Arctodus pristinus

A. pristinus specimens closely overlap the size of Tremarctos floridanus, with the some males of A. pristinus overlapping in size with the females of A. simus.^[1] A. pristinus individuals recovered from the Leisey Shell Pit, Florida, were calculated to an average of ~133 kilograms (293 lb).^[32] However, as the dimensions of some individuals from Port Kennedy Bone Cave and Aguascalientes are within A. simus range, northern and western A. pristinus may have been larger than Floridan A. pristinus.^[7]

Arctodus simus

Some A. simus individuals might have been the largest land-dwelling specimens of Carnivora that ever lived in North America. Standing up on its hind legs, A. simus stood 2.4–3.4 m (8–11 ft).^[33][34] When walking on all fours, A. simus stood 1–1.67 m (3.3–5.5 ft) high at the shoulder, with the largest males being tall enough to look an adult human in the eye.^[35][34][36] The average weight of A. simus was ~625 kilograms (1,378 lb), with the maximum recorded at 957 kilograms (2,110 lb).^[37]

Sexual dimorphism

Arctodus simus skeletal at the Field Museum of Natural History, Chicago.

There is much variation in adult size among specimens- the lack of finds, sexual dimorphism, individual variation and potentially ecomorphs could be augmenting the average size of both species of Arctodus.^[2][7][44] Size differences between specimens of Arctodus simus (such as skull and long bone dimensions) have led to the suggestion of a larger northern/central subspecies (A. s. yukonensis) and southern subspecies (A. s. simus).^[37][7][31] Indeed, Kurtén hypothesized the evolution of A.s. yukonensis in the Irvingtonian, with A.s. simus emerging in the Rancholabrean.^[31][7] However, the discovery of a very large southern Arctodus simus in Florida (deep within the supposed range of A. s. simus),^[18] & possibly Rancho La Brea,^[7] and a notably small specimen from the Yukon,^[16] put doubt on this designation. Perceived ecomorphologies are possibly due to the low number of specimens, and sex-biased sampling.^[18]

For example, only one baculum (penis bone) has been recovered from over 100 giant short-faced bear sites in North America (Potter Cave). The majority of large individuals are from open sites, where usually only a few elements were recovered. In contrast, cave passages produced numerous examples of small, yet relatively complete individuals where bacula would likely be found if they had been present. Both the small size of recovered individuals and the lack of bacula from cave deposits suggest that mostly female individuals of A. simus were using caves.^[45][46][7] 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.^[18][47][29] Sexual dimorphism may also explain A. simus teeth (from multiple individuals from the same site) generally clustering into two sizes.^[7]

Skull

Male A. simus skull, photographed at the Cleveland Museum of Natural History in Cleveland, Ohio.

Members of the Tremarctinae 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, potentially giving Arctodus a more felid-like appearance. This broad snout could have housed a highly developed olfactory apparatus, or accommodated a larger throat passage to bolt down large food items, akin to spotted hyenas.^{[38][23][54][55]} 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.^[37][50] 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.^[56][57]

The orbits 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 brown bear, suggesting that stereoscopic vision was not a priority.^[40][50][58] As with Tremarctos ornatus, specimens with a large sagittal crest were likely male, whereas females had a reduced or no sagittal crest.^[2] Although there are limited samples, the middle ear bones of A. simus are proportionally larger than modern ursine bears, suggesting the species was particularly attuned to low-frequency sounds.^[59]

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). Because herbivorous carnivorans lack an efficient digestive tract for breaking down plant matter via microbial action, they must break down plant matter via extensive chewing or grinding, and thus possess features to create a high mechanical advantage of the jaw, which was present in Arctodus.^[23][60][61] However, the low mandibular condyle relative to the tooth row (and therefore potential wide gape) of Arctodus has been inferred as an adaptation for carnivory.^[31][62][63] An analysis of the mandibular morphology of tremarctine bears found that Arctodus pristinus and Arctodus simus were divergent in the dimensions of their cranial anatomy, with Arctodus simus clustering tightly with Arctotherium angustidens, suggesting a similar foraging strategy. A. simus specimens have a concave jaw, large masseter and temporalis muscles, deeper horizontal ramus and a reduced slicing dentition length, when compared to A. pristinus. However, both species of Arctodus and Arctotherium angustidens were still found to be comfortably in the "omnivorous" bear cranio-morphotype.^[50]

Dentition

Right lower second molar (M2) of Arctodus simus, from Rancho La Brea, California.

