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

It had a body resembling that of an otter with the head of a seal, a long tail and webbed feet adapted for swimming, say the researchers, who report the find today in the British journal Nature.

It was "the least specialised for swimming" of its known peers, the authors write. "The presence of enlarged, probably webbed feet, robust forelimbs and an unspecialised tail suggests that Puijila swam quadrupedally using its webbed fore- and hind-feet for propulsion."

Lake dweller

"It was almost certainly not specialised for swimming under water using simultaneous pelvic paddling," like modern seals that oscillate their hind feet side to side or their fore flippers, in a movement akin to flying.

It predominantly lived and hunted in freshwater lakes that would freeze over in the winter.

But early populations may also have frequented marine shore environments more than their counterparts based further south, said the researchers, because a warmer Arctic Ocean would have remained open when access to lakes was restricted by ice cover.