''Arctodus'' is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of short-faced bear that inhabited

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

during the

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 ...

(~2.6 Mya until 12,800 years ago). There are two recognized

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

: the lesser short-faced bear (''Arctodus pristinus'') and the giant short-faced bear (''Arctodus simus''). Of these species, ''A. simus'' was larger, is known from more complete remains, and is considered one of the best known members of North America's extinct Ice Age

megafauna In zoology, megafauna (from Ancient Greek, Greek μέγας ''megas'' "large" and Neo-Latin ''fauna'' "animal life") are large animals. The precise definition of the term varies widely, though a common threshold is approximately , this lower en ...

. ''A. pristinus'' was largely restricted to the Early Pleistocene of the

eastern United States The Eastern United States, often abbreviated as simply the East, is a macroregion of the United States located to the east of the Mississippi River. It includes 17–26 states and Washington, D.C., the national capital. As of 2011, the Eastern ...

, whereas ''A. simus'' had a broader range, with most finds being from the Late Pleistocene of the United States, Mexico and Canada. ''A. simus'' evolved from ''A. pristinus'', but both species likely overlapped in the Middle Pleistocene. Both species are relatively rare in the fossil record. Today considered to be an enormous omnivore, ''Arctodus simus'' is believed to be one of the largest known terrestrial

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 ...

ns that has ever existed. However, ''Arctodus'', like other bears, was highly sexually dimorphic. Adult ''A. simus'' ranged between , with females clustering at ≤, and males around . The largest males stood at at the shoulder, and up to tall on their rear legs. Studies suggest that ''Arctodus simus'' browsed on C₃ vegetation and consumed browsing herbivores such as

deer A deer (: deer) or true deer is a hoofed ruminant ungulate of the family Cervidae (informally the deer family). Cervidae is divided into subfamilies Cervinae (which includes, among others, muntjac, elk (wapiti), red deer, and fallow deer) ...

camelids Camelids are members of the biological family Camelidae, the only currently living family in the suborder Tylopoda. The seven extant members of this group are: dromedary camels, Bactrian camels, wild Bactrian camels, llamas, alpacas, vicuñas, ...

, and

tapir Tapirs ( ) are large, herbivorous mammals belonging to the family Tapiridae. They are similar in shape to a Suidae, pig, with a short, prehensile nose trunk (proboscis). Tapirs inhabit jungle and forest regions of South America, South and Centr ...

. ''A. simus'' preferred temperate open woodlands but was an adaptable species, taking advantage of many habitats and feeding opportunities. ''Arctodus'' belongs to the

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 ( ...

subfamily of bears, which are endemic to the

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 ...

. Of these short-faced bears, ''Arctodus'' was the most widespread in North America. However, the genus was restricted to the Pleistocene. ''A. pristinus'' went extinct around 300,000 years ago, with ''A. simus'' disappearing ~12,800 years ago in the

Late Pleistocene extinctions The Late Pleistocene to the beginning of the Holocene saw the extinction of the majority of the world's megafauna, typically defined as animal species having body masses over , which resulted in a collapse in faunal density and diversity acro ...

. The cause behind these extinctions is unclear, but in the case of ''A. pristinus'', this was likely due to climate change and competition with other ursids, such as the

black bear Black bear or Blackbear may refer to: Animals * American black bear (''Ursus americanus''), a North American bear species * Asian black bear (''Ursus thibetanus''), an Asian bear species Music * Black Bear (band), a Canadian First Nations group ...

and ''

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 ...

''. ''A. simus'' likely went extinct due to ecological collapse disrupting the vegetation and prey it relied on.

Taxonomy

''Arctodus'' was first described by

Joseph Leidy Joseph Mellick Leidy (September 9, 1823 – April 30, 1891) was an American paleontologist, parasitologist and anatomist. Leidy was professor of anatomy at the University of Pennsylvania, later becoming a professor of natural history at Swarth ...

in 1854, with finds of ''A. pristinus'' from the Ashley Phosphate Beds, South Carolina. The scientific name of the genus, ''Arctodus'', derives from Greek, and means "bear tooth". The first fossils of ''Arctodus simus'' were found in the Potter Creek Cave,

Shasta County Shasta County (), officially the County of Shasta, is a county located in the northern portion of the U.S. state of California. Its population is 182,155 as of the 2020 census, up from 177,223 from the 2010 census. The county seat is Redding. ...

, California, by J. A. Richardson in 1878, and were initially described as ''Arctotherium simum'' by

Edward Drinker Cope Edward Drinker Cope (July 28, 1840 – April 12, 1897) was an American zoologist, paleontology, paleontologist, comparative anatomy, comparative anatomist, herpetology, herpetologist, and ichthyology, ichthyologist. Born to a wealthy Quaker fam ...

in 1879. Historically, all specimens were grouped together under ''A. pristinus'', until a revision by

Björn Kurtén Björn Kurtén (19 November 1924 – 28 December 1988) was a Finnish vertebrate paleontologist and science fiction writer. Early life and education Kurtén was born in Vaasa in 1924. He was a member of the Swedish-speaking minority in Finland ...

in 1967. In the 19th and early 20th centuries, specimens of ''Arctodus'' were occasionally referred to ''

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 ...

'', and vice versa. However, today neither genera are considered to have overlapped, with the closest point of contact being México, with the giant ''Arctodus simus'' in Valsequillo,

Puebla Puebla, officially the Free and Sovereign State of Puebla, is one of the 31 states that, along with Mexico City, comprise the Federal Entities of Mexico. It is divided into 217 municipalities and its capital is Puebla City. Part of east-centr ...

, and the smaller ''Arctotherium wingei'' in the

Yucatán Peninsula The Yucatán Peninsula ( , ; ) is a large peninsula in southeast Mexico and adjacent portions of Belize and Guatemala. The peninsula extends towards the northeast, separating the Gulf of Mexico to the north and west of the peninsula from the C ...

. Other early researchers believed ''Arctodus'' to be a sister lineage of the agriotheriin ''

Indarctos ''Indarctos'' is an extinct genus of bear, present in Africa, North America, and Eurasia during the Miocene. It was present from ~11.1 to 5.3 Annum, Ma, existing for approximately . The oldest member is from Arizona (~11.1—7.7 Ma) and youngest ...

''. Sometimes described as the "American cave bear", ''Arctodus'' should not be mistaken for the similarly large Eurasian cave bear (''Ursus spelaeus''). As an ursine, the Eurasian cave bear last shared a common ancestor with the tremarctine ''Arctodus'' circa 13.4 million years ago. Fossils of ''Arctodus pristinus'' can be confused with the similarly sized, partially contemporaneous short-faced bear, ''Tremarctos floridanus''. ''Arctodus'' has higher crowned and considerably larger teeth than its relative ''Tremarctos''. ''A. pristinus'' can be distinguished by broader and taller molars on average, but as they are often worn, differentiation can be difficult. Moreover, diagnosing isolated ''A. simus'' remains (such as femora, scapulae, certain vertebrae, ribs, podials) from brown bears can be challenging, as some large brown bears overlap in dimensions with small ''Arctodus simus''. Beyond standard differences between tremarctine and ursine bears, ''A. simus'' has a more anterior

protocone A cusp is a pointed, projecting, or elevated feature. In animals, it is usually used to refer to raised points on the crowns of teeth. The concept is also used with regard to the leaflets of the four heart valves. The mitral valve, which has two ...

& extended enamel ridge forming a shearing blade on the maxillary P4. The molars are also shorter & broader in ''Arctodus'' than brown bears.

Evolution

''Arctodus'' belongs to the subfamily Tremarctinae, which appeared in North America during the earliest parts of the late Miocene epoch in the form of ''

Plionarctos ''Plionarctos'' is an extinct genus of short-faced bear endemic to North America from the Miocene to the Pliocene. Taxonomy Described by Childs Frick in 1926, ''Plionarctos'' is the oldest known genus within the subfamily of the short-faced b ...

''. The medium-sized ''Arctodus pristinus,'' ''

'' and ''Arctotherium sp.'' evolved from ''Plionarctos'' in the Blancan age of North America. The genetic divergence date for ''Arctodus'' is ~5 million years ago, around the

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 2.58C₄ 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 70–80% of North American genera. ''Arctodus'' first appears in the early Late Blancan (Early Pleistocene), with the earliest finds being ''A. pristinus'' from the

Kissimmee River The Kissimmee River is a river in south-central Florida, United States that forms the north part of the Everglades wetlands area. The river begins at East Lake Tohopekaliga south of Orlando, flowing south through Lake Kissimmee into the large ...

6 and Santa Fe River 1 sites in Florida, dated to between 2.6 - 2.3 Mya, and ''Arctodus sp.'' from 111 Ranch (~2.6 Mya) and San Simon (~2.2Mya) in Arizona, and

La Union La Union (), officially the Province of La Union (; ; ; ; ; ), is a coastal province in the Philippines situated in the Ilocos Region on the island of Luzon. The province's capital, the San Fernando, La Union, City of San Fernando, is the most ...

in New Mexico (Mesilla Fauna B, 2.2 - 1.8 Mya). This appearance coincides with the start of the

Quaternary Glaciation The Quaternary glaciation, also known as the Pleistocene glaciation, is an alternating series of glacial period, glacial and interglacial, interglacial periods during the Quaternary period that began 2.58 Year#SI prefix multipliers, Ma (million ...

, and the second phase of the

Great American Biotic 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 lan ...

, with the first records of the main South American faunal wave into the United States. ''Arctodus pristinus'' was mostly restricted to the more densely forested thermal enclave in

eastern North America The Nearctic realm is one of the eight biogeographic realms constituting the Earth's land surface. The Nearctic realm covers most of North America, including Greenland, Central Florida, and the highlands of Mexico. The parts of North America t ...

. with the greatest concentration of fossils being in Florida. During the early Irvingtonian faunal stage, a western population of ''A. pristinus'' evolved into the enormous ''A. simus,'' with the earliest confirmed records being at least 780,000 years old from the Irvington type locality in California. Correspondingly, ''A. simus'' is most plentiful from western North America, albeit preferring mixed habitat such as temperate open woodlands. Their ranges may have met in the Middle Pleistocene of Kansas, with ''A. simus'' migrating east in the Late Pleistocene (around the extinction of ''A. pristinus''). Although both ''Arctodus'' species co-inhabited North America for at least half a million years during the Middle Pleistocene (''A. pristinus'' went extinct ~300,000 BP), there is no direct evidence of

overlap Overlap may refer to: * In set theory, an overlap of elements shared between sets is called an intersection, as in a Venn diagram. * In music theory, overlap is a synonym for reinterpretation of a chord at the boundary of two musical phrases * Ove ...

competition Competition is a rivalry where two or more parties strive for a common goal which cannot be shared: where one's gain is the other's loss (an example of which is a zero-sum game). Competition can arise between entities such as organisms, indi ...

in the fossil record as of yet, as both species established largely separate ranges. Irvingtonian age (1,900,000 BP - 250,000 BP) specimens of ''Arctodus simus'' are particularly sparse. Finds are mostly from California, with additional remains from Texas, Kansas, Nebraska, and Montana. However, ''A. simus'' became a pan-continental species in the Rancholabrean faunal stage (Late Pleistocene), sharing that distinction with the

. Despite ''Arctodus simus'' large temporal and geographic range, fossil remains are comparatively rare (109 finds as of 2010, in otherwise well-sampled localities).

