The team uncovered the skeleton in the Kala Chitta Hills in Pakistan. The American and Pakistani researchers say the find strengthens their conviction that this was the place where whales made the transition from land to sea. 'They learnt to swim right there,' according to the team leader, Hans Thewissen, of Northeastern Ohio Universities College of Medicine.
Scientists have known for about a century that the ancestors of whales were meat-eating land animals resembling wolves. There is also much fossil evidence of early whales that were completely aquatic, animals about 50ft (16 metres) long with short hind limbs and flippers.
'The importance of our find is that it is an animal that is making the transition between land animals and fully aquatic whales,' said Dr Thewissen. Such transition fossils - missing links - have been found before, but none has comprised enough bones to show how the creature moved.
The whale was about the size of a male sea-lion, with a clumsy gait on land but apparently a fast, adept swimmer.
Dr Thewissen said the fossil indicates that archaic whales swam by undulating their spines like otters and ambled around on land like seals. The creature had enormous paddle-like feet. One of its toes was about 17 centimetres (7in) long. 'When it was stationary on land it would rest its chest and abdomen on the ground with its fingers and toes planted outwards in a sprawl. Its forelimbs were not long enough to push its whole body off the ground,' he said. The toes sprawled because they were so long and flipper-like and they would have become entangled if it had walked with its feet facing forwards.
Scientists are not certain why whales took to the sea; the most likely explanation is that they were seeking an abundant and reliable food source.
In an article in today's issue of Science magazine, the team says it has placed the skeleton in a new genus and species: Ambulocetus natans.
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