Tremarctos is a genus of the monophyletic bear subfamily Tremarctinae, endemic to Americas from the Pliocene to recent. The northern species, the Florida short-faced bear (T. floridanus), went extinct in the Late Pleistocene. The sole living Tremarctos species is the South American spectacled bear (T. ornatus). Tremarctos is also the only living genus under the Tremarctinae subfamily, with the other short-faced bears (Plionarctos, Arctodus, and Arctotherium) all being extinct.
Taxonomy
The genus name Tremarctos was created in 1855 by Alfred Gervais. While his suggestion for making Tremarctos a separate genus was rejected, after further studies comparing skeletal structures of the teeth and jaws of Tremarctos to other bear species his suggestion was later accepted.
The word Tremarctos is derived from the Greek language and translates to "hole bear" in English, due to the entepicondylar foramen on the humerus. However, the name "hole bear" could also come from the fact that the tan facial markings appear on the face except around the bear's eyes, making it look like the bear has holes for eyes, giving it the name Jukumari in Andean languages.
Diagnostics
Within Ursidae
Unlike almost all other extant bears (from the Ursinae subfamily), tremarctine bears like Tremarctos appear to have a disproportionately shorter snouts, 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 shorter and taller skulls, but not a shorter face than most living bears. In addition to being brachycephalic, tremarctine bears' skulls possessed well developed zygomatic arches and glenoid mandibular fossas, with tremarctine bears often possessing an entepicondylar foramen on the humerus. Moreover, tremarctine bears' orbits are also bigger, more rounded and lateralized. Cranial differences between tremarctine and ursine bears also include an extra lateral cusp between the trigonid and talonid on the m1 molar,
Tremarctos
T. floridanus and T. ornatus are morphologically very similar, and are considered sister species. However, T. floridanus was around twice as big as T. ornatus, being around the size of a larger American black bear. Though both species have short rostrums in comparison with ursine bears, the rostrum of T. floridanus is relatively narrow compared with T. ornatus. Additionally, unlike T. ornatus, T. floridanus possesses a signature "glabella" (dome-like protrusion) on the frontal bone of the cranium. Both species share practically identical dentitions (particularly behind the canines), while T. ornatus does not, suggesting T. ornatus potentially possesses a larger gape. The lower jaws of T. floridanus are larger; while the ramus of the mandible is taller in T. floridanus, the relative height of the mandible's coronoid process is the same in both species. the hindlimbs of T. floridanus are the same length as their forelimbs. The genetic divergence date for Arctodus is between 5.5 million years ago and 4.8 million years ago,
The medium-sized Arctodus pristinus, Tremarctos floridanus and Arctotherium sp. evolved from Plionarctos in the Blancan faunal age of North America, being first recorded ca. 2.6Ma. Researchers believe that Arctotherium was a sister lineage to Tremarctos, or even emerged from the Tremarctos genus. These first appearances near the Plio-Pleistocene boundary coincide with the start of the Quaternary Glaciation, the formation of the Panama Land Bridge, and the second phase of the Great American Biotic Interchange, with the first records of the main South American faunal wave into the United States. A Plionarctos harroldum specimen from Taunton (Washington, 2.9Ma)
Genetic research on the mitochondrial DNA of tremarctine bears indicates Tremarctos was more closely related to Arctotherium than Arctodus. However, a preliminary investigation of tremarctine bear's nuclear DNA suggests an extensive history of hybridization between Tremarctos and Arctodus in North America, although hybridization with Arctotherium (likely A. wingei) as the Tremarctos genus migrated southwards into South America is also possible. Evidence of gene flow between Tremarctos and an ursine bear was also uncovered, most likely due to the extensive overlap between Tremarctos and the ancestors of the American black bear in Pleistocene North America.
Tremarctos genus
The Intermontane Plateaus of North America preserve the oldest possible remains of Tremarctos (T. floridanus), being from Palm Spring Formation (Anza-Borrego, California, ~2.7Ma), Grand View fauna (Glenns Ferry Formation, Idaho, 2.3Ma), and San Simon (Arizona ca. 2.2Ma), although the Grand View specimen may instead represent Plionarctos. researchers suggest that indeterminate ursid from the mid-Blancan Buckhorn fauna (New Mexico, 4Ma - 3Ma) may represent either Tremarctos sp. or Protarctos abtrusus.
The North American species T. floridanus is believed to have been the ancestor of T. ornatus. However, the fossil record of T. ornatus is unknown, as T. ornatus remains do not appear until the Holocene of South America. This suggests that the extant T. ornatus descends from an independent, later dispersal event from North America to that of Arctotherium, possibly after Arctotherium wingei became extinct in the Americas.
The last reliable direct radiocarbon date for T. floridanus is 23,230 ± 490 BP, from Laubach Cave No. III, Texas, although the chronology of megafaunal extinctions in the Late Pleistocene of southeastern North America (the native range of Tremarctos floridanus) has been disputed. The canalis semicircularis lateral suggests that T. floridanus had a head posture of 38°, which is more oblique than its sister species T. ornatus (29°); as T. ornatus inhabits densely vegetated areas, the more oblique head posture in T. floridanus could infer a greater capacity for long distance vision.
Dentition
The species under the Tremarctos genus have short mandibles as well as short crania. Even though they are omnivores, the evidence of having short mandibles and crania indicates they feed mostly on plants and fruits and only sometimes eat meat. Because of this they also have resilient teeth near their cheeks. They also have small canines and large molars showing they eat meat as well. They have 42 teeth.
Habitat
Unlike its modern Neotropical sister species T. ornatus, T. floridanus was a temperate species that has almost entirely been recovered from Nearctic sites (southern North America). T. ornatus however lives in South America, in the highlands associated with the Andes Mountains of Bolivia, Colombia, Peru and Venezuela. They are also able to live in other types of biomes such as temperate grasslands and deciduous forests.