The premolars and first molars of Arctodus pristinus are relatively smaller and more widely spaced than those of Arctodus simus.^[1][18] In A. pristinus, the features of the dentition can be quite variable, particularly the M2 molar.^[2] The teeth of A. pristinus transition between undulating to acute-angled Hunter-Schreger bands while A. simus exhibited a transition between undulating to zigzag bands, demonstrating an evolution towards reinforced tooth enamel. This has been convergently evolved with giant pandas, Agriotheriin bears, and Hemicyon.^[64]

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, and the possible carnassial shear 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.^[23][65]

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.^[7][66] Additionally, while A. simus evolved from the smaller A. pristinus, their teeth remained generally the same size.^[7] 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.^[57]

Post-cranial

Limbs

A. simus compared with a human.

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.^[37][67]

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.^[51][68] 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,^[23][36] as the spectacled bear (Tremarctos ornatus), the closest living relative of Arctodus, shares various adaptations for herbivory with Arctodus.^[37]

Researchers also disagree when interpreting the humerus of Arctodus simus.^[51] 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 brown bear or in Panthera. This is due to a weak medial epicondyle and reduced development of the pronator teres muscle.^[40] Furthermore, the forelimb of Arctodus could have been in the early stages of cursorial 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 and brown bears.^[69][51]

However, some researchers argue that the epicondyles were still well developed.^[51] The shape of the elbow joint, along with an well-developed medial epicondyle which forms an angle with the condyle, and shallower olecranon fossa, would have given Arctodus a higher degree of forelimb dexterity. Originally evolved to facilitate arboreality, researchers believe that the terrestrial Arctodus (along with Arctotherium and the giant panda) retained this characteristic to assist in foraging for vegetation.^[48][23][70] Other researchers believe that this wide range of ulna rotation suggests that forearms of Arctodus were powerful and could subdue large prey.^[51]

Paws

The paws (metapodials and 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 (hallux) of Arctodus was positioned more closely and parallel to the other four digits (i.e. with straight toes, Arctodus had less lateral splaying).^[67][36]

For comparison, the manus of the spectacled bear has five digits arrayed in a shallow arc.^[71]

This theory is potentially contradicted by trackways tentatively attributed to Arctodus simus from near Lakeview in Oregon. The trackways exhibit extreme toe splaying, with three centrally aligned, evenly spaced toes at the front, and two almost perpendicular 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.^[72] 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.^[73][74] 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 trait. Absent in ursine bears, the false thumb of the spectacled bear has been suggested to assist in herbivorous food manipulation (such as extracting mast, e.g. bromelaid/palm hearts), or arboreality.^[75]

Locomotion

Paul Matheus proposed that Arctodus may have moved in a highly efficient, moderate-speed pacing 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 the tendons, and increased stride length, making Arctodus built more for endurance than for great speed.^[33][68] His calculations suggested that Arctodus likely had a top speed of 40–45 kilometres per hour (25–28 mph), and based on hyaenid proportions, would shift from singlefoot locomotion to a pace at 8.5 km/h (5.3 mph), and would begin to gallop at 18.5 km/h (11.5 mph), a fairly high speed. Based on other mammals, the optimal pace speed of Arctodus would have been 13.7 km/h (8.5 mph), which would have also been rather fast for moderate speed travel. For comparison, hyenas cross country ~10 km/h (6.2 mph).^[68]

Maturity

Examinations on a mostly full sized young individual of Arctodus simus from an Ozark cave suggest that Arctodus, like other ursids, reached sexual maturity 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, and epiphyseal plates, along with tooth eruption, have been used to determine adulthood in Arctodus.^[45][16]

Haplotype cladogram

Below is a cladogram exploring the relationships between the mitochondrial haplogroups of Arctodus simus. Other than the specimen from Chiquihuite Cave, all haplotypes form a single monophyletic clade.^[16][76]

Paleoecology

Although smaller than its descendant, Arctodus pristinus was still a relatively large tremarctine bear.^[1] Sometimes referred to as the eastern short-faced bear,^[78] A. pristinus has been found in Florida, Kansas, Maryland and Pennsylvania, South Carolina, and West Virginia in the US, and Aguascalientes in Mexico.^{[2][79][80][81]} A. pristinus is particularly well known from Florida, especially from the Leisey Shell Pit.^[82] Like A. simus and other tremarctine bears, A. pristinus had adaptations for herbivory, and was likely largely herbivorous itself.^[2]

Competition with other ursids

In the Early Pleistocene, Arctodus pristinus was much more populous the south-east of North America, whereas the black bear was more common in the north-east.^[83] A partially contemporary tremarctine bear inhabiting western North America, Tremarctos floridanus are very similar to A. pristinus in terms of size, skeletal anatomy, and dietary preferences.^[2]

