Ursus maritimus tyrannus (Latin for tyrant sea bear) was recognized as the first subspecies of polar bear in 1964 by
Björn Kurtén. Fossils were found at the gravels of the Thames at Kew Bridge, London, England.
It is believed to have evolved between 250,000 and 100,000 years ago when a large number of Siberian brown bears became separated by glaciers.
The habitat of the early polar bear started out as far south as England and the northern coast of Spain. As they moved farther and farther north,
they adapted to the harsh environment of the Arctic.
Tyrannus was markedly larger and more Arctos-like than the present polar bear. Their bone
structure was similar to that of brown bears only much larger. They measured about 6 feet at the shoulder, over 12 feet in length, and weighed on average
over 2,500 pounds making it the largest carnivorous land mammal ever.
Though the diet of this early polar bear isn't really known, one can probably assume that it started out pretty much like that of the Siberien brown bear,
and became more and more like that of the modern polar bear, adopting a more specialized and
carnivorous diet. Its size and strength would have allowed this bear to take down large animals like buffalo and woolly mammoths. However, it would have
also been capable of running any predators away from their kill, allowing it to be a scavenger.
Ursus maritimus tyrannus, was essentially a brown bear subspecies with many brown bear features. This group of Siberian bears from which they evolved is
believed to have quickly decreased in number as a result of selective pressures allowing survival only to those capable of adapting to the changing environment.
To adapt to the Arctic, they developed a dense, white, oily coat to camouflage them and repel water. Their carnivorous diet provided them with a thick layer of
body fat which gave them insulation and buoyancy. Their paws became larger and partially webbed to use as paddles in the water. To emit as little heat as
possible, their ears and tail became smaller. Eventually, these early polar bears evolved into the
modern polar bears of today.