Missing link between fishes and land animals found!

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Science & Medicine


Fossil Said to Be Missing Link Between Sea, Land Creatures

By Thomas H. Maugh II, Times Staff Writer
12:40 PM PDT, April 5, 2006

U.S. researchers have found the missing evolutionary link between fish and land animals — fossils of a strange creature that first crawled onto the shore about 375 million years ago.

The fossils, found on Ellesmere Island in Arctic Canada, have the skull, neck, ribs and limb bones of four-limbed animals, but also the primitive jaw, fin and scales of fish, according to a report to be published Thursday in the journal Nature.

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"This really is what our ancestors looked like when they began to leave the water," according to an editorial accompanying the report. The newly discovered species, called Tiktaalik roseae, "blurs the boundary between fish and land-living animal both in terms of its anatomy and its way of life," said biologist Neil Shubin of the University of Chicago, one of the co-leaders of the expedition.

The creature lived in shallow waterways, where it hunted for prey with its mammal-like snout and sharp teeth, but it was able to pull itself out of the water for short periods of time and move around on its limb-like fins.

The fossils, ranging in length from four to nine feet, were remarkably well preserved, allowing the team to examine the joints carefully and to conclude that the shoulder, elbow and wrist joints were sufficiently strong to support the animal's body on land.

"Human comprehension of the history of life on Earth is taking a major leap forward," said H. Richard Lane of the National Science Foundation, which funded the research along with the National Geographic Society and others.

"These exciting discoveries are providing fossil Rosetta stones for a deeper understanding of this evolutionary milestone — fish to land-roving tetrapods," he said.

In the Late Devonian period nearly 400 million years ago, the landmass where the fossils were found straddled the Equator and had a climate much like that now found in the Amazonian Basin. It was a flat coastal plain with shallow, slow-moving rivers that meandered to the sea.

"This kind of shallow stream system seems to be the place where many features of land-living animals first arose," said expedition co-leader Ted Daeschler of the Academy of Natural Sciences in Philadelphia.

But as the Earth's continental plates shifted, the mass was carried far north, to Canada's Nunavut Territory, more than 600 miles north of the Arctic Circle. Finding and extracting the fossils presented major challenges, starting with the need to helicopter into the region.

Freezing temperatures and high winds limited the amount of time the team could work each day, and the near-constant precipitation prevented the plaster used in the fossil-preservation process from drying.

"And we were always looking over our shoulders for polar bears. We saw lots of their tracks," Shubin said.

Team members all carried guns for protection.

The key breakthrough came on the 2004 expedition — one of five yearly trips — when team members spied the front end of a fish skull sticking out of the bluff. "That's ideal — having the snout sticking out — because in the cliff behind it is likely the rest of the animal," Shubin said.

The team ultimately found three nearly complete specimens, but they weren't totally sure of what they had until they returned to the lab and started studying the bones.

"As each piece to Tiktaalik's anatomy was exposed, we began to see just how wonderfully intermediate this animal's features were between land and water," Shubin said.

The creature had a very flat skull, like that of a crocodile, but it had armor like a fish. While crocodiles are reptiles, Tiktaalik also has features of a fish.

Moreover, it had a neck, making it the only fish known to have one. "The neck was one of the biggest surprises," Daeschler said. "This freed the skull from the shoulder girdle and gave the animal extra mobility."

Third, at the ends of the powerful fins, the team found wrists and bones similar to fingers. But the fins also contained the thin rods found in fish fins. "Here is a creature with fins that can do push-ups," Shubin said.

Finally, instead of the tiny rod-like ribs of a fish, Tiktaalik had full-fledged ribs that overlapped one another like those of an anteater. "Ribs like that produce a stiff trunk," said Farish A. Jenkins of Harvard University, another expedition leader.

"Fish that stay in the water are buoyant and don't need that, so this animal must have developed these structures for life in the shallows and making excursions onto land," he said.

"This animal is both fish and tetrapod," Shubin said. "We jokingly call it a fishapod."

Rather than follow the conventional protocol of using Latin for a new species name, the research team asked the Nunavut Council of Elders for suggestions. They recommended Tiktaalik, which means "a large, shallow-water fish" in the Inuktitut language. Roseae honors an anonymous donor.