Tremarctinae

The Tremarctinae or short-faced bears is a subfamily of Ursidae that contains one living representative, the spectacled bear (Tremarctos ornatus) of South America, and several extinct species from four genera: the Florida spectacled bear (Tremarctos floridanus), the North American giant short-faced bears Arctodus (A. pristinus and A. simus), the South American giant short-faced bear Arctotherium (including A. angustidens, A. vetustum, A. bonariense, A. wingei, and A. tarijense) as well as Plionarctos (P. edensis and P. harroldorum), which is thought to be ancestral to the other three genera. Of these, the giant short-faced bears (Arctodus simus and Arctotherium angustidens) may have been the largest ever carnivorans in the Americas. The group is thought to have originated in eastern North America, and then invaded South America as part of the Great American Interchange. Most short-faced bears became extinct at the end of the Pleistocene.

Evolution
Tremarctinae originate with their common ancestor, Plionarctos, in the Middle Hemphillian (earliest Late Miocene, ~10Ma) of North America; Plionarctos is last recorded in the early Blancan (Early Pliocene, ~3.3Ma). Around the Miocene-Pliocene boundary (~5 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. Correspondingly, a 2016 study suggested that the mean divergence dates for Arctotherium, Arctodus and Tremarctos were 4.8 Ma, and between Arctotherium and Tremarctos at 4.1 Ma. A further study calculated the divergence date between Arctodus and Tremarctos at 5.5 Ma. All three genera are first recorded from the Blancan (Pliocene/Pleistocene boundary) of North America.

An indeterminate Arctotherium is first recorded from the late Pliocene of El Salvador. In the Early Pleistocene, short-faced bears began to establish themselves more thoroughly in both North and South America. The medium sized Arctodus pristinus inhabited a broad range in the North American continent, with Tremarctos floridanus endemic to the Gulf Coast. The first records of Arctotherium in South America come in the form of the gigantic Arctotherium angustidens, possibly the largest carnivorous land mammal ever, in Argentina circa 1 Ma. What the evolutionary history of Arctotherium in the previous 1.5 million years, and their history in South America, is unclear. In the Middle Pleistocene, both Arctodus and Arctotherium gave way to new forms; Arctodus pristinus gave way to the huge Arctodus simus, which inhabited a pan-continental range, from Alaska to Mexico. Arctotherium angustidens, on the other hand, was replaced by a series of smaller, medium-sized species- Arctotherium vetustum, then shortly thereafter by Arctotherium bonariense, and Arctotherium tarijense. Although the smaller Arctotherium wingei is only known from Late Pleistocene records, the species' more archaic position in the Arctotherium family tree also suggests an origin in the Middle Pleistocene. Arctotherium wingei was the only known species of Arctotherium to principally inhabit a range north of the Southern Cone, and to reinvade Central America.

By the terminal Pleistocene, Arctodus simus, Tremarctos floridanus, Arctotherium tarijense and Arctotherium wingei collectively occupied a range from Alaska to southernmost Patagonia. All of these forms were extinct by the end of the early Holocene. Around this time, Tremarctos ornatus, otherwise known as the spectacled bear, starts appearing in the South American fossil record. Scholars suggest that the spectacled bear migrated into Central and South America upon the extinction of Arctotherium wingei, if Pleistocene records of Andean Arctotherium sp. aren't confirmed as the spectacled bear.

Anatomy
Tremarctinae appear to have a disproportionately shorter snouts compared with most modern bears, giving them the name "short-faced." This apparent shortness is an illusion caused by the deep snouts and short nasal bones of tremarctine bears compared with ursine bears; Tremarctinae had a deeper but not a shorter face than most living bears. Osteological differences between tremarctine and ursine bears include an extra lateral cusp between the trigonid and talonid on the m1 molar, a premasseteric fossa on the mandible, and often an entepicondylar foramen on the humerus of tremarctine bears. Additionally, tremarctine bears' skulls are deeper and more brachycephalic, their zygomatic arches and glenoid fossas are well developed, and they have larger molars in comparison with ursines. Moreover, tremarctine bears' orbits are also bigger, more rounded and lateralized.

Paleobiology
Tremarctines inhabited a wide range of niches- from small and mostly herbivorous bears inhabiting more forested habitat, such as Arctotherium wingei and Tremarctos ornatus, to the colossal Arctotherium angustidens and Arctodus simus; plains adapted omnivores with a penchant for large quantities of meat. Although the two giant species appear superficially similar, both species had key, significant differences. While Arctodus simus had a wide range across North America for 800,000 years, Arctotherium angustidens appears to be limited to the Southern Cone, in open plains habitat. Furthermore, whereas Arctodus simus varied its diet between quasi-carnivory in Alaska to classic omnivory, Arctotherium angustidens had similar rates of carnivory across specimens, according to isotope studies. Additionally, the much more gracile form of Arctodus, in contrast with the robust Arctotherium angustidens, has puzzled researchers. However, it has been posited that the Pliocene extinctions of scavenger-niche mega-carnivores, such as the procyonid Chapalmalania in South America, and both Borophagus and Agriotherium in North America, was a shared impetus for gigantism in Arctodus and Arctotherium.

Arctodus and Tremarctos share characteristics common to herbivorous bears. This includes cheek teeth with large surface areas, a deep mandible, and large mandibular muscle attachments. Because herbivorous carnivorans lack an efficient digestive tract for breaking down plant matter via microbial action, they must break down plant matter via extensive chewing or grinding, and thus possess features to create a high mechanical advantage of the jaw. This presents the possibility that these traits may be an ancestral condition of the group, if not an indication of their preferred dietary habits.

Systematics
Traditionally, analyses of the phylogenetic inner relationships of tremarctines had Plionarctos and Tremarctos as basal groups with respect to a short-faced bear clade of Arctodus and Arctotherium. A study of the affinities of bears belonging to Arctotherium indicates that they were more closely related to the spectacled bear than to Arctodus, implying convergent evolution of large size in the two lineages.

Tremarctines are occasionally referred to as arctodonts or arctotheres in older scientific literature, although this has fallen out of fashion.

Taxonomy
The following taxonomy of the tremarctine bears follow by Mitchell et al. (2016):
 * Subfamily Tremarctinae Merriam & Stock, 1925
 * †Plionarctos Frick, 1926
 * †Plionarctos harroldorum Tedfored & Martin, 2001
 * †Plionarctos edensis Frick, 1926
 * †Arctodus Leidy, 1854
 * †Arctodus simus Cope, 1879
 * †Arctodus pristinus Leidy, 1854
 * †Arctotherium Burmeister, 1879
 * †Arctotherium angustidens Gervais & Ameghino, 1880
 * †Arctotherium vetustum Ameghino, 188)
 * †Arctotherium wingei Ameghino, 1902
 * †Arctotherium bonariense (Gervais, 1852)
 * †Arctotherium tarijense Ameghino, 1902
 * Tremarctos Gervais, 1855
 * †Tremarctos floridanus (Gildey, 1928)
 * Tremarctos ornatus (Cuvier, 1825) – spectacled bear