I am the walrus

A newly unearthed fossil of a semi-aquatic carnivore in the Canadian Arctic has provided the missing link in the origin of seals, sea lions and walruses – or pinnipeds – researchers report.

The fossil, named Puijila darwini, is the earliest pinniped skeleton found so far. It is known that pinnipeds evolved from terrestrial mammals – and in fact are thought by some to have a sister relationship with ursoids (bears) or musteloids (skunks, badgers and otters) – but so far no strong evidence has been found of the transition stages.

The earliest pinniped fossil previously known was Enaliarctos, which already had well-developed flippers, limb adaptations for swimming. Puijila darwini, however, has webbed feet rather than flippers.

The research team, led by Natalia Rybczynski from the Canadian Museum of Nature in Ottawa, describes a long tail and fore and hindlimbs that are more like modern land mammals than pinnipeds, but it also has traits indicative of semi-aquatic adaptation including a shortened femur and flattened phalanges, suggestive of webbing.

The location of the find is interesting as well, as it suggests that the evolution of pinnipeds included a freshwater transition stage, as hypothesised by Charles Darwin.

The fossil was found in what was a crater lake on Devon Island, in Nunavut, northern Canada. It is approximately 23 million years of age, placing it in the early Miocene.

Botanic fossils found in the area from the Miocene suggest a cool, coastal temperate climate, in which freshwater lakes would freeze in winter. This means it is likely that Puijila would have traveled over land to find food in the sea.

Darwin wrote about the transition from freshwater to saltwater in semi-aquatic mammals in On the Origin of Species, theorizing that “a strictly terrestrial animal, by occasionally hunting for food in shallow water, then in streams or lakes, might at last be converted in an animal so thoroughly as to brace the open ocean”.

The findings are published in Nature.