WASHINGTON–The most complete fossils ever found of a key transitional fish have been unearthed on Canada’s Ellesmere Island. The fish, a new species that blurs the distinction between fish and land-living animals, paints the clearest picture yet of the paleontological moment when life crawled from water to land, some 375 million years ago.
The three nearly complete specimens were found in 2004 by a team led by Neil Shubin of the University of Chicago and Ted Daeschler of the Academy of Natural Sciences in Philadelphia in collaboration with Farish A. Jenkins of Harvard University. The remarkable discovery is reported in the April 6 issue of the journal Nature. The research was partly supported by the National Geographic Society.
The specimens are exceptionally well-preserved and relatively complete. The new animal is a mosaic: It has the scales and fins of a fish, but the ribs, neck, head and appendage bones are like those of a land animal. Shubin said the animal stands at a critical evolutionary point — one of the crucial events in the story of life on earth. “This animal represents the transition from water to land — the part of history that includes ourselves,” Shubin said. “It’s as much a part of our history as, say, Australopithecus africanus. When we talk about the fish’s wrist, we’re talking about the origin of parts of our own wrist.”
The new species has been named Tiktaalik roseae. The genus name “Tiktaalik” — which means “large freshwater fish” — was supplied by the elders of Nunavut Territory, which encompasses Ellesmere Island in the Canadian Arctic. The specimens have been dated to an age of 375 million years.
“Tiktaalik is a significant and exciting discovery that demonstrates Nunavut’s great potential for scientific research. I wish the team a successful field season in 2006 and look forward to news of more exciting discoveries.” said Louis Tapardjuk, Minister of Culture, Language, Elders and Youth for the Government of Nunavut.
Reaching the tundra site by a series of aircraft including helicopter, the team battled freezing temperatures and high winds to pull the fossils from the frozen rock; 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 carried guns for protection.
The discovery culminated a five-year, 400-mile search in the Arctic for fossils of life from the late Devonian era by Shubin and Daeschler. After learning that Canadian Devonian-age deposits had never been explored, Shubin, Daeschler and Jenkins set out in 1999 to search for elpistostegid fish, a group considered to be most closely related to tetrapods. Tetrapods are animals with limbs and include amphibians, reptiles, birds and mammals — including humans.
Preliminary evidence was found in 2002, and the paleontologists returned to the site two years later with a team to excavate through overlying layers of a rock bluff and collect fish bones piled atop each other by the hundreds.
On the third day of the 2004 expedition, bad weather prevented team members from working at the bluff, so they took a walk. When they sat down for lunch on hills well above the bluff, Shubin spotted in the rock next to him some fragments of elpistostegids. The team realized it was getting close to its target.
The next day at the bluff, while digging in ice-covered rock, Shubin knocked off a piece of rock the size of a large coin and noticed distinctive scales. “I was holding in my hand a half-dollar-sized piece of the side wall of an articulated elpistostegid,” he said.
Later at the bluff team members spied the front end of a fish skull protruding from the rock. “That’s ideal — having the snout sticking out — because in the cliff behind it is likely the rest of the animal,” Shubin said. It was. And within about 25 feet of that specimen, another near-complete specimen was found, this one some 9 feet in length — twice as long as the first. “Within two weeks, we had a total of three articulated specimens of a creature we knew was sitting at the cusp of the transition between aquatic and land-living animals,” Shubin said.
But the most significant discoveries were to come in the laboratory as technicians teased the fossilized bones from the rock. “As each piece of Tiktaalik’s anatomy was exposed, we began to see just how wonderfully intermediate this animal’s features were between land and water,” Shubin said.
First, Tiktaalik had a very flat skull, like that of a crocodile. Yet, it had armor like a fish’s. Second, the fish had a neck, the only fish known to have this feature. “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, when the fossil technicians got to the ends of the animal’s fins, they found wrists and bones similar to fingers.
Finally, in April 2005, the preparation work began to unlock the secrets of Tiktaalik’s trunk. Instead of the tiny rod-like ribs of a fish, the scientists discovered full-fledged ribs — overlapping one another like those of an anteater. “Ribs like that produce a stiff trunk,” said Jenkins. “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.”
That land was a flat coastal plain with shallow, slow-moving rivers that meandered their way to the sea. The area resembled today’s Mississippi River Delta, Jenkins said, and had a sub-tropical climate.
Besides Shubin, Daeschler and Jenkins, members of the 2004 expedition team included Stephen M. Gatesy of Brown University, Jack Conrad, graduate student at University of Chicago, and Corwin Sullivan, graduate student at Harvard.
The research was supported by the National Geographic Society, National Science Foundation, Academy of Natural Sciences, University of Chicago, Putnam Expeditionary Fund of Harvard and an anonymous donor.