Ursus maritimus tyrannus
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Ursus maritimus tyrannus was a very large fossil subspecies of polar bear, descended from an Arctic population of brown bears. Its name in Latin means tyrannt sea bear. The species is mentioned by Björn Kurtén, who assigned it to a Polar bear subspecies, U. m. tyrannus. Its bones have been found in contemporary England.
U. m. tyrannus was the first polar bear and evolved sometime in the Middle Pleistocene. While the oldest fossil is 100,000 years old, the species is thought to have evolved between 100,000 and 250,000 years ago from a population of brown bear likely isolated by glaciation. That population is believed to have diminished in numbers quickly into a much smaller population, with selection pressures favouring those individuals who adapted better to the changed environment. Over time, subject to intense selective pressures, the small population evolved the characteristics of the first polar bears.
Initially the isolated brown bears displayed the same traits as brown bears of that time period. Because litters of cubs can show significant variations in hair color and hair thickness, this gave certain individuals a survival advantage passed on with each generation. Eventually skull changes and even changes in dentition occurred, leading to the smooth and rather quick evolution of U. m. tyrannus.
The few bones that had been found of U. m. tyrannus were very similar to the brown bear bones, but were considerably larger in dimensions. If everything is scaled out correctly from its remains, it would have been 1.83 m (6 ft 0 in) at the shoulders, 3.7 m (12 ft 2 in) long, more than 4 m (13 ft 1 in) on the hind legs and with an average weight of 1.2 tons or more. These measurements would make U. m. tyrannus the largest bear ever to live, as well as the largest mammalian carnivore to ever hunt on land.
Hunting and diet
The diet and hunting behaviour of U. m. tyrannus are virtually unknown, though the similarity of its anatomy to that of the brown bears may suggest that its diet and hunting behaviour were more like those of brown bears than polar bears.The development of the dentition displayed by modern polar bears began as late as 10,000 years ago, as polar bears' diets became more specialized and carnivorous; the process may have started with U. m. tyrannus. In the Pleistocene much of Europe, including England, was covered with ice sheets, and large herbivores such as the woolly mammoth would have been abundant. The prevalence of these prey species, and the general lack of plant material in its environment, might have forced U. m. tyrannus to adopt a more carnivorous diet. The similarity of U. m. tyrannus's build to that of modern brown and polar bears, and its distinctness from species like Arctodus simus, would have made the animal much more suitable for taking on very large prey species, using its massive forelimbs to grab and wrestle down prey. Modern polar bears have been observed to wrestle down and kill animals much heavier than themselves, such as bull walruses..
However, its diet as well as its behaviour remains a great mystery, so searching for further fossils may help shed more light on this extinct bear.