Science - Walking fish reveals fresh evolutionary insights

By Will Dunham

WASHINGTON (Reuters) - An extraordinary fish that existed 375 million years ago had unique features in its head that helped pave the way for vertebrate animals to live on land, scientists said on Wednesday.

Scientists for the first time described features in the underside of the skull of Tiktaalik roseae, the so-called "walking fish" discovered in the Canadian Arctic in 2004. It is considered an important transitional animal in the evolution of fish into amphibians, the first land-dwelling vertebrates.

The findings showed that the migration from water to land was more complicated than merely having a fish's fins transform into legs, the scientists wrote in the journal Nature.

The head showed changes from more primitive fish that helped adapt to the new feeding and breathing conditions presented by a terrestrial environment, the scientists said. Like some other fish of its time, it had gills and lungs.

"It's not to say that Tiktaalik itself is a terrestrial animal. It spent most of its time in water, for sure," Jason Downs of the Academy of Natural Sciences in Philadelphia, one of the researchers, said in a telephone interview.

"So what it's really demonstrating is that many of these changes that are occurring and things that we once associated with terrestrial life are turning out, in fact, to be adaptations for life in shallow water settings that Tiktaalik might had found himself in," Downs added.

PREDATOR

It likely inhabited the mudflats of freshwater flood plains of a subtropical environment. It was a large aquatic predator, measuring up to 9 feet long, with sharp teeth and a flattened head like a crocodile and unlike primitive fish.

It may have been able to exit the water for short jaunts on land.

"Fish in the water, insects on land -- it could feed on all of those if you look at the skull," said Neil Shubin of the University of Chicago, another of the researchers.

Tiktaalik is seen as a forerunner of all land vertebrates including amphibians, reptiles, mammals and eventually people.

The scientists described key features in its head and braincase and the decline in size of a bone called the hyomandibula. In fish, this bone links the braincase, roof of the mouth and gill structures and coordinates their motions during underwater feeding and respiration.

As land animals evolved, the hyomandibula eventually became the stapes, one of the tiny bones in the middle ear.

Tiktaalik has features of some of the more primitive fish it lived with as well as features of the first four-legged amphibians that lived on land. Its fins had discernible wrists and elbows in an evolutionary step toward legs that could be used to walk around on dry land.

The underside of the skull remained encased in rock at the time Tiktaalik's discovery was announced. Using a needle to remove rock grain by grain under a microscope, scientists have painstakingly studied the inside of the creature's skull.

(Editing by Xavier Briand)