'Missing link' fossil seal walked 北极化石补上海豹行走“缺失的一环”

It may look like a cross between a seal and an otter¹; but an Arctic fossil could, scientists say, hold the secret of seal evolution in its feet.

海豹和水獭的进化之间可能有交叉，但是科学家称，北极发现的一具化石可能揭开海豹的腿进化的秘密。

The scientists suggest this proto-seal lived on land and in fresh water

A skeleton unearthed² in northern Canada shows a creature with feet that were probably webbed（有蹼的）, 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 Pujilla darwini.

Pujilla 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 contention⁴（争论，争辩） that land mammals would naturally move into the marine⁵ environment via a fresh water stage, just as pinnipeds（鳍足类的） - seals, sealions and walruses⁶（海象） - have apparently⁷ 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."

Reconstructing the skeleton of the "walking seal"

Flip³ side

The skeleton was about 65% complete, which enabled the researchers to reconstruct what the animal would have looked like in remarkable⁸ detail.

The legs suggest it would have walked upright on land; but the foot bones hint strongly at webbed feet. The fact that the remains⁹ were found in a former crater¹⁰ lake（火山口形成的深湖） that has also yielded fossil fish from the same period was additional evidence for a semi-aquatic¹¹ past.

"The remarkably¹² preserved skeleton of Pujilla had heavy limbs, indicative of well developed muscles, and flattened¹³ phalanges（趾骨） (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 adept¹⁴ at（熟练于） both swimming and walking on land. Pujilla is the evolutionary¹⁵ evidence we have been lacking for so long."

Until now, the most primitive¹⁶（原始的） 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.

Enaliarctos had flippers, but may have had to bring its prey¹⁷ to the shore for eating, whereas modern pinnipeds manage it at sea.

Intriguingly¹⁸（有趣地）, different species of present-day seal swim in different ways - either rotating their flippers, or waving their hind¹⁹-quarters from side to side, using the hind limbs（后肢） for propulsion（推进，拖动力）.

Enaliarctos appears to have been capable of both modes of swimming - and as a four-legged animal with four webbed feet, Pujilla 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 seminal²⁰（精液的，生殖的） work On the Origin of Species, published 150 years ago this year.

"A strictly²¹ terrestrial animal（陆生动物）, by occasionally hunting for food in shallow water, then in streams or lakes, might at last be converted in an animal so thoroughly²² aquatic（水生的） as to brace²³ the open ocean," he wrote.