Natalia Rybczynski (left) and Mary Dawson. Credit: M. LIPMAN

At the end of another long Arctic summer day on Canada's Devon Island, Natalia Rybczynski's palaeontology field crew was disheartened, having once again found nothing new. Then, to make matters worse, their all-terrain vehicle ran out of petrol. Undergraduate researcher Elizabeth Ross, who had been in charge of fuel, sheepishly kicked around in the dirt while Rybczynski ran back to camp.

Uncovering a piece of bone, Ross immediately showed it to palaeontologist Mary Dawson of the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania. By the time Rybczynski returned, the two had uncovered handfuls of bones from an animal that turned out to be a missing link in the evolution of pinnipeds — seals, sea lions and the walrus.

Of all the mammals that have marched back into the sea during their evolution, the ancestors of pinnipeds have been the most elusive. Other ancient specimens, of the genus Enaliarctos, have previously been found on the northwestern coast of the United States and already had fully formed flippers. On the basis of these remains, many researchers in the field assumed that pinnipeds had evolved from terrestrial forms in the same region.

Rybczynski, a palaeobiologist at the Canadian Museum of Nature in Ottawa, Ontario, and her team uncovered 65% of the animal's skeleton within a couple of days. Nestled in freshwater lake deposits in an impact crater, the skeleton dates to roughly 20 million years ago.

“Even in the field, we were seeing evidence of specialization for swimming,” recalls Rybczynski. This included powerful shoulder muscles and flattened phalange bones that indicate webbing of the feet. The animal's teeth and skull revealed that it was a carnivore that shared characteristics with pinnipeds. Through phylogenetic analysis, the team confirmed it to be a pinniped (see page 1021).

“It's the kind of animal that can be once-in-a-career. It's a transitional fossil that's filling a gap in our understanding,” says Rybczynski. That alone would have been enough to put it on the map of scientific merit, but add to that the fact that it was found in an unexpected locale — the Arctic Circle — and in a freshwater setting, and “it's really phenomenal”, she adds.

Named Puijila darwini (pronounced pew-yee-la, for an Inuit word for young seal), it's the most terrestrial of all known pinniped fossils. Rybczynski likens it to a “swimming wolverine” that was a powerful hunter both on land and in the water. And it may have hunted in seawater as well as freshwater, opting for the former when lakes froze over in winter. Puijila's habitat would have been a forested, relatively temperate area, but subject to a great deal of seasonal variation owing to its high latitude. The setting brings new ideas about pinniped evolution.

“What would it have been like hunting in the near-total darkness of the Arctic winter?” Rybczynski wonders. Biologists have generally associated the evolution of pinnipeds' large eyes with deep diving, she says, but it could instead be linked to the Arctic Circle's long, dark winters.

The discovery also addresses scepticism about the migration of pinniped ancestors from the Arctic to the Pacific — the Bering land bridge that existed at the time would have presented a barrier to swimmers. But, Rybczynski notes, the flipperless Puijila could have walked across.