January 7, 2010

Scientists Reconsider When Animals Left Water For Land

Fossilized footprints of a mysterious, long-extinct creature in a Polish quarry have caused paleontologists to reconsider traditional thinking of how sea-based vertebrates moved to land.

Until now, scientists have thought they understood the evolutionary transition from fin to foot fairly well.

One of the key theories in evolutionary biology is that tetrapods, four-legged animals with a spine, came from fish that had pairs of lobed fins.

Fish called elpistostegids were the intermediate stage in the process. Its head and body were shaped like a tetrapod, but it still retained fins instead of hands and feet, according to this theory.

The most well known creature from this stage is Tiktaalik, a large shallow-water fish whose fossilized remains were discovered in 2006 just 600 miles short of the North Pole, dug out from river sediments on Canada's Ellesmere Island.

This particular fish existed about 375 million years ago, although elpistostegids dating back 385 million years have also been found.

However, trackways found at a disused quarry at Zachelmie in the Holy Cross Mountains of southeastern Poland have caused scientists to rethink the timeline and role of the elpistostegids.

A team headed by tetrapod expert Per Ahlberg of Uppsala University, Sweden, report in the British weekly journal Nature the finding of a dozen distinctive "hand" and "foot" prints from a creature that lived around 395 million years ago.

This means that they are 18 million years older than the earliest tetrapod fossils found yet, and 10 million years earlier than the first known elpistostegids.

The imprints are as wide as 10.3 inches, indicating that the tetrapod was approximately 8.1 feet long.

Paleontologists saw no signs of body drag, which suggests that the tetrapod would have had to float on the water while walking on the muddy bottom.

The find blows all traditional thought out of the water since it implies that tetrapods emerged much earlier than was previously thought.

Rather than inhabiting river deltas and lakes before tracking onto land, they flourished in shallow seas, traveling through the mud of coral-reef lagoons.

The new theory suggests that the elpistostegids unearthed so far were merely a failed branch rather than part of the stem from which all land vertebrates, including us, evolved.

The prints "force a radical reassessment of the timing, ecology and environmental setting of the fish-tetrapod transition, as well as the completeness of the body fossil record," says the study.


Image Caption: Life restoration of Tiktaalik roseae, a transitional fossil ("missing link") between sarcopterygian fishes and tetrapods from the late Devonian period of North America. Zina Deretsky, National Science Foundation


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