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Walking Fish Helps Fill Evolutionary Gap

October 16, 2008

Some 375 million years ago, a unique fish existed with features in its head that helped pave the way for vertebrate animals to live on land, scientists said on Wednesday.

Now, new research is providing the first glimpse at the internal head skeleton of Tiktaalik roseae.

The transition from aquatic to terrestrial lifestyle involved complex changes not only to appendages (fins to limbs) but also to the internal head skeleton, researchers report in the recent issue of the journal Nature.

“Exquisite specimens of Tiktaalik roseae discovered several years ago continue to function as rosetta stones for understanding the emergence of quadripeds on land,” said H. Richard Lane, program director in the National Science Foundation (NSF)’s Division of Earth Sciences, which funded the research.

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.

They discovered 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.

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

“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.

The Tiktaalik was likely up to 9 feet in length with sharp teeth and a flattened head like a crocodile.

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.

“We used to think of this transition of the neck and skull as a rapid event, largely because we lacked information about the intermediate animals,” said Neil Shubin of the University of Chicago, who co-led the team that discovered Tiktaalik roseae. “Tiktaalik neatly fills this morphological gap, and helps to resolve the timing of this complex transition.”

It took more than a year for fossil preparators C. Frederick Mullison, of The Academy in Philadelphia, and Bob Masek, of the University of Chicago, to expose and preserve the delicate details in the fossil head skeleton.

The public can see a cast and a reconstruction of Tiktaalik roseae on permanent display in The Academy’s museum.

Image 1: The head of a fossil specimen of Tiktaalik roseae. Credit: Ted Daeschler

Image 2: Fossil fish bridges the evolutionary gap between animals of land and sea. Credit: Zina Deretsky, National Science Foundation

Image 3: Scientists look for fossil evidence of Tiktaalik on Ellesmere Island, Nunavut Territory, Canada. Credit: Ted Daeschler

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