Description

Size

''Arctodus pristinus''

Around the size of

grizzly bear The grizzly bear (''Ursus arctos horribilis''), also known as the North American brown bear or simply grizzly, is a population or subspecies of the brown bear inhabiting North America. In addition to the mainland grizzly (''Ursus arctos horr ...

s, ''A. pristinus'' specimens closely overlap the size of ''Tremarctos floridanus'', with some males of ''A. pristinus'' overlapping in size with the females of ''A. simus.'' Floridan ''A. pristinus'' individuals were calculated to an average of ~. However, the dimensions of some individuals from

Port Kennedy Bone Cave The Port Kennedy Bone Cave is a limestone cave in the Port Kennedy section of Valley Forge National Historical Park, Pennsylvania, USA. The Bone Cave "contained one of the most important middle Pleistocene (Irvingtonian, approximately 750,000 y ...

and

Aguascalientes Aguascalientes, officially the Free and Sovereign State of Aguascalientes, is one of the 32 states which comprise the Political divisions of Mexico, Federal Entities of Mexico. At 22°N and with an average altitude of above sea level it is pre ...

suggest that northern and western ''A. pristinus'' may have been larger than Floridan ''A. pristinus'', being up to .

''Arctodus simus''

Some ''A. simus'' individuals might have been the largest land-dwelling specimens of

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 ...

that ever lived in North America. Standing up on its hind legs, ''A. simus'' stood , with a maximum vertical arm reach of . When walking on all fours, ''A. simus'' stood high at the shoulder, with the largest males being tall enough to look an adult human in the eye. The average weight of ''A. simus'' was ~, with the maximum recorded at .

= Studies

= In a 2010 study, the mass of six ''A. simus'' specimens was estimated; half of the specimens weighed between and , with a mean weight of ~850 kg, suggesting larger specimens were probably more common than previously thought. However, the other specimens were calculated to be less than . The weight range calculated from all examined specimens was between 957 kg and . Hypothetically, the largest individuals of ''A. simus'' may have approached , or even . However, a 2006 study argued that the maximum size of ''Arctodus'' was ~, based on the largest known skull. Additionally, a 1998 study calculated the average weight of ''Arctodus'' specimens from the

La Brea Tar Pits La Brea Tar Pits comprise an active Paleontological site, paleontological research site in urban Los Angeles. Hancock Park was formed around a group of tar pits where natural Bitumen, asphalt (also called asphaltum, bitumen, or pitch; ''brea'' ...

at ~, smaller than recovered

brown bear The brown bear (''Ursus arctos'') is a large bear native to Eurasia and North America. Of the land carnivorans, it is rivaled in size only by its closest relative, the polar bear, which is much less variable in size and slightly bigger on av ...

remains (~, although these remains postdate ''Arctodus''). A 1999 study by Per Christiansen calculated a mean weight of ~ from seven male ''A. simus'' limb bones, suggesting large males weighed between and .

Sexual dimorphism

There is much variation in adult size among specimens- the lack of finds,

sexual dimorphism Sexual dimorphism is the condition where sexes of the same species exhibit different Morphology (biology), morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecy, di ...

, individual variation and potentially ecomorphs could be augmenting the average size of both species of ''Arctodus''. Size differences between specimens of ''Arctodus simus'' (such as skull and long bone dimensions) led Kurtén to suggest a larger northern/central subspecies (''A. s. yukonensis'') and a southern subspecies (''A. s. simus''). evolving in the Irvingtonian and Rancholabrean respectively. However, the discovery of a very large southern ''Arctodus simus'' in Florida and New Mexico (deep within the supposed range of ''A. s. simus''), & possibly Rancho La Brea, and notably small specimens from the

Yukon Yukon () is a Provinces and territories of Canada, territory of Canada, bordering British Columbia to the south, the Northwest Territories to the east, the Beaufort Sea to the north, and the U.S. state of Alaska to the west. It is Canada’s we ...

and

Vancouver Island Vancouver Island is an island in the northeastern Pacific Ocean and part of the Canadian province of British Columbia. The island is in length, in width at its widest point, and in total area, while are of land. The island is the largest ...

, put doubt on this designation. Perceived ecomorphologies are possibly due to the low number of specimens, and sex-biased sampling. For example, only one

baculum The baculum (: bacula), also known as the penis bone, penile bone, ''os penis'', ''os genitale'', or ''os priapi'', is a bone in the penis of many placental mammals. It is not present in humans, but is present in the penises of some primates, ...

(penis bone) has been recovered from over 100 giant short-faced bear sites in North America, although it may belong to a black bear (Potter Cave). None of the specimens assigned to the larger morph (''A. s. yukonensis'') is from a cave passage, being usually isolated remains from open sites. Furthermore, over 70% of the smaller specimens (once assigned as the ''A. s. simus'' subspecies) are from cave deposits where bacula would likely be found if present, suggesting that mostly female individuals of ''A. simus'' were using caves. Therefore, in conjunction with ursid sexual dimorphism (e.g. male spectacled bears are 30% - 40% larger than females), the larger, massive ''Arctodus'' individuals are often considered male, particularly older males, with the smaller, more lightly built individuals being females. Sexual dimorphism may also explain ''A. simus'' teeth (from multiple individuals at the same site) generally clustering into two sizes. In 2025, Salis and colleagues sequenced mitochondrial genomes from 34 specimens of ''A. simus'', all of which were virtually complete and represented at least 31 individuals from 28 deposits in various regions of North America. They assigned 16 specimens to males and 13 specimens to females, of which 11 were categorized into a specific size class, with 7 large specimens belonging to males and 4 small specimens belonging to females. They also found no evidence supporting the subspecies designation for ''A. simus'', with no apparent genetic diversity and

phylogeographic Phylogeography is the study of the historical processes that may be responsible for the past to present geographic distributions of genealogical lineages. This is accomplished by considering the geographic distribution of individuals in light of ge ...

structure found in the analyzed specimens. Additionally, the most recent common ancestor of the mitochondrial lineages of ''A. simus'' was suggested be either the Middle Pleistocene (209,100 years ago) or the Late Pleistocene (73,600 years ago) based on phylogenetic analyses of the sampled specimens.

Anatomy

The two species of ''Arctodus'' are differentiated not only by size, but also by the shorter snout, greater

prognathism Prognathism is a positional relationship of the mandible or maxilla to the skeletal base where either of the jaws protrudes beyond a predetermined imaginary line in the coronal plane of the skull. In the case of ''mandibular'' prognathism (nev ...

, more robust teeth and longer limbs of ''A. simus,'' and the relative proportions of each species' molars and premolars. ''Arctodus pristinus'' is distinguished from ''A. simus'' smaller, narrower, and less crowded teeth. However, the morphologies of both species are otherwise very similar. As a result, differentiating ''Arctodus simus'' from ''Arctodus pristinus'' can be difficult, as male individuals of ''Arctodus pristinus'' can overlap in size with female individuals of ''Arctodus simus''. ''Arctodus simus'' superficially resembled living

hyaenids Hyenas or hyaenas ( ; from Ancient Greek , ) are feliformia, feliform carnivoran mammals belonging to the Family (biology), family Hyaenidae (). With just four extant species (each in its own genus), it is the fifth-smallest family in the orde ...

in skull shape and relative lengths of the trunk, back and limbs. The most nearly complete skeleton of ''A. simus'' found in the United States was unearthed in Fulton County, Indiana; the original bones are in the

Field Museum of Natural History The Field Museum of Natural History (FMNH), also known as The Field Museum, is a natural history museum in Chicago, Illinois, and is one of the largest such museums in the world. The museum is popular for the size and quality of its educationa ...

, Chicago.

Skull

Members of the

subfamily of bears appear to have a disproportionately short snout compared with most modern bears, giving them the name "short-faced". ''Arctodus'' has also been argued to exhibit a wide and shortened

rostrum Rostrum may refer to: * Any kind of a platform for a speaker: **dais **pulpit ** podium * Rostrum (anatomy), a beak, or anatomical structure resembling a beak, as in the mouthparts of many sucking insects * Rostrum (ship), a form of bow on naval ...

, potentially giving ''Arctodus'' a more

felid Felidae ( ) is the Family (biology), family of mammals in the Order (biology), order Carnivora colloquially referred to as cats. A member of this family is also called a felid ( ). The 41 extant taxon, extant Felidae species exhibit the gre ...

-like appearance. Matheus suggested that a broad

snout A snout is the protruding portion of an animal's face, consisting of its nose, mouth, and jaw. In many animals, the structure is called a muzzle, Rostrum (anatomy), rostrum, beak or proboscis. The wet furless surface around the nostrils of the n ...

could have housed a highly developed olfactory apparatus, or accommodated a larger throat passage to bolt down large food items, akin to spotted hyenas. However, this apparent shortness is an illusion caused by the deep snouts and short nasal bones of tremarctine bears compared with ursine bears; ''Arctodus'' has a deeper but not a shorter face than most living bears. This characteristic is also shared by the only living tremarctine bear, the omni-herbivorous

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 ...

. Snout deepness could be variable, as specimens from Huntington Reservoir in Utah, and the Hill-Shuler locality, Texas, were noted as being distinctly "short-faced" in comparison with other ''Arctodus simus'' individuals. The

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 ...

of ''Arctodus'' are proportionally small compared to the size of the skull, and somewhat laterally orientated (a characteristic of tremarctine bears), more so than actively predatory carnivorans or even the

, suggesting that

stereoscopic vision Binocular vision is seeing with two eyes, which increases the size of the visual field. If the visual fields of the two eyes overlap, binocular depth can be seen. This allows objects to be recognized more quickly, camouflage to be detected, spa ...

was not a priority. The

optic canal The ''optic foramen'' is the opening to the optic canal. The canal is located in the sphenoid bone; it is bounded medially by the body of the sphenoid and laterally by the lesser wing of the sphenoid. The superior surface of the sphenoid bone is ...

and other sphenoidal openings crowd together more in ''A. simus'' than in '' Ursus''. As with ''Tremarctos ornatus'', specimens with a large

sagittal crest A sagittal crest is a ridge of bone running lengthwise along the midline of the top of the skull (at the sagittal suture) of many mammalian and reptilian skulls, among others. The presence of this ridge of bone indicates that there are excepti ...

were likely male, whereas females had a reduced or no sagittal crest. Although there are limited samples, the

middle ear The middle ear is the portion of the ear medial to the eardrum, and distal to the oval window of the cochlea (of the inner ear). The mammalian middle ear contains three ossicles (malleus, incus, and stapes), which transfer the vibrations ...

bones A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, an ...

of ''A. simus'' are proportionally larger than modern ursine bears, suggesting the species was particularly attuned to low-frequency sounds. The canalis semicircularis lateral suggests that ''A. pristinus'' had a head posture of 48°, which being more oblique than several ''Arctotherium'' and ''Tremarctos'' species, could also infer a greater capacity for long distance vision. Morphologically, ''Arctodus simus'' exhibits masticular characteristics common to herbivorous bears. This includes cheek teeth with large, blunt surface areas, a deep mandible, and large mandibular muscle attachments (which are rare in carnivorous mammals). As herbivorous carnivorans such as ''Arctodus'' lack the

gut microbiota Gut microbiota, gut microbiome, or gut flora are the microorganisms, including bacteria, archaea, fungi, and viruses, that live in the digestive tracts of animals. The gastrointestinal metagenome is the aggregate of all the genomes of the g ...

to efficiently break down plant matter, these features created 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 to break down plant matter via extensive chewing or grinding. Although the low mandibular

condyle A condyle (;Entry "condyle"
in

relative to the tooth row (and therefore potential wide gape) of ''Arctodus simus'' has been inferred as an adaptation for carnivory, it is also present in the omni-herbivorous spectacled bear. However, both ''Arctodus pristinus'' and ''Tremarctos floridanus'' have condyles raised well above the plane of the teeth. The purpose of the highly vaulted

calvarium The calvaria is the top part of the skull. It is the superior part of the neurocranium and covers the cranial cavity containing the brain. It forms the main component of the skull roof. The calvaria is made up of the superior portions of the f ...

and straight cheek bones of ''Arctodus simus'' have been similarly disputed. A 2009 analysis of the

mandibular 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 ...

morphology of tremarctine bears found notable differences between ''Arctodus pristinus'' and ''Arctodus simus'', with ''A. simus'' specimens possessing a concave jaw, large

masseter In anatomy, the masseter is one of the muscles of mastication. Found only in mammals, it is particularly powerful in herbivores to facilitate chewing of plant matter. The most obvious muscle of mastication is the masseter muscle, since it is the ...

and

temporalis In anatomy, the temporalis muscle, also known as the temporal muscle, is one of the muscles of mastication (chewing). It is a broad, fan-shaped convergent muscle on each side of the head that fills the temporal fossa, superior to the zygomatic ...

muscles, deeper horizontal ramus and a reduced slicing dentition length when compared to ''A. pristinus''. Instead, ''Arctodus simus'' was most similar to ''Arctotherium angustidens-'' however, both species of ''Arctodus'' and ''Arctotherium angustidens'' were still comfortably in the "omnivorous" bear cranio-morphotype.

