April 22, 2009

Arctic Fossil Explains How Seals Evolved

Scientists from Canada and the U.S. have discovered the skeleton of a previously unknown web-footed carnivore in Canada's Arctic. The researchers said their discovery sheds light on how seals developed from land-based mammals.

The primitive animal, known as Puijila darwini, measured around 43 inches from nose to tail, and had a body similar to that of an otter, but a skull akin to a seal.

New research suggests Puijila is a "missing link" in the evolution of the group that today includes seals, sea lions, and the walrus.

Modern seals, sea lions, and walruses all have flippers - adaptations that evolved over time as some terrestrial animals moved to a semi-aquatic lifestyle. However, until now, the morphological evidence for this transition from land to water was sparse.

"The remarkably preserved skeleton of Puijila had heavy limbs, indicative of well developed muscles, and flattened phalanges which suggests that the feet were webbed, but not flippers. This animal was likely adept at both swimming and walking on land. For swimming it paddled with both front and hind limbs. Puijila is the evolutionary evidence we have been lacking for so long," said Mary Dawson, curator emeritus of Carnegie Museum of Natural History.

Portions of the Puijila darwini fossil were discovered in 2007 in deposits that accumulated in what was a crater lake in coastal Devon Island, Nunavut, Canada.  A follow up visit in 2008 yielded the basicranium, an important structure for determining taxonomic relationships.

Paleobotanic fossils indicate this location during the Miocene had a cool, coastal temperate environment, similar to present-day New Jersey.   Given that freshwater lakes would freeze in the winter, it is likely that Puijila would travel over land to the sea for food.

The transition from freshwater to saltwater in semi-aquatic mammals has been hypothesized for some time, originally by Charles Darwin.

"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 aquatic as to brace the open ocean," wrote Darwin in On the Origin of Species by the Means of Natural Selection.

"The find suggests that pinnipeds went through a freshwater phase in their evolution. It also provides us with a glimpse of what pinnipeds looked like before they had flippers," said Natalia Rybczynski, who led the field expedition.

The animal is described as having a long tail with forelimbs proportionate to modern carnivorous land animals, as opposed to pinnipeds. 

The earliest well-represented pinniped, known as Enaliarctos, was a marine form with flippers, had been found on northern Pacific shores of North America.  It was theorized for some time that pinniped evolution had been centered on the Arctic, a hypothesis supported by the Puijila discovery.

The analysis of the skeleton and support for the hypotheses that pinniped origins can be found in the Arctic will be described in the April 23 issue of the journal Nature.


Image 1: Life reconstruction of Puijila darwini swimming in crater lake. Credit: Mark A. Klingler/Carnegie Museum of Natural History

Image 2: Skeletal illustration of Puijila darwini. Credit: Mark A. Klingler/Carnegie Museum of Natural History

Image 3: Reconstruction of skeleton showing preserved bones in dark grey. Credit: Mark A. Klingler/Carnegie Museum of Natural History


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