This fossil fish bridges the evolutionary gap between animals of land and sea. Click on image for full size (331 Kb) Courtesy of Zina Deretsky, National Science Foundation

New research has provided the first detailed look at the internal head skeleton of Tiktaalik roseae, the 375-million-year-old fossil animal that represents an important intermediate step in the evolutionary transition from fish to animals that walked on land.

Results of the study, published in this week's issue of the journal Nature, show that the transition from aquatic to terrestrial lifestyle involved complex changes not only to appendages (fins to limbs) but also to the internal head skeleton.

"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.

A team co-led by scientist Ted Daeschler at the Academy of Natural Sciences in Philadelphia discovered Tiktaalik roseae (tik-TAHL-ik RO-zay) in 2004, in Devonian-age rock on Ellesmere Island in Canada, more than 700 miles above the Arctic Circle.

The creature was a large aquatic predator with a flattened head and body.

The body plan and nature of the deposits where the fossils were found suggest an animal that lived on the bottom in shallow water, and perhaps out of the water for short periods.

Tiktaalik roseae has features of the skull, neck, ribs and appendages that are shared with the earliest limbed animals (tetrapods), as well as fishlike features such as scales and fin rays. This mosaic of features makes it a textbook example of a transitional fossil, say paleontologists.

Jason Downs, a scientist at the Academy of Natural Sciences and lead author of this week's paper, said the examination of the internal head skeleton further demonstrates the intermediacy of Tiktaalik roseae.

"The braincase, palate and gill arches of Tiktaalik help reveal the pattern of evolutionary change in this part of the skeleton," said Downs. "We see that cranial features once associated with land-living animals were in fact the first adaptations for life in shallow water."

"The gradual evolutionary transition from fish to tetrapod, and the transition from aquatic to terrestrial lifestyles required much more than the evolution of limbs," said Daeschler. "The head of these animals was becoming more solidly constructed and, at the same time, more mobile with respect to the body across this transition."

Trends in head shape include a flattening of the skull and a lengthening of the snout.

Using several well-preserved specimens of Tiktaalik roseae, the research helps document the relative timing of the particular skeletal changes associated with changes in head shape.

"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."

During this transition, interactions among the different parts of the head skeleton also were changing.

"Fish in deep water move and feed in three-dimensional space, and can easily orient their bodies in the direction of their prey," said Farish Jenkins, Jr., an evolutionary biologist at Harvard University and co-author of the paper. "A mobile neck is advantageous in settings where the body is relatively fixed, as is the case in shallow water and on land."

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.

The fossil research in Nunavut is carried out with authorization from the Department of Culture, Language, Elders and Youth, Government of Nunavut. All fossils are the property of the people of Nunavut and will be returned to Canada after they are studied.

In addition to NSF, the research was supported by private donors, the Academy of Natural Sciences, the Putnam Expeditionary Fund (Harvard University), the University of Chicago and the National Geographic Society.

Text above is courtesy of the National Science Foundation

This fossil fish bridges the evolutionary gap between animals of land and sea. Click on image for full size (331 Kb) Courtesy of Zina Deretsky, National Science Foundation

Scientists have gotten inside the head of a 375-million-year-old fossil animal.

They have been studying the inside of the head skeleton of Tiktaalik roseae, a fossil animal that was important link in the transition from fish to animals that walked on land.

Tiktaalik roseae (tik-TAHL-ik RO-zay) was a large aquatic predator with a flattened head and body. It was somewhat similar to the earliest four-footed animals that lived on land, but it also had features of fish, like scales and fin rays. Evidence suggests that the creature lived on the bottom in shallow water, and perhaps out of the water for short periods too.

The transition from life in the water to on land involved complex changes. Animals evolved from having fins to having limbs that can walk on land. And their heads changed too.

It took more than a year for scientists to get the delicate fossil head skeleton out of the sedimentary rock in which it was preserved. They found that the head had features that are usually associated with land animals like a flat skull and a long snout. The scientists suspect that these features helped Tiktaalik roseae survive in shallow water.

"Fish in deep water move and feed in three-dimensional space, and can easily orient their bodies in the direction of their prey," said biologist Farish Jenkins, Jr. "A mobile neck is advantageous in settings where the body is relatively fixed, as is the case in shallow water and on land."

This fossil fish bridges the evolutionary gap between animals of land and sea. Click on image for full size (331 Kb) Courtesy of Zina Deretsky, National Science Foundation

Scientists have been studying the head of a very interesting fossil animal.

The animal was large and had a flattened head and body. It lived in shallow water 375-million-years-ago.

It was a bit like the first four-footed animals that lived on land. And it was also a bit like fish that live in the water. It had scales and fins. The scientists think that the animal lived on the bottom in shallow water. It may have been able to get out of the water sometimes too.

It took more than a year for scientists to get the fossil head skeleton out of the surrounding sedimentary rock. Once they did, they found that the head had a flat skull and a long snout, much like some land animals. The scientists suspect that these features helped the animal survive in shallow water.