= Dentition

The

premolar The premolars, also called premolar Tooth (human), teeth, or bicuspids, are transitional teeth located between the Canine tooth, canine and Molar (tooth), molar teeth. In humans, there are two premolars per dental terminology#Quadrant, quadrant in ...

s and first molars of ''Arctodus pristinus'' are relatively smaller and more widely spaced than those of ''Arctodus simus''. In ''A. pristinus'', the features of the dentition can be quite variable, particularly the M2 molar. An analysis of the Hunter-Schreger bands from the teeth of ''A. pristinus'' and ''A. simus'' demonstrated an evolutionary trend towards partially reinforced

tooth enamel Tooth enamel is one of the four major Tissue (biology), tissues that make up the tooth in humans and many animals, including some species of fish. It makes up the normally visible part of the tooth, covering the Crown (tooth), crown. The other ...

. This has been convergently evolved with giant pandas, agriotheriin bears, and ''

Hemicyon ''Hemicyon'', also known as the "dog-bear" (literally "half dog", from Greek (half) + (dog)), is an extinct genus of hemicyonine bear, which probably originated in Eurasia but was found in Europe, Asia and North America during the Miocene e ...

''. The dentition of ''A. simus'' has been used as evidence of a predatory lifestyle- in particular the large canines, the high-crowned lower

first molar 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 the possible

carnassial shear Carnassials are paired upper and lower teeth modified in such a way as to allow enlarged and often self-sharpening edges to pass by each other in a shearing manner. This adaptation is found in carnivorans, where the carnassials are the modified f ...

with the upper fourth premolar. However, the wearing of the molars to a relatively flat & blunt loph (suitable as a crushing platform as per modern omnivorous bears), small shear facet, and the flattened cusps across age ranges (unlike carnivores, which instead have carnassial shears) disagrees with this hypothesis. Dentition can be a poor indicator of size in ''A. simus'', as some medium-size individuals have teeth that surpass the size of those with the largest skeletons. Additionally, while ''A. simus'' evolved from the smaller ''A. pristinus'', their teeth remained generally the same size. A specimen of ''A. simus'' from the Seale Pit of the Hill-Shuler locality, Texas, with only two premolars, crowding of the anterior premolar out of line, and a wider and shorter muzzle, was suggested to be an undescribed form of ''Arctodus''.

Post-cranial

= Limbs

Researchers have differing interpretations on the limb morphology of ''Arctodus''. A comprehensive 2010 study concluded that the legs of ''Arctodus'' weren't proportionally longer than modern bears would be expected to have, and that bears in general are long-limbed animals, obscured in life by their girth and fur. The study concluded the supposed "long-legged" appearance of the bear is largely an illusion created by the animal's relatively shorter back and torso. In fact, ''Arctodus'' probably had an even shorter back than other bears, due the necessary ratio between body length and body mass of the huge bear. However, other researchers argue that the limb bones of ''Arctodus simus'' are proportionally longer than those of other bears, leading to a "gracile" appearance. Although longer, the proportions still overlap with ''Ursus'', and the limb bones are stouter than in the large-bodied felids (''Panthera''). Rather than for running, these elongated limb bones may have evolved for increased locomotor efficiency during prolonged travel. This stiff-legged, swinging gait could have been similar to a polar bear's. Some researchers suggest that proportionally longer limbs may be an adaptation for increased vision over tall ground cover in an open habitat, or were used in tearing and pulling down vegetation. Researchers also disagree when interpreting 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 ''Arctodus simus''. Sorkin argued that the pronation of the forearm and the flexion of the wrist and digits, and more lightly muscled forelimbs, all of which are crucial to grasping a large prey animal with the forepaws, were probably less powerful in ''Arctodus'' than in either the

or in ''

Panthera ''Panthera'' is a genus within the family (biology), family Felidae, and one of two extant genera in the subfamily Pantherinae. It contains the largest living members of the cat family. There are five living species: the jaguar, leopard, lion, ...

.'' This is due to a weak medial epicondyle and reduced development of the

pronator teres The pronator teres is a muscle (located mainly in the forearm) that, along with the pronator quadratus, serves to pronate the forearm (turning it so that the palm faces posteriorly when from the anatomical position). It has two origins, at th ...

muscle. The forelimb of ''Arctodus'' could have been in the early stages of

cursorial A cursorial organism is one that is adapted specifically to run. An animal can be considered cursorial if it has the ability to run fast (e.g. cheetah) or if it can keep a constant speed for a long distance (high endurance). "Cursorial" is often ...

evolution, being capable of more efficient and high-speed straight-line locomotion (relative to extant bears), and was possibly more adept at pursuing large prey than

polar Polar may refer to: Geography * Geographical pole, either of the two points on Earth where its axis of rotation intersects its surface ** Polar climate, the climate common in polar regions ** Polar regions of Earth, locations within the polar circ ...

and

s. On the other hand, some researchers argue that the epicondyles were still well developed, with this wide range of ulna rotation suggests that forearms of ''Arctodus'' were powerful and could subdue large prey. A 2013 examination of Rancho La Brean specimens found that they did not possess distally elongated limbs, which discredited cursoriality. Furthermore, the relatively broad humeral & femoral

epicondyle An epicondyle () is a rounded eminence on a bone that lies upon a condyle ('' epi-'', "upon" + ''condyle'', from a root meaning "knuckle" or "rounded articular area"). There are various epicondyles in the human skeleton The human skeleton is ...

s were characteristic of

diggers The Diggers were a group of religious and political dissidents in England, associated with a political ideology and programme resembling what would later be called agrarian socialism.; ; ; Gerrard Winstanley and William Everard (Digger), Will ...

and polar bears, and suggested ''Arctodus simus'' could have foraged for roots, tubers and

ground squirrel Ground squirrels are rodents of the squirrel family (Sciuridae) that generally live on the ground or in burrows, rather than in trees like the tree squirrels. The term is most often used for the medium-sized ground squirrels, as the larger ones ar ...

s and/or had developed forelimb muscles to immobilze moving prey. The shape of the

elbow joint The elbow is the region between the upper arm and the forearm that surrounds the elbow joint. The elbow includes prominent landmarks such as the olecranon, the cubital fossa (also called the chelidon, or the elbow pit), and the lateral and the ...

, along with an well-developed medial epicondyle which forms an angle with the

condyle A condyle (;Entry "condyle"
in

, and shallower

olecranon fossa The olecranon fossa is a deep triangular depression on the posterior side of the humerus, superior to the trochlea. It provides space for the olecranon of the ulna during extension of the forearm. Structure The olecranon fossa is located ...

, would have given ''Arctodus'' a higher degree of forelimb dexterity. Originally evolved to facilitate

arboreality Arboreal locomotion is the locomotion of animals in trees. In habitats in which trees are present, animals have evolved to move in them. Some animals may scale trees only occasionally (scansorial), but others are exclusively arboreal. The habi ...

, other researchers believe that the terrestrial ''Arctodus'' (along with ''Arctotherium'' and the

giant panda The giant panda (''Ailuropoda melanoleuca''), also known as the panda bear or simply panda, is a bear species endemic to China. It is characterised by its white animal coat, coat with black patches around the eyes, ears, legs and shoulders. ...

) retained this characteristic to assist in foraging for vegetation.

= Paws

= The

paw A paw is the soft foot-like part of a mammal, generally a quadruped, that has claws. Common characteristics The paw is characterised by thin, pigmented, keratinised, hairless epidermis covering subcutaneous collagenous and adipose tissue, whi ...

s (

metapodial Metapodials are long bone The long bones are those that are longer than they are wide. They are one of five types of bones: long, short, flat, irregular and sesamoid. Long bones, especially the femur and tibia, are subjected to most of the l ...

s and

phalanges The phalanges (: phalanx ) are digit (anatomy), digital bones in the hands and foot, feet of most vertebrates. In primates, the Thumb, thumbs and Hallux, big toes have two phalanges while the other Digit (anatomy), digits have three phalanges. ...

) of ''Arctodus'' were characteristically long, slender, and more elongated along the third and fourth digits compared to ursine bears. ''Arctodus''' paws were therefore more symmetrical than ursine bears, whose feet have axes aligned with the most lateral (fifth) digit. Also, the first digit of ''Arctodus'' was positioned more closely and parallel to the other four digits (i.e. with straight toes, ''Arctodus'' had less lateral splaying). However this is potentially contradicted by possible ''Arctodus simus'' trackways from near Lakeview, Oregon, with strong toe splaying, three centrally aligned & evenly spaced toes at the front, and two almost perpendicular lateral toes (80° from the axis of the foot on either side). The trackways suggest that ''Arctodus'' had an oval-shaped, undivided pad on its sole, front paws that were slightly larger than its back paws, possessed long claws, and had its hind foot overstep the forefoot when walking, like modern bears. An additional ''A. simus'' paw print measuring long and wide has been recovered from

White Sands National Park White Sands National Park is a national park of the United States located in New Mexico and completely surrounded by the White Sands Missile Range. The park covers in the Tularosa Basin, including the southern 41% of a field of white sand ...

, New Mexico. Some claw marks attributed to ''Arctodus simus'' at Riverbluff Cave (as they were four meters above the floor of the cave) were nearly 20 cm in width. The presence of a partial false thumb in ''Arctodus simus'' is a characteristic shared with ''Tremarctos floridanus'' and the spectacled bear, and is possibly an

ancestral An ancestor, also known as a forefather, fore-elder, or a forebear, is a parent or ( recursively) the parent of an antecedent (i.e., a grandparent, great-grandparent, great-great-grandparent and so forth). ''Ancestor'' is "any person from ...

trait. Absent in ursine bears, the false thumb of the spectacled bear has been suggested to assist in herbivorous food manipulation (such as

bromeliads The Bromeliaceae (the bromeliads) are a family of monocot flowering plants of about 80 genera and 3700 known species, native mainly to the tropical Americas, with several species found in the American subtropics and one in tropical west Africa, ...

, leaves, berries, tree bark & fruits, cactus fruits & pulp, palm hearts & fronds), or

Paleopathology

Beyond carbohydrate-associated dental pathologies present in the genus, extensive pathologies have been preserved on the most nearly complete skeleton of ''Arctodus''. The leading hypothesis suggests the Fulton County ''Arctodus'' specimen suffered from a

syphilis Syphilis () is a sexually transmitted infection caused by the bacterium ''Treponema pallidum'' subspecies ''pallidum''. The signs and symptoms depend on the stage it presents: primary, secondary, latent syphilis, latent or tertiary. The prim ...

