The genus nameTremarctos was created in 1855 by Alfred Gervais.[1] While his suggestion for makingTremarctos a separate genus was rejected, after further studies comparing skeletal structures of the teeth and jaws ofTremarctos to other bear species his suggestion was later accepted.[2]
The wordTremarctos is derived from the Greek language and translates to "hole bear" in English, due to theentepicondylar foramen on thehumerus. 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 nameJukumari in Andean languages.[3]
Unlike almost all other extant bears (from theUrsinae subfamily), tremarctine bears likeTremarctos 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 withursine bears; Tremarctinae had shorter and taller skulls, but not a shorter face than most living bears.[4] In addition to beingbrachycephalic, tremarctine bears' skulls possessed well developedzygomatic arches andglenoid mandibular fossas,[4] a premasseteric fossa on themandible (except forPlionarctos),[5] with tremarctine bears often possessing anentepicondylar foramen on thehumerus.[6] Moreover, tremarctine bears'orbits are also bigger, more rounded and lateralized.[4] Unlike tremarctine bears, ursine bears have only one masseteric fossa on their mandible and more slender and elongated skulls, with generally narrower molars (with the exception of polar bears).[7][8] Cranial differences between tremarctine andursine bears also include an extra lateral cusp between thetrigonid andtalonid on them1 molar,[6] with tremarctines possessing larger molars in comparison with ursines.[4] Within the Tremarctinae subfamily,Tremarctos is noteworthy for its relatively smaller teeth, the presence of anterior premolars, well-defined masseteric fossae, and a W-shaped cusp pattern on the lower m1 molar.[9]
T. floridanus andT. ornatus are morphologically very similar, and are consideredsister species. However,T. floridanus was around twice as big asT. ornatus, being around the size of a larger American black bear.[10] Though both species have short rostrums in comparison with ursine bears, the rostrum ofT. floridanus is relatively narrow compared withT. ornatus. Additionally, unlikeT. ornatus,T. floridanus possesses a signature "glabella" (dome-like protrusion) on the frontal bone of the cranium.[11] Both species share practically identical dentitions (particularly behind the canines),[12][11] though the dentition ofT. floridanus was larger, often with a reduced number of premolars and relatively longer molars.[9]Tremarctos floridanus has mandibular condyles raised well above the plane of the teeth,[13] whileT. ornatus does not, suggestingT. ornatus potentially possesses a larger gape.[14] The lower jaws ofT. floridanus are larger; while theramus of the mandible is taller inT. floridanus, the relative height of themandible's coronoid process is the same in both species.[9]
Both species also differ slightly in their post-cranial proportions.T. floridanus has been described as a relatively long-limbed species compared toT. ornatus, with the humerus, femur and neck being notably longer compared to body length.[11] While the forelimbs ofT. ornatus are longer than their hindlimbs (likely due to arboreal activity),[15] the hindlimbs ofT. floridanus are the same length as their forelimbs.[11] Additionally,T. floridanus has also been described as possessing much more robust limb bones.[12] However, the paws ofT. floridanus are proportionally shorter and smaller thanT. ornatus.[11] Kurtén compared the differences between theTremarctos species as the differences between brown bears and Eurasian cave bears.[12]
Tremarctos belongs to the subfamily Tremarctinae, which first appeared asPlionarctos during thelate Miocene epoch of North America.Plionarctos is also believed to be the ancestor of all subsequent Tremarctine bears (Arctodus,Arctotherium andTremarctos).[16] The genetic divergence date forArctodus is between 5.5 million years ago and 4.8 million years ago,[17][16] and betweenArctotherium andTremarctos at 4.1 million years ago.[16] This genetic divergence occurred around theMiocene-Pliocene boundary, when tremarctine bears, along with other ursids, experienced an explosive radiation in diversity, asC4 vegetation (grasses) and open habitats dominated. The world experienced a major temperature drop and increased seasonality, and a faunal turnover which extinguished 70–80% of North American genera.[18][19]
The medium-sizedArctodus pristinus,Tremarctos floridanus andArctotherium sp. evolved fromPlionarctos in theBlancan faunal age of North America, being first recorded ca. 2.6Ma.[20][21][22] Researchers believe thatArctotherium was asister lineage toTremarctos, or even emerged from theTremarctos genus.[23] These first appearances near the Plio-Pleistocene boundary coincide with the start of theQuaternary Glaciation, the formation of thePanama Land Bridge, and the second phase of theGreat American Biotic Interchange, with the first records of the main South American faunal wave into the United States.[24] APlionarctos harroldum specimen fromTaunton (Washington, 2.9Ma)[24] appears evolutionarily intermediate betweenPlionarctos harroldum andTremarctos floridanus, affirming thatPlionarctos harroldum is the likely ancestor ofTremarctos.[25]
Genetic research on the mitochondrial DNA of tremarctine bears indicatesTremarctos was more closely related toArctotherium thanArctodus.[26] However, a preliminary investigation of tremarctine bear's nuclear DNA suggests an extensive history of hybridization betweenTremarctos andArctodus in North America, although hybridization withArctotherium (likelyA. wingei) as theTremarctos genus migrated southwards into South America is also possible. Evidence of gene flow betweenTremarctos and an ursine bear was also uncovered, most likely due to the extensive overlap betweenTremarctos and the ancestors of theAmerican black bear in Pleistocene North America.[27]
The North American speciesT. floridanus is believed to have been the ancestor ofT. ornatus.[38][39] However, the fossil record ofT. ornatus is unknown, asT. ornatus remains do not appear until theHolocene of South America.[40] This suggests that the extantT. ornatus descends from an independent, later dispersal event from North America to that ofArctotherium,[41][42][43] possibly afterArctotherium wingei became extinct inthe Americas.[44] Current scholarly analysis asserts thatA. wingei may have restricted the range of theT. floridanus outside of Central & South America until the extinction ofA. wingei, where subsequentlyTremarctos begins to be found in the South America.[45][46][47] Genetic research suggests a possible history of hybridization betweenTremarctos andArctotherium (likelyA. wingei) either in Central America throughout the Pleistocene or as theTremarctos genus migrated into South America at the end of the Pleistocene.[27]
However, as the montane niche (highland forests between 1,800m and 3100m) was otherwise open in South America,T. ornatus may have been present in the Pleistocene South America (asArctotherium preferred lowlands). Since bothA. wingei andA. tarijense have been recovered from a maximum altitude of 1860m (Tarija, Bolivia), their ranges would have had minimal overlap.[48]
The last reliable direct radiocarbon date forT. floridanus is 23,230 ± 490 BP, fromLaubach Cave No. III, Texas,[49][50] although the chronology ofmegafaunal extinctions in the Late Pleistocene of southeastern North America (the native range ofTremarctos floridanus) has been disputed.[50] Statistical analyses suggest that a later survival (until the end of theRancholabrean epoch ca. 11,700 BP) in theAtlantic Plains of the United States is possible, based on sampling biases associated with uncommon fauna, and a lack of reliable dates from the humid Atlantic plain due to poor preservation.[50]
WhileT. floridanus may have disappeared in North America, the species may haveevolved intoT. ornatus in South America, either in the Pleistocene or the Holocene.[38][39] Molecular evidence from Colombian, Ecuadorian and VenezuelanT. ornatus specimens suggests a population divergence occurred between 15,000 and 25,000 thousand years ago.[25] The earliest known remains of the spectacled bear are from a male fromChaquil Cave, north central Peru, and have been dated to 6,790 years ago (5,980 radiocarbon years).[25] Further finds are from archeological sites in Colombia (dated to 4,030 BP & 2,725 BP respectively) and an archeological site in Peru dated to 1,500 BP.[48][25]
Species under theTremarctos genus are about average-sized bears, similar in size to theAmerican black bear. They can weigh anywhere from 150 to 400 lbs.T. floridanus however, was bigger thanT. ornatus. Both species aresexually dimorphic, withT. floridanus males being ~25% larger than the females.[11] This also results in the males having protein blood levels higher than the females' levels.[15] Unlike most bears, the forelimbs are either longer than its hindlimbs (T. ornatus),[15] or of equal length (T. floridanus).[11]
Tremarctos has long curved claws andplantigrade feet.[2] Additionally,Tremarctos possesses proportionally shorter feet than its Tremarctine relatives (e.g.Arctodus).[51] As for their fur, they are usually black with a tannish marking that extends from their chest to around their face. WhileTremarctos floridanus's exact appearance is unknown, they presumably resembled theTremarctos ornatus. Although they have large, deep skulls, they are the smallest in length among the bear species.[15] The premassateric fossa has been used to detect maturity in tremarctine bears, with onlyT. ornatus adults possessing fully developed fossa.[52] Thecanalis semicircularis lateral suggests thatT. floridanus had a head posture of 38°, which is more oblique than its sister speciesT. ornatus (29°); asT. ornatus inhabits densely vegetated areas, the more oblique head posture inT. floridanus could infer a greater capacity for long distance vision.[53]
The species under theTremarctos genus have shortmandibles 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 smallcanines and largemolars showing they eat meat as well.[54] They have 42 teeth.
