A gigantic Short-faced bear (Arctotherium angustidens Gervais and Ameghino, 1880, Ursidae) from the early Pleistocene of La Plata, Buenos Aires, Argentina.

SOIBELZON, L.H.; SCHUBERT B.W.

Río de Janeiro

Simposio; VII Simposio Brasileiro de Paleontología de Vertebrados; 2010

Arctotherium angustidens Gervais and Ameghino (the giant South American short-faced bear) belongs to the Tremarctinae subfamily, a group of bears (Ursidae, Mammalia) that have inhabited the Americas since the Late Miocene. Tremarctines display a wide range in body size, from gigantic (e.g., Arctotherium angustidens and Arctodus simus (Cope)) to medium-sized (e.g., Arctotherium tarijense Ameghino and Arctodus pristinus Leidy) and relatively small (e.g., Arctotherium wingei Ameghino and Tremarctos ornatus (Cuvier)). The South American fossil bears are all considered to be in the genus Arctotherium Burmeister, which is divided into five species that span from Early to Late Pleistocene. Two evolutionary patterns are described for this genus: 1) a body size reduction tendency; and 2) a dietary change from a carnivorous omnivore to an omnivore-herbivore.  In this context we report here an extraordinarily large Arctotherium angustidens exhumed from Ensenadan sediments (Early to Middle Pleistocene) during the building of a hospital in La Plata, Buenos Aires (Argentina).   To our knowledge, this specimen represents the largest bear ever recorded.  Further, we compare the body size tendency of South American fossil bears with those described for others Pleistocene bears and discuss possible causes of the observed differences.