New fish fossils reveal how hind legs evolved


Scientists have discovered the rear end of an ancient fish - and it provides a "key link" in the evolution of hind legs.

Tiktaalik roseae is a 375 million-year-old fossil fish — nicknamed 'the fishapod' by scientists as it's a transitional species between fish and land-dwelling tetrapods. It was discovered in the Canadian Arctic in 2004.

At the time there was no fossil evidence for the rear end of the fish, but now additional materials have been discovered, which suggest it may have been able to use its hind fins not only as paddles, but also to walk.

In fact the well-preserved pelves and partial pelvic fin from Tiktaalik roseae reveal that the evolution of hind legs actually began as enhanced hind fins!

"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," said team leader Neil Shubin, of the University of Chicago. "But it looks like this shift actually began to happen in fish, not in limbed animals."

With a broad flat head and sharp teeth, Tiktaalik roseae looked like a cross between a fish and a crocodile, growing up to a length of 9' as it hunted in shallow freshwater environments. It had gills, scales and fins, but also a mobile neck, robust ribcage and primitive lungs. In particular, its large forefins had shoulders, elbows and partial wrists, which allowed it to support itself on ground.

The new fossils include the complete pelvis of the original 'type' specimen, making it possible to directly compare the front and rear appendages of the animal.

The scientists were immediately struck by the pelvis — comparable to those of some early tetrapods — and particularly the hip socket, which was very different from anything in the lineage leading up to limbed vertebrates.

Tiktaalik pelves were still clearly fish-like, but the expanded size, mobility and robustness of the pelvic girdle, hip joint and fin of Tiktaalik made a wide range of motor behaviours possible.

 

"It’s reasonable to suppose with those big fin rays that Tiktaalik used its hind fins to swim like a paddle," said Shubin (pictured above). "But it’s possible it could walk with them as well. African lungfish living today have similarly large pelves, and we showed in 2011 that they walk underwater on the bottom.

"Regardless of the gait Tiktaalik used, it’s clear that the emphasis on hind appendages and pelvic-propelled locomotion is a trend that began in fish, and was later exaggerated during the origin of tetrapods," Shubin said.

The new material is described in a paper by Neil Shubin, Ted Daeschler, and Farish A. Jenkins, Jr. in the Proceedings of the National Academy of Sciences.

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