Short-faced bear

**Short-faced bears** belong to the **Tremarctinae** subfamily of bears. There is one living representative, the spectacled bear (_Tremarctos ornatus_) of South America. Extinct members include _Arctodus, Arctotherium_, _Plionarctos_ and the Florida spectacled bear (_Tremarctos floridanus_). Of these, the **giant short-faced bears** (_Arctodus simus_ and _Arctotherium angustidens_) may have been the largest ever predators in the Americas.

## Evolution

The group evolved in eastern North America from _Plionarctos_, and then spread across the Americas as part of the Great American Interchange. Most short-faced bears became extinct at the end of the Pleistocene.

The medium sized _Arctodus pristinus_ and _Tremarctos floridanus_ inhabited much of North America in the Early Pleistocene. While a fragmentary _Arctotherium_ is first found from El Salvador, the next fossils are of the gigantic _Arctotherium angustidens_ in Argentina, from around 1 million years ago. _Arctotherium angustidens_ was possibly the largest predatory land mammal ever.

In the Middle Pleistocene, _Arctodus pristinus_ evolved into _Arctodus simus_, which inhabited most of North America, from Alaska to Mexico. _Arctotherium angustidens_ was replaced by smaller, medium-sized species- first _Arctotherium vetustum_, then by _Arctotherium bonariense_ and _Arctotherium tarijense_. _Arctotherium wingei_ was the only known species of _Arctotherium_ to inhabit tropical South America and Central America.

## Biology

Short-faced bears were either smaller and mostly herbivorous bears inhabiting forested habitats, such as _Arctotherium wingei_ and _Tremarctos ornatus_, or omnivores adapted for more open habitats, such as _Arctotherium angustidens_ and _Arctodus simus_.

Although the two giant species appear similar, both species had key differences. While _Arctodus simus_ inhabited most of North America for over a million years, _Arctotherium angustidens_ has only been found in the Southern Cone, from open plains habitat. Also, while _Arctodus simus_ varied its diet between mostly eating meat in Alaska to omnivory elsewhere, _Arctotherium angustidens_ had similar rates of eating meat across specimens, according to studies of its bone chemistry.

Additionally, the much more slender bones of _Arctodus_, in contrast with the robust _Arctotherium angustidens_, have puzzled researchers. However, the extinctions scavenger-niche mega-carnivores in their habitats could have been a shared reason for evolving gigantism in _Arctodus_ and _Arctotherium_.

_Arctodus_ and _Tremarctos_ share characteristics common to herbivorous bears. These were either ancestral traits of the group, or clues to their preferred diets. _Arctotherium_ was more closely related to the spectacled bear than to _Arctodus_, implying convergent evolution of large size in the two lineages.

## Taxonomy

The following taxonomy of the short-faced bears follow Mitchell et al. (2016):

- Subfamily Tremarctinae
    - †_Plionarctos_
        - †_Plionarctos edensis_
        - †_Plionarctos harroldorum_
    - †_Arctodus_
        - †_Arctodus pristinus_
        - †_Arctodus simus_
    - †_Arctotherium_
        - †_Arctotherium angustidens_
        - †_Arctotherium bonariense_
        - †_Arctotherium tarijense_
        - †_Arctotherium vetustum_
        - †_Arctotherium wingei_
    - _Tremarctos_
        - †_Tremarctos floridanus_
        - _Tremarctos ornatus_ – spectacled bear