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.^[2] T. floridanus was possibly being an ecological replacement of A. pristinus, with T. floridanus finds being widespread in Rancholabrean Florida and the wider southeastern United States.^[2][22][28]

The evolution of Arctodus simus, competition with Tremarctos floridanus and black bears, and possibly the transitioning of Pleistocene 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 Middle Pleistocene (300,000 BP).^[18][28] There are unreliable records of A. pristinus in South Carolina, California and Florida in the Late Pleistocene, suggesting a possible survival as a relict species in refugia until the Quaternary extinction event.^[84][79][28]

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), and Cumberland Cave, Maryland, often in association with the black bear. This suggests a close association with the biome.^[2][81]

Paleoecology

Open woodlands, such as those in the Mexican highlands, would have presented ample foraging opportunities for Arctodus.

Evolving from the smaller A. pristinus around 1.36 million years ago, scholars today mostly conclude that Arctodus simus was a colossal, opportunistic omnivore, with a flexible, locally adapted diet akin to the brown bear.^{[37][85][50][63]} If Arctodus simus wasn't largely herbivorous,^[2][23] the scavenging of megaherbivore carcasses, and the occasional predatory kill would have complimented the large amounts of vegetation consumed when available.^[37][40] Carbon-13 (δ¹³C) isotope data gathered from Arctodus specimens from Beringia, Vancouver Island, California, Mexico and Texas, indicates that Arctodus simus had a diet based on C₃ resources, primarily from mixed habitats (such as open woodlands).^{[25][26][86][87][88][89]} This includes C₃ vegetation (leaves, stems, fruits, bark, and flowers from trees, shrubs, and cool season grasses)^[23] and the browsers that fed on them, such as deer, camelids, tapir, bison and ground sloths.^[26][37]

Primarily inhabiting a range from southern Canada to central Mexico in the west, to Pennsylvania and Florida in the east,^[7][18][90] A. simus also inhabited Beringia between ~50,000 BP to ~23,000 BP. Finds today span from northern Alaska to the Yukon.^[16][18][90] Based on the wide distribution of the species, Arctodus simus inhabited a diversity of climatic conditions and environments,^[18][23][91] ranging from boreal forests and moist tundra in the north,^[92] the montane woodlands of the US Interior Highlands,^[28] the pluvial lakes and piñon–juniper woodland of the inland western USA,^[93] subtropical woodlands and mixed savannas of the south-western USA and Mexican Plateau,^[22][25][26] to the pine–oak forests of the Trans-Mexican Volcanic Belt, the boundary of the Nearctic realm.^[13]

Sometimes referred to as the bulldog bear,^[94][95] or great short-faced bear,^[96] 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.^[16] However, A. simus was relatively plentiful in western North America, with over 50% of specimens from the contiguous United States (<40,000 BP).^[28][91] Arctodus simus was integral to what has been referred to as the Camelops fauna, or alternatively Camelops/Odocoileus lucasi ("Navahoceros") fauna, a faunal province centered in western North America. The Camelops fauna was also characterized by shrub-ox, prairie dogs, dwarf pronghorns, 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.^[24][97][98]

Competition with ursine bears

Arctodus simus reconstruction at the Hot Springs Mammoth Site, South Dakota.

Black bears have inhabited North America since at least the Middle Pleistocene, whereas brown bears (along with lions, bison and red foxes) first emigrated to North America via Beringia during the Illinoian Glaciation. The North American lineage of brown bears developed around ~177,000 BP, with fossils being first recovered ~111,000 BP from East Beringia. Genetic divergences suggest brown bears first migrated south during MIS-5 (~92,000-83,000 BP) upon the opening of the ice-free corridor,^[92][99] with the first fossils being near Edmonton (26,000 BP).^[63]

The most commonly accepted ecological parallel of Arctodus simus in scientific literature is the brown bear.^[37][50][63] Both brown bears and Arctodus simus exhibit a high degree of dietary variability, and while largely herbivorous, meat can be an important dietary element to certain populations of both species.^[87] Additionally, the potential of habitual kleptoparasitism is often noted in Arctodus, with brown bears being opportunistic, curious, and regularly stealing kills from smaller predators.^[67][87]

One 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.^[42] However, this has been refuted by more recent research, as new dates establish an extended coexistence.^[92][63] On a continent-wide scale, although the brown bear and Arctodus simus were sympatric at times as brown bears spread through North America, Arctodus simus may typically have dominated competitive interactions, and displaced brown bears from specific localities. 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.^[63] Brown bears and Arctodus have been discovered together in Alaska (then Beringia) between 50,000 BP-34,000 BP,^[92] and in later Pleistocene deposits in Vancouver Island, California, Wyoming and Nevada.^[7][63]

