NEW YORK -- Researchers have found a fossil fish with a crocodile-like head that apparently moved on land like a seal -- an important discovery for understanding how fish evolved into land animals with four limbs and a backbone.
Scientists have long known that these land animals evolved from fish, but the details of that transformation have been a mystery. The new discovery isn't the first fossil fish from that time period, but "it's the best one," said one expert not involved in the work.
The specimens, found in Canada above the Arctic Circle, are remarkably complete. The creature looks more like a land-dweller than previously known transitional fish, said the expert, Robert Carroll of McGill University in Montreal.
"It's an important new contribution to (understanding) a very, very important transition in the history of life," said Carroll, curator of vertebrate paleontology at McGill's Redpath Museum.
The fish, which lived 375 million years ago, "sort of blurs the distinction between fish and land-living animals," said one of its discoverers, paleontologist Neil Shubin of the University of Chicago.
The new find includes specimens, 4 to 9 feet long, found on Canada's Ellesmere Island. The discovery is reported in today's issue of the journal Nature by Shubin, Ted Daeschler of the Academy of Natural Sciences in Philadelphia, and Farish A. Jenkins Jr. of Harvard.
In its day, the creature swam in shallow streams in what was then a subtropical climate, researchers say. A meat-eater, it lived mostly in water.
Yet, its front fins had bones that correspond to a shoulder, upper arm, elbow, forearm and a primitive version of a wrist, Shubin said. From the shoulder to the wrist area, "it basically looks like a scale-covered arm," he said.
"Here's a creature that has a fin that can do push-ups," he said. "This is clearly an animal that is able to support itself on the ground," probably both in very shallow water and for brief excursions on dry land. On land, it apparently moved like a seal, he said.
The name chosen for it is Tiktaalik (pronounced "tic-TAH-lick") roseae, chosen from the traditional language used in Nunavut Territory where it was found. It refers to a large freshwater fish found in the shallows.
The researchers have not yet dug up any remains from the tail end of Tiktaalik's body, so they don't know exactly what the hind fins and tail might have looked like.
As for its head, it was crocodile-shaped like that of early amphibians, with eyes on the top rather than the side. Unlike other fish, it could move its head independently of its shoulders like a land animal. The back of its head also had features like those of land-dwellers. It probably had lungs as well as gills, and it had overlapping ribs that could be used to support the body against gravity, Shubin said.
Yet, the creature's jaws and snout remained very fishlike, showing that "evolution proceeds slowly; it proceeds in a mosaic pattern with some elements changing while others stay the same," Daeschler said.
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