The Greenland shark, one of two species of sleeper sharks, was the obvious suspect, she said, so they performed autopsies to see what they had been eating. That's when they found the polar bear bone.

Even a young, two- or three-year-old bear would be a ferocious opponent for a Greenland shark, which can grow to up to 7 meters (23 feet) and weigh more than a tonne.

Polar bears find most of their food on barren sea ice, in between the months of April and July. During the rest of the year, when they are stranded on land, they fast for long periods of time, as ...