New Ancient Fossils Reveal Link in Hind Limb Evolution

Scientists may have just uncovered new evidence of hind limb evolution. They've found well-preserved pelves and a partial pelvic fin from a 375 million-year-old transitional species between fish and legged animals, revealing a bit more about how fish began to walk on 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," said Neil Shubin, one of the researchers, in a news release. "But it looks like this shift actually began to happen in fish, not in limbed animals."

The fossil that the researchers discovered is that of Tiktaalik roseae. This lobe-finned fish had a broad flat head and sharp teeth, looking a bit like a cross between a fish and a crocodile. It grew up to 9 feet in length and hunted in shallow, freshwater environments. While it had gills, scales and fins, it also had a mobile neck, robust ribcage and primitive lungs. More specifically, its large forefins had shoulders, elbows and partial wrists, which allowed it to support itself on ground.

Yet what is more interesting is the rear portion of this ancient animal. The new discovery shows that the pelvis of this creature is comparable to those of some earlier tetrapods. It possesses a prominent ball and socket hip joint that connects to a highly mobile femur that can extend beneath the body.

"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," said Daeschler, one of the researchers, in a news release. "Tiktaalik was a combination of primitive and advanced features. Here, not only were the features distinct, but they suggest an advanced function. They appear to have used the fin in a way that's more suggestive of the way a limb gets used."

It's very likely that the species used its hind fins to swim like a paddle, but it's also possible it could walk with them. The fact that there's so much emphasis on hind appendages means that it's likely this feature was exaggerated when tetrapods evolved. The findings reveal a little bit more about hind limb evolution.

The findings are published in the journal Proceedings of the National Academy of Sciences.