-like ( treponemal) disease, or

yaws Yaws is a tropical infection of the skin, bones, and joints caused by the spirochete bacterium ''Treponema pallidum pertenue''. The disease begins with a round, hard swelling of the skin, in diameter. The center may break open and form an ulc ...

, based on the various

lesion A lesion is any damage or abnormal change in the tissue of an organism, usually caused by injury or diseases. The term ''Lesion'' is derived from the Latin meaning "injury". Lesions may occur in both plants and animals. Types There is no de ...

s present. The same individual records a pathological growth distorting the right

, with

abscesses An abscess is a collection of pus that has built up within the tissue of the body, usually caused by bacterial infection. Signs and symptoms of abscesses include redness, pain, warmth, and swelling. The swelling may feel fluid-filled when pres ...

are noted between the molars and on both ulna. Hypotheses include syphilis,

osteoarthritis Osteoarthritis is a type of degenerative joint disease that results from breakdown of articular cartilage, joint cartilage and underlying bone. A form of arthritis, it is believed to be the fourth leading cause of disability in the world, affect ...

, a fungal infection in addition to long term syphilis, or an infected wound. Several specimens from Fairbanks, Alaska, also exhibit either pathological growths or

periodontal disease Periodontal disease, also known as gum disease, is a set of inflammatory conditions affecting the tissues surrounding the teeth. In its early stage, called gingivitis, the gums become swollen and red and may bleed. It is considered the main c ...

, along with a healed toe bone from Big Bear Cave, Missouri.

Paleobiology

Locomotion

Paul Matheus proposed that ''Arctodus simus'' may have moved in a highly efficient, moderate-speed

pacing Pacing may refer to: In sport * Pacing, an athletic technique of spreading one's effort out over longer-distance track and field races (also in swimming) * Pacing (horse gait), a horse gait used in standardbred horse races * Motor-paced racing ...

gait, more specialized than modern bears. His research concluded that the large body size, taller front legs, high shoulders, short and sloping back, and long legs of ''Arctodus'' also compounded locomotive efficiency, as these traits swelled the amount of usable

elastic strain energy In physics, the elastic potential energy gained by a wire during elongation with a tensile (stretching) or compressive (contractile) force is called strain energy. For linearly elastic materials, strain energy is: : U = \frac 1 2 V \sigma \var ...

in the tendons, and increased stride length, making ''Arctodus'' built more for endurance than for great speed. His calculations suggested that ''Arctodus'' likely had a top speed of , and based on hyaenid proportions, would shift from singlefoot locomotion to a pace at , and would begin to gallop at , a fairly high speed. Based on other mammals, the optimal pace speed of ''Arctodus'' would have been . For comparison, hyenas cross country ~. This mobility would have facilitated travelling across a large home range, which Mattson suggests may have topped . Swimming has also been presented as a hypothesis for the colonization of Vancouver Island by ''Arctodus simus''.

Maturity

Examinations on a mostly full sized young individual of ''Arctodus simus'' from an

Ozark The Ozarks, also known as the Ozark Mountains, Ozark Highlands or Ozark Plateau, is a physiographic region in the U.S. states of Missouri, Arkansas, and Oklahoma, as well as a small area in the southeastern corner of Kansas. The Ozarks cov ...

cave suggest that ''Arctodus'', like other ursids, reached

sexual maturity Sexual maturity is the capability of an organism to reproduce. In humans, it is related to both puberty and adulthood. ''Puberty'' is the biological process of sexual maturation, while ''adulthood'', the condition of being socially recognized ...

well before full maturity. Comparisons with black bears suggest the ''Arctodus'' specimen was either 4–6 years of age if female, or 6–8 years if the specimen was male. Additionally, wear patterns on the individual's teeth are similar to a 4-6 year old ''Ursus americanus''. Fused sutures,

epiphyses An epiphysis (; : epiphyses) is one of the rounded ends or tips of a long bone that ossify from one or more secondary centers of ossification. Between the epiphysis and diaphysis (the long midsection of the long bone) lies the metaphysis, inc ...

, and

epiphyseal plate The epiphyseal plate, epiphysial plate, physis, or growth plate is a hyaline cartilage plate in the metaphysis at each end of a long bone. It is the part of a long bone where new bone growth takes place; that is, the whole bone is alive, with ma ...

s, along with

tooth eruption Tooth eruption is a process in tooth development in which the teeth enter the mouth and become visible. It is currently believed that the periodontal ligament plays an important role in tooth eruption. The first human teeth to appear, the decidu ...

, have been used to determine adulthood in ''Arctodus.''

Genetic diversity

An examination of

mitochondrial DNA Mitochondrial DNA (mtDNA and mDNA) is the DNA located in the mitochondrion, mitochondria organelles in a eukaryotic cell that converts chemical energy from food into adenosine triphosphate (ATP). Mitochondrial DNA is a small portion of the D ...

sequenced from specimens of ''Arctodus simus'' from Alaska, Alberta, Ohio and the Yukon suggest an extremely low level of genetic diversity among the 23 individuals studied (≤ 44,000 ¹⁴C BP), with only seven

haplotype A haplotype (haploid genotype) is a group of alleles in an organism that are inherited together from a single parent. Many organisms contain genetic material (DNA) which is inherited from two parents. Normally these organisms have their DNA orga ...

s recovered. Genetic diversity was comparable to modern endangered fauna, such as the brown kiwi and African cheetah. Explanations include a

genetic bottleneck A population bottleneck or genetic bottleneck is a sharp reduction in the size of a population due to environmental events such as famines, earthquakes, floods, fires, disease, and droughts; or human activities such as genocide, speciocide, wid ...

before 44,000 ¹⁴C BP, or a low level of genetic diversity being a feature of a species which was primarily solitary, with a large home range and relatively small population size. However, this does not entirely preclude genetic diversity in ''Arctodus simus'', with genetic samples from

Chiquihuite Cave Chiquihuite Cave is a possible Upper Paleolithic archaeological site in the Astillero Mountains, Zacatecas State, in North-Central Mexico. Chiquihuite Cave may be evidence of early human presence in the Western Hemisphere up to 33,000 years ago. I ...

, Zacatecas indicating a deep divergence with previously studied specimens of ''A. simus''. Additional specimens from the California Channel Islands and Wyoming have been sequenced, but are unassigned.

Haplotype cladogram

Below is a cladogram exploring the relationships between the mitochondrial

haplogroup A haplotype is a group of alleles in an organism that are inherited together from a single parent, and a haplogroup (haploid from the , ''haploûs'', "onefold, simple" and ) is a group of similar haplotypes that share a common ancestor with a sing ...

s of ''Arctodus simus''. Other than the specimen from Chiquihuite Cave, all haplotypes form a single

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

Diet

Herbivory

The fact that ''Arctodus'' did not significantly differ in dentition or build from modern bears has led most authors to support the hypothesis that the ''A. simus'' was omnivorous, like most modern bears, and would have eaten significant amounts of plant matter. Morphologically, ''Arctodus simus'' exhibits masticular and dental characteristics which confirms that short-faced bears such as the

and ''Arctodus'' were adapted to and actively consumed vegetation. This is affirmed by a lack of dental damage associated with carnivory amongst specimens of ''Arctodus''. Dental pathologies which have been found, such as

incisor Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, wher ...

wear & supragingival dental calculus in a young individual from Missouri, and

cavities Cavity may refer to: Biology and healthcare *Body cavity, a fluid-filled space in many animals where organs typically develop ** Gastrovascular cavity, the primary organ of digestion and circulation in cnidarians and flatworms * Dental cavity or t ...

associated with

carbohydrate A carbohydrate () is a biomolecule composed of carbon (C), hydrogen (H), and oxygen (O) atoms. The typical hydrogen-to-oxygen atomic ratio is 2:1, analogous to that of water, and is represented by the empirical formula (where ''m'' and ''n'' ...

consumption in individuals from the La Brea Tar Pits & Pellucidar Cave (Vancouver Island), further suggest an omnivorous diet for ''Arctodus simus''. Additional morphological adaptations include dexterous forelimbs and a partial false thumb, which would have assisted in foraging for vegetation, along with the body size of large ''Arctodus simus'' (~1000 kg) matching or exceeding the expected upper limitations for a terrestrial carnivore (based on the more restrictive energy base for a carnivorous diet). While features of ''Arctodus simus'' morphology suggest herbivory, their close phylogenetic relationship to the omni-herbivorous

presents the possibility that these traits may be an ancestral condition of the group. A browsing diet foraged from the canopies of trees and shrubs could have been difficult with the large and flattened rostrum and incisor build of ''Arctodus'', while evidence of digging adaptations in ''Arctodus''' forelimbs and claws (e.g. for rooting) is mixed. Regardless, gross tooth wear suggests consumption of plant matter in the diet of ''Arctodus simus''. The diet of individuals from La Brea was most similar to the spectacled bear, which consumes tough leafy matter, seeded & pitted fruits and occasional protein. ''Arctodus''' tooth wear remained consistent throughout the Pleistocene in La Brea. This indicated a less generalized diet than modern omni-herbivorous black bears, with none of the dental evidence of hard food consumption (such as carcasses or nuts) found in polar bears, black bears and hyenas. Comparisons with the dental microwear of '' Ursus speleaus'' suggest dietary differences between the species, with cave bears consuming tougher vegetation than ''A. simus''. Although some researchers argue that herbivory should be more obvious from the isotope data gathered from northern ''Arctodus'', several ''Arctodus''

coprolite A coprolite (also known as a coprolith) is fossilized feces. Coprolites are classified as trace fossils as opposed to body fossils, as they give evidence for the animal's behaviour (in this case, diet) rather than morphology. The name ...

s from

The Mammoth Site The Mammoth Site is a museum and paleontological site near Hot Springs, South Dakota, in the Black Hills. It is an active paleontological excavation site at which research and excavations are continuing. The facility encloses a prehistoric sin ...

in South Dakota and Meander Cave at Ni'iinlii'njik Territorial Park, Yukon contain ''Juniperus'' seeds (toxic to black & brown bears).

Opportunistic carnivory

Evidence suggests that ''Arctodus'' also consumed meat, as evidenced by elevated nitrogen-15 isotope levels (corresponding to protein consumption) and bone damage on contemporary fauna. Additionally, elevated carbon-13 levels (corresponding to C₃ resources) from many localities (Alaska, California,

San Luis Potosí San Luis Potosí, officially the Free and Sovereign State of San Luis Potosí, is one of the 32 states which compose the Federal Entities of Mexico. It is divided in 59 municipalities and is named after its capital city, San Luis Potosí. It ...

, Texas, Vancouver Island, and the Yukon) largely suggest browsers (and browsed vegetation) were the core of ''A. simus''' diet. ''Arctodus simus''' status as a predator is questioned by its

gracility Gracility is slenderness, the condition of being gracile, which means slender. It derives from the Latin adjective ''gracilis'' (Masculine (grammar), masculine or Feminine (grammar), feminine), or ''gracile'' (Grammatical gender, neuter), which ...

and lack of agility, which could have complicated predation upon adult mega-herbivores, and hindered the chasing down of nimbler prey. Nevertheless, larger (male) ''Arctodus simus'' are suggested to have been more carnivorous than females, as very large brown bears may not be able to sustain themselves on a vegetarian diet. Furthermore, the much larger frame of ''A. simus'' would have provided an advantage in disputes over carcasses. Studies establish that ''Arctodus simus'' would have had a varied diet across its range, and was outcompeted and/or more herbivorous with increased competition from other predators. The extinction of cursorial, hyper-carnivorous ''

Borophagus ''Borophagus'' ("gluttonous eater") is an extinct genus of the subfamily Borophaginae, a group of canids endemic to North America from the Middle Miocene epoch through the Early Pleistocene epoch 12—1.8 Annum, Mya. Evolution ''Borophagus'', ...

