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 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 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