When the polar bear was originally documented, two subspecies were identified: the American polar bear ("Ursus maritimus maritimus") by Constantine J. Phipps in 1774, and the Siberian polar bear ("Ursus maritimus marinus") by Peter Simon Pallas in 1776. This distinction has since been invalidated. One alleged fossil subspecies has been identified: "Ursus maritimus tyrannus", which became extinct during the Pleistocene. "U.m. tyrannus" was significantly larger than the living subspecies. However, recent reanalysis of the fossil suggests that it was actually a brown bear.