U.S. researchers have found fossils of what they say is a missing evolutionary link between fish and land animals -- a strange creature that first crawled onto the shore about 375 million years ago.
The fossils, found on Ellesmere Island in Arctic Canada, suggest the limbs, skull, neck and ribs of four-limbed animals, but also the primitive jaw, fin and scales of fish, scientists reported today in the journal Nature.
"This really is what our ancestors looked like when they began to leave the water," said an accompanying editorial by zoologist Jennifer A. Clack of the University of Cambridge and biologist Per Erik Ahlberg of Uppsala University in Sweden.
The newly discovered species, called Tiktaalik roseae, "blurs the boundary between fish and land-living animal both in terms of its anatomy and its way of life," said biologist Neil Shubin of the University of Chicago, a co-leader of the expedition.
The creature lived in shallow waterways, where it hunted for prey with its crocodile-like snout and sharp teeth, but was able to pull itself out of the water for short periods of time and move around on its limb-like fins, scientists said.
The specimens, ranging from 4 to 9 feet long, were remarkably well-preserved. The scientists were able to examine the joints and to conclude that the shoulder, elbow and wrist joints were strong enough to support the creature's body on land.
"Human comprehension of the history of life on Earth is taking a major leap forward," said H. Richard Lane of the National Science Foundation, which funded the research along with the National Geographic Society and other groups.
"These exciting discoveries are providing fossil Rosetta Stones for a deeper understanding of this evolutionary milestone -- fish to land-roving tetrapods," he said.
In the late Devonian period, nearly 400 million years ago, the landmass where the fossils were found straddled the equator and had a climate much like that now found in the Amazonian basin. It was a flat coastal plain with shallow, slow-moving rivers that meandered to the sea.
"This kind of shallow stream system seems to be the place where many features of land-living animals first arose," said expedition co-leader Ted Daeschler of the Academy of Natural Sciences in Philadelphia.
But as Earth's continental plates shifted, the mass was carried north to Canada's Nunavut territory in the Arctic Ocean.
Finding and extracting the fossils presented major challenges, such as the need to travel by helicopter into the region.
Freezing temperatures and high winds limited the amount of time the team could work each day, and the near-constant precipitation prevented the plaster used in the fossil-preservation process from drying. "And we were always looking over our shoulders for polar bears. We saw lots of their tracks," Shubin said. Team members all carried guns for protection.
The key breakthrough came on a 2004 fossil hunting expedition -- one of five yearly trips -- when team members spied the front end of what looked like a fish skull sticking out of the bluff. "That's ideal -- having the snout sticking out -- because in the cliff behind it is likely the rest of the animal," Shubin said.
The researchers ultimately found three nearly complete specimens, but they weren't sure what they had until they returned to the lab and studied the bones.
"As each piece to Tiktaalik's anatomy was exposed, we began to see just how wonderfully intermediate this animal's features were between land and water," Shubin said.
The creature had a flat skull, like that of a crocodile, but it had armor like a fish, scientists said. And it had a neck, they said, making it the only fish known to have one. "The neck was one of the biggest surprises," Daeschler said. "This freed the skull from the shoulder girdle and gave the animal extra mobility."
At the ends of the powerful fins, the team found wrists and bones similar to fingers. But the fins also contained the thin rods found in fish fins. "Here is a creature with fins that can do push-ups," Shubin said.
And instead of the tiny rod-like ribs of a fish, Tiktaalik had full-fledged ribs that overlapped one another like those of an anteater. "Ribs like that produce a stiff trunk," said Farish A. Jenkins of Harvard University, another expedition leader.
"Fish that stay in the water are buoyant and don't need that, so this animal must have developed these structures for life in the shallows and making excursions onto land," he said.
Shubin said: "This animal is both fish and tetrapod. We jokingly call it a 'fishapod.' "
Rather than following the convention of using Latin for a species' name, the research team asked the Nunavut elders council for suggestions. It recommended Tiktaalik, which means "a large shallow-water fish" in the Inuktitut language. Roseae honors an anonymous donor.