Arctotherium


Arctotherium is an extinct genus of Pleistocene South American short-faced bears within Ursidae. 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 confirmed remains are those of A. angustidens from Buenos Aires, Argentina, dating to the Ensenadan epoch, 0.98 to 1.76 Ma old, within the Early to Middle Pleistocene, with a tooth possibly belonging to Arctotherium dating about 2.588 Mya. They are closer genetically to the spectacled bear, than to Arctodus of North America, implying the two extinct forms evolved large size in a convergent manner, perhaps to facilitate dominating other carnivores in the competition for the biggest carcasses. The northernmost species, A. wingei, known from Venezuela in South America, apparently invaded Central America and reached as far north as the Yucatán.

Description

Arctotherium was named by Hermann Burmeister in 1879. A specimen of A. angustidens from Buenos Aires shows an individual estimated, using the humerus, to weigh between, though the authors consider the upper limit as improbable and say that is more likely. An estimated standing height for Arctotherium is 3.4–4.3 meters. It would still make the genus the largest bear ever found and contender for the largest carnivorous land mammal known.
Its large size has been attributed to increased competition from other, later-arriving or evolving carnivorans, such as jaguars, 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, probably quickly dominated the predatory niches formerly occupied by the native typical South American groups such as metatherian sparassodont and avian phorusrhacid carnivores.

Ecology

There is evidence to suggest that Arctotherium had dens.

Distribution

Fossils of Arctotherium have been found in:
;Blancan
;Quaternary