'' and ''Huracan'' in the more open western North America left a vacant niche, possibly contributing to the evolution of ''A. simus'' (along with changes to the herbivore guild).

Bone damage

The bite marks found on many bones of ground sloths ('' Northrotheriops texanus'') and young

proboscidea Proboscidea (; , ) is a taxonomic order of afrotherian mammals containing one living family (Elephantidae) and several extinct families. First described by J. Illiger in 1811, it encompasses the elephants and their close relatives. Three l ...

ns at Leisey Shell Pit in Florida matched the size of the canine teeth of ''Arctodus pristinus''. It is not known if these bite marks are the result of active predation or scavenging. Additionally, ''Arctodus pristinus'' was the most common large predator from Port Kennedy Cave, Pennsylvania, where the majority of mastodon remains were juveniles and likely represent accumulated prey. ''Arctodus simus'' has been found in association with

n remains near Frankstown, Pennsylvania (juvenile

mastodon A mastodon, from Ancient Greek μαστός (''mastós''), meaning "breast", and ὀδούς (''odoús'') "tooth", is a member of the genus ''Mammut'' (German for 'mammoth'), which was endemic to North America and lived from the late Miocene to ...

), and at

, South Dakota ( Columbian mammoths). However, questions remain as to whether these finds determine a predatory or scavenging relationship, or whether they were simply preserved at the same deposit (possibly at different times). On the other hand, a

woolly mammoth The woolly mammoth (''Mammuthus primigenius'') is an extinct species of mammoth that lived from the Middle Pleistocene until its extinction in the Holocene epoch. It was one of the last in a line of mammoth species, beginning with the African ...

specimen from

Saltville Saltville is a town in Smyth and Washington counties in the U.S. state of Virginia. The population was 1,824 at the 2020 census. It is part of the Kingsport– Bristol (TN)– Bristol (VA) Metropolitan Statistical Area, which is a co ...

, Virginia was likely scavenged on by ''Arctodus simus'', as evidenced by a

canine Canine may refer to: Zoology and anatomy * Animals of the family Canidae, more specifically the subfamily Caninae, which includes dogs, wolves, foxes, jackals and coyotes ** ''Canis'', a genus that includes dogs, wolves, coyotes, and jackals ** Do ...

gouge through the

calcaneus In humans and many other primates, the calcaneus (; from the Latin ''calcaneus'' or ''calcaneum'', meaning heel; : calcanei or calcanea) or heel bone is a bone of the Tarsus (skeleton), tarsus of the foot which constitutes the heel. In some other ...

. Several Columbian mammoth bones from a cave near Huntington Reservoir, Utah also record ursid gnaw marks attributed to ''Arctodus'', with an ''Arctodus'' specimen preserved in association with the remains. Importantly, the canines of '' Panthera atrox'' overlap in size with ''Arctodus simus'', complicating the identification of tooth marks. However, this is not to discredit all tooth marks attributed to ''Arctodus'', as damaged bones from near

Tanana River The Tanana River (Lower Tanana language, Lower Tanana: Tth'eetoo', Upper Tanana language, Upper Tanana: ''Tth’iitu’ Niign'') is a tributary of the Yukon River in the U.S. state of Alaska. According to linguist and anthropologist William Brig ...

, Alaska suggest that ''Arctodus'' transported megafaunal longbones back to a cave-like den and chewed on them, at a time when lions had a limited overlap with ''Arctodus'' in Beringia. Furthermore, a perforated

peccary Peccaries (also javelinas or skunk pigs) are pig-like ungulates of the family Tayassuidae (New World pigs). They are found throughout Central and South America, Trinidad in the Caribbean, and in the southwestern area of North America. Peccari ...

ilium from

Sheriden Cave Sheriden Cave is a Paleo-Indian archaeological site from the late Ice age in Wyandot County, Ohio, United States. Glacial deposits sealed off the cave more than 10,000 years ago. Sheriden Cave is a karst sinkhole on a dolomite ridge that crosses ...

has also been hypothesized as being scavenged by ''Arctodus simus''. Bone damage on a cranial fragment (and possibly the humerus) of an ''Arctodus'' individual on

may have been due to

cannibalism Cannibalism is the act of consuming another individual of the same species as food. Cannibalism is a common ecological interaction in the animal kingdom and has been recorded in more than 1,500 species. Human cannibalism is also well document ...

Beringia

Analysis of bones from Alaska showed high concentrations of nitrogen-15, a nitrogen isotope accumulated most strongly in carnivores. Although few specimens exist, there is currently no evidence of the same carbohydrate-related dental pathologies evident in southern populations of ''Arctodus simus''. Based on this evidence, ''A. simus'' was suggested to have been more carnivorous in

Beringia Beringia is defined today as the land and maritime area bounded on the west by the Lena River in Russia; on the east by the Mackenzie River in Canada; on the north by 70th parallel north, 72° north latitude in the Chukchi Sea; and on the south ...

than the rest of North America (with a preference for herbivores which consumed C₃ vegetation, particularly

caribou The reindeer or caribou (''Rangifer tarandus'') is a species of deer with circumpolar distribution, native to Arctic, subarctic, tundra, boreal, and mountainous regions of Northern Europe, Siberia, and North America. It is the only represe ...

). Increased carnivory may be due to a lower proportion of competitors and probably a lower availability of carbohydrate-rich food supplies across the year in the far northern latitudes. Survival during the cold season for some northern populations of ''A. simus'' could have depended on the regular scavenging of

ungulate Ungulates ( ) are members of the diverse clade Euungulata ("true ungulates"), which primarily consists of large mammals with Hoof, hooves. Once part of the clade "Ungulata" along with the clade Paenungulata, "Ungulata" has since been determined ...

carcasses, as is the case with Alaskan brown bears. Ultimately, an opportunistic foraging strategy including up to 50% vegetation, and the meat of reindeer, muskox,

carrion Carrion (), also known as a carcass, is the decaying flesh of dead animals. Overview Carrion is an important food source for large carnivores and omnivores in most ecosystems. Examples of carrion-eaters (or scavengers) include crows, vultures ...

, and possibly some predators, is consistent with the isotopic data and the conclusions of the ecomorphological studies.

Carbon isotope studies

Although elevated nitrogen-15 levels have been argued to indicate carnivory, even the isotope data of the most carnivorous Beringian ''Arctodus'' overlapped with modern, typically omni-herbivorous brown bears from Europe, eastern

Wyoming Wyoming ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States, Western United States. It borders Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho t ...

, and central

Montana Montana ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Idaho to the west, North Dakota to the east, South Dakota to the southeast, Wyoming to the south, an ...

, demonstrating that isotope data cannot distinguish between

hypercarnivore A hypercarnivore is an animal that has a diet that is more than 70% meat, either via active predation or by scavenging. The remaining non-meat diet may consist of non-animal foods such as fungi, fruits or other plant material. Some extant exampl ...

s and omnivores which eat a significant amount of animal matter. Studies are also complicated by a lack of compound-specific data, and isotope data being variable in carbon-13, and nitrogen-15 (due to individual/evolving prey and plant choices, the isotopic composition of the local environment, and nutritional stress).

Carbon-13 Carbon-13 (13C) is a natural, stable isotope of carbon with a nucleus containing six protons and seven neutrons. As one of the environmental isotopes, it makes up about 1.1% of all natural carbon on Earth. Detection by mass spectrometry A m ...

levels in ''Arctodus simus'' (enriched by both plants and prey matter) consistently reflect a diet based on C₃ resources, typically found in closed to mixed habitats with at least some tree cover (such as open woodlands). This includes C₃ vegetation (

leaves A leaf (: leaves) is a principal appendage of the stem of a vascular plant, usually borne laterally above ground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, stem, ...

, stems,

fruits In botany, a fruit is the seed-bearing structure in flowering plants (angiosperms) that is formed from the ovary after flowering. Fruits are the means by which angiosperms disseminate their seeds. Edible fruits in particular have long propaga ...

bark Bark may refer to: Common meanings * Bark (botany), an outer layer of a woody plant such as a tree or stick * Bark (sound), a vocalization of some animals (which is commonly the dog) Arts and entertainment * ''Bark'' (Jefferson Airplane album), ...

, and

flower Flowers, also known as blooms and blossoms, are the reproductive structures of flowering plants ( angiosperms). Typically, they are structured in four circular levels, called whorls, around the end of a stalk. These whorls include: calyx, m ...

s from trees, shrubs, and cool season

) and the browsers that fed on them, such as

bison A bison (: bison) is a large bovine in the genus ''Bison'' (from Greek, meaning 'wild ox') within the tribe Bovini. Two extant taxon, extant and numerous extinction, extinct species are recognised. Of the two surviving species, the American ...

and

ground sloth Ground sloths are a diverse group of extinct sloths in the mammalian superorder Xenarthra. They varied widely in size with the largest, belonging to genera '' Lestodon'', ''Eremotherium'' and ''Megatherium'', being around the size of elephants. ...

Hibernation

''Arctodus pristinus'' specimens have been found in caves such as Port Kennedy (Pennsylvania, where fossils from as many as 36 individuals have been found), Cumberland Cave (Maryland) and Hamilton Cave (West Virginia), often in association with the black bear. This suggests a close association with the biome. According to a 2003 study, in karst regions, fossils of ''Arctodus simus'' have been recovered almost exclusively from cave sites. In the contiguous United States, that ~38% of all sites are from caves (possibly ~50% in western USA) suggests a close association between this species and cave environments. Metabolic denning (

hibernation Hibernation is a state of minimal activity and metabolic reduction entered by some animal species. Hibernation is a seasonal heterothermy characterized by low body-temperature, slow breathing and heart-rate, and low metabolic rate. It is mos ...

/torpor) is unclear in ''Arctodus''. Like polar bears, male and unmated female ''A. simus'' may have forgone denning, leaving maternal denning by females as the preferred explanation behind the recovery of the small, yet relatively complete individuals recovered from caves. However, to date, there are no records of adults with associated offspring from caves. Regardless, '' Arctotherium angustidens'', a fellow

giant short-faced bear 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 ('' ...

, has been recovered from a cave in

Argentina Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...

with offspring. At Riverbluff Cave, the most abundant claw marks are from ''Arctodus simus''. They are most abundant at the bear beds and their associated passageways, indicating a close relationship with denning. Numerous "bear" beds often preserve ''Arctodus simus'' and both Pleistocene and modern

American black bear The American black bear (''Ursus americanus''), or simply black bear, is a species of medium-sized bear which is Endemism, endemic to North America. It is the continent's smallest and most widely distributed bear species. It is an omnivore, with ...

s in association (''U.a. amplidens'' and ''U. a. americanus'')- such deposits have been found in

Missouri Missouri (''see #Etymology and pronunciation, pronunciation'') is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. Ranking List of U.S. states and territories by area, 21st in land area, it border ...

