Based on fossils found a decade ago, scientists had assumed that the earliest known species of fish to make the momentous transition to four-legged animals, capable of living and moving about on dry land, had developed a kind of front-wheel drive. They used enhanced forward fins to crawl out of shallow waters, and only later adapted their rear fins into limbs.

But that was before subsequent discoveries turned up well-preserved hindquarters of Tiktaalik roseae, a transitional species that lived 375 million years ago. New findings, reported Monday in Proceedings of the National Academy of Sciences, challenge the theory that vertebrates did not gain four-limbed mobility until well after they had settled the land.

Five new specimens of Tiktaalik (pronounced tic-TAH-lick) fossils found in the Canadian Arctic reveal that the modification of fins into four limbs actually began as adaptations for life in shallow water, according to a research team led by Neil H. Shubin, an evolutionary biologist at the University of Chicago. The change may have enabled the fish to walk on a lake floor, paddle about and even make brief forays on land, the scientists said.

In an interview, Dr. Shubin said that comparisons of the upper and lower anatomy of a single Tiktaalik fossil showed the hind appendage to be at least as long and complex as the forward appendage. The pelvic girdle, though still fishlike, appeared to be larger and more robust to support strong rear limbs, and the hip joint made a wide range of movements possible.