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ABSTRACT -
The South American giant short-faced bear (Arctotherium angustidens Gervais and Ameghino, 1880) is one of five described Arctotherium species endemic to South America and it is known for being the earliest, largest, and most carnivorous member of the genus. Here we report an extraordinarily large A. angustidens individual exhumed from Ensenadan sediments (early to middle Pleistocene) at Buenos Aires Province, Argentina. Based on overall size, degree of epiphyseal fusion, and pathologies, this bear was an old-aged male that sustained serious injuries during life. Body mass of the bear is estimated and compared to other ursid species based on a series of allometric equations. To our knowledge, this specimen now represents the largest bear ever recorded. In light of this discovery, we discuss the evolution of body size in Arctotherium (from large-to-small) and compare this to bears that exhibited different evolutionary trajectories. We suggest that the larger size and more carnivorous nature of A. angustidens, compared to later members of the genus, may reflect the relative lack of other large carnivores and abundance of herbivores in South America just after the Great American Biotic Interchange.
INTRODUCTION
Arctotherium angustidens Gervais and Ameghino, 1 880 belongs to the Tremarctinae subfamily (Carnivora: Ursidae), a diverse and endemic group of American bears that includes small to gigantic species recorded from late Miocene to recent times (Fig. 1; see Kurten, 1966, 1967; Tedford and Martin, 2001; Soibelzon, 2004a; Soibelzon et al., 2005; Schubert et al., 2010, Schubert, 2010). Of these, 1) Plionarctos Frick, 1926 is recorded from the late Miocene to the early Pliocene of North America with two described species, P. edensis Frick, 1926 and P. harroldorum Tedford and Martin, 2001; 2) Aretodus Leidy, 1854 of North America is known from late Pliocene until the end of the Pleistocene and contains two species, Aretodus pristinus Leidy, 1854 and Aretodus sitnus (Cope, 1879); 3) Arctotherium Burmeister, 1879 comprises five South American species, all recorded during the Pleistocene; and 4) Tremarctos Gervais, 1855 includes two species, T floridanus Gildey, 1928 from the late Pliocene and Pleistocene of North America and the only living tremarctine, T. ornatus (Cuvier, 1825) of South America (Soibelzon, 2004a; Soibelzon et al, 2005). Aretodus and Arctotherium, sister taxa commonly known as giant shortfaced bears, contain...