Tremarctinae

Evolution

Neogene origins

Tremarctinae originate with their common ancestor, Plionarctos, in the Hemphillian faunal stage (Late Miocene) of North America, with the oldest confirmed record of this genus dating to ~5.6Ma; Plionarctos is last recorded in the early Blancan faunal stage (Pliocene, ~3Ma). Around the Miocene-Pliocene boundary (~5.3 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, genetic studies suggest that the mean divergence dates for Arctotherium, Arctodus and Tremarctos were between 5.5Ma and 4.8Ma, and between Arctotherium and Tremarctos at 4.1 Ma. All three genera are first recorded from the Blancan (Pliocene/Pleistocene boundary) of North America. Preliminary research on the carnassial teeth of tremarctine bears demonstrate rapid and diverse morphological changes across the genera, suggesting rapid evolutionary change and specialization in comparison with other lineages of bears.

Quaternary expansion

In the Early Pleistocene, short-faced bears began to establish themselves more thoroughly in both North and South America. Indeterminate Arctotherium sp. are first recorded from the Plio-Pleistocene El Breal de Orocual tar pits in Venezuela, and the earliest Pleistocene (Gelasian) of El Salvador. The medium sized Arctodus pristinus inhabited a broad range on 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.

Decline

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.

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

The following taxonomy of the tremarctine bears follow by Mitchell et al. (2016):

• Subfamily Tremarctinae Merriam & Stock, 1925
  • †Plionarctos Frick, 1926
    • †Plionarctos edensis Frick, 1926
    • †Plionarctos harroldorum Tedford & Martin, 2001
  • †Arctodus Leidy, 1854
    • †Arctodus pristinus Leidy, 1854
    • †Arctodus simus Cope, 1879
  • †Arctotherium Burmeister, 1879
    • †Arctotherium angustidens Gervais & Ameghino, 1880
    • †Arctotherium bonariense Gervais, 1852
    • †Arctotherium tarijense Ameghino, 1902
    • †Arctotherium vetustum Ameghino, 1885
    • †Arctotherium wingei Ameghino, 1902
  • Tremarctos Gervais, 1855
    • †Tremarctos floridanus (Gildey, 1928)
    • Tremarctos ornatus (Cuvier, 1825) – spectacled bear