Template:Infobox MammalArctotherium is an extinct genus of South American short-faced bears within Ursidae of the late Pliocene through the end of the Pleistocene. Their ancestors migrated from North America to South America during the Great American Interchange, following the formation of the Isthmus of Panama. They were endemic to South America, living from ~2.0–0.01 Million years ago, existing for approximately 1.99 million years. Their closest relatives were the North American short-faced bears of genus Arctodus (A. pristinus and A. simus). The closest living relative would be the spectacled bear (Tremarctos ornatus).
Arctotherium was named by Hermann Burmeister in 1879. It was assigned to Tremarctinae by Krause et al. 2008. A humerus of A. angustidens from Buenos Aires indicates that the big males of this species would have weighed 2.2 tonnes, standing 14 feet tall, making it the largest bear and largest carnivorous land mammal known. In contrast to their North American cousins, South American short-faced bears showed a trend of declining size and carnivory over time. This has been attributed to increased competition from other, later-arriving or evolving carnivorans, like jaguars or lions, following the early dispersal of short-faced bears to South America. (The North American carnivorans that invaded South America, including short-faced bears and Smilodon, quickly dominated the predatory niches formerly occupied by South America's native metatherian sparassodont and avian phorusrhacid carnivores.)
Description
Actotherium was the largest bear, known at this time. Compared with Arctodus simus, it had a more powerful physique with the same linear dimensions and was also very much robustly build. Representatives of this species reached 4.7 m in length and weighed about 2,200 kg. . The calculations were made on the basis of seven different measurements of animal bones. Arctotherium angustidens lived during the Pleistocene, the Argentine plains. At one time (2 million - 500 thousand years ago) it was the largest predator on the planet. It appeared to have been omnivorous, but large herbivores were the basis of its diet. Some competition it could make is with the sabre-toothed cats, who were also at the top of the food chain. The closest relative of A. angustidens is that of the living spectacled bear, which is significantly lost in size compared to its distant ancestor.