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"Long before it's in the papers"
August 03, 2010

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“Missing link” walking-fish fossils awe scientists

April 5, 2006
Special to World Science

Scientists are hailing a set of newfound fossils as a “missing link” that documents one of evolution’s most spectacular transitions: the shift from water to land.

The fossils come from a fierce fish with bones in its fins, which gave it enough strength to walk on land a bit, researchers say.

A Tiktaalik fossil. (Credit: Ted Daeschler)

Tiktaalik, center, is believed to fill an evolutionary gap between two species it is shown between: an earlier fish of 380 million years ago, of an ancestral group known as lobe-finned fish (bottom right) and a later land-dwelling animal of 360 million years ago (top left). (Credit: Kalliopi Monoyios)

An artist's interpretation of the animal. (Courtesy National Geographic Society)

A map of the site where the fossils were found. (Credit: Kalliopi Monoyios)

“Human comprehension of the history of life on Earth is taking a major leap forward,” said H. Richard Lane, director of sedimentary geology and paleobiology at the U.S. National Science Foundation in Arlington, Va., which helped fund the research.

Dubbed Tiktaalik roseae, the species “blurs the boundary between fish and land-living animal,” said the University of Chicago’s Neil Shubin, co-leader of the research team. 

The group published its findings in two papers in the April 6 issue of the research journal Nature.

Tiktaalik was a predator with sharp teeth, a crocodile-like head and a flattened body that seems to have lived in shallow streams, scientists said. Well-preserved fossils from several specimens indicated a length from 4 to 9 feet long. 

About 375 million years old, it had a skull, neck, ribs and part of a fin like the earliest limbed animals, but also fins, scales and jaws like a fish, the researchers said.

The fossils, scientists argued, show how the fish’s pectoral fins—those behind the head—evolved into the limbs of tetrapods, or four-limbed animals. 

The fins contain bones that correspond to the upper arm, forearm and primitive parts of the hand of land-living animals, they explained.

The skeleton “indicates that it could support its body under the force of gravity whether in very shallow water or on land,” said Farish Jenkins of Harvard University and a co-author of the papers describing the research. “This represents a critical early phase in the evolution of all limbed animals.” 

“Most of the major joints of the fin are functional in this fish,” Shubin said. “The shoulder, elbow and even parts of the wrist are already there and working in ways similar to the earliest land-living animals.” 

“When we talk about the fish’s wrist, we’re talking about the origin of parts of our own wrist,” he added. “This animal is both fish and tetrapod.”

The fins have bones indicative of powerful, mobile appendages, the researchers said, but also contain in reduced form the thin rods found in normal fish fins. The wide, flattened body was described as tetrapod-like, but covered by scales like those of fish.

The fossils were found on Ellesmere Island in Arctic Canada, which during the animal’s time had a warm, subtropical climate like that of the Amazon basin today, the scientists explained. The species would have lived in the small streams of this system, an environment conducive to the water-land transition. 

“We knew that the rocks on Ellesmere Island offered a glimpse into the right time period and the right ancient environments” for such fossils, said Ted Daeschler of the Academy of Natural Sciences in Philadelphia, a co-leader of the project. 

But it wasn’t easy to find them in the remote, rugged terrain, he added.

Paleontologists collected the specimens during four summers of exploration in Canada’s Nunavut Territory, 600 miles from the North Pole. The fossils turned up in a remote valley of the island, where scientists said tantalizing fragments found in 2000 convinced them to return to the site.

Reaching the tundra site by helicopter and other aircraft, researchers said they battled frigid cold and biting winds to pull the fossils from frozen rock. Researchers carried guns because “we were always looking over our shoulders for polar bears—we saw lots of their tracks,” Shubin said.

The finds came from layered rock of the so-called Fram Formation, deposits of meandering stream systems thought to have formed about 375 million years ago when North America was part of a supercontinent straddling the equator. These fossils and previously known fossil relatives suggest the evolution from fish to tetrapod occurred on this land mass, the researchers added.

Rather than using the traditional Latin or Greek to name the fossil, the scientists said, they asked local Nunavut residents to suggest a name. Community elders suggested Tiktaalik (tic-TA-lick), meaning large, shallow-water fish in the Inuktikuk language. The second part of the name, roseae, honors someone whose name is being kept under wraps because he or she helped fund the project on condition of anonymity.

Other fish species, notably the coelacanth, have gotten great attention among scientists and the public as representatives of ancient lineages close to the water-land transition in evolution. But coelacanths represent a class of fish thought to have preceded Tiktaalik evolutionarily, and thus aren’t seen as the missing link in the land-water transition. 

Nor are they the closest to such that has been found before: until now, the most tetrapod-like fish known was a vaguely crocodile-shaped predator from about 385 million years ago, Panderichthys, according to a commentary in the journal by Per Erik Ahlberg of Uppsala University in Sweden and Jennifer Clack of Cambridge University in the U.K. Fossils suggest this creature “was beginning to ‘walk,’ but perhaps in shallow water rather than on land,” they wrote.

On the other side of the transition, they wrote, the earliest fragmentary tetrapods come from about 10 million years later, so that was the length of the “gap” for which fossils were missing. “Into this gap drops Tiktaalik,” they wrote.

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WORLD SCIENCE

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