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12-Nov-2004

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Ancient brown bear migrations



Lateral (upper) and inferior (lower) view of brown bear fossil P98.5.374 recovered from fluvial gravels near Edmonton, Alberta. Scale bar is 3 cm. Image Science.
Click here for a high resolution photograph.

Some of the large mammals that lived in North America during the last ice age, such as brown bears, originally migrated from Siberia. They reached North America by crossing over on a short stretch of land called the "Bering Strait" that was exposed during times of low sea level.



The fossil fragments described by the authors next to the skull of a male Brown Bear, also known as a Grizzly, from Jasper National Park, Alberta. The skull weighs 231.5 kilograms. The Provincial Museum of Alberta.
Click here for a high resolution photograph.

A mystery for researchers who study this time period is how and when the animals spread across the continent, since giant ice sheets covered parts of North America for much of this time. Now, scientists have analyzed fossil pieces of a brown bear skull that are helping them clear up the confusion. They describe their findings in the 12 November 2004 issue of the journal Science.

Some evidence has suggested that after the brown bears crossed the Bering Strait between 50,000 and 100,000 years ago, they were stuck up north, in what's now Alaska and the Yukon territory. That's because the ice sheet that had once covered the northeastern part of the continent would have spread all the way to the west coast, blocking any bear travel down to the south.



The fossil fragments described by the authors next to the skull of a male Brown Bear, also known as a Grizzly, from Jasper National Park, Alberta. The skull weighs 231.5 kilograms. [The Provincial Museum of Alberta]
Click here for a high resolution photograph.

According to this picture, around 12,000 or 13,000 years ago, the ice sheet retreated enough to allow wildlife to travel south along an ice-free corridor connecting Alaska and the Yukon with the lower part of North America.

The problem, though, is that the ice sheet didn't spread all the way west until after the bears probably arrived in North America. Could they have travelled south before the ice sheet barrier formed?

Paul Matheus of the University of Alaska Fairbanks and his colleagues have analyzed a fossil that suggests the answer to this question is yes. They found the upper and lower halves of a brown bear jaw, which turned out to be approximately 25,000 years old.

When the researchers analyzed the fossils' genetic material, they found that this bear was probably closely related to the bears that live in southern Canada and the northern United States today.

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