Beringia

Isotope values (δ¹³C and δ¹⁵N) in numerous Beringian Arctodus simus specimens suggests A. simus usually occupied a higher trophic level compared with invading brown bears. While some Beringian brown bears consumed salmon as well as vegetation, reduced signatures of terrestrial meat consumption were noted. On the other hand, data from Beringian specimens of Arctodus suggest that while omnivorous, only terrestrial sources of meat were important for Beringian Arctodus.^[54]

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.^[100][101] When Arctodus went extinct in Beringia ~23,000 BP, brown bears recolonized Beringia, 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.^[92][63]

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 permanent home range, female Arctodus simus may not have (at least not to the same extent).^[76][92]

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.^[67][87] Where they overlap, brown bears are observed to take over the higher trophic niche, with lower population densities, much smaller territorial ranges, and the seasonal migration of black bears in areas where brown bears are also present.^[99]

The brown bear was a direct competitor of Arctodus simus.

Hibernation

Arctodus specimens have been particularly plentiful from caves in the montane woodlands of the US Interior Highlands, such as the Ozarks.

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, 26 of 69 Arctodus simus sites (~38%) are in caves. That greater than one-third of all sites are caves suggests a close association between this species and cave environments. Furthermore, over 70% of the smaller specimens (once assigned as the A. s. simus subspecies) are from cave deposits. Not one of the specimens assigned to the larger morph (A. s. yukonensis) is from a cave passage. Taking into account that female ursids are smaller and more prone to den in caves, it is likely that the majority of Arctodus simus from such deposits were females and may have been denning when they perished.^[45] At Riverbluff Cave, the most abundant claw marks are from Arctodus simus- some are up to 4 meters high on the cave walls. They are most abundant at the bear beds and their associated passageways, indicating a close relationship with denning.^[73]

Female specimens of Arctodus simus have been inferred to have been exhibiting maternal denning, however the expression of metabolic denning (hibernation/torpor) is unclear in Arctodus.^[46] Moreover, to date, there are no records of adults with associated offspring from caves.^[47] However, Arctotherium angustidens, a fellow giant short-faced bear, has been recovered from a cave in Argentina with offspring.^[102]

Numerous "bear" beds often preserve Arctodus simus and both Pleistocene and modern American black bears in association (U.a. amplidens and U. a. americanus)- such deposits have been found in Missouri, Oklahoma and Potter Creek Cave, California. These mixed deposits are assumed to have accumulated over time as individual bears (including Arctodus) died during winter sleep.^{[5][103][104]} Furthermore, environmental DNA suggests that Arctodus and black bears shared a cave in Chiquihuite cave, Zacatecas.^[16]

At the Labor-of-Love cave in Nevada, both American black bears and brown bears have been found in association with Arctodus simus. A study in 1985 noted that sympatry between Arctodus and brown bears preserved in caves is rare, with only Little Box Elder Cave in Wyoming and Fairbanks II site in Alaska hosting similar remains.^[23][42]

Paleopathology

Beyond carbohydrate-associated dental pathologies present in the genus,^[45][91] extensive pathologies have been preserved on the most nearly complete skeleton of Arctodus. One hypothesis suggests the Fulton County Arctodus specimen suffered from a syphilis-like (treponemal) disease, or yaws, based on the various lesions present.^{[35][105][106]} However, alternate hypotheses include tuberculosis, osteomyelitis, arthritis or a fungal infection, either singularly or in combination with other causes. The same individual records a pathological growth distorting the right humerus,^[40] with abscesses are noted between the molars and on both ulna. Hypotheses include syphilis, osteoarthritis, a fungal infection in addition to long term syphilis, or an infected wound.^[35][107] Several specimens from Fairbanks, Alaska, also exhibit either pathological growths or periodontal disease.^[7]

Arctodus pristinus

Titanis was likely the apex predator of Arctodus pristinus' habitat in Blancan Florida.

Arctodus simus

Pacific Mountain System

Arctodus simus inhabited Californian savannas for over a million years.

