Water supports the bodies of submerged fish, making strong ribs largely unecessary, "so this animal must have developed these structures for life in the shallows and making excursions on to land," Jenkins said.
Shubin says the fish's wide head and sharp teeth suggest it hunted much like a crocodile and that it also breathed air.
"Look at the side of the snout. It has a nice big pair of external nostrils," he said.
Tiktaalik could become an icon of evolution in action, write paleontologists Per Ahlberg of Sweden's Uppsala University and Jennifer A. Clack of the University of Cambridge in the United Kingdom in an accompanying commentary.
The paleontologists say the new fish form goes a long way toward filling the evolutionary gap between fish and the earliest amphibians.
"Our remote ancestors were large, flattish, predatory fishes," they write. "Strong limblike pectoral fins enabled them to haul themselves out of the water."
Evolutionary scientists agree that all four-limbed land vertebrates, including dinosaurs and mammals, are descended from lobe-fins, a group of primitive fishes with fins suggesting limbs.
Living lobe-fins include lungfish, which have gills but can also breathe air using modified swim bladders that act as lungs.
Tiktaalik would have breathed like a lungfish, says Clack, senior assistant curator at Cambridge's University Museum of Zoology.
"It's increasing its reliance on air, so it's not purely a gill-breather," she said.
This freshwater fish needed a large, wide head to pump air into its simple lungs.
"It's a sort of bellows arrangement," Clack explained. "The more air you can get in with a mouthful, the better."
Like a Croc
The creature's long snout seems to be adapted for snapping at prey and hunting with its head above water like a crocodile, Clack said.
"Snapping underwater is less efficient, because water pressure gets in the way," she added. "There would probably have been some large invertebrates around the water margins, or it might still have been feeding on small fishes and things in the water."
The creature also lacked the rigid bony covering over the head and shoulders that most fish have, effectively giving Tiktaalik a neck.
"The animal would have been able to lift its head from the water," Clack said.
The new fossils are so complete and well preserved that they "answer questions that previous material has been unable to answer," she added.
Ted Daeschler, co-leader of the fossil hunt, called the discovery "a dream come true."
"We knew that the rocks on Ellesmere Island offered a glimpse into the right time period and were formed in the right kinds of environments to provide the potential for finding [such] fossils," said Daeschler, curator of vertebrate biology at the Academy of Natural Sciences in Philadelphia, Pennsylvania.
Ellesmere Island is more than 600 miles (970 kilometers) north of the Arctic Circle in Canada's Nunavut territory.
Polar bears roam the now frigid region. But Nunavut's fossil-bearing rocks were formed when North America was part of a giant supercontinent that straddled the Equator.
Huge predators would have lurked in Tiktaalik's rivers and lakes, study co-leader Shubin saysperhaps one reason why Tiktaalik appears to have been headed for land.
"Land had no predators, and it also had food in the form of invertebrates," Shubin said.
"Put this all together and the shallows and mudflats might have been a good place to make a living."
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