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Landlubber. The skeleton of Puijila darwini fills a fossil gap that helps researchers to envision what pinnipeds looked like (inset) before they entered the sea.

Alex Tirabasso, Canadian Museum of Nature; (inset) Stefan Thompson

Arctic Fossil Reveals a "Walking Seal"

Scientists have found the first skeleton of a land-dwelling relative of seals, sea lions, and walruses. The 20-million- to 24-million-year-old Arctic fossil sports webbed feet instead of flippers, providing a long-sought glimpse of what such animals looked like before they dove into the sea.

Before marine mammals swam through the world's oceans, their ancestors meandered on land. Researchers have founds several intermediate fossils that trace the transition from land to water in whales and manatees, but they have no such record for pinnipeds--seals, sea lions, and walruses. Much to scientists' dismay, the most primitive pinniped fossils, which date to between 20 million and 28 million years ago, had full flippers, making it hard to pinpoint how the animals evolved to live in an aquatic environment.

It took a balky, all-terrain vehicle with a broken gas gauge to help reveal the missing piece. In 2007, vertebrate paleontologist Natalia Rybczynski of the Canadian Museum of Nature in Ottawa and her colleagues were returning to their camp from a long day of fieldwork in the Canadian Arctic when they ran out of fuel. While Rybczynski left to get more gas, her graduate student, who was supposed to make sure that the vehicle was refueled, scuffed the ground in irritation and uncovered part of a shin bone. By the time Rybczynski returned, her colleagues' hands were filled with little black bones, and they were doing what Rybczynski refers to as "the fossil dance." Within a few days, the team had recovered most of the skeleton.

At first, they thought they had uncovered some sort of strange otter, but when they examined the fossils back in the lab, the teeth and parts of the skull hinted that the animal was a flipper-less, primitive pinniped. They couldn't be sure, because they lacked the diagnostic portion of the skull. Within minutes of returning to the field in 2008, they found what they were looking for and celebrated with another round of fossil dances. The skeleton represents the most primitive pinniped to date, the team reports today in Nature.

The 110-cm-long animal, Puijila darwini, resembled an otter, with a long tail, doglike teeth, and webbed feet. Like an otter, its body was adapted for swimming but spent most of its time on land. Rybczynski says that although the creature wasn't the direct ancestor of modern pinnipeds, it reveals what these ancestors might have looked like before they became fully flippered. She also says that finding this primitive fossil in the Arctic suggests that this area could be the center of pinniped evolution, contradicting the prevailing idea that the mammals evolved along the western coast of North America.

"I think it really is a very fabulous discovery that fills a critical evolutionary gap," says Annalisa Berta, an evolutionary biologist at San Diego State University in California. John Flynn, a vertebrate paleontologist at the American Museum of Natural History in New York City, adds that the work "points out the remarkable value of doing remote expeditionary work in difficult places." But Flynn notes that to understand the significance of the find for pinniped evolutionary history, researchers need to determine where Puijila sits in the pinniped family tree by conducting a more comprehensive skeletal analysis that includes modern species.