Walking seal: Puijila had a body resembling an otter, with the head of a seal, a long tail and webbed feet adapted for swimming, say the researchers.

Credit: Stefan Thompson

OTTAWA: A chance discovery has turned up the fossil of a prehistoric ancestor to seals, which shows that their large eyes may have evolved to help them see in the dark conditions of the Arctic.

Palaeontologists working in Canada's remote north discovered the skeleton in an ancient lake bed – and it has given them a glance of the animal's land-to-sea transition, which had been difficult to study because few fossils are known.

The find offers the "first glimpse into the earliest stages of this important evolutionary transition," lead researcher Natalia Rybczynski said.

Low-light not deep-sea

It has also helped the team refute the prevailing theory that seals evolved further south on North America's northwest shores, and suggests that their large eyes were adapted to hunt in dark Arctic winters, not low-light, deep-sea diving.

"It changes our thinking about how and where the evolution of this animal took place," said Rybczynski, a palaeontologist at Carleton University in Ottawa.

"We knew that pinnipeds [seals, sea lions and walruses] came from a terrestrial ancestor, but we had no idea how that land-to-sea transition occurred," or where, she explained. "If it was in the high Arctic, then we also have to consider the Arctic and the conditions there as contributing to its evolution."

The fossil skeleton, thought to be 20 to 24 million years old, was found in 2007 during an expedition to a meteor impact crater that once formed a lake on Devon Island, Nunavut.

"Element of chance"

Past expeditions to the crater had uncovered remains of rabbits, freshwater fish, a bird, a shrew, a rhinoceros and a small hoofed mammal that once lived in the then cool, temperate, environment.

Researchers believe the light conditions for the seal ancestor would have been similar to today with 24-hour darkness for part of the year and 24-hour daylight for some of the remainder.

Rybczynski's team happened upon the exact spot of the seal ancestor's bones after their all-terrain vehicle ran out of fuel, Rybczynski said. Within two days they had dug up 65% of the skeleton while waiting for fuel.

"There's always an element of chance in palaeontology," she quipped.

The so-called "walking seal" was named Puijila darwini, meaning young sea mammal in Inuktitut – the language of the Inuit people – and in homage to Charles Darwin.