375 million-year-old fish fossil reveals evolution of hind limbs

Researchers have discovered a well-preserved pelves and a partial pelvic fin from Tiktaalik roseae, a 375 million-year-old transitional species between fish and the first legged animals, which has revealed that the evolution of hind legs actually began as enhanced hind fins.

This study challenges existing theory that large, mobile hind appendages were developed only after vertebrates transitioned to land.

"Previous theories, based on the best available data, propose that a shift occurred from 'front-wheel drive' locomotion in fish to more of a 'four-wheel drive' in tetrapods. But it looks like this shift actually began to happen in fish, not in limbed animals," Neil Shubin, PhD, Robert R. Bensley Distinguished Service Professor of Anatomy at the University of Chicago, said.

A lobe-finned fish with a broad flat head and sharp teeth, Tiktaalik looked like a cross between a fish and a crocodile, growing up to a length of 9 feet as it hunted in shallow freshwater environments.

The fossils included the complete pelvis of the original 'type' specimen, making a direct comparison of the front and rear appendages of a single animal possible.

The scientists were immediately struck by the pelvis, which was comparable to those of some early tetrapods. The Tiktaalik pelvic girdle was nearly identical in size to its shoulder girdle, a tetrapod-like characteristic.

It possessed a prominent ball and socket hip joint, which connected to a highly mobile femur that could extend beneath the body. Crests on the hip for muscle attachment indicated strength and advanced fin function.

And although no femur bone was found, pelvic fin material, including long fin rays, indicated the hind fin was at least as long and as complex as its forefin.

The study was published in the Proceedings of the National Academy of Sciences.