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

Details of Evolutionary Transition from Fish to Land Animals Revealed
News story originally written on October 15, 2008

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

Last modified January 16, 2009 by Lisa Gardiner.

You might also be interested in:

Traveling Nitrogen Classroom Activity Kit

Check out our online store - minerals, fossils, books, activities, jewelry, and household items!...more

What Is a Fossil?

Fossils are evidence of ancient life preserved within sedimentary rocks. They are clues to what living things, ecosystems, and environments were like since life has existed on this planet. The oldest...more

Evidence of Evolution

This Windows to the Universe Exploratour examines the scientific evidence of biological evolution. Take the tour to travel through 10 web pages about the scientific theory that explains how and why living...more

The Arctic: Earth's North Polar Region

North of the Arctic Circle (at 66.5°N latitude) you will find the Arctic Ocean surrounded by the northernmost parts of the continents of Europe, Asia, and North America. You will find the geographic North...more

Triggers of Volcanic Eruptions in Oregon's Mount Hood Investigated

A new study has found that a mixing of two different types of magma is the key to the historic eruptions of Mount Hood, Oregon's tallest mountain, and that eruptions often happen in a relatively short...more

Oldest Earth Mantle Reservoir Discovered

Researchers have found a primitive Earth mantle reservoir on Baffin Island in the Canadian Arctic. Geologist Matthew Jackson and his colleagues from a multi-institution collaboration report the finding--the...more

It’s Not Your Fault – A Typical Fault, Geologically Speaking, That Is

Some geologic faults that appear strong and stable, slip and slide like weak faults. Now an international team of researchers has laboratory evidence showing why some faults that 'should not' slip are...more

Extended Period of Lower Solar Activity Linked to Changes in Sun's Conveyor Belt

A new analysis of the unusually long solar cycle that ended in 2008 suggests that one reason for the long cycle could be a stretching of the sun's conveyor belt, a current of plasma that circulates between...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA