subfamily In biological classification, a subfamily (Latin: ', plural ') is an auxiliary (intermediate) taxonomic rank, next below family but more inclusive than genus. Standard nomenclature rules end botanical subfamily names with "-oideae", and zo ...

Ursidae Bears are carnivoran mammals of the family (biology), family Ursidae (). They are classified as caniforms, or doglike carnivorans. Although only eight species of bears are extant, they are widespread, appearing in a wide variety of habitats ...

spectacled bear The spectacled bear (''Tremarctos ornatus''), also known as the South American bear, Andean bear, Andean short-faced bear or mountain bear and locally as jukumari ( Aymara and Quechua), ukumari ( Quechua) or ukuku, is a species of bear native to ...

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...

Arctodus ''Arctodus'' is an extinct genus of short-faced bear that inhabited North America during the Pleistocene (~2.6 Year#mya, Mya until 12,800 years ago). There are two recognized species: the lesser short-faced bear (''Arctodus pristinus'') and the gia ...

Arctotherium ''Arctotherium'' ("bear beast") is an extinct genus of the Pleistocene Tremarctinae, short-faced bears endemic to Central America, Central and South America. ''Arctotherium'' migrated from North America to South America during the Great American In ...

carnivora Carnivora ( ) is an order of placental mammals specialized primarily in eating flesh, whose members are formally referred to as carnivorans. The order Carnivora is the sixth largest order of mammals, comprising at least 279 species. Carnivor ...

Americas The Americas, sometimes collectively called America, are a landmass comprising the totality of North America and South America.''Webster's New World College Dictionary'', 2010 by Wiley Publishing, Inc., Cleveland, Ohio. When viewed as a sing ...

Great American Interchange The Great American Biotic Interchange (commonly abbreviated as GABI), also known as the Great American Interchange and the Great American Faunal Interchange, was an important late Cenozoic paleozoogeographic biotic interchange event in which land ...

Pleistocene The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...

Late Miocene The Late Miocene (also known as Upper Miocene) is a sub-epoch of the Miocene epoch (geology), Epoch made up of two faunal stage, stages. The Tortonian and Messinian stages comprise the Late Miocene sub-epoch, which lasted from 11.63 Ma (million ye ...

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58ursids, experienced an explosive radiation in diversity, as C4 vegetation (

grasses Poaceae ( ), also called Gramineae ( ), is a large and nearly ubiquitous family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos, the grasses of natural grassland and species cultivated in ...

) 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 The Blancan North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), typically set from 4,750,000 to 1,806,000 years BP, a period of . An indeterminate ''

Arctotherium ''Arctotherium'' ("bear beast") is an extinct genus of the Pleistocene Tremarctinae, short-faced bears endemic to Central America, Central and South America. ''Arctotherium'' migrated from North America to South America during the Great American In ...

'' is first recorded from the late Pliocene of

El Salvador El Salvador, officially the Republic of El Salvador, is a country in Central America. It is bordered on the northeast by Honduras, on the northwest by Guatemala, and on the south by the Pacific Ocean. El Salvador's capital and largest city is S ...

. 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 ''Tremarctos floridanus'' is an extinct species of bear in the family Ursidae, subfamily Tremarctinae. ''T. floridanus'' became extinct at the end of the last ice age, 11,000 years ago. Its fossils have been found throughout the Southeastern U ...

'' 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 ''Arctodus'' is an extinct genus of short-faced bear that inhabited North America during the Pleistocene (~2.6 Year#mya, Mya until 12,800 years ago). There are two recognized species: the lesser short-faced bear (''Arctodus pristinus'') and the gia ...

'' and ''

Arctotherium ''Arctotherium'' ("bear beast") is an extinct genus of the Pleistocene Tremarctinae, short-faced bears endemic to Central America, Central and South America. ''Arctotherium'' migrated from North America to South America during the Great American In ...

