Arctodus is an extinct genus of short-faced bears that inhabited North America during the Pleistocene (about 2.58 Mya until 12,800 years ago). The two recognized species are 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. A. pristinus was largely restricted to the Early Pleistocene of the eastern United States, 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 carnivorans that has ever existed. 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 A. simus browsed on C3 vegetation and consumed browsing herbivores such as deer, camelids, and tapir. The species preferred temperate open woodlands, but was adaptable, taking advantage of many habitats and feeding opportunities.
Arctodus belongs to the Tremarctinae subfamily of bears, which are endemic to the Americas. Of these short-faced bears, Arctodus was the most widespread in North America, and was present in the Blancan, Irvingtonian and Rancholabrean faunal stages. A. pristinus went extinct around 300,000 years ago, with A. simus disappearing about 12,800 years ago in the Late Pleistocene extinctions. The cause behind these extinctions is unclear, but in the case of A. pristinus, was likely due to climate change and competition with other ursids, such as the black bear and Tremarctos floridanus. A. simus likely went extinct due to ecological collapse disrupting the vegetation and prey on which it relied.
Taxonomy
Arctodus was first described by Joseph Leidy in 1854, with finds of A. pristinus from the Ashley Phosphate Beds, Charleston County, South Carolina. The scientific name of the genus, Arctodus, derives from Greek, and means "bear tooth". The first fossils of A. simus were found in the Potter Creek Cave, Shasta County, California, by J. A. Richardson in 1878, and were initially described as Arctotherium simum by Edward Drinker Cope in 1879. Historically, all specimens were grouped together under A. pristinus until a revision by Björn Kurtén in 1967. The (lost) holotype and neotype of A. pristinus are both from South Carolina, with the specimen from Potter Creek Cave now considered the holotype for A. simus.
In the 19th and early 20th centuries, specimens of Arctodus were occasionally referred to Arctotherium, and vice versa. However, Arctodus and Arctotherium did not overlap biogeographically, with the southernmost specimens of Arctodus being from the Trans-Mexican Volcanic Belt, and the northernmost records of Arctotherium (A. wingei) being from the Yucatán Peninsula. Other early researchers believed Arctodus to be a sister lineage of the agriotheriin Indarctos. 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 Mya.
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 A. simus specimens. Beyond standard differences between tremarctine and ursine bears, A. simus has a more anterior protocone and extended enamel ridge forming a shearing blade on the maxillary P4. The molars are also shorter and broader in Arctodus than brown bears.
Evolution
Arctodus belongs to the subfamily Tremarctinae, which appeared in North America during the late Miocene epoch in the form of Plionarctos. The medium-sized Arctodus pristinus, Tremarctos floridanus, and Arctotherium species evolved from Plionarctos in the Blancan faunal stage of North America. The genetic divergence date for Arctodus is between 5.5 million and 4.8 million years ago, around the Miocene-Pliocene boundary, when tremarctine bears, along with other ursids, experienced an explosive radiation in diversity, as C4 vegetation (grasses) and open habitats dominated. The world experienced a major temperature drop and increased seasonality, and a faunal turnover, which extinguished 70–80% of North American genera.
Arctodus first appears in the early Late Blancan faunal stage (Gelasian), with the earliest finds being A. pristinus from the Kissimmee River 6 and Santa Fe River 1 sites in Florida, dated from 2.6-2.3 Mya, and Arctodus sp. from 111 Ranch (~2.6 Mya) and San Simon (about 2.2 Mya) in Arizona, and La Union in New Mexico (Mesilla Fauna B, 2.2-1.8 Mya). This appearance coincides with the start of the Quaternary glaciation, and the second phase of the great American biotic interchange, with the first records of the main South American faunal wave into the United States. A. pristinus was mostly restricted to the more densely forested thermal enclave in eastern North America. with the greatest concentration of fossils being in Florida.
