Arctodus
Arctodus is an extinct genus of short-faced bears that inhabited North America during the Pleistocene. The two recognized species are the lesser short-faced bear and the giant short-faced bear. 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, but was restricted to the Pleistocene. 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, 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 holotype and neotype of A. pristinus are both from South Carolina.In the 19th and early 20th centuries, specimens of Arctodus were occasionally referred to Arctotherium, and vice versa. Today, however, the genera are not considered to have overlapped, with the closest point of contact being México, with the giant Arctodus simus in Valsequillo, Puebla, and the smaller Arctotherium wingei in 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. 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 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 age 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 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, 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 and San Simon in Arizona, and La Union in New Mexico. 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, a western population of A. pristinus evolved into the enormous A. simus, although an anomalous Blancan record of A. simus exists from the Palm Spring Formation. 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. Although both Arctodus species co-inhabited North America for at least half a million years during the Middle Pleistocene, no direct evidence of overlap or competition has been found in the fossil record as of yet, as both species established largely separate ranges.
Irvingtonian age specimens of A. simus are particularly sparse. Finds are mostly from California, with additional remains from Kansas, Nebraska, Montana and Texas, and potentially Irvingtonian trace fossils from Missouri, and Oregon. However, A. simus became a pancontinental species in the Rancholabrean faunal stage, sharing that distinction with the American black bear. Despite A. simus large temporal and geographic range, fossil remains are comparatively rare.
The mitochondrial genome of Tremarctos ornatus 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 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.