Fossil find fills a key missing link
U. of C. scientist says the remains of an ancient fish his team found in the Canadian Arctic show how fins developed into limbs
By Peter Gorner
Tribune science reporter
Published April 6, 2006
Paleontologists announced Wednesday the discovery of a creature they say bridges the gap between life in the water and life on land: the "missing evolutionary link" connecting fish and the first animals that crawled ashore more than 300 million years ago.
The animal, described in the journal Nature, stands at a critical juncture in the story of life on Earth. Scientists believe the transition from fins to limbs was a major evolutionary change that eventually led to the existence of humankind.
Three specimens of the creature, called Tiktaalik roseae, were unearthed from frozen river sediments on Ellesmere Island in the Canadian Arctic. The fossils, 375 million years old, show evidence not only of fish scales and fins, but also of primitive wrists, fingers, ribs and a neck--qualities shared with four-limbed land animals, called tetrapods. Its fins could have flexed and extended like an arm, leg or wing.
Some observers say the animal, which its finders jokingly dubbed a "fishapod," could be an icon of evolution as potent as Archaeopteryx, the feathery fossil showing the transition between reptiles and birds.
"It represents the transition from water to land--the part of history that includes ourselves," said Neil Shubin, the University of Chicago scientist who co-led the expedition that found it. "When we talk about the fish's wrist, we're talking about the origin of parts of our own wrist."
According to H. Richard Lane, program director of the National Science Foundation's division of earth sciences, "human comprehension of the history of life on Earth is taking a major leap forward" with this discovery. The foundation was a major sponsor of the project, along with the National Geographic Society and the researchers' home institutions.
The genus name Tiktaalik (tick-TAH-lick) means "large freshwater fish" in Inuit and was supplied from the elders of Nunavut Territory, which encompasses Ellesmere Island, just 600 miles from the North Pole. "Roseae" came from the name of an otherwise anonymous donor.
Shubin, Ted Daeschler of the Academy of Natural Sciences in Philadelphia and Farish Jenkins of Harvard reported the discovery. The fossil-hunters spent six years searching in the icy desert of the Canadian tundra, so inhospitable they could only work there for one month a year.
Looking for fossils from the late Devonian Period, 380 to 365 million years ago, the team endured 24-hour daylight and sleeting Arctic storms. They armed themselves with shotguns against the possibility of polar bears. "We were always looking over our shoulders; we saw lots of tracks," Shubin said.
Scientists have known that fish evolved into land creatures with backbones and four legs more than 365 million years ago, but the fossil record held large gaps about how exactly this occurred.
After learning that Devonian-age deposits in Canada had never been explored, the scientists set out in 1999 to search for elpistostege fish, the group considered to be most closely related to tetrapods. They worked 12-hour days, often walking 10 to 15 miles a day, scouring the rocks for signs of fossils.
Tantalizing fragments uncovered in 2000 persuaded the scientists to return to the site. More evidence was found in 2002, and the team returned two years later to excavate layers of a rock bluff and to look at fish bones piled there by the hundreds.
On the third day of the 2004 expedition, the team members spotted something embedded in the bluff--a crocodile-like snout.
"We did a few high fives when we uncovered the fossil, but there's only so much celebrating you can do in the Arctic," said Shubin, chairman of organismal biology at the U. of C.
It wasn't until the scientists got the material back to their lab and teased the fossil from the rock that they fully realized what they had.
"What we saw was a real mosaic between characteristics of fish and those previously thought to be only in land animals. The fossil was showing us how creatures were assembled over time to live on land," Shubin said.
Unlike any fish known to exist before or after it, Tiktaalik had jointed bones in its pectoral fins making up parts of an elbow and a wrist and primitive parts of a hand.
The creature would have been capable of a sort of pushup, Shubin said, but its mobility on land would have been limited.
"The skeleton of Tiktaalik indicates that it could support its body under the force of gravity whether in very shallow water or on land," Jenkins said.
Researchers ultimately collected well-preserved material from several specimens ranging from 4 to 9 feet long and determined Tiktaalik was a predator with sharp teeth, a crocodile-like head and a flattened body.
They speculate that crawling out of the water allowed the animal to escape pursuit by other, fiercer aquatic meat-eaters.
"This is an example of an important and extremely interesting transitional form between fish and tetrapods. It's wonderful material. The surprise is that the material is so well-preserved you can study that transition in detail," said John Bolt, curator of fossil amphibians and reptiles at the Field Museum.
The discovery suggests the transformation from life in water to life on land happened gradually in fish living in shallow water.
When Tiktaalik was alive, the land where the scientists found it was located at the equator and had a subtropical climate.
Over millions of years, drifting continental plates brought the area north to the Arctic.
"It's the best fish-fossil found so far to show how the first land animals evolved," said Jenny Clack, a land-animal evolution expert from Cambridge University who contributed an article in Nature explaining the significance of the find and comparing it to Archaeopteryx.
"It confirms everything we thought and also tells us about the order in which certain changes occurred."
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pgorner@tribune.com