The Pacific Mountain System seems to represent a cradle of evolution for Arctodus simus. The earliest finds of Arctodus simus are from California, from early and middle Irvingtonian age sites such as Fairmead,^[110] Irvington,^[111] Riverside County,^[29] and Vallecito Creek.^[112]

Despite the shift to aridified environments from the Early to Late Pleistocene of the Central Valley (1.1Ma to ~15,000 BP), Arctodus simus remained consistent with the consumption of C₃ resources. This period saw the evolution from wetter mixed woodland-grassland and marsh/prairie C₃ dominated environs at Irvington and Fairmead, to the more arid, mixed C₃-C₄ savannas of the McKittrick Tar Pits. Dire wolves and Arctodus simus were consistent members of the local predator guild throughout the Pleistocene, whereas jaguars, Homotherium, Miracinonyx and Smilodon transitioned to Panthera atrox and coyotes.^[25] In addition to browsing, Arctodus could have competed with other closed habitat browser specialists such as Smilodon and Panthera atrox for deer (Cervus & Odocoileus), camelids (Hemiauchenia & Camelops), Paramylodon, and peccaries.^[25] However, specimens collected from the La Brea Tar Pits show evidence that A. simus preferred a specialized, herbivorous diet. A. simus is particularly famous from fossils found in the La Brea Tar Pits, with at least 30 individuals recovered.^[113][60] Many more finds come from across California,^[7][5] and Vancouver Island,^[63][86] where the semi-arid woodland/scrub transitioned to forest-steppe,^[114] and open grasslands/heath.^[86]

Comparatively, the Rocky Mountain System had the fewest number of specimens of Arctodus simus in western North America.^[28] 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 Arctodus simus, in addition to other contemporary high-elevation alpine fauna such as Colombian mammoths, mastodon, horses, and giant bison ≤11,400 BP (10,000 ¹⁴C BP).^{[56][24][115]} Other remains have been found from Wyoming (such as Natural Trap Cave),^[116][117] and Montana.^[118][119]

Intermontane Plateaus

A reconstruction of Rancholabrean New Mexico (White Sands).

The Intermontane Plateaus had the highest number of Arctodus simus specimens south of the ice sheets.^[28] 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.^[44] In contrast with other parts of North America, the 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, woodlands and pluvial lakes where desert exists today.^[114] The mid-Wisconsian U-Bar Cave, New Mexico, was vegetated by sagebrush, grasses, and woodlands. Notable fauna which lived alongside Arctodus simus included Shasta ground sloth, shrub-ox, pronghorns (Stockoceros, Capromeryx), Camelops, Odocoileus, horses, Lynx, puma, black bear, mountain goats, prairie dogs, and Stock's vampire bat.^[27][120] Dire wolves were also found in association with Arctodus simus, and both species are the most common carnivorans of Rancholabrean New Mexico.^[121] Beyond Utah and New Mexico,^{[122][123][124][125][126]} other important US specimens have also been found in Arizona,^[7] eastern California,^[127] Idaho,^[7] Nevada,^[128] and eastern Oregon.^{[129][130][131]}

The Intermontane Plateaus extended deep into Mexico, where tremarctine bears (Arctodus simus & Tremarctos floridanus) were dominant.^[28] Arctodus simus was limited to the Mexican plateau, which was generally occupied by tropical thorn scrub and scrub woodland.^[132][114] An Arctodus simus individual from Cedral, San Luis Potosí, inhabited closed vegetation, based on the individual's δ¹³C signature. Consuming C₃ resources, its diet may have incorporated local C₃ specialists such as tapir, llamas, camels, and Shasta ground sloth along with browsed vegetation. The site, incorporating trees, herbs and cacti, hosted an open gallery forest near grassland or scrub with a humid climate. This biodiverse forest-savanna mosaic was part of the wider mesic savanna and piñon–juniper woodland ecoregion which Arctodus inhabited in the Late Pleistocene central Mexico and southwestern USA.^{[26][133][134]} Similar highland remains have been recovered from Jalisco,^[135] Michoacán,^[132] Puebla,^[7] State of Mexico,^[136][137] and Zacatecas.^[16]

Interior Plains

The Interior Plains were composed of temperate steppe grassland,^[114] and among the specimens yielded from this region is one the largest Arctodus simus currently on record, from the banks of the Kansas river.^[138] The late Irvingtonian Doeden gravel pits in Montana preserves an open grassland habitat, with riparian woodlands, and likely some shrublands.^[139] Arctodus simus co-existed with ground sloths (Megalonyx, Paramylodon), Pacific mastodon, camels, and Bootherium.^{[140][141][30]} As bison were yet to migrate into North America, Columbian mammoths and horses dominated these early Illinoian grasslands.^[142]

