Walking away from a fish-eat-fish world.

Hynerpeton bassetti lacked the tough body and huge teeth so prevalent among the heavies in Pennsylvania swamps 365 million years ago. But this newly discovered creature did sport an edge over the fierce fish it competed against. H. bassetti had lungs and limbs capable of carrying it onto land -- making it one of the earliest vertebrates to conquer the continents.

The H. bassetti fossil found last year is the oldest known tetrapod -- or four-legged vertebrate -- in North America and only the second oldest in the world. Despite its pioneering position, however, the animal displayed adaptations to land locomotion even more advanced than those seen in amphibians that came several million years later, says Edward B. Daeschler of the Academy of Natural Sciences of Philadelphia. Daeschler and his coworkers describe the animal in the July 29 Science.

The researchers have thus far unearthed part of the skull and a well-preserved shoulder, which reveal critical information about the capabilities of H. bassetti. "We were very surprised, because we did not expect a tetrapod this early to have this level of specialization," says Neil H. Shubin of the University of Pennsylvania in Philadelphia, who collaborated with Daeschler.

H. bassetti had robust shoulder bones with scars that indicate the attachment point of well-developed muscles. The animal could have made strong limb motions both up and down and front to back in a decidedly unfishlike manner, says Daeschler, who hedges on the question of exactly how this animal moved when on dry land. "We haven't said it was a walker, but it was capable of pretty good motion on land," he says.

At the time the creature lived, in the late Devonian period, Pennsylvania lay near the equator on the coast of a broad sea that filled much of ancestral North America. Previous discoveries have turned up early tetrapods in Greenland, Scotland, Russia, Latvia, and Australia. By extending the record to North America, the Pennsylvania find demonstrates that different groups of amphibians were crawling up on land across much of the equatorial world.

"This contributes to us moving away from the naive picture so often caricatured in cartoons of one beach and one fish with its little flag saying' This is where fishes invaded the land,'" says paleontologist Michael I. Coates of the Museum of Zoology at the University of Cambridge in England.

For decades, researchers have speculated that tetrapods evolved because the harsh climate often dried up lakes and rivers, bestowing an advantage on those that could migrate across land to new water holes. But more recently some have reasoned that evolution may have favored amphibians because they could escape the competition in the water and exploit new resources on land.

Judging from the giant fish that inhabited the same swamps as H. bassetti, Shubin favors the later interpretation. "It's an arms race in these swamps. Everything is getting bigger, developing huge teeth and armor. What this thing seems to be doing is not getting bigger or more armored. It seems to be leaving the fray. It's winning the battle by avoiding it," he say.