Unlike its modernNeotropical sister speciesT. ornatus,T. floridanus was a temperate species that has almost entirely been recovered fromNearctic sites (southern North America).[55][56]T. ornatus however lives in South America,[57] in the highlands associated with theAndes Mountains of Bolivia, Colombia, Peru and Venezuela.[2][15]Tremarctos orantus is the only species of bear to live in South America. They like to build nests that they use to sleep and eat in, in trees of highland humid tropicalrain forests.[58] They are also able to live in other types of biomes such astemperate grasslands anddeciduous forests.[2]
The diet ofTremarctos bears is varied. They are consideredomnivores as they eat fruit, corn, plants, insects, farm animals, and more.[58] However, the extreme wear of the teeth in the earliest known remains ofT. ornatus (6,790 BP) suggests a primarily carnivorous diet for that male individual.[25]
Species under this genus are able to use their paws to manipulate food. They can strip leaves off of trees, pick apart fruits, grasp food, etc. They do this with the help of having a radiussesamoid, also referred to as a "false thumb".[1]
Hunting and eating take up most of aTremarctos bears activity levels.Tremarctos bears arediturnal meaning that they will hunt during either the day, evening, or night, meaning they do not hibernate. It depends on where the bear is located and what the current season is that determines when they are the most active.[15]
Courtship occurs between male and femaleT. ornatus and this usually involves vocalizations or playful fighting.T. ornatus are polygamous, with male bears breeding with multiple female bears. They are alsopolyesterous, meaning the bears have specific breeding seasons with twins being the most common litter. Cubs are usually born during seasons when fruits and vegetation are growing and becoming ripe.[15]
The mothering bears create dens for raising their offspring. When the cubs are born, their eyes are closed and they lack teeth.[3] The mothers will care for the cubs for about three years and after which, they separate.[15] MaleT. ornatus tend to live longer than females.[2]
While theT. ornatus is higher up on the food chain, there are a couple of predators and other things that threaten them. WithTremarctos floridanus extinct,Tremarctos ornatus is the only living species of this genus, their population is slowly declining and they are considered vulnerable on the Threatened Species List. One of the biggest reasons for their decline is due to humans poaching and causing their habitat loss. In some locations they encounter predators such asjaguars andcougars. They are also susceptible to parasites such asticks andnematodes as well as diseases such asalopecia andneoplasia.[3]
^Cassiliano, Michael L. (1999). "Biostratigraphy of Blancan and Irvingtonian Mammals in the Fish Creek-Vallecito Creek Section, Southern California, and a Review of the Blancan-Irvingtonian Boundary".Journal of Vertebrate Paleontology.19 (1):169–186.doi:10.1080/02724634.1999.10011131.ISSN0272-4634.JSTOR4523978.
^Morgan, Gary S.; White Jr., Richard S. (2005)."Miocene and Pliocene vertebrates from Arizona".Vertebrate Paleontology in Arizona. New Mexico Museum of Natural History and Science Bulletin.29 – via ResearchGate.
^Prevosti, Francisco J.; Soibelzon, Leopoldo H.; Prieto, Alfredo; Roman, Manuel San; Morello, Flavia (2003). "The Southernmost Bear: Pararctotherium (Carnivora, Ursidae, Tremarctinae) in the Latest Pleistocene of Southern Patagonia, Chile".Journal of Vertebrate Paleontology.23 (3):709–712.doi:10.1671/0272-4634(2003)023[0709:TSBPCU]2.0.CO;2.ISSN0272-4634.JSTOR4524368.S2CID86116001.
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