Some authors consider Bison occidentalis to be an intermediate species between Bison antiquus and modern American bison (Bison bison), spanning from the Late Pleistocene-Early Holocene and including remains from central North America.[3] However, as the type specimen of B. occidentalis is from Alaska, other authors have argued that the species should be restricted to remains that region, with suggestions that true B. occidentalis evolved locally in the region from steppe bison (Bison priscus) around 13,000 years ago.[4] Due to this uncertainty, specimens that likely represent intermediates between B. antiquus and B. bison in central North America have been referred to as B. "occidentalis".[3] Some authors have suggested that B. bison descends from hybridization between true B. occidentalis migrating into central North America from Alaska and central North American B. antiquus, though this suggestion has been rejected by other authors who suggest that B. bison evolved from B. antiquus alone.[4]
^C. G Van Zyll de Jong , 1986, A systematic study of recent bison, with particular consideration of the wood bison (Bison bison athabascae Rhoads 1898), p.53, National Museum of Natural Sciences