Global warming may not be the only threat to the polar bear. Scientists are puzzling over the discovery of the jawbone of a young polar bear in the stomach of a Greenland shark, a species that thrives in the cold waters of the far north.

The find suggests that the polar bear may have a serious challenger to its place at the top of the Arctic food chain. Until now, only killer whales were thought to offer a threat to Ursus maritimus as the Arctic's top predator.

Kit Kovacs, a seal expert at the Norwegian Polar Institute in Tromso, stumbled across the polar bear remains while attempting to find out who or what was killing large numbers of harbour seals in the Arctic archipelago of Svalbard.

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. "We were so shocked we were laughing," Ms Kovacs said.

The prospect of a marine battle between the world's largest land predator and the Greenland shark which can grow to a length of more than six metres, is a true clash of the titans. And it is one that is likely to have wildlife film-makers rushing for the North Pole.

Climate change has been melting polar ice cover and shrinking the natural habitat for the polar bear, now regarded as an endangered species. With polar sea ice expected to disappear altogether during the Arctic summer within a generation there is speculation that the bears may be spending longer in the water, while hunting or moving between icebergs. This could make them a potential target for large marine predators. Warmer waters may also be tempting more and larger sharks further north.

Shark experts were unconvinced and think it more likely that the shark would have fed on a bear's carcass, rather than killed a live bear, as even a young animal would be a fierce opponent.

Steve Campana, head of the Canadian shark research laboratory at the Department of Fisheries and Oceans, said he had not heard of sharks attacking a bear before. "It sounds like a scavenge," he told Reuters, adding that it was a "million-dollar question" for researchers as to whether Greenland sharks were preying on polar bears.

Ms Kovacs continued, however, to stand by her theory. Sleeper sharks, which are among the longest living of their species, can descend to depths of 2,200 metres, but seal blubber found in their stomachs by researchers indicates that they are seeking food in shallower waters. "We didn't know they went to the surface to feed," she said. "We can't say whether or not the shark took a swimming young bear or ate a carcass."

One clue may come from what the scientists did not find in their examination of the shark: anthropods, tiny creatures that feast on carcasses, gaining access through eyes or belly buttons. Scavengers are usually full of anthropods but none were found in the stomach with the polar bear.

"We don't know how active these sharks are as predators," Ms Kovacs said. That is the question scientists will seek to answer.

Sleeper sharks are among the most numerous large sharks in the world and are prolific feeders which have thrived while other shark populations have plummeted.

What Ms Kovacs and her research team have found suggests that they are also much more skilled hunters than was previously believed. Although no one has witnessed a kill, the scientists' findings appear to show that sleeper sharks can catch live seals, which are fast and agile prey.ball."

The discovery of the polar bear's jawbone is likely to leave scientists guessing for some time, but Ms Kovacs said: "I won't be going swimming there again... They are incredibly cryptic, dark sharks, who move slowly and get in close." "

Battle for top of the Arctic food chain

Weight: Adults Greenland sharks and mature polar bears weigh in at more than one tonne. The bears just have the edge.

Age: These sharks are among the oldest on the planet, some living for 200 years. Polar bears rarely live beyond 25.

Range: Polar bears can swim for up to 60 miles in open water. The shark can descend to depths of 2,200m.

Weapons: The bears' claws are for digging in ice, its teeth are its main weapon. The shark has rows of sharp upper teeth.