A fossil skeleton that helps to explain how seals and walruses developed flippers has been discovered in the Canadian Arctic, shedding light on the mammals' transition from land to water.
The otter-like creature, which lived about 23 million years ago, fills an important gap in the fossil record, showing how seals and their cousins evolved from small carnivores that hunted on land and in water.
It has been named after Charles Darwin, because it fulfils a prediction that he made 150 years ago in his work On the Origin of Species.
He 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."
Puijila darwini, which takes the first part of its name from the Inuit word for "young sea mammal", marked an intermediate phase in just such a process.
Modern seals, sea lions and walruses belong to a group called the pinnipeds, or fin-footed mammals, which are descended from land-living ancestors and evolved flippers in place of limbs as they adapted to water.
This evolutionary process has been difficult to study precisely because the earliest known pinniped, a creature called Enaliarctos, already had flippers, and scientists did not have access to transitional forms in the fossil record.
That has changed with the discovery of Puijila, a nearly complete fossil skeleton that is one of these missing links. The creature, about 110cm long, shares some features with modern pinnipeds and has been identified as the earliest known member of the group, but it also has anatomical characteristics that are found in modern bears, skunks, badgers, weasels and otters.
"The remarkably preserved skeleton of Puijila had heavy limbs, indicative of well-developed muscles, and flattened phalanges (finger and toe bones) which suggests that the feet were webbed, but not flippers," said Mary Dawson, of the Carnegie Museum of Natural History in Pittsburgh, a member of the discovery team.
"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."
Natalia Rybczynski, of the Canadian Museum of Nature in Ottawa, who led the expedition, said: "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."
The fossil, details of which are published in the journal Nature, was discovered in the summer of 2007 during fieldwork on Devon Island, in the northern Canadian province of Nunavut. It was discovered in rock exposed by a meteor impact crater.
During the Miocene period, when Puijila lived, this region had a cool temperate climate, and its main habitat would have been fresh water. The research team suggests that, as freshwater lakes would have frozen in winter, it would have migrated over land to the sea in order to hunt for food.
Over many generations, as Darwin predicted, the descendants of Puijila or a similar species would gradually have evolved adaptations to swimming in the sea, leading to the emergence of modern pinnipeds such as seals and walruses.
The discovery supports a longstanding hypothesis that pinnipeds emerged in the Arctic
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ANI
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