Oklahoma Oklahoma ( ; Choctaw language, Choctaw: , ) is a landlocked U.S. state, state in the South Central United States, South Central region of the United States. It borders Texas to the south and west, Kansas to the north, Missouri to the northea ...

and Potter Creek Cave, California. These mixed deposits are assumed to have accumulated over time as individual bears (including ''Arctodus'') died during winter sleep. Furthermore,

environmental DNA Environmental DNA or eDNA is DNA that is collected from a variety of environmental samples such as soil, seawater, snow or air, rather than directly sampled from an individual organism. As various organisms interact with the environment, DNA ...

suggests that ''Arctodus'' and black bears shared a cave in

Chiquihuite cave Chiquihuite Cave is a possible Upper Paleolithic archaeological site in the Astillero Mountains, Zacatecas State, in North-Central Mexico. Chiquihuite Cave may be evidence of early human presence in the Western Hemisphere up to 33,000 years ago. I ...

, Zacatecas. At Labor-of-Love Cave, Nevada, both American black bears and brown bears have been found in association with ''Arctodus simus''. A study in 1985 noted that

sympatry In biology, two closely related species or populations are considered sympatric when they exist in the same geographic area and thus frequently encounter each other. An initially interbreeding population that splits into two or more distinct spe ...

between ''Arctodus'' and brown bears preserved in caves is rare, with only Little Box Elder Cave, Wyoming and Fairbanks II, Alaska hosting similar remains.

Paleoecology

''Arctodus pristinus''

South Florida Museum - Big Carnivore Skeleton

Endemic to the late Blancan faunal stage and Irvingtonian faunal stage, ''Arctodus pristinus'' was a relatively large tremarctine bear. Sometimes referred to as the eastern short-faced bear, ''A. pristinus'' has been found in Florida, Kansas, Maryland, New Mexico, Pennsylvania, South Carolina, and West Virginia in the US, and

in Mexico. Possible remains have also been recovered from Arizona. ''A. pristinus'' is particularly well known from Florida, especially from the Leisey Shell Pit. Like ''A. simus'' and other tremarctine bears, ''A. pristinus'' had adaptations for herbivory, and was likely largely herbivorous itself, although ''Arctodus'' has been suggested to be generally more carnivorous than contemporary bears.

Eastern North America

''Arctodus pristinus'' is considered a biochronological indicator for the period between the Late Blancan and late Irvingtonian periods of Pleistocene Florida- more fossils of ''Arctodus pristinus'' are known from Florida (about 150) than anywhere else. In the

Early Pleistocene The Early Pleistocene is an unofficial epoch (geology), sub-epoch in the international geologic timescale in chronostratigraphy, representing the earliest division of the Pleistocene Epoch within the ongoing Quaternary Period. It is currently esti ...

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 .Santa Fe River 1 site (~2.2 Ma), which ''Arctodus pristinus'' inhabited, was a fairly open grassland environment dotted with karst sinks and springs and dominated by longleaf pine flatwoods. ''Arctodus pristinus'' co-existed with

terror birds Phorusrhacids, colloquially known as terror birds, are an extinct family of large carnivorous, mostly flightless birds that were among the largest apex predators in South America during the Cenozoic era. Their definitive fossil records range from ...

, sabertooth cats, giant sloths (''

Eremotherium ''Eremotherium'' (from Greek for "steppe" or "desert" "beast": ἔρημος "steppe or desert" and θηρίον "beast") is an extinct genus of giant ground sloth in the family Megatheriidae. ''Eremotherium'' lived in southern North America, Cen ...

'', ''

Megalonyx ''Megalonyx'' (Greek, "great-claw") is an extinct genus of ground sloths of the family Megalonychidae, native to North America. It evolved during the Pliocene Epoch and became extinct at the end of the Late Pleistocene, living from ~5 million ...

'', ''

Paramylodon ''Paramylodon'' is an extinct genus of ground sloth of the family Mylodontidae endemic to North America during the Pliocene through Pleistocene epochs, living from around ~4.9 Mya–12,000 years ago. Within the genus only two species are recog ...

''), giant armadillos (''

Glyptotherium ''Glyptotherium'' (from Ancient Greek for 'grooved or carved beast') is a genus of glyptodont (an extinct group of large, herbivorous armadillos) in the family Chlamyphoridae that lived from the Early Pliocene, about 5 million years ago, to th ...

,'' ''

Holmesina ''Holmesina'' is an extinct genus of pampathere, a group of armadillo-like xenarthrans that were distantly related to extant armadillos. Like armadillos, and unlike the other extinct branch of megafaunal cingulates the glyptodonts, the shell w ...

'', ''

Pachyarmatherium ''Pachyarmatherium'' is a genus of extinct large armadillo-like cingulates found in North and South America from the Pliocene and Pleistocene epochs, related to the extant armadillos and the extinct pampatheres and glyptodonts. It was present ...

''),

gomphotheres Gomphotheres are an extinct group of proboscideans related to modern Elephant, elephants. First appearing in Africa during the Oligocene, they dispersed into Eurasia and North America during the Miocene and arrived in South America during the Ple ...

hyenas Hyenas or hyaenas ( ; from Ancient Greek , ) are feliformia, feliform carnivoran mammals belonging to the Family (biology), family Hyaenidae (). With just four extant species (each in its own genus), it is the fifth-smallest family in the orde ...

, canids (''

'', ''

Canis lepophagus ''Canis lepophagus'' (Latin: : 'dog'; : 'hare' or 'rabbit'; suffix : '-eating'; hence hare-eating dog) is an extinct species of canid which was endemic to much of North America during the Early Pliocene. It is notable because its lineage is prop ...

''), Platygonus, peccaries, Hemiauchenia, llamas, Capromeryx, dwarf pronghorns, and Nannippus, three-toed horses. Smaller fauna included condors, Rail (bird), rails, ducks, Erethizon, porcupines, and alligators.

''Arctodus simus''

Evolving from the smaller ''A. pristinus'' in the Irvingtonian, early Irvingtonian faunal stage, scholars today mostly conclude that ''Arctodus simus'' was a colossal, opportunistic omnivore, with a flexible, locally adapted diet akin to the

. If ''Arctodus simus'' wasn't largely herbivorous, the scavenging of megaherbivore carcasses, and the occasional predatory kill would have complimented the large amounts of vegetation consumed when available. Sometimes referred to as the bulldog bear, or great short-faced bear, ''Arctodus simus'' has been recovered from a comparatively small number of finds in relation to other large carnivorans, with the species suggested to have lived in low population densities. Matheus argues that unlike other Nearctic realm, Nearctic carnivorans, ''A. simus'' did not appear to have an ecological equivalent ("super-huge bear") in the Palearctic realm. ''Arctodus simus'' was initially restricted to the western United States during the Irvingtonian. However, in the Rancholabrean faunal stage, ''A. simus'' expanded its range from southern Canada to Trans-Mexican Volcanic Belt pine–oak forests, central Mexico in the west, and to Pennsylvania and Florida in the east. ''A. simus'' also inhabited eastern

at times, with finds today spanning from northern Alaska to the

. Based on the wide distribution of the species, ''Arctodus simus'' inhabited a diversity of climatic conditions and environments. A 2009 study examining megafaunal extinctions in Northern America noted 12 records (<40,000 BP) of ''Arctodus simus'' from the Intermontane Plateaus, 7 from the Pacific Mountain System, 6 each from the Interior Plains and U.S. Interior Highlands, Interior Highlands, 3 each from the Atlantic Plains and Rocky Mountain System, and 1 from the Appalachian Highlands. ''A. simus'' was relatively plentiful in western North America, with over 50% of specimens from the western contiguous United States (<40,000 BP). ''Arctodus simus'' was integral to what has been referred to as the ''Camelops'' fauna, or alternatively ''Camelops''/''American mountain deer, "Navahoceros"'' fauna, a faunal province centered in western North America. The ''Camelops'' fauna was also characterized by Euceratherium, shrub-ox, prairie dogs, Capromeryx, dwarf pronghorns, Nothrotheriops, Shasta ground sloths, and American lions. The diverse flora of the ''Camelops'' faunal province included montane conifers and oak parklands, shrub and grassland that stretched across the North American Cordillera south of Canada, to the Valley of Mexico. This faunal province supported a variety of large grazing and browsing mammals.

Western Mountains

The Pacific Mountain System seems to represent a cradle of evolution for ''Arctodus simus''. The earliest confirmed finds of ''Arctodus simus'' are from Fremont, California#Irvington District, Irvington, California, which are at least 780,000 years old, but may be older than 1.2Mya. Other Irvingtonian age sites come from California, such as Temecula Basin, Elsinore, Fairmead, California, Fairmead, and Murrieta, California, Murrieta. Older yet disputed remains come from San Timoteo Formation, El Casco (1.4Mya). Despite the shift to aridified, mixed C₃-C₄ habitats between the Early and Late Pleistocene of the Central Valley (California), Central Valley (~1Mya to ~15,000 BP), ''Arctodus simus'' remained consistent with the consumption of C₃ resources. dire wolf, Dire wolves and ''Arctodus simus'' were ever present members of the local predator guild throughout the Pleistocene, whereas jaguars, ''Homotherium'', ''Miracinonyx'' and ''Smilodon'' (Fairmead & Irvington) transitioned to ''Panthera atrox'' and coyotes (McKittrick Tar Pits). Although ''Arctodus'' could have hunted other closed habitat browsers such as deer (Elk, ''Cervus'' & ''Odocoileus''), camelids (''Hemiauchenia'' & ''Camelops''), ''Paramylodon'', and Platygonus, peccaries, specimens collected from the

suggest ''A. simus'' preferred a herbivorous diet. ''A. simus'' is particularly famous from fossils found in the La Brea Tar Pits, with 33 individuals recovered (the most of any locality). As only one juvenile has been found from La Brea, ''A. simus'' is suggested to have been solitary. Many more finds come from across California, Vancouver Island, and Washington (state), Washington, where the semi-arid woodland/scrub transitioned to Forest steppe, forest-steppe, and open grasslands/heath. Comparatively, the Rocky Mountain System had the fewest number of specimens of ''Arctodus simus'' in western North America. However, one of the youngest dated ''Arctodus simus'' is from a cave near Huntington Reservoir, Utah, which sits at an elevation of 2,740m (~9,000 ft). The central and southern Rocky Mountains may have acted as refugia for boreal parkland megafauna from the plateau such as ''Arctodus simus'', with the Huntington specimen being the only confirmed extinct megafauna dated to the Younger Dryas of the Great Basin. Other remains have been found from Wyoming (such as Natural Trap Cave), and Montana.

