[6][8][9][10][11] However, today neither genera are considered to have overlapped, with the closest point of contact being México, with the giant Arctodus simus in Valsequillo, Puebla,[7][12][13] and the smaller Arctotherium wingei in the Yucatán Peninsula.

[2][19][20] The genetic divergence date for Arctodus is ~5 million years ago,[17][21] around the Miocene-Pliocene boundary, when tremarctine bears, along with other ursids, experienced an explosive radiation in diversity, as C4 vegetation (grasses) and open habitats dominated.

[38][61] Matheus suggested that a broad snout could have housed a highly developed olfactory apparatus, or accommodated a larger throat passage to bolt down large food items, akin to spotted hyenas.

[74] Sorkin argued that the pronation of the forearm and the flexion of the wrist and digits, and more lightly muscled forelimbs, all of which are crucial to grasping a large prey animal with the forepaws, were probably less powerful in Arctodus than in either the brown bear or in Panthera.

Furthermore, the relatively broad humeral & femoral epicondyles were characteristic of diggers and polar bears, and suggested Arctodus simus could have foraged for roots, tubers and ground squirrels and/or had developed forelimb muscles to immobilze moving prey.

Additionally, elevated carbon-13 levels (corresponding to C3 resources) from many localities (Alaska,[101] California,[30][94] San Luis Potosí,[31] Texas,[102] Vancouver Island,[92][103] and the Yukon)[104] largely suggest browsers (and browsed vegetation) were the core of A. simus' diet.

[88][103][101][94] The extinction of cursorial, hyper-carnivorous Borophagus and Huracan in the more open western North America left a vacant niche, possibly contributing to the evolution of A. simus (along with changes to the herbivore guild).

[21][74] The bite marks found on many bones of ground sloths (Northrotheriops texanus) and young proboscideans at Leisey Shell Pit in Florida matched the size of the canine teeth of Arctodus pristinus.

[104][114] Carbon-13 levels in Arctodus simus (enriched by both plants and prey matter) consistently reflect a diet based on C3 resources, typically found in closed to mixed habitats with at least some tree cover (such as open woodlands).

[30][31][92][101][94][102] This includes C3 vegetation (leaves, stems, fruits, bark, and flowers from trees, shrubs, and cool season grasses)[28] and the browsers that fed on them, such as deer, camelids, tapir, bison and ground sloths.

[2] In the Early Pleistocene of Blancan Florida, the Santa Fe River 1 site (~2.2 Ma), which Arctodus pristinus inhabited,[1][2] was a fairly open grassland environment dotted with karst sinks and springs and dominated by longleaf pine flatwoods.

[127][128] Evolving from the smaller A. pristinus in the early Irvingtonian faunal stage,[7][27] scholars today mostly conclude that Arctodus simus was a colossal, opportunistic omnivore, with a flexible, locally adapted diet akin to the brown bear.

The diverse flora of the Camelops faunal province included montane conifers and oak parklands, shrub and grassland that stretched across the North American Cordillera south of Canada, to the Valley of Mexico.

Dire wolves and Arctodus simus were ever present members of the local predator guild throughout the Pleistocene, whereas jaguars, Homotherium, Miracinonyx and Smilodon (Fairmead & Irvington) transitioned to Panthera atrox and coyotes (McKittrick Tar Pits).

[30] Although Arctodus could have hunted other closed habitat browsers such as deer (Cervus & Odocoileus), camelids (Hemiauchenia & Camelops), Paramylodon, and peccaries,[30] specimens collected from the La Brea Tar Pits suggest A. simus preferred a herbivorous diet.

Notable fauna which lived alongside Arctodus simus included Shasta ground sloth, shrub-ox, pronghorns (Stockoceros, Capromeryx), Camelops, Odocoileus, horses, Lynx, puma, black bear, mountain goats, prairie dogs, and Stock's vampire bat.

[36][38][7]In the Rancholabrean age, Arctodus simus, grey wolves and coyotes were part of a predator guild throughout the great plains, and were joined by Columbian mammoths, camels, Hemiauchenia, and American pronghorns.

At the terminal Pleistocene Sheriden Cave, Ohio, a mosaic habitat consisting of marsh, open woodland, and patchy grassland was home to Arctodus simus, Cervalces scotti, caribou, peccaries (Platygonus, Mylohyus), giant beaver, porcupine, and American pine marten.

Inhabiting in this C3 resource dominated environment were Arctodus simus, mastodon, (southernmost) woolly mammoths, Bootherium, horses, caribou, Megalonyx, dire wolves, beavers, Cervalces, and a variety of warm-adapted reptiles, suggesting a more mesic and less seasonal climate than today.

At the Rainbow River and Lake Rousseau localities in Rancholabrean Florida, three Arctodus simus specimens have been recovered, alongside Smilodon, dire wolves, jaguars, ground sloths (Megalonyx, Paramylodon), llamas (Hemiauchenia, Palaeolama), Vero's tapir, giant beaver, capybara, Holmesina, horses, Bison antiquus, mastodon, Columbian mammoths and Tremarctos floridanus, in a climate similar to today's.

[49][205] The North Slope of Alaska <40,000 BP (Ikpikpuk and Titaluk rivers) preserves an upland and floodplain environment, with horses, bison then caribou being the most populous herbivores, and woolly mammoths, muskox, elk and saiga antelope more scarce.

[101][206] Additionally, upon the flooding of the Bering Strait and expansion of moist tundra and peatlands in eastern Beringia during MIS-3, lions, brown bears and Homotherium went regionally extinct ~35,000 BP, whereas wolves and Arctodus persisted.

[49] Together with great size, the two species converged on several adaptations, including a skull with a short broad rostrums, premasseteric fossa on the mandible, possible carnassial shears (P4 and m1), and long limbs (relative to body length).

[222] In the late 1980s, Val Geist hypothesized that "specialist, aggressive, competitive Rancholabrean fauna" such as Arctodus were a barrier for humans (along with other Siberian megafauna such as moose, grey wolves and brown bears) when migrating into North America (both Beringia and below the ice sheets).

[88] Various factors, including the depletion in number of large herbivores,[50][46] the diminishing nutritional quality of plants during climate change, and competition with fellow omnivores (humans and brown bears) for food resources, have been suggested as the cause of Arctodus simus' extinction.

For example, on Vancouver Island (~13,500 BP), vegetation changed rapidly from open woodlands with abundant lodgepole pine to increasingly closed forests with shade-tolerant spruce, mountain hemlock, and red alder.

[93][244] That the individual from Sheriden Cave, Ohio was very closely related to Beringian specimens may further support this idea, as these populations had possibly been isolated from before the Last Glacial Maximum (tens of thousands of years).

[28][49][74] Furthermore, the lack of definitive predatory adaptions (such as the absence of laterally compressed canines, and carnassials built for crushing and grinding rather than shearing meat) puts doubt to any species-wide hyper-carnivorous interpretations of Arctodus.

[72] Although the only living hyper-carnivorous ursid, the polar bear, also lacks carnassial shears, the species' specialization on small prey and reliance on blubber (rather than coarser flesh) invalidates this comparison with Arctodus.

[42] Under this model, A. simus was ill-equipped to be an active predator, having evolved as a specialized scavenger adapted to cover an extremely large home range in order to seek out broadly and unevenly distributed mega-mammal carcasses.