Seals and sea-lions gracefully careen through today’s oceans with the help of legs that have become wide, flat flippers. But it was not always this way. Seals evolved from carnivorous ancestors that walked on land with sturdy legs; only later did these evolve into the flippers that the family is known for. Now, a beautifully new fossil called Puijila illustrates just what such early steps in seal evolution looked like. With four legs and a long tail, it must have resembled a large otter but it was, in fact, a walking seal.
Natalia Rybczynski unearthed the new animal at Devon Island, Canada and worked out that it must have swam through the waters of the Arctic circle around 20-24 million years ago. She named it Puijila darwini after an Inuit word referring to a young seal, and some obscure biologist. The skeleton has been beautifully preserved, with over 65% of the animal intact, including its limbs and most of its skull.
Puijila is a massive boon for biologists trying to understand the evolution of pinnipeds, the group that includes seals, sea lions and walruses. It’s not itself a direct ancestor, having branched off the evolutionary path that led to modern pinnipeds. It did, however, retain many of the same features that a direct ancestor would have had. “Puijila is a transitional fossil,” Rybczynski explains. “It gives us a glimpse of what the earliest stages of pinniped evolution looked like, before pinnipeds had flippers. And it suggests that in the land-to-sea transition, pinnipeds went through a freshwater phase.”
This familiar group evolved from land-dwelling carnivores and their closest living relatives are the bears and the mustelids (otters, weasels, skunks and badgers). For other marine mammals like whales and dolphins, the fossil record has given us dramatic visuals for the gradual transformation from land-dweller to full-time swimmer. But for pinnipeds, that transition is much murkier because until now, the earliest known seal Enaliarctos already had a full set of true flippers. Puijila changes all of that.
In the Origin of
the Species, the ever-prescient Darwin wrote, “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”. This year, on the 150th anniversary of the book’s publication, the walking seal that bears his name pays a fitting tribute to Darwin’s insight.