Fish’s fins evolved into limbs in water

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 Jan. 13 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 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.

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.

“It’s clear that the emphasis on hind appendages and pelvic-propelled locomotion is a trend that began in fish,” he said, adding that the trend was “later exaggerated” during the origin of four-legged animals known as tetrapods in the Late Devonian period, 395 million to 362 million years ago.

Another author of the journal article, Edward B. Daeschler of the Academy of Natural Sciences at Drexel University in Philadelphia, said the pelvis, particularly the hip socket, was “very different from anything that we knew of in the lineage leading up to limbed vertebrates.”

Tiktaalik, which grew as long as nine feet, looked like a cross between a fish and a crocodile. In addition to gills, Tiktaalik had primitive lungs. It also had a robust rib cage, and its large forefin had shoulders, elbows and partial wrists that allowed it to support itself on the ground.

– New York Times