Arctodus also roamed the southern mixed grasslands of 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. Mammoth site, South Dakota),^[143] the southern plains were a parkland with riparian hackberry forests, and large expanses of mixed grass prairie grasslands grading into wet meadows, with limited seasonality. In the south (Lubbock Lake, Texas), this fauna was joined by Smilodon, dire wolves, grey fox and red fox, preying upon prairie dogs, horses (Equus & Haringtonhippus), peccaries, Odocoileus, Capromeryx, Bison antiquus and Holmesina.^[143][144] Beyond Texas,^[145] Arctodus has also been found in Iowa,^[146] Kansas,^[7][147] Nebraska,^[7] and southern Canada (Alberta & Saskatchewan),^{[148][149][150]} which when unglaciated, would have formed a tundra ecosystem with an ice-free corridor to Beringia.^[151]

In the lowlands of the eastern Interior plains, the plains transitioned to closed habitat. At the terminal Pleistocene Sheriden Cave, Ohio, a mosaic habitat consisting of marsh, open woodland, and patchy grassland was home to Arctodus simus, Cervalces scotti, caribou, peccaries (Platygonus, Mylohyus), giant beaver, porcupine, and American pine marten.^[152][153] Similar remains have been found in Indiana,^[35] and Kentucky.^[154][155]

Eastern USA

Lake Rousseau, Florida, is the south-easternmost locality which Arctodus simus is known to have inhabited.

Compared to other regions, Arctodus simus was relatively rare in eastern North America.^[18][28][91] To the north, the Appalachian Highlands were dominated by taiga.^[114] Post-LGM Saltville, Virginia, was a mosaic of grassy/herb laden open areas intermixed with open canopy boreal woodlands (oaks, pines, spruce, birch, firs) and marshes. Inhabiting in this C₃ resource dominated environment were Arctodus simus, mastodon, (southernmost) woolly mammoths, Bootherium, horses, caribou, Megalonyx, dire wolves, beavers, Cervalces, and a variety of warm-adapted reptiles, suggesting that a more mesic and less seasonal climate. Heavy bone damage on a mammoth carcass by both dire wolves and Arctodus suggests a potentially competitive scavenging relationship ^[162][52] Additional remains have been found in Virginia,^[47] and Pennsylvania.^[7][163]

To the south, the Atlantic Plains covered a great expanse of lowland, from the open deciduous woodlands of the 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 wolves, jaguars, ground sloths (Paramylodon, Megalonyx), llamas (Palaeolama, Hemiauchenia), Vero's tapir, giant beaver, capybara, Holmesina, horses, Bison antiquus, mastodon, Colombian 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.^[18] Additional finds of south-eastern Arctodus simus are from Alabama,^[164] Arkansas,^[165] Mississippi,^{[166][167][168]} South Carolina,^[169] and Texas.^[57][170]

Beringia

Arctodus is suggested to have had a kleptoparasitic relationship with Beringian wolves, akin to modern wolves and brown bears.

Mostly isolated by the Cordilleran and Laurentide ice sheets, Beringia is considered ecologically separate to the rest of North America, being largely an extension of the Eurasian mammoth steppe.^[171] However, the occasional opening of an ice-free corridor, and the migration barrier of the Beringian gap, meant that Eastern Beringia (Alaska and the Yukon) supported a unique assemblage of fauna, with many endemic North American fauna flourishing.^[172] This mostly open and treeless steppe-tundra, dominated by grasses, sedges, Artemisia spp., and a range of other forbs had a cold, dry climate, which prevented glaciation. 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.^[92][63] However, additional undated remains may be of Sangamonian age.^[173] The largest known skull of A. simus was recovered from the Yukon, and may represent the largest specimen known.^[40][174]

The 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. Cave lions, bears (Ursus arctos and Arctodus simus), and Beringian wolves made up the megafaunal predator guild.^[175][176] 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 dominates today) was the preferred habitat of Arctodus in Beringia.^[87][175]

Additionally, upon the flooding of the Bering Strait and expansion of moist tundra and peatlands in Eastern Beringia during MIS-3, lions, brown bears and Homotherium went regionally extinct ~35,000 BP, whereas wolves and Arctodus persisted. Simultaneously, most megafaunal herbivores in Beringia experienced population bottlenecks, whilst 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 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.^{[92][87][100][101][177]} The idea that Arctodus had a kleptoparasitic relationship with wolves and Homotherium in Beringia has been explored,^[87] with the additional possibility that Arctodus successfully competed against brown bears and Homotherium for access to caribou pre-LGM.^[178]

The local extinction of Arctodus in Beringia ~23,000 BP (possibly due to sharp climatic cooling associated with Heinrich Event-2),^[92][63] was much earlier than in other parts of its range. While recolonized by cave lions and brown bears, Arctodus did not repopulate Beringia once the ice-free corridor to the south re-opened later in the Pleistocene.^[92][179]

