Ursus maritimus tyrannus is an extinct subspecies of polar bear, known from a single fragmentary ulna found in the gravels of the Thames at Kew Bridge, London. It was named by the Finnish paleontologist Björn Kurtén in 1964 and is interpreted to represent a relatively large subadult individual: the ulna is estimated to have been 48.5 cm (19 in) long when complete. For comparison, modern subadult polar bear ulnae are 36–43 cm (14–17 in) long. An unpublished reinvestigation of the fossil suggests that the fossil is actually a brown bear.