By Richard Black
Environment correspondent, BBC News website
It may look like a cross between a seal and an otter; but an Arctic fossil could, scientists say, hold the secret of seal evolution in its feet.
A skeleton unearthed in northern Canada shows a creature with feet that were probably webbed, but were not flippers.
Writing in the journal Nature, scientists suggest the 23 million-year-old proto-seal would have walked on land and swum in fresh water.
It is the oldest seal ancestor found so far and has been named Pujilla darwini.
Pujilla is the term for “young sea mammal” in the Inuktitut language, spoken by Inuit groups in Devon Island where the fossil was found.
Mary Dawson Carnegie Museum of Natural History
And the reference to Charles Darwin honours the famous biologist’s contention that land mammals would naturally move into the marine environment via a fresh water stage, just as pinnipeds – seals, sealions and walruses – have apparently done.
“The find suggests that pinnipeds went through a fresh water phase in their evolution,” said Natalia Rybczynski from the Canadian Museum of Nature (CMN) in Ottawa, who led the fieldwork.
“It also provides us with a glimpse of what pinnipeds looked like before they had flippers.”
The skeleton was about 65% complete, which enabled the researchers to reconstruct what the animal would have looked like in remarkable detail.
The legs suggest it would have walked upright on land; but the foot bones hint strongly at webbed feet. The fact that the remains were found in a former crater lake that has also yielded fossil fish from the same period was additional evidence for a semi-aquatic past.
“The remarkably preserved skeleton of Pujilla had heavy limbs, indicative of well developed muscles, and flattened phalanges (finger or toe bones) which suggest that the feet were webbed – but not flippers,” said Mary Dawson from the Carnegie Museum of Natural History in Pittsburgh, US, another of the scientists involved.
“This animal was likely adept at both swimming and walking on land. Pujilla is the evolutionary evidence we have been lacking for so long.”
Until now, the most primitive fossil pinniped was a creature called Enaliarctos that dates from about the same period and appears to have lived in the sea along the northwestern coasts of North America.
Enaliarctos had flippers, but may have had to bring its prey to the shore for eating, whereas modern pinnipeds manage it at sea.
Intriguingly, different species of present-day seal swim in different ways – either rotating their flippers, or waving their hind-quarters from side to side, using the hind limbs for propulsion.
Enaliarctos appears to have been capable of both modes of swimming – and as a four-legged animal with four webbed feet, Pujilla is a logical fore-runner of this creature which could swim with all four limbs.
The new discovery also shows, the scientists say, that seals, sealions and walruses very likely had their origins in the Arctic.
Darwin forecast the transition from land to sea via fresh water in his seminal work On the Origin of Species, published 150 years ago this year.
“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,” he wrote.
Story from BBC NEWS:
Published: 2009/04/22 19:27:50 GMT
A web of intrigue in the Arctic
Thu, Apr 23, 2009
RESEARCHERS working in the far north of Canada have recovered the fossil remains of a unique seal-like creature that has paws rather than flippers. They believe it may be a “missing link” that fills in a huge gap in the family tree for seals and related animals, writes DICK AHLSTROM
Modern seals are ungainly on land, staggering along on short flippers not best designed for terrestrial use. Once in the water, however, seals and their cousins, sea lions and walruses, become graceful swimmers.
Scientists are excited about the find, made in a crater lake on Devon Island in Nunavut, Canada. Located well inside the Arctic Circle, due north of the Great Lakes, the near-complete skeleton dates back between 21 and and 24 million years. Devon Island today is under ice most of the time but back then it was a temperate haven, with a coastal climate much like Ireland’s today and covered with conifer forests.
The animal, Puijila darwini, measured about 110cm from head to the tip of its longish tail, and although it had a body and muzzle akin to a seal’s, it had legs like an otter with webbed paws that were ideal for swimming. This meant it was swift of foot on land but also in the water, according to the research team from the Canadian Museum of Nature, the Carnegie Museum of Natural History in Pittsburgh and the American Museum of Natural History, who describe their findings this morning in in the journal Nature.
“The remarkably preserved skeleton of Puijila has heavy limbs, indicative of well- developed muscles and flattened phalanges , which suggests that the feet were webbed but not flippers,” according to Dr Mary Dawson, curator emeritus of Carnegie.
SEALS, SEA LIONS and walruses are part of a family known as the pinnipeds. The earliest known example, Enaliarctos , recovered along the north Pacific coast, already had flippers. But the authors note in their report that the modern seal also has physical characteristics that match up with land meat-eaters such as bears in one group or skunks, badgers, weasels and otters in another.
They believe that Puijila was semi-aquatic and a carnivore.
“The discovery suggests that the evolution of pinnipeds included a freshwater transitional phase, and may support the hypothesis that the Arctic was an early centre of pinniped evolution,” they write.
The theory holds that the Puijila hunted on land but also swam about in the temperate lakes during the summer, though it may have had to travel further when these froze over during the winter. They probably made for the nearby coast to hunt for fish to sustain them during the winter.This, over time, could have caused a gradual transition from fresh water to salt water for the evolving pinnipeds, ultimately leading to the flipper adaptation, the authors maintain.
The finding fills in a gap in the family tree and provides useful evidence in arguing for this slow evolutionary modification, which changed the web-footed land animal into a wholly aquatic carnivore.
The creature was the first meat-eating carnivore found at the site, but it yielded up other interesting findings from the early Miocene epoch.
These included two families of freshwater fishes, one bird and four other mammalian groups, including shrews, rabbits, rhinoceroses and “artiodactyl”, small short-legged plant-eaters who were the early ancestors of modern giraffes and deer.
Puijila darwini takes its name from Charles Darwin, appropriately so given this prescient observation, quoted by the authors from his On the Origin of Species by the Means of Natural Selection : “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.”
© 2009 The Irish Times
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