The fossil was discovered in sediments of what had been a lake when the Arctic was much warmer, showing Puijila was at home in fresh water as well as on land. Besides eating fish, it probably ate rodents and ducks with dog-like teeth set in powerful jaws, Rybczynski said.

Annalisa Berta, a biology professor at San Diego State University who wasn't involved in the work, welcomed the find.

"This is a fantastic discovery that fills a critical evolutionary gap (from) when terrestrial carnivores traded limbs for fins and moved from land to sea," she wrote in an e-mail.

Apart from the poorly preserved ribs, researchers recovered about 65 percent of the skeleton. Berta said this completeness will help scientists sort out the order in which key pinniped features appeared.

Puijila's fossilized remains were found in 2007 and 2008. Its Arctic location was "completely unexpected" because most scientists believed pinnipeds evolved on the west coast of North America, Rybczynski said. The discovery lends some credence to a hypothesis that the Arctic was an early center for these mammals' evolution, she said.

Not all experts agreed. Maybe Puijila arrived in the Arctic from elsewhere, finding an area where its competitors could not survive, said Tom Demere, curator of paleontology at the San Diego Natural History Museum.

He noted that an older fossil that the study authors consider a pinniped had been discovered in Mongolia. (That fossil included only head and jaw remnants and so its body features are not known).

So the new evidence for Arctic evolution is "inconclusive at best," he said.

ChinaDaily Mobile News