Carbon isotope studies

Evidence from the carbon isotope values of an Arctodus simus specimen from Cedral, San Luis Potosí, México, showed that this individual fed upon C₃ resources. In fact, that Arctodus individual had the strongest δ¹³C value of its local fauna. Owing to having only one sample of Arctodus simus from Cedral and the lack of nitrogen isotopic values, the study found it difficult to infer whether Arctodus simus was consuming animal protein. Arctodus' carbon isotope value did not overlap with, but was closest to values from the tapir and Hemiauchenia.^[26]

For specimens from inland California (Fairmead Landfill) from the Middle Pleistocene, a 2012 study proposed that Arctodus simus consumed Colombian mammoth, and large ungulates.^[185] However, the author republished in 2015 with colleagues, recalculating Arctodus' δ¹³C values to be closest to C₃ vegetation consuming Cervus and Mammut, although again no nitrogen isotope values were recovered.^[25] In the later Californian McKittrick Tar Pits, Arctodus simus had a diet which likely included deer and tapir, similar to the one inferred for the Cedral individual.^[26]

Alaskan specimens were thought to also largely predate upon similar megafauna as proposed for the Fairmead individuals in the 2012 study,^[67] but nitrogen & carbon isotope data suggests reindeer, muskox and possibly fellow predators and their kills, were regularly consumed.^[87] Arctodus' nitrogen-15 levels are higher in the Yukon, suggesting that Arctodus possibly occupied an even higher trophic level than other Arctodus in Beringia. However, this contrast likely reflects subtle differences in the isotopic composition of primary producers in the region.^[54][186] A specimen from Cowichan Head, Vancouver Island, had isotopes suggesting a terrestrial diet at a relatively high trophic level, aligning with Beringian specimens.^[86]

A single find from the Channel Islands of California with nitrogen isotope signatures indicating a ~19% consumption rate of seals (along with bison and camels) bolsters the suggestion that A. simus would have had a flexible diet across its range. The Arctodus fossil in the Channel Islands was likely transported post-mortem from the mainland; the partial reliance on marine resources has been suggested to be as a result of a competitive carnivore guild in mainland California. The marine signal was in between island foxes and bald eagles, most closely resembling Late Pleistocene California condors.^[88] A specimen from Friesenhahn Cave, Texas, had a nitrogen-15 sample closest to striped skunks.^[89]

Bone damage

Arctodus may have found young proboscideans to be suitable prey.

The bite marks found on many bones of ground sloths (Northrotheriops texanus) and young proboscideans 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.^[1]

Arctodus simus has been found in association with proboscidean remains near Frankstown, Pennsylvania (juvenile mastodon), and at The Mammoth Site, 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).^[58][163] On the other hand, a woolly mammoth specimen from Saltville, Virginia was likely scavenged on by Arctodus simus, as evidenced by a canine gouge through the calcaneus.^[52] 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.^[56] A mastodon humerus from the Snowmastodon site in Colorado bears tooth marks also suggested to be from Arctodus.

Importantly, the canines of Panthera atrox overlap in size with Arctodus simus, complicating the identification of tooth marks.^[52] However, this is not to discredit all tooth marks attributed to Arctodus, as damaged bones from an Arctodus den site in Alaska suggest that Arctodus transported megafaunal longbones back to a cave-like den and chewed on them,^[187] at a time when lions had a limited overlap with Arctodus in Beringia.^[92][91] Furthermore, a perforated peccary ilium from Sheriden Cave has also been hypothesized as being scavenged by Arctodus simus.^[153] Bone damage on a cranial fragment (and possibly the humerus) of an Arctodus individual on Vancouver Island has been attributed to cannibalism, as Arctodus may have been the only large terrestrial carnivoran in the area.^[63][99]

Beringia

Analysis of bones from Alaska showed high concentrations of nitrogen-15, a stable nitrogen isotope accumulated by carnivores. Although few specimens exist, there is currently no evidence of the same carbohydrate-related dental pathologies evident in southern populations of Arctodus simus.^[91] Based on this evidence, A. simus was suggested to have been more carnivorous in Beringia than the rest of North America (with a preference for herbivores which consumed C₃ vegetation, particularly caribou).^[87][178] 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.^[91]

A 2015 study suggests that caribou could not account for the high levels of carbon-13 and nitrogen-15 in some Arctodus individuals in Beringia. The study suggests that the consumption of tundra muskox, which sometimes express high proportions of these isotopes, and possibly other predators in its Beringian range, may explain the data.^[87]

It would be reasonable to assume that meat and bone marrow were likely to be the primary food resources for some northern populations of A. simus, in which the survival during the cold season could have depended on the regular scavenging of ungulate carcasses, as is the case with Alaskan brown bears.^[37] Ultimately, an opportunistic foraging strategy including up to 50% vegetation, and the meat of reindeer, muskox, carrion, and possibly some predators, is consistent with the isotopic data and the conclusions of the ecomorphological studies.^[87]

Rebuttal

Clues from Arctodus' dentition, such as the absence of molar damage associated with processing bone, dental cavities, and the lack of specialisation in the canines, discourages a hyper-carnivorous interpretation of Arctodus.

