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*.

## 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

## References