It's long been assumed that marine mammals in the pinniped group – seals, sea lions and walrus – evolved from a land-based common ancestor, but until now, no definitive fossil evidence had materialized.

A newly discovered species, Puijila darwini, which lived in the Artic during the Miocene (23 to 5 million years ago), promises to be that missing link, reports a study published online today in Nature. The well-preserved skeleton was found on Devon Island in Nunavut, Canada, an area that would have had a much more temperate climate when the P. darwini roamed the region.

"Puijila is the evolutionary evidence we have been lacking for so long," Mary Dawson, a curator emeritus of vertebrate paleontology at the Carnegie Museum of Natural History in Pittsburgh, said in a statement.

"This animal was likely adept at both swimming and walking on land," said Dawson.

From head to tail, it would have measured about 3.6 feet (110 centimeters) and probably had had webbed feet instead of the flippers its modern descendents flaunt. The webbed feet would have allowed it to spend part of its time hunting food in fresh water lakes while making land travel less awkward than flippers in winter, when frozen lakes would have forced it to find food on solid land. A long tail and short legs gave it a look not unlike the modern otter, probably most closely resembling today's river otter (Lontra canadensis).

Although land animals are assumed to have initially evolved from sea-dwellers, some – such as the ancestors of whales, manatees and walruses – eventually crept back into the watery habitat, which makes these transitional species like Puijila an important glimpse into that evolutionary process.

The name for the new species is an apt one. Puijila is Inuktitut for "young sea mammal," and darwini is in honor of Charles Darwin, who theorized about these very evolutionary forces in The Origin of Species, noting: "A strictly terrestrial animal, by occasionally hunting for food in shallow water, then in streams or lakes, might at last be converted into an animal so thoroughly aquatic as to brave the open ocean."

Reconstruction of Puijila darwini swimming in a crater lake © Mark A. Klingler/Carnegie Museum of Natural History