The pelvis of a 375 million-year-old transitional species between fish and the first legged animals has been discovered by scientists in northern Canada.
Tiktaalik Roseae looked like a cross between a crocodile and a fish. It could grow up to nine feet in length and hunted in shallow freshwater environments.
The creature had gills, scales and fins, but also tetrapod-like features, such as a mobile neck, strong ribcage and primitive lungs. Its large fore-fins also had shoulders, elbows and partial wrists, meaning it could support itself on the ground.
Researchers at the University of Chicago discovered a number of well-preserved pelvises and a partial pelvic fin from the Canadian site and have said it shows the evolution of hind legs began as enhanced hind fins – challenging the theory that large, mobile hind appendages evolved after animals moved onto land.
Published in the Proceedings of the National Academy of Sciences, corresponding author Neil Shubin said: "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."
Before the pelvis and pelvic fin, only the front portion of Tiktaalik had been described.
Looking at the new specimens, researchers said they were surprised at how comparable the pelvis was with that of early tetrapods. It had a prominent ball and socket hip joint that connected to a highly mobile femur, meaning it could extend beneath the body.
Study co-author Edward Daeschler said: "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.
"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."
Shubin added: "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. African lungfish living today have similarly large pelvis, 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."