Details of evolutionary transition from fish to land animals revealed

Head skeleton sheds light on intermediate steps

Fossil fish bridges the evolutionary gap between animals of land and sea. Click here for more information.

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.

The head of a fossil specimen of Tiktaalik roseae. Click here for more information.

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.