Within Arctotherium, two clades are thought to exist- A. bonariense and A. tarijense have been described as the most derived species of the genus,[1][8] whilst A. vetustum and A. wingei are regarded the most archaic, even more so than A.

[10] 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.

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.

[21] An extraordinarily large specimen of A. angustidens recovered in 2011 from Buenos Aires shows an individual estimated, using the humerus, to weigh between 983 and 2,042 kg (2,167 and 4,502 lb).

A. bonariense A. tarijense 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.

[10] The exceptions are the Tarija formation in southern Bolivia where three species have been recovered,[9] a possible record of A. vetustum in Brazil,[1] unassigned Arctotherium sp.

postcranial remains from Rio Grande do Sul,[22] a Blancan-age unassigned Arctotherium tooth from El Salvador,[6] and A. wingei, which almost exclusively inhabited a more northern range.

[20][30] 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.

[19] Smilodon fatalis, Arctotherium bonariense, Canis nehringi, maned wolves, and humans would have also joined this predator guild at various stages of the Lujanian.

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.

[21] 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.

[30][24] 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.

[30] Moreover, pathologies found on a huge specimen of A. angustidens, being multiple deep injuries which had long healed despite infection,[2] demonstrate a lifestyle of conflict.

[38] 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.

[43] The oldest known specimen of Arctotherium consists of a baby tooth (dp4 molar) found in the Río Tomayate locality of Cuscatlán Formation of El Salvador, along with a partial Borophagus skull, dated to the latest Pliocene (2.588 Ma).

[2][6] 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 25–35 kg (55–77 lb) canids, and several smaller (<15 kg (33 lb)) mustelids, canids, felids and mephitids.

[2] In the Ensenadan, A. angustidens was only rivalled in size by Smilodon populator, with Theriodictis platensis, Canis gezi,[45] Protocyon scagliorum, Panthera onca and pumas rounding out the predator guild in the Early Pleistocene Argentina.

[4][20] 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.

[1] 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.

[21] For example, 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.

[20][47] A. tarijense competed against Smilodon populator, giant jaguars (Panthera onca mesembrina), pumas, Lycalopex, Cerdocyon, Conepatus, Didelphis, and Dusicyon avus in Late Pleistocene Argentina.

[19] Smilodon fatalis, Arctotherium bonariense, Canis nehringi, maned wolves, and humans would have also joined this predator guild at various stages of the Lujanian.