Fossil fills gap in move from sea to land

Paleontologists have discovered the fossil remains of a 375-million-year-old fish that fills a crucial gap in the evolutionary history of Earth's four-limbed creatures.

Among the fish's most distinguishing features: the skeletal beginnings of shoulders, wrists, and legs that signal a future move to land.

Gently scratched from soil layers on desolate Ellesmere Island in Canada's Nunavut Territory, the new fossils could join the ancient bird archaeopteryx as an icon for evolution in action, some researchers say.

"It's really an amazing find," says Hans-Dieter Sues, associate director of science at the Smithsonian Institution's National Museum of Natural History in Washington.

Scientists have long surmised that four-limbed land animals evolved from early fish. In recent years, they have been slowly closing the fossil gap between these fish and the first land vertebrates. But existing fossils have failed to fill an important, 10-million-year hole in the record, when the fish were undergoing significant physical changes.

Thus, the appearance of the new species is likely to become "a textbook example of the transition" from fish to four-limbers, known as tetrapods, says Edward Daeschler, a paleontologist with the Academy of Natural Sciences in Philadelphia. He and Neil Shubin, of the University of Chicago, led the team that made the discovery, which will publish its findings Thursday in the journal Nature.

Scientists can look at fossils on either side of the gap, then sit back "and philosophize on what intermediate forms should look like," Dr. Sues says. This single find, he continues, clears much of the fog from that view.

The team named the species Tiktaalik roseae, using an Inuit word for large, shallow-water fish. During its heyday, it lived in freshwater streams and ponds under conditions far warmer and more lush than today - at the time, what is now Ellesmere Island was part of the continent Laurentia, which straddled the equator.

The sharp-toothed meat-eater's head gives it a crocodile-like profile - suggesting a fish even Julia Child would have found tough to turn into chowder. But because of the excellent state of preservation, the researchers say they have been able to identify several key features that serve as significant way marks along the path between fish and tetrapod.

Like its immediate ancestors, Tiktaalik is scaly and displays gill and fin features that indicate it lived mostly in the water. But it lost its gill cover and its snout grew, suggesting that changes were under way in the creature's breathing mechanism and in its food sources. It boasts a heftier rib cage than its ancestors, presumably to support it when it leaves the water. It's skull has lost related bones associated with fish but missing in tetrapods. Another gill feature, a tiny slit that became part of the ear in tetrapods, has grown wider. And the bones in the fins along its sides point to proto-limbs with enough strength and flexibility at the shoulder, elbow, and wrist to allow Tiktaalik to lift itself off the bottom and perhaps temporarily move about on land.

The blending of fish and tetrapod features prompted the team to tag the species with the street name "fishapod," according to Dr. Shubin.

The discovery caps a search that spans five field trips over six years aimed at filling this gap in the fossil records. The team knew the kind of rock formations they needed to search, pored over detailed geological maps, then settled on a set of sites to search. Twin Otter aircraft carried the team and its gear from Resolute Bay to the island, then a helicopter ferried them the rest of the way.

"It's always an intimidating thing" when the helicopter's thumpa-thumpa-thumpa fades to silence, Dr. Daeschler recalls. "You're hundreds of miles from anywhere ... working for five to six weeks with temperatures in the 30s and 40s and it's often drizzly and sleety. But the landscapes are naked geology. You're seeing all these processes - not only in the ancient rocks but the modern river erosion and glacial valleys. Every day is exciting because you don't know what the day will hold."