Intermontane Plateaus

The Intermontane Plateaus had the highest number of ''Arctodus simus'' specimens south of the ice sheets. The region has yielded some of the largest specimens of ''A. simus,'' including what was once the largest specimen on record, from Salt Lake Valley, Utah. Disputed Irvingtonian remains from Deserts of California, eastern California (Victorville, California, Victorville and Vallecito Creek (California), Vallecito Creek) may be as old as 2Mya. In contrast with other parts of North America, the Intermountain West, plateaus received more rainfall during the Late Pleistocene, as glacially cooled air collided with hot desert air. As a result, this greatly expanded the range of subalpine parkland, Pinyon–juniper woodland, piñon-juniper & Ponderosa pine forest, ponderosa woodlands, Sagebrush steppe, sagebrush grasslands and pluvial lakes where desert exists today. The mid-Wisconsin glaciation, Wisconsian U-Bar Cave, New Mexico, was vegetated by Artemisia tridentata, sagebrush, grasses, and woodlands. Notable fauna which lived alongside ''Arctodus simus'' included Shasta ground sloth, Euceratherium, shrub-ox, pronghorns (''Stockoceros,'' ''Capromeryx''), ''Camelops'', ''Odocoileus'', horses, ''Lynx'', Cougar, puma, black bear, Oreamnos, mountain goats'','' prairie dogs, and Stock's vampire bat. Dire wolf, Dire wolves were also found in association with ''Arctodus simus,'' and both species are the most common large carnivorans of Rancholabrean New Mexico. Beyond Utah and New Mexico, other important US specimens have also been found in Arizona, eastern California, Idaho, Nevada, and eastern Fossil Lake (Oregon), Oregon. The Intermontane Plateaus extended into central Mexico, with the Mexican Plateau sharing the Late Pleistocene Mesic habitat, mesic savanna and Pinyon–juniper woodland, piñon–juniper woodland ecoregion with the Southwestern United States, southwestern USA. While ''Arctodus'' was limited to the Mexican plateau, the typical tropical thorn scrub and scrub woodland of the plateau was seemingly prime habitat for tremarctine bears. An ''Arctodus simus'' individual from Cedral, San Luis Potosí, Cedral, San Luis Potosí, inhabited closed vegetation, based on the individual's Δ13C, ''δ''¹³C signature. Consuming C₃ resources, its diet may have incorporated local C₃ specialists such as tapir, Hemiauchenia, llamas, Camelops, camels, and Nothrotheriops, Shasta ground sloth along with browsed vegetation. The site, incorporating trees, herbs and cacti, hosted an open gallery forest near Savanna, grassland or Shrubland, scrub with a Humid subtropical climate, humid climate. Similar highland remains have been recovered from Zacoalco de Torres, Jalisco, Lake Cuitzeo, Michoacán, Hueyatlaco, Puebla, Tequixquiac, State of Mexico, and Chiquihuite cave, Zacatecas.

Interior Plains

The Interior Plains were composed of temperate steppe grassland, and among the specimens yielded from this region is one of the largest ''Arctodus simus'' currently on record, from the banks of the Kansas river. The late Irvingtonian Miles City, Montana, Doeden gravel pits in Montana preserves an open grassland habitat, with riparian woodlands, and likely some shrublands. ''Arctodus simus'' co-existed with ground sloths (''

'', ''Paramylodon''), Pacific

, Camelops, camels, and ''Bootherium''. As bison were yet to migrate into North America, Columbian mammoths and horses dominated these Illinoian (stage), early Illinoian grasslands. Additional Irvingtonian remains are from Arkalon, Kansas, Kansas, Nebraska and Briscoe County, Texas, Texas.In the Rancholabrean age, ''Arctodus simus'', grey wolves and coyotes were part of a predator guild throughout the great plains, and were joined by Columbian mammoths, camels, ''Hemiauchenia'', and American pronghorns. While the northern plains aridified into cold steppe (e.g. The Mammoth Site, Mammoth site, South Dakota), the southern plains were a parkland with riparian Celtis, hackberry forests, and large expanses of mixed grass prairie grasslands grading into wet meadows, with limited seasonality. In the south (Lubbock Lake Landmark, Lubbock Lake, Texas), this fauna was joined by ''Smilodon'', dire wolf, dire wolves, grey fox and red fox, preying upon prairie dogs, horses (''Equus'' & ''Haringtonhippus''), peccaries, ''Odocoileus'', ''Capromeryx'', ''Bison antiquus'' and ''Holmesina''. Beyond Texas, ''Arctodus'' has also been found in Iowa, Kansas, Nebraska, and southern Canada (Alberta & Saskatchewan), which when unglaciated, would have formed a tundra ecosystem with an ice-free corridor to Beringia. In the lowlands of the eastern Interior plains, the plains transitioned to closed habitat. At the terminal Pleistocene

, Ohio, a mosaic habitat consisting of marsh, open woodland, and patchy grassland was home to ''Arctodus simus'', ''Cervalces scotti'', caribou, peccaries (Platygonus compressus, ''Platygonus'', Long-nosed peccary, ''Mylohyus''), Castoroides, giant beaver, North American porcupine, porcupine, and American marten, American pine marten. Similar remains have been found in Indiana, and Kentucky.

Interior Highlands

To the south, the Interior Highlands had a very high density of ''Arctodus simus'' specimens (second only to the black bear), due to the high rate of preservation in the cave-rich region. Sympatry between the two species is most apparent in

- ''Arctodus simus'' has been found in association with black bears at Riverbluff, Bat and Big Bear caves. Big Bear Cave preserves fossilized hair associated with ''Arctodus''. During the Last Glacial Maximum, both bears were joined by Dire wolf, dire wolves, coyotes, jaguars, snowshoe hare, groundhogs and beavers at Bat Cave, which also records thousands of ''Platygonus'' remains. These fauna inhabited well-watered forest-grassland ecotone with a strong Taiga of North America, taiga influence, although the region did occasionally cycle through drier, grassier periods. These Montane ecosystems#Temperate climate, open woodlands were dominated by pines and spruce, and to a lesser extent by oaks''.'' Additional finds have been recovered from Adair County, Oklahoma, Oklahoma.

Eastern USA

Compared to other regions, ''Arctodus simus'' was relatively rare in eastern North America. To the north, the Appalachian Highlands were dominated by taiga. Post-LGM Saltville (archaeological site), Saltville, Virginia, was a mosaic of grassy/herb laden open areas intermixed with open canopy Taiga, boreal woodlands (oaks, pines, spruce, birch, firs) and marshes. Inhabiting in this C3 carbon fixation, C₃ resource dominated environment were ''Arctodus simus'',

, (southernmost)

s, ''Bootherium,'' horses, caribou, ''

'', dire wolf, dire wolves, beavers, ''Cervalces'', and a variety of warm-adapted reptiles, suggesting a more Mesic habitat, mesic and less seasonal climate than today. Heavy bone damage on a mammoth carcass by both dire wolves and ''Arctodus'' suggests a potentially competitive scavenging relationship Beyond Covington, Virginia, Virginia, additional remains have been found in Frankstown Township, Blair County, Pennsylvania, Pennsylvania. To the south, the Subtropics, subtropical Atlantic Plains covered a great expanse of lowland, from the open deciduous woodlands of the Atlantic coastal plain, Atlantic coast, to the semi-arid woodland/scrub of Florida, to the spruce-fir conifer forests and open habitat of the Gulf Coastal Plain. Although scarce, this contrast of habitats highlights the adaptability of ''Arctodus simus''. At the Rainbow River and Lake Rousseau localities in Rancholabrean Florida, three ''Arctodus simus'' specimens have been recovered, alongside ''Smilodon'', dire wolf, dire wolves, jaguars, ground sloths (''

'', ''Paramylodon''), llamas (''Hemiauchenia'', ''Palaeolama''), Tapirus veroensis, Vero's tapir, giant beaver, Neochoerus pinckneyi, capybara, ''Holmesina'', horses, ''Bison antiquus'',

, Columbian mammoths and ''Tremarctos floridanus'', in a climate similar to today's. Furthermore, the abundance of black bears, and particularly ''Tremarctos floridanus'' in Florida, has led to a theorized niche partitioning of ursids in Florida, with ''Tremarctos floridanus'' being herbivorous, and black bears and ''Arctodus simus'' being omnivorous, with ''Arctodus'' being possibly more inclined towards carnivory. Additional finds of south-eastern ''Arctodus simus'' are from Alabama, Arkansas, Mississippi, South Carolina, and Texas.

Beringia

Largely isolated by the Cordilleran and Laurentide ice sheets,

is considered ecologically separate to the rest of North America, being largely an extension of the mostly open and treeless Eurasian mammoth steppe. However, the occasional opening of an ice-free corridor, and the migration barrier of the Beringian gap, meant that eastern Beringia (Alaska and the

) supported a unique assemblage of fauna, with many endemic North American fauna flourishing. Currently, all specimens of ''A. simus'' in Beringia have been dated to a 27,000 year window (50,000 BP - 23,000 BP) from eastern Beringia, while additional undated remains may be of Sangamonian age. Unlike contemporary Beringian carnivorans, ''A. simus'' apparently never inhabited western Beringia (and therefore Asia). The largest known skull of ''A. simus'' was recovered from the Yukon, and may represent the largest specimen known. The Alaska North Slope, North Slope of Alaska <40,000 BP (Ikpikpuk and Titaluk rivers) preserves an upland and floodplain environment, with horses, bison then caribou being the most populous herbivores, and woolly mammoths, muskox, elk and saiga antelope more scarce. Panthera spelaea, Cave lions, bears (''Ursus arctos'' and ''Arctodus simus''), and Beringian wolf, Beringian wolves made up the megafaunal predator guild. Isotope data implies that caribou and muskox were principal components of the carnivorous portion of ''Arctodus simus''' Arctic diet, suggesting that the warmer, wetter vegetation on the margins of the dry mammoth steppe (similar to the moist acidic tundra vegetation which Tundra of North America, dominates today) was the preferred habitat of ''Arctodus'' in Beringia. Additionally, upon the flooding of the Bering Strait and Paludification, expansion of moist tundra and Mire, peatlands in eastern Beringia during MIS 3, MIS-3, Panthera spelaea, lions,

s and ''Homotherium'' went regionally extinct ~35,000 BP, whereas wolves and ''Arctodus'' persisted. Simultaneously, most megafaunal herbivores in Beringia experienced population bottlenecks, whilst Woolly mammoth, mammoth populations steadily declined. This restriction of prey and habitat could explain the extinctions. However, genetically distinct cave lions and brown bears appear in MIS 2, MIS-2 circa the extinction of ''Arctodus'' in a re-emerged Beringia ~23,000 BP, opening up the possibility that some level of competition was at play. The idea that ''Arctodus'' had a Kleptoparasitism, kleptoparasitic relationship with Beringian wolf, wolves and ''Homotherium'' in Beringia has been explored, with the additional possibility that ''Arctodus'' successfully competed against brown bears and ''Homotherium'' for access to

pre-Last Glacial Maximum, LGM. The local extinction of ''Arctodus'' in Beringia ~23,000 BP (possibly due to sharp climatic cooling associated with Heinrich event, Heinrich Event-2), was much earlier than in other parts of its range. While recolonized by Panthera spelaea, cave lions and

s from Eurasia, ''Arctodus'' did not repopulate Beringia once the ice-free corridor to the south re-opened later in the Pleistocene.

Map of fossil localities

Relationships with other bears

''Arctodus pristinus''

In the Early Pleistocene, ''Arctodus pristinus'' was much more populous the south-east of North America, whereas the

was more common in the north-east. The black bear has inhabited North America since at least the Mid-Pleistocene Transition, Middle Pleistocene, while ''

'', a tremarctine bear inhabiting western North America at the time, is very similar to ''A. pristinus'' in terms of size, skeletal anatomy, and dietary preferences. Despite this, generally speaking large tremarctine fossils from the Early and Middle Pleistocene of Florida are considered to be ''A. pristinus'', whereas those from the Late Pleistocene of Florida are considered to be ''T. floridanus''. Indeed, black bears and ''Tremarctos floridanus'' are believed to have only colonized Florida with the extinction of ''A. pristinus'' (both of which only appear in Florida in the Late Pleistocene), however, ''T. floridanus'' could yet still be found from older sites in Florida. ''T. floridanus'' was possibly an ecological replacement of ''A. pristinus'', with ''T. floridanus'' finds being widespread in Rancholabrean Florida and the wider southeastern United States.