The kleptoparasite hypothesis has been repeatedly challenged. The short, broad rostrum of Arctodus is a characteristic also shared with the sun bear and the spectacled bear, which are both omnivorous.^[37] Specialized scavengers like hyenas show distinctive patterns of molar damage from cracking bones. Based on lack of "bone-cracking" wear in specimens from Rancho La Brea, researchers in 2013 concluded that Arctodus simus was not a specialized scavenger. Of living bears, this population of A. simus showed the most similar tooth wear patterns to its closest living relative, the spectacled bear, which can have a highly varied diet ranging between omnivory to almost pure herbivory.^[23][60]

Additionally, severe tooth crown fractures and alveolar infections were found in the South American giant short-faced bear (Arctotherium angustidens). These were interpreted as evidence of feeding on hard materials (e.g. bones), which could tentatively indicate for these bears the regular scavenging of ungulate carcasses obtained through kleptoparasitism. However, such dental pathologies were not observed in various specimens of A. simus, other than the strong wear facets of old individuals.^[37] Instead, recovered dental damage (incisor wear, dental calculus & cavities) is herbivorous in origin.^[45][91] Moreover, researchers in 2015 reviewing links between canine breakage, microwear texture patterns and carnivorans from La Brea found that A. simus consumed foods softer and tougher than black bears and polar bears, avoided hard/brittle foods, and reaffirmed affinities between A. simus and modern, largely herbivorous spectacled bears.^[181]

Furthermore, the relative lack of Arctodus remains at predator traps such as the La Brea Tar Pits, suggests that Arctodus did not compete for carcasses.^[91] Although La Brea has produced more Arctodus simus specimens than any other site (presumably due to the quality of preservation with tar), they are only 1% of all carnivorans in the pits,^[181] which is a similar rate to brown bears and black bears, both omnivorous ursids which lean towards herbivory.^[192] A similar rate (~0.9%) of relative abundance was calculated for Arctodus compared to other megafauna at the Natural Trap Cave in Wyoming by 1993.^[193] Additionally, isotope analyses of Beringian Arctodus specimens suggest that Arctodus had a low consumption rate of horses and mammoths in Beringia, despite those species making up ~50% of the available biomass in Beringia.^[87] Further evidence comes from the evolution of brain size relative to body size- bears with high caloric diets and which do not exhibit dormancy showed a weak but significant correlation with bigger relative brain size. Arctodus simus plotted in between the likely hypercarnivorous Cephalogale, and the almost exclusively herbivorous Eurasian cave bear and Indarctos, suggesting omnivory.^[194]

Beringia

Beringia during the Last Glacial Maximum

Humans migrated to North America via the Siberian mammoth steppe, arriving at Eastern Beringia (Alaska and the Yukon). However, the migration was halted at the North American Ice Sheet, which separated Beringia and southern North America for most of the Late Pleistocene.^[203] Both humans and Arctodus are first dated to ~50,000 BP in Beringia, both from sites in the Yukon, and co-existed until Arctodus 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 lions, brown bears and saber-tooth cats.^[92] Important sites of pre-LGM human occupation in Beringia include Old Crow Flats & the Klondike,^[204][205] Kuparuk River Valley,^[206] & the Bluefish Caves.^[207][208]

Contiguous North America

The human colonization of North America south of the ice sheets further disproves the idea that Arctodus was a migration barrier. Pre-LGM sites across the Americas such as Chiquihuite Cave, Valsequillo, El Cedral, Santa Elina, Calico, Hartley Mammoth Site, and Pendejo Cave suggest that humans co-existed with Arctodus for many thousands, if not tens of thousands of years. This extensive overlap with Arctodus across North America puts significant doubt to the migration barrier hypothesis.^{[209][210][211][212][213]}

The earliest universally accepted pre-Clovis site south of Beringia are the White Sands footprints in New Mexico, dated to ~22,000 BP. Younger dates from Saltville, La Sena, Meadowcroft, Topper, Triquet Island, Cactus Hill, and Buttermilk Creek in the USA, and Monte Verde in Chile, further solidify a rapid human expansion across the Americas despite competitive pressure from North America's predators.^[63][202]