Tiktaalik roseae shows signs of rear-leg development

Fossils of an ancient fish species discovered in the Canadian Arctic show that hind legs evolved from fins some 375 million years ago, challenging a theory that they developed only after vertebrates took to land, according to research published this week.

Ancient fish, discovered in 2004 on southern Ellesmere Island, grew up to 2.7 metres long

The remains of the Tiktaalik roseae were discovered in 2004 on southern Ellesmere Island in northern Canada. (University of Chicago/Beth Rooney/Associated Press)

Fossils of an ancient fish species discovered in the Canadian Arctic show that hind legs evolved from fins some 375 million years ago, challenging a theory that they developed only after vertebrates took to land, according to research published this week.

The remains of the fish, the Tiktaalik roseae, were discovered in 2004 on southern Ellesmere Island in northern Canada by a team of scientists that included a now-deceased Harvard University professor, Farish Jenkins, according to the findings published in the Proceedings of the National Academy of Sciences, a scientific journal.

Researchers had initially focused only on specimens containing the front portion of the fish but subsequently discovered pelvises and partial pelvic fin material from the rear portion that were comparable to those of some early four-legged animals, with a ball-and-socket hip joint.

"This is an amazing pelvis, particularly the hip socket, which is very different from anything that we knew of in the lineage leading up to limbed vertebrates," Ted Daeschler, associate curator of vertebrate zoology at Drexel University in Philadelphia and a co-author of the study, said in a statement.

The prevailing scientific understanding, Daeschler said, was based on "a 'front-wheel drive' idea that these animals were primarily using their front limbs for locomotion," while hind limbs were small and not necessarily used for movement.

"It looks like this shift actually began to happen in fish, not in limbed animals," Neil Shubin, a University of Chicago professor and co-author of the study, said in the statement. "It's reasonable to suppose with those big fin rays that Tiktaalik used its hind fins to swim like a paddle. But it's possible it could walk with them as well."

The Tiktaalik roseae looked like a cross between a fish and a crocodile, with lobe fins, a broad flat head and sharp teeth, according to Drexel. It hunted in shallow freshwater environments and grew to a length of up to 2.7 metres.

It had gills, scales and fins, but also features associated with four-legged animals — a mobile neck, a robust ribcage and primitive lungs — the university said. Its large forefins had shoulders, elbows and partial wrists, allowing it to support itself on ground.

Such features make the fish one of the best-known transitional species between fish and land-dwelling tetrapods, or animals with two pairs of limbs, the university said.

Comments

To encourage thoughtful and respectful conversations, first and last names will appear with each submission to CBC/Radio-Canada's online communities (except in children and youth-oriented communities). Pseudonyms will no longer be permitted.

By submitting a comment, you accept that CBC has the right to reproduce and publish that comment in whole or in part, in any manner CBC chooses. Please note that CBC does not endorse the opinions expressed in comments. Comments on this story are moderated according to our Submission Guidelines. Comments are welcome while open. We reserve the right to close comments at any time.