''Arctodus simus''

The most commonly accepted ecological parallel of ''Arctodus simus'' in scientific literature is the brown bear. Both brown bears and ''Arctodus simus'' exhibit a high degree of dietary variability, and while Dietary biology of the brown bear, largely herbivorous, meat can be an important dietary element to certain populations of both species. Additionally, the potential of habitual kleptoparasitism is often noted in ''Arctodus'', with brown bears being opportunistic, curious, and regularly stealing kills from smaller predators. One past theory behind the extinction of ''Arctodus simus'' is that ''A. simus'' may have been out-competed by brown bears as the latter expanded southwards from eastern Beringia ~13,000 BP, and gradually established itself in North America. However this has been refuted as new dates establish an extended coexistence, with some isolated ''A. simus'' remains being re-evaluated as brown bears. Brown bears (along with American lion, lions,

and red foxes) first emigrated to North America via

during the Illinoian (stage), Illinoian Glaciation, with brown bears first arriving between ~177,000 BP and ~111,000 BP in eastern Beringia. Genetic divergences suggest brown bears first migrated south during Marine Isotope Stage 5, MIS-5 (~92,000 - 83,000 BP) upon the opening of the ice-free corridor, with the first fossils being near Edmonton (26,000 BP). On a continent-wide scale, although the brown bear and ''Arctodus simus'' were Sympatry, sympatric at times as brown bears spread into North America, ''Arctodus simus'' may typically have dominated competitive interactions, and displaced brown bears from specific localities. Additionally, ''Arctodus''' prolonged co-existence with black bears may have put significant constraints on the black bear's evolution. At the end of the Pleistocene, one reason brown bears persisted where ''Arctodus simus'' went extinct was because ''Arctodus'' may have been less flexible in adapting to new and rapidly changing environments that impacted the availability or quality of food and habitat. Brown bears and ''Arctodus'' have been discovered together in Alaska (then Beringia) between 50,000 BP and 34,000 BP, and in later Pleistocene deposits in

, Maricopa, California, California, Wyoming and Nevada.

''Beringia''

Isotope values (''Δ13C, δ''^{Δ13C, 13}Δ13C, C and Δ15N, ''δ''¹⁵N) in numerous

n ''Arctodus simus'' specimens suggests ''A. simus'' usually occupied a higher trophic level compared with invading brown bears. While some Beringian brown bears consumed salmon, data from Beringian specimens of ''Arctodus'' clustered much more tightly, and suggested that only terrestrial sources of meat were important for Beringian ''Arctodus''. The forcing of a smaller bear into a more herbivorous diet has been compared to the modern relationship between brown bears and American black bears. Where they overlap, black bears take the lower trophic niche, with lower population densities, much smaller territorial ranges, and seasonal migrations. That ''Arctodus simus'' (along with local climate change) may have excluded brown bears from eastern Beringia from ~34,000 to ~23,000 BP further suggests that ''Arctodus'' may typically have been dominant over brown bears. When ''Arctodus'' went extinct in Beringia ~23,000 BP, brown bears recolonized

, but had more carnivorous diets than their Beringian kin pre ~34,000 BP. This bolsters the idea that these bears competed for similar resources and niches. Extinction and repopulation is further evidenced by the high genetic (mitochondrial) diversity of Beringian brown bears in contrast with Beringian ''Arctodus simus''. This contrast in genetic diversity has also been hypothesized to suggest that while female brown bears have a Philopatry, permanent home range, female ''Arctodus simus'' may not have (at least not to the same extent).

''Vancouver Island''

Brown bears, black bears and ''Arctodus simus'' all co-existed on Vancouver Island once the island de-glaciated ~14,500 BP. According to an isotope analysis, all three bears relied on terrestrial resources, ''Arctodus'' holding an intermediate trophic position between the brown and black bears. This may be an underestimate, as the ''Arctodus'' specimens from Vancouver Island are believed to be female; as per brown and black bears, female ''A. simus'' may have had a significant decrease in protein consumption compared with male ''A. simus'' when co-existing with brown bears. Additionally, an analysis of ''Arctodus''' data suggested that when consuming protein, Threonine, meat was preferred. While Niche partition, niche-partitioning on Vancouver Island was possible, both ''Arctodus simus'' and brown bears appeared to have preferred more open habitats.

Convergent evolution

Both Tremarctinae, giant short-faced bears ''Arctodus simus'' and '' Arctotherium angustidens'' reached huge body sizes, in an example of convergent evolution. However, beyond gigantism, there are notable differences between the species. Not only did ''Arctotherium angustidens'' reach a higher maximum weight (an exceptional specimen was calculated at ~), ''A. angustidens'' was a much more robust animal, in contrast with the Gracility, gracile ''Arctodus simus''. Excluding the exceptional specimen, ''Arctotherium angustidens'' had been calculated to a weight range between and , with the largest specimens of either species being said to be comparable to one another. The panda-relative ''Agriotherium, Agriotherium africanum'' has also been suggested to share ecomorphological convergences with ''Arctodus simus.'' Together with great size, the two species converged on several adaptations, including a skull with a short broad rostrums, premasseteric fossa on the mandible, possible Carnassial, carnassial shears (P4 and m1), and long limbs (relative to body length). These features were also shared by other extinct bears (''Agriotherium'', ''Huracan (mammal), Huracan'' and ''Arctotherium, Arctotherium bonariense''). However, while ''Agriotherium'' and ''Huracan'' have definitive adaptions for meat-heavy diets stemming from a running, predatory lifestyle, ''Arctodus simus'' lacks similar adaptations beyond proportionally longer limbs.

Interactions with humans

One documented interaction with Clovis culture, Clovis people is present at the Lubbock Lake Landmark, Texas. A likely already deceased ''Arctodus simus'' was processed for subsistence (butchery marks indicated skinning, de-fleshing and disarticulation) and tool production, much in the same way as a Lehner Mammoth-Kill Site, mammoth carcass (~13,000 BP / 11,100 Carbon-14, ¹⁴C BP ). Additionally, other remains of the ''Arctodus simus'' have been found in association with Paleo-Indians, Paleo-Indian artifacts in

, Ohio, and Huntington Dam, Utah, with an ''A. simus'' Metatarsal bones, footbone fragment from Nez Perce National Historical Park, Spalding, Idaho also being charred. The direct relationship between humans and some associated ''Arctodus'' remains has been debated. Human hunting and butchery of large megafauna, particularly mammoths and mastodon, would likely have put people in competition with ''Arctodus simus''. Defense against these large bears and the abandonment of carcasses are plausible outcomes, along with the possible caching and disposal of carcass remains underwater to mask its odor from ''Arctodus''.

Migration barrier hypothesis

In the late 1980s, Valerius Geist, Val Geist hypothesized that "specialist, aggressive, competitive Rancholabrean fauna" such as ''Arctodus simus'' were a barrier for humans (along with other Siberian megafauna such as moose, grey wolves and brown bears) when migrating into North America (both Beringia and Laurentide Ice Sheet, below the ice sheets). Male ''A. simus'' were the largest and most powerful carnivorous land mammals in North America, with the potential specialization in obtaining and dominating distant and scarce resources. Humans in this hypothesis, though familiar with brown bears, would not have been able to avoid predation or effectively compete with ''Arctodus simus'' and other large Pleistocene North American carnivores, making human expansion difficult in Beringia and impossible south of the ice sheets. However, this theory has Fringe theory, never been accepted by anthropologists. Paul Matheus argues that there were negligible ecological differences across the mammoth steppe, and that humans successfully competed against and even hunted territorial cave bears, cave hyenas, Panthera spelaea, cave lions, leopards, tigers and wolves in Eurasia before reaching eastern Beringia, making the solitary ''Arctodus'' an unlikely impediment to expansion. Indeed, new dates establish an extended co-existence of humans and megafauna such as ''Arctodus'' across North America.

Beringia

Humans migrated to North America via the Siberian mammoth steppe, arriving at Beringia, eastern Beringia (Alaska and the Yukon). However, the migration was halted at the Wisconsin glaciation, North American Ice Sheet, which separated Beringia and southern North America for most of the Late Pleistocene. Both humans and ''A. simus'' are first dated to ~50,000 BP in Beringia, both from sites in the Yukon, and co-existed until ''A. simus'' went extinct in Beringia ~23,000 BP during the Last Glacial Maximum. This co-existence continued through the regional extinction of other Beringian predators such as cave lions, brown bears and Homotherium, saber-tooth cats. Important sites of pre-LGM human occupation in Beringia include Old Crow Flats and the Klondike, Yukon, Klondike, Kuparuk River, Kuparuk River Valley, and the Bluefish Caves.

Contiguous North America

The human colonization of North America south of the ice sheets further disproves the idea that ''Arctodus'' was a migration barrier. The earliest universally accepted pre-Clovis site south of Beringia are the White Sands National Park#Native people, White Sands footprints in New Mexico, dated to ~22,000 cal. BP. Other pre-Last Glacial Maximum, LGM sites across the Americas, such as

, Valsequillo, El Cedral, Santa Elina, Gault (archaeological site), Gault, and Hartley Mammoth Site, affirm that humans proliferated alongside megafauna (such as ''Arctodus'') in southern North America for more than ten thousand years. Humans were definitively widespread across the Americas by at least 15,000 BP.

Extinction

''Arctodus pristinus''

''Arctodus pristinus'' went extinct in the Chibanian, Middle Pleistocene (300,000 years ago), being last recorded from the Coleman 2A site, Florida. The evolution of ''Arctodus simus'', competition with ''

'' and black bears, and possibly the transitioning of

Florida from a hot, wet, densely forested habitat to a still hot, but drier and much more open biome are thought to be factors behind the gradual disappearance of ''Arctodus pristinus'' in the Irvingtonian, late Irvingtonian faunal stage. There are dubious records of ''A. pristinus'' in South Carolina and California from the Sangamonian, Late Pleistocene, however these are heavily disputed. Modern research establishes ''A. pristinus'' as existing between the Pliocene-Pleistocene boundary and the Middle Pleistocene.

''Arctodus simus''

With the extinction of ''Arctodus pristinus'', ''Arctodus simus'' became the final representative of the genus. ''Arctodus simus'' went extinct around 12,800 years ago, and is one of the most recently dated megafauna to go extinct in North America, being reliably dated to within the Late Pleistocene extinctions, Pleistocene-Holocene boundary (13,800 BP - 11,400 BP). Both local and regionalized dietary flexibility has been a factor suggested for the species' longevity. Various factors, including the depletion in number of large herbivores, the diminishing nutritional quality of plants during climate change, and competition with fellow omnivores (humans and brown bears) for food resources, have been suggested as the cause of ''Arctodus simus''' extinction. However, multiple studies put doubt on brown bears being culpable in ''Arctodus simus''' extinction, with the brown bear being more of an ecological replacement that was more adaptable to change.

Taxonomy

Evolution

Description

Size

''Arctodus pristinus''

''Arctodus simus''

= Studies

Sexual dimorphism

Anatomy

Skull

= Dentition

Post-cranial

= Limbs

= Paws

Paleopathology

Paleobiology

Locomotion

Maturity

Genetic diversity

Haplotype cladogram

Diet

Herbivory

Opportunistic carnivory

Bone damage

Beringia

Carbon isotope studies

Hibernation

Paleoecology

''Arctodus pristinus''

Eastern North America

''Arctodus simus''

Western Mountains

Intermontane Plateaus

Interior Plains

Interior Highlands

Eastern USA

Beringia

Map of fossil localities

Relationships with other bears

''Arctodus pristinus''

''Arctodus simus''

''Beringia''

''Vancouver Island''

Convergent evolution

Interactions with humans

Migration barrier hypothesis

Beringia

Contiguous North America

Extinction

''Arctodus pristinus''

''Arctodus simus''

Climate change

Low genetic diversity

Last dates

Revival efforts

History of research

"Super predator" hypothesis

Specialist kleptoparasite vs Omnivore

Rebuttal

See also

References

External links