By Richard Black
Environment correspondent, BBC News website
It may look like a cross between a seal and an otter; but an Arctic fossil could, scientists say, hold the secret of seal evolution in its feet.
A skeleton unearthed in northern Canada shows a creature with feet that were probably webbed, but were not flippers.
Writing in the journal Nature, scientists suggest the 23 million-year-old proto-seal would have walked on land and swum in fresh water.
It is the oldest seal ancestor found so far and has been named Puijila darwini.
is the term for "young sea mammal" in the Inuktitut language, spoken by Inuit groups in Devon Island where the fossil was found.
And the reference to Charles Darwin honours the famous biologist's contention that land mammals would naturally move into the marine environment via a fresh water stage, just as pinnipeds - seals, sealions and walruses - have apparently done.
"The find suggests that pinnipeds went through a fresh water phase in their evolution," said Natalia Rybczynski from the Canadian Museum of Nature (CMN) in Ottawa, who led the fieldwork.
"It also provides us with a glimpse of what pinnipeds looked like before they had flippers."
The skeleton was about 65% complete, which enabled the researchers to reconstruct what the animal would have looked like in remarkable detail.
The legs suggest it would have walked upright on land; but the foot bones hint strongly at webbed feet. The fact that the remains were found in a former crater lake that has also yielded fossil fish from the same period was additional evidence for a semi-aquatic past.
"The remarkably preserved skeleton of Puijila
had heavy limbs, indicative of well developed muscles, and flattened phalanges (finger or toe bones) which suggest that the feet were webbed - but not flippers," said Mary Dawson from the Carnegie Museum of Natural History in Pittsburgh, US, another of the scientists involved.
"This animal was likely adept at both swimming and walking on land. Puijila
is the evolutionary evidence we have been lacking for so long."
The scientists suggest this proto-seal lived on land and in fresh water
Until now, the most primitive fossil pinniped was a creature called Enaliarctos
that dates from about the same period and appears to have lived in the sea along the northwestern coasts of North America.
had flippers, but may have had to bring its prey to the shore for eating, whereas modern pinnipeds manage it at sea.
Intriguingly, different species of present-day seal swim in different ways - either rotating their flippers, or waving their hind-quarters from side to side, using the hind limbs for propulsion.
appears to have been capable of both modes of swimming - and as a four-legged animal with four webbed feet, Puijila
is a logical fore-runner of this creature which could swim with all four limbs.
The new discovery also shows, the scientists say, that seals, sealions and walruses very likely had their origins in the Arctic.
Darwin forecast the transition from land to sea via fresh water in his seminal work On the Origin of Species, published 150 years ago this year.
"A strictly terrestrial animal, by occasionally hunting for food in shallow water, then in streams or lakes, might at last be converted in an animal so thoroughly aquatic as to brave the open ocean," he wrote.