Wednesday, April 5, 2006

arctic crawling fish discovered ...

`Missing Link' Fossil Discovered of Fish Come Ashore

April 5 (Bloomberg) -- A team of scientists working in the Canadian Arctic dug up skeletons of a huge fish that could crawl across land, a discovery that fills a gap in the fossil record.

The creature lived 375 million years ago in a subtropical wetland, said Edward Daeschler of the Academy of Natural Sciences in Philadelphia, lead author of one of the two papers announcing the find in this week's issue of the journal Nature.

Every limbed creature probably traces its ancestry to one type of fish that started leaving the water for short periods, possibly to get to other ponds or to escape predators, Daeschler said. This fish, the Tiktaalik roseae, is the best-preserved example of such a creature, he said.

``If we were on earth watching for the 10 million years it took for certain fishes to develop limbs, we wouldn't be able to say when it happened,'' Daeschler said. ``It was very gradual.''

Zoologists say arms and legs have internal skeletal bones rather than the fan of bones seen in fish fins. Tiktaalik had both kinds of bones in its front fins. They could bend outward to help it crawl across the ground, possibly like small sea lion flippers.

The animal, which the scientists estimate was as long as 9 feet (274 centimeters), was still distinctly a fish, Daeschler said. It had gills, scales and fins and ate fish rather than a land-based diet. Its jaws were as much as a foot long, set in a triangular, flat head. The front end looked like a crocodile with much stubbier legs. The scientists are unsure how the tail looked.

Preserved in 3-D

Daeschler, along with co-lead author Neil Shubin of the University of Chicago and also Farish Jenkins Jr. of Harvard University, found the fossils in a formerly swampy area that seems to have been flooded, possibly when a riverbank broke open.

At least 10 individual Tiktaaliks were buried quickly, preserving them in three dimensions and keeping most of the bones together, he said.

The eight-member team found the site in 2002 while examining stream sediments on Ellesmere Island in Canada's Nunavut territory. They were there because the rock in that area dates to the period when fish first developed limbs, between 370 million and 380 million years ago.

At the time, northern Canada, Greenland and some of Europe were part of a supercontinent on or near the equator.

The scientists dug up the skeletons of Tiktaalik in 2004. They plan to return this summer to search for more recent skeletons that might show the further disappearance of fin bones as well as to try to determine why this fish found it useful to leave the water.