'' gave way to new forms; ''Arctodus pristinus'' gave way to the huge ''Arctodus simus'', which inhabited a pan-continental range, from

Alaska Alaska ( ) is a non-contiguous U.S. state on the northwest extremity of North America. Part of the Western United States region, it is one of the two non-contiguous U.S. states, alongside Hawaii. Alaska is also considered to be the north ...

to

Mexico Mexico, officially the United Mexican States, is a country in North America. It is the northernmost country in Latin America, and borders the United States to the north, and Guatemala and Belize to the southeast; while having maritime boundar ...

. ''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 The Southern Cone (, ) is a geographical and cultural subregion composed of the southernmost areas of South America, mostly south of the Tropic of Capricorn. Traditionally, it covers Argentina, Chile, and Uruguay, bounded on the west by the Pac ...

, and to reinvade

Central America Central America is a subregion of North America. Its political boundaries are defined as bordering Mexico to the north, Colombia to the southeast, the Caribbean to the east, and the Pacific Ocean to the southwest. Central America is usually ...

. By the terminal Pleistocene, ''Arctodus simus'', ''Tremarctos floridanus'', ''Arctotherium tarijense'' and ''Arctotherium wingei'' collectively occupied a range from Alaska to southernmost

Patagonia Patagonia () is a geographical region that includes parts of Argentina and Chile at the southern end of South America. The region includes the southern section of the Andes mountain chain with lakes, fjords, temperate rainforests, and glaciers ...

. All of these forms were extinct by the end of the early Holocene. Around this time, '' Tremarctos ornatus'', otherwise known as the

spectacled bear The spectacled bear (''Tremarctos ornatus''), also known as the South American bear, Andean bear, Andean short-faced bear or mountain bear and locally as jukumari ( Aymara and Quechua), ukumari ( Quechua) or ukuku, is a species of bear native to ...

, 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 The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name ''molar'' derives from Latin, ''molaris dens'', meaning "millstone tooth ...

and

talonid The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name ''molar'' derives from Latin, ''molaris dens'', meaning "millstone tooth ...

on the m1 molar, a premasseteric fossa on the

mandible In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...

, and often an

entepicondylar foramen The entepicondylar foramen is an opening in the distal (far) end of the humerus (upper arm bone) present in some mammals. It is often present in primitive placentals, such as the enigmatic Madagascan '' Plesiorycteropus''. In most Neotominae and a ...

on the

humerus The humerus (; : humeri) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius (bone), radius and ulna, and consists of three sections. The humeral upper extrem ...

of tremarctine bears. Additionally, tremarctine bears' skulls are deeper and more

brachycephalic Brachycephaly (derived from the Ancient Greek '' βραχύς'', 'short' and '' κεφαλή'', 'head') is the shape of a skull shorter than average in its species. It is perceived as a cosmetically desirable trait in some domesticated dog and ...

, their

zygomatic arch In anatomy, the zygomatic arch (colloquially known as the cheek bone), is a part of the skull formed by the zygomatic process of temporal bone, zygomatic process of the temporal bone (a bone extending forward from the side of the skull, over the ...

es and glenoid fossas are well developed, and they have larger molars in comparison with ursines. Moreover, tremarctine bears'

orbits In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an physical body, object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an satellite, artificia ...

are also bigger, more rounded and lateralized. The

ichnotaxon An ichnotaxon (plural ichnotaxa) is "a taxon based on the fossilized work of an organism", i.e. the non-human equivalent of an artifact. ''Ichnotaxon'' comes from the Ancient Greek (''íchnos'') meaning "track" and English , itself derived from ...

''Ursichnus sudamericanus'' is referred to ''Arctotherium tarijense''; unlike the footprints of ursine bears, the heel impression is absent in the manus, and the digit III imprint is longer than the remaining digits. Being very similar to extant spectacled bears, ''Ursichnus sudamericanus'' suggests all tremarctines had long claws and five digits whose tips arrayed in a shallow arc.

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 The Southern Cone (, ) is a geographical and cultural subregion composed of the southernmost areas of South America, mostly south of the Tropic of Capricorn. Traditionally, it covers Argentina, Chile, and Uruguay, bounded on the west by the Pac ...

