The Prehistoric Bear Thread- Anything Ursidae related

The connection with forest habitats explains why U. arctos Pleistocene finds are rare in the north of Eastern Siberia, since open tundro-steppe and steppe landscapes dominated here.

U. arctos Pleistocene remains are found on the territory of Yakutia since the beginning of the middle Pleistocene. It was assumed that at this time there existed a large bear close to U. a. kamiensis- at the end of the middle and beginning of the late Pleistocene the smaller U. a. priscus*, and at the end of the Pleistocene the latter was replaced by a small bear, U. arctos spp, similar in size to the modern Yakut brown bear.

In 2015–2016 in the north of Yakutia, new fossils of several specimens of exceptionally large brown bear have been found, which are described in this article.

All described finds belong to adult individuals, probably males. Their mineralization is insignificant and is characteristic of bone remains from the frost-holes of Yakutia during the Karginsky-Sartan period. The surface color of bones varies from yellowish light brown to dark brown. The mandibular branch of MM 1661 is more strongly mineralized, its color is dark brown, in places it is almost black, which is typical for bones of an earlier time, middle-beginning of the late Pleistocene.

The remains originate from the Upper Pleistocene sediments and are found together with the remains of typical mammals of the mammoth fauna, the woolly mammoth Mammuthus primigenius (Blum.), steppe bison Bison priscus (Boj.) and cave lion Panthera spelaea.

The brown bear fossils in many ways exceed the size of modern U. arctos from Yakutia. Among the new finds are CM1 and AN8004, the sizes of which are exceptionally large- the majority of their measurements are far superior to those of not only modern brown bears from Yakutia but also to the maximum sizes of the largest Eurasian subspecies, U. a. beringianus and U. a. piscator.

U. arctos can reach extremely large sizes in the presence of a rich forage base. The modern Yakutia brown bear lives in harsh climactic conditions, and its diet is rather poor. It eats less animal food and low-calorie vegetable matter prevails in its diet. Under unfavorable conditions, an ecomorph of smaller bears came to be, characterized by an adaptive feature- a long winter sleep which lasts for 6.5 to 7.5 months.

It is difficult to explain the reasons for gigantism in the fossil brown bear of northern Yakutia. Probably it was due to the peculiarities of the Karginian warm period, which saw an increase in the diversity of vegetation, and accordingly, of edible plants. In addition, at this time (the bears would've had access to) the carcasses of large animals- like mammoths and woolly rhinos- and would've hunted their young.

In modern day U. arctos the tendency towards predation increases during periods of lack of vegetable food. The corpses of adult mammoths, which could weigh up to 4.000-5.000 kg, would provide a brown bear with high protein feed for a long time, since for example, a modern bear that feeds on a moose weighing up to 400-500 kg will take between 7 and 10 days to consume it fully.

The consumption of mammoth meat has been indicated by isotope analysis data from eastern Beringia. This increase in available food apparently caused the increase of size in the Karginian brown bears from Yakutia.

The cave bear is one of the best known extinct large mammals that inhabited Europe during the “Ice Age,” becoming extinct ≈24,000 years ago along with other members of the Pleistocene megafauna. Long-standing hypotheses speculate that many cave bears died during their long hibernation periods, which were necessary to overcome the severe and prolonged winters of the Last Glacial. Here, we investigate how long hibernation periods in cave bears would have directly affected their feeding biomechanics using CT-based biomechanical simulations of skulls of cave and extant bears. Our results demonstrate that although large paranasal sinuses were necessary for, and consistent with, long hibernation periods, trade-offs in sinus-associated cranial biomechanical traits restricted cave bears to feed exclusively on low energetic vegetal resources during the predormancy period. This biomechanical trade-off constitutes a new key factor to mechanistically explain the demise of this dominant Pleistocene megafaunal species as a direct consequence of climate cooling.