Fossil Fish

Scientists have long known that fish evolved into the first creatures on land with four legs and backbones more than 365 million years ago, but they've had precious little fossil evidence to document how it happened.
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Some 375 million years ago, the creature looked like a cross between a fish and a crocodile. It swam in shallow, gently meandering 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.

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The creature was dubbed Tiktaalik (pronounced "tic-TAH-lick") roseae, and also had the crocodile-shaped head 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 were still 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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