, 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 ''Arctodus'' is an extinct genus of short-faced bear that inhabited North America during the Pleistocene (~2.6 Year#mya, Mya until 12,800 years ago). There are two recognized species: the lesser short-faced bear (''Arctodus pristinus'') and the gia ...

'', in contrast with the robust '' Arctotherium angustidens'', has puzzled researchers. However, it has been posited that the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58procyonid Procyonidae ( ) is a New World family (biology), family of the order (biology), order Carnivora. It includes the Procyon (genus), raccoons, ringtails, cacomistles, coatis, kinkajous, olingos, and olinguitos. Procyonids inhabit a wide range of e ...

'' Chapalmalania'' in South America, and both '' Borophagus'' and '' Agriotherium'' in North America, was a shared impetus for gigantism in ''

Arctodus ''Arctodus'' is an extinct genus of short-faced bear that inhabited North America during the Pleistocene (~2.6 Year#mya, Mya until 12,800 years ago). There are two recognized species: the lesser short-faced bear (''Arctodus pristinus'') and the gia ...

'' and ''

Arctotherium ''Arctotherium'' ("bear beast") is an extinct genus of the Pleistocene Tremarctinae, short-faced bears endemic to Central America, Central and South America. ''Arctotherium'' migrated from North America to South America during the Great American In ...

''. ''

Arctodus ''Arctodus'' is an extinct genus of short-faced bear that inhabited North America during the Pleistocene (~2.6 Year#mya, Mya until 12,800 years ago). There are two recognized species: the lesser short-faced bear (''Arctodus pristinus'') and the gia ...

'' 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 Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. The device trades off input forces against movement to obtain a desired amplification in the output force. The model for ...

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 ''Arctodus'' is an extinct genus of short-faced bear that inhabited North America during the Pleistocene (~2.6 Year#mya, Mya until 12,800 years ago). There are two recognized species: the lesser short-faced bear (''Arctodus pristinus'') and the gia ...

'' and ''

Arctotherium ''Arctotherium'' ("bear beast") is an extinct genus of the Pleistocene Tremarctinae, short-faced bears endemic to Central America, Central and South America. ''Arctotherium'' migrated from North America to South America during the Great American In ...

''. A study of the affinities of bears belonging to ''Arctotherium'' indicates that they were more closely related to the

spectacled bear The spectacled bear (''Tremarctos ornatus''), also known as the South American bear, Andean bear, Andean short-faced bear or mountain bear and locally as jukumari ( Aymara and Quechua), ukumari ( Quechua) or ukuku, is a species of bear native to ...

than to ''Arctodus'', implying

convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...

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 ''Arctodus'' is an extinct genus of short-faced bear that inhabited North America during the Pleistocene (~2.6 Year#mya, Mya until 12,800 years ago). There are two recognized species: the lesser short-faced bear (''Arctodus pristinus'') and the gia ...

'' Leidy, 1854 *** †''Arctodus simus'' Cope, 1879 *** †''Arctodus pristinus'' Leidy, 1854 ** †''

Arctotherium ''Arctotherium'' ("bear beast") is an extinct genus of the Pleistocene Tremarctinae, short-faced bears endemic to Central America, Central and South America. ''Arctotherium'' migrated from North America to South America during the Great American In ...

'' 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 ''Tremarctos floridanus'' is an extinct species of bear in the family Ursidae, subfamily Tremarctinae. ''T. floridanus'' became extinct at the end of the last ice age, 11,000 years ago. Its fossils have been found throughout the Southeastern U ...

'' (Gildey, 1928) *** '' Tremarctos ornatus'' (Cuvier, 1825) – spectacled bear

References

External links

Yukon Beringia Interpretive Centre - North American Short-Faced Bear
{{Taxonbar, from=Q337197 Bears Mammal subfamilies Fossil taxa described in 1925

Evolution

Anatomy

Paleobiology

Systematics

Taxonomy

See also

References

External links