Arctotherium ("bear beast") 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 earliest recovered remains of Arctotherium are from Central America and northern South America, with specimens referred to Arctotherium c.f. wingei and Arctotherium sp. from the Plio-Pleistocene boundary of Venezuela, and Arctotherium sp. from the Early Pleistocene of El Salvador, dated to between 3Ma and 1.8Ma. Arctotherium reappears later as Arctotherium angustidens ca. 1Ma, which subsequently remains as the only confirmed species for the rest of the Early Pleistocene and early Middle Pleistocene. Around 400,000 years ago, Arctotherium angustidens is replaced in the fossil record by all other species of Arctotherium. Correspondingly, Arctotherium is known from the Blancan and Rancholabrean faunal stages in North America, and the Marplatan, Ensenadan and Lujanian faunal stages of South America.
Endemic to the Neotropical realm, Arctotherium is believed to have preferred open habitats with arid and semi-arid climates. While Arctotherium is suggested to have diversified in the tropical Americas, specimens are concentrated in the Southern Cone of South America. 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 a highly carnivorous apex predator, while A. wingei was largely herbivorous.
Arctotherium angustidens is one of the largest known bears and possibly the largest terrestrial carnivorous mammal ever, with some individuals suggested to exceed 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. The last species of the genus (A. bonariense, A. tarijense and A. wingei) went extinct around 13,000 - 9,000 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 (North American short faced bears), Plionarctos and Tremarctos (the Floridian and modern spectacled bear). 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 (~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.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 (likely A. wingei) is possible, either in Central America throughout the Pleistocene or as Tremarctos migrated southwards into South America at the end of the Pleistocene.
North America Some researchers suggest 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, USA (2.9Ma) appears to be evolutionarily intermediate between P. harroldum and T. floridanus.
Genetic research suggests an origin for Arctotherium in the Early Pliocene, with the mean genetic divergence dates for Arctotherium, Arctodus and Tremarctos between 5.5Ma and 4.8 Ma, and between Arctotherium and Tremarctos at 4.1 Ma. 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, before the establishment of a direct land connection between North and South America approximately 3 Ma.
All three genera are first recorded in North America from the Early Pleistocene of the Late Blancan faunal stage (with Arctotherium sp. being recovered from El Salvador), however the first possible records of Arctotherium are from the Plio-Pleistocene boundary of Venezuela (ca. 3Ma). While Tremarctos floridanus was once described from the Yucatán Peninsula, subsequent research reassigned these specimens to Arctotherium wingei, and established Arctotherium as being the only known bear from the Plio-Pleistocene of the Neotropical realm (Central and South America).
South America The first possible records of Arctotherium sp. and A. c.f. wingei are from the El Breal de Orocual tar pits in Venezuela, dated to the Late Pliocene and Early Pleistocene (ca. 3Ma), discrediting the hypothesis that ursids were only part of the second phase of the Great American Interchange ca. 1.8Ma. Tropical South America is believed to have been central to the diversification of Arctotherium, as almost all species have been recovered from at least one locality in the tropics. Beyond these early remains, the only confirmed Arctotherium species in South America from the Early Pleistocene is the gigantic A. angustidens from Buenos Aires, Argentina (1Ma). What the evolutionary history of Arctotherium was beforehand, particularly regarding its sudden significant size, is unclear. While early Arctotherium sp. from El Salvador and Venezuela have been recorded as sharing affinities with A. angustidens, A. angustidens remains have been only been dated to between 1Ma to 0.4 Ma of the Pleistocene, which corresponds with the Ensenadan faunal stage.
A. angustidens went extinct at the start of the Lujanian faunal stage in the late Middle Pleistocene (~400,000 years ago), and was replaced by medium-sized Arctotherium species. The first recorded successor species was 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. wingei'''s more ancestral position in Arctotherium, and tentatively assigned remains from the Plio-Pleistocene boundary of Venezuela, suggests an origin from at least the Middle Pleistocene.
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.
In comparison with other studied Tremarctine bears (A. angustidens, A. bonariense, T. floridanus & T. ornatus) 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 (32°) and A. bonariense (24°) 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 (found in association with an adult), 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 (as for the giant panda) 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.
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.