During the early Irvingtonian faunal stage (1.9 Mya to 850,000 BP), a western population of A. pristinus evolved into the enormous A. simus. Early Irvingtonian sites mostly come from California, being from Elsinore, the Irvington type locality, and possibly El Casco, with additional footprints from Oregon. However, anomalous records from the late Blancan faunal stage of southern California hail from Anza-Borrego (Palm Spring Formation, ~2Mya), and possibly Murrieta and Victorville. Correspondingly, A. simus is most plentiful from western North America, albeit preferring mixed habitat such as temperate open woodlands.
Irvingtonian age specimens of A. simus are particularly sparse, with additional later Irvingtonian remains being from California, Kansas, Nebraska, Montana and Texas, and potentially Irvingtonian trace fossils from Missouri. However, A. simus became a pancontinental species in the proceeding Rancholabrean faunal stage (190,000 BP - 12,000 BP), sharing that distinction with the American black bear. Despite A. simus large temporal and geographic range, fossil remains are comparatively rare (109 finds as of 2010, in otherwise well-sampled localities).
Although both Arctodus species certainly co-inhabited North America during the Middle Pleistocene (A. pristinus became extinct about 300,000 BP), no robust evidence of overlap or competition has been recovered, with both species appearing to establish largely separate ranges. However, anomalous Late Pleistocene records of A. pristinus suggest co-existence in the Trans-Mexican Volcanic Belt of Mexico. In the United States, 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).
The mitochondrial genome of Tremarctos ornatus (the modern spectacled bear) is more closely Arctotherium than Arctodus. However, a preliminary investigation of the nuclear DNA of short faced bears suggests either an extensive history of hybridization between Arctodus and Tremarctos in North America, or hybridization between Tremarctos and Arctotherium (likely A. wingei) in either Central America or South America. Regardless, no evidence of hybridization between Arctodus and ursine bears was found despite an overlap with black bears and brown bears in Pleistocene North America.
Description Size
Arctodus pristinus Around the size of grizzly bears, 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 around . The dimensions of some individuals from Port Kennedy Bone Cave and Aguascalientes, though, suggest that northern and western A. pristinus populations 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 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 about , with the maximum recorded at .
Sexual dimorphism
Arctodus has been described as very sexually dimorphic; A. simus males were sometimes twice as large as females. Akin to its relative, the spectacled bear (with male spectacled bears being 30-40% larger than females), the larger, massive Arctodus individuals are considered male, particularly older males, with the smaller, more lightly built individuals being females. As with T. ornatus, specimens with a large sagittal crest were likely male, whereas females had a reduced or no sagittal crests. A 2025 mitochondrial DNA study affirmed sexual dimorphism in A. simus, with sexually dimorphic size classes and a uniform population being described from at least 31 individuals recovered from 28 sites across the United States and Canada.
Studies In a 2010 study, the mass of six A. simus specimens was estimated; half of the specimens weighed between , with a mean weight around 850 kg, suggesting larger (male) specimens were probably more common than previously thought. The other (female) specimens were calculated to be less than . The weight range calculated from all examined specimens was between 957 kg and . A 1999 study by Per Christiansen calculated a mean weight of from seven male A. simus limb bones, suggesting large males weighed between . Hypothetically, the largest A. simus males may have approached , or even . However, a 2006 study argued that the maximum size of Arctodus was roughly , based on the largest known skull.
Anatomy The two species of Arctodus are differentiated not only by size, but also by the shorter snout, greater prognathism, more robust teeth, and longer limbs of A. simus, and the relative proportions of each species' molars and premolars. A. pristinus is distinguished from A. simus by smaller, narrower, and less crowded teeth. The morphologies of both species are otherwise very similar. As a result, differentiating A. simus from A. pristinus can be difficult, as male individuals of A. pristinus can overlap in size with females of A. simus. Arctodus simus superficially resembled living hyaenids 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, Chicago. Historically, individual specimens of A. simus from Missouri and Texas have been suggested to represent new species, due to limb and cranial characteristics respectively.
Skull