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By looking at the fossilized skull of Tiktaalik, scientists can tell how the fish was able to breathe air.T. Daeschler

Nine years ago, a team of scientists who study fossils set out in search of a puzzle’s missing piece. This team of paleontologists traveled to a remote island in the Canadian Arctic. They were looking for fossils that would help them understand a prehistoric mystery. They wanted to learn how fish living millions of years ago began to change in ways that allowed later animals to live on land.

Fossils found already suggested that all the land-living backboned animals, or vertebrates, evolved from a type of ancient fish. Many changes over a long period of time led this kind of fish to change, or evolve, over millions of years. These changes eventually led to reptiles, amphibians, birds and mammals. These animals are called tetrapods, a name that refers to their four limbs. 

According to the fossil record, the ancestor, or relative, of the tetrapods was a lobe-finned fish. This fish lived in the water and had fins with features similar to the tetrapods’ four limbs. The fins of the ancient fish were positioned along the side of the body, much like a salamander's legs. And the fins were strong enough to support the weight of the animal's body.

Lobe-finned fish are almost 400 million years old. They are older than tetrapods by about 17 million years. Scientists think that a "missing link" animal lived during this 17-million-year gap. They believe that this animal may have had characteristics of both the lobe-finned fish and the first tetrapods.

Paleontologist Neil Shubin of the University of Chicago hoped to find evidence of this missing link. He and other paleontologists thought that missing link fossils would be found in rocks from the 17-year-gap.

By studying detailed maps and other geologic records, Shubin and his colleagues learned the rocks in the Canadian Arctic were between 360 and 380 million years old. They were the perfect age for beginning the search for the mystery fossils.

After several years of digging, the team began to uncover fish fossils. In 2004, they found 375-million-year-old fossils new to scientists. The fossils belonged to an animal that had gills and other features of lobe-finned fish. But the new animal also had some of the features of tetrapods, such as wrist bones.

The age of the fossils suggests the creature may have been transitional –  halfway between a fish and a tetrapod. Shubin and his colleagues named the animal “Tiktaalik.” The name means "large, freshwater fish" in the language spoken by native people of the Canadian Arctic.

Discovering these fossils was one step in figuring out what kinds of body changes happened as lobe-finned fish adapted to life on land. Just last month – October 2008 – the team reported another important finding. It turns out Tiktaalik had the first neck of all vertebrates. That may not sound quite as impressive as the first legs or lungs. But that neck gave Tiktaalik a huge advantage over its neckless fishy ancestors. A neck allowed Tiktaalik to turn its head. Tiktaalik also had both gills and lungs. Both a neck and lungs let Tiktaalik poke its head out of the water and look around for predators. "Tiktaalik was already specializing in breathing air," Shubin says.

These findings about Tiktaalik suggest it's an important missing link between fish and land vertebrates. But all the new information also makes scientists wonder what they haven’t discovered yet. “The world is peppered with large areas of unexplored rocks of the right age to yield more transitional fossils,” says Jennifer A. Clack, a paleontologist at the University of Cambridge in England.

Power Words

Evolution: gradual change in living things over time

Fossil: hardened remains of plants or animals that lived long ago

Paleontologist: a scientist who studies prehistoric life forms by using evidence such as fossils

Prehistoric: the time period before written records were kept

Tetrapod: a term meaning "four-footed;" tetrapods are a group of animals that evolved from fish. The group includes reptiles, amphibians, birds and mammals.

Vertebrate: an animal with a backbone

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