Arctodus is an extinct genus of short-faced bear that inhabited North America during the Pleistocene (~2.5 Mya until 12,000 years ago). There are two recognized species: the lesser short-faced bear (Arctodus pristinus) and the giant short-faced bear (Arctodus simus). Both species are relatively rare in the fossil record. 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. Of these species, A. simus was larger, is known from more complete remains, and is considered one of the most charismatic of North America's megafauna.
Today considered to be an enormous omnivore, Arctodus simus is believed to be one of the largest known terrestrial mammalian carnivorans 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, camelids, and tapir. A. simus preferred open woodlands but was an adaptable species, 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. However, both species went extinct in the Pleistocene. A. pristinus went extinct around 300,000 years ago, with A. simus disappearing ~12,000 years ago in the Quaternary extinction event, being one of the last recorded megafauna to go extinct in North America. 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 and Tremarctos floridanus. 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 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, California, by J. A. Richardson in 1878, and were initially described as 'Arctotherium simum by Edward Drinker Cope in 1879. The most nearly complete skeleton of A. simus found in the US was unearthed in Fulton County, Indiana; the original bones are in the Field Museum of Natural History, Chicago.
In the 19th and early 20th centuries, specimens of Arctodus were occasionally referred to Arctotherium, and vice versa. However, today neither genera are considered to have overlapped, with the closest point of contact being México; the giant Arctodus simus in Valsequillo, Puebla, and the smaller Arctotherium wingei in the Yucatán Peninsula. Conversely, fossils of Arctodus pristinus are often confused with the similarly sized, partially contemporaneous short-faced bear, Tremarctos floridanus. 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.
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, a genus considered ancestral to Tremarctinae. Plionarctos gave way to the medium-sized Arctodus pristinus, Tremarctos floridanus and Arctotherium sp. in the Blancan age of North America, with the genetic divergence date for Arctodus being ~5.5 million years ago. Correspondingly, around the Miocene-Pliocene boundary (~5 Ma), 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.