Temporal range: Early Pleistocene-Early Holocene, 2.0–0.011 Ma
|Life restoration of Arctotherium bonariense|
Arctotherium is an extinct genus of South American short-faced bears within Ursidae 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 during the late Pliocene. The oldest remains are from the Ensenadan epoch within the early-middle Pleistocene 1.2 Mya. Their closest relatives were the North American short-faced bears of genus Arctodus (A. pristinus and A. simus). The closest living relative is the spectacled bear (Tremarctos ornatus).
Arctotherium was named by Hermann Burmeister in 1879. It was assigned to the Tremarctinae by Krause et al. 2008. A specimen of A. angustidens from Buenos Aires shows an individual estimated, using the humerus, to weigh between 983 and 2,042 kg (2,167 and 4,502 lb), though the authors consider the upper limit as improbable and say that 1,588 kg (3,501 lb) is more likely. It is still possibly the largest bear ever found and contender for the largest carnivorous land mammal known to science. In contrast to their North American relatives, 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, such as jaguars, lions or Smilodon, 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.)
Arctotherium could have hunted large prey such as ground sloths, glyptodonts, litopterns, notoungulates, horses, tapirs, camels, and young and/or weak gomphotheres using its massive size and strength as well as eating plants.
- Soibelzon, L. H.; Tonni, E. P.; Bond, M. (October 2005). "The fossil record of South American short-faced bears (Ursidae, Tremarctinae)". Journal of South American Earth Sciences 20 (1–2): 105–113. doi:10.1016/j.jsames.2005.07.005.
- Krause, J.; Unger, T.; Noçon, A.; Malaspinas, A.; Kolokotronis, S.; Stiller, M.; Soibelzon, L.; Spriggs, H.; Dear, P. H.; Briggs, A. W.; Bray, S. C. E.; O'Brien, S. J.; Rabeder, G.; Matheus, P.; Cooper, A.; Slatkin, M.; Pääbo, S.; Hofreiter, M. (2008-07-28). "Mitochondrial genomes reveal an explosive radiation of extinct and extant bears near the Miocene-Pliocene boundary". BMC Evolutionary Biology 8: 220. doi:10.1186/1471-2148-8-220. PMC 2518930. PMID 18662376.
- Dell'Amore, C. (2011): Biggest Bear Ever Found, National Geographic News, Published February 3, 2011
- Soibelzon, L. H.; Schubert, B. W. (January 2011). "The Largest Known Bear, Arctotherium angustidens, from the Early Pleistocene Pampean Region of Argentina: With a Discussion of Size and Diet Trends in Bears". Journal of Paleontology (Paleontological Society) 85 (1): 69–75. doi:10.1666/10-037.1. Retrieved 2011-06-01.
- Hodge, A.-M. (2011-03-31). "Updated Range of Immensity for Arctotherium: New Record for Largest Known Bear". nature.com blogs. Nature Publishing Group. Retrieved 2011-06-01. External link in