistocene South America, and stretched into Central America. This was A. wingeiA. wingei has been recovered from cenotes & cave systems across the Yucatán; the Hoyo Negro also preserves ground sloths, Cuvieronius, Baird's tapir, Smilodon fatalis, Protocyon, puma, bobcat, collared peccary, and white-nosed coati from a tropical forest, 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
Southern Cone
Arctotherium only reappears in the fossil record 1 million years ago as A. angustidens, from the Buenos Aires Province of Argentina in the Southern Cone. A. angustidens is the only known species of Arctotherium from the Early Pleistocene.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, 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''A. bonariense'', ''A. tarijense & A. vetustum''
A. bonariense, A. tarijense and A. vetustum appeared during the Bonaerian phase of the Lujanian faunal stage, in what is referred to as the Megatherium americanum biozone.A. tarijense competed against Smilodon populator, giant jaguars, pumas, Lycalopex, Cerdocyon, Conepatus, Didelphis, and Dusicyon avus in Late Pleistocene Argentina. Although carnivory increased the further south Arctotherium lived, A. tarijense
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 Smilodon populatorThe Tarija Valley of Bolivia is the only locality which records three species of Arctotherium; A. angustidens and A. tarijense & A. wingei. Although Tarija preserves many typical Pampean species, the higher frequency of gomphothere, horse, capybara and tapir remains is unique, with the latter two being restricted today to warmer and wetter regions.
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 and consumed, leaving only skeletal remains. These behaviours have been suggested as ArctotheriumInteraction with humans
The remains of A. wingei in the Hoyo Negro of the Yucatán appear to be in association with human remains.Extinction
The last known records of Arctotherium are an ambiguous find of A. bonariense from Uruguay, 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.