A 375 million-year-old fossil has shed new light on the theory of evolution, challenging the widespread view that large hind appendages first appeared after vertebrates transitioned from the water to land.
A report published in the Proceedings of the National Academy of Sciences said the well-preserved pelves and partial pelvic fin of a Tiktaalik roseae - which looked like a cross between a crocodile and a fish - indicated that hind legs actually began as hind fins.
First discovered in 2004, Tiktaalik roseae are the best-known transitional species which bridged the leap between fish and land-dwelling tetrapods.
Studies of the species had previously indicated that the creature grew up to three metres in length and hunted in shallow freshwater environments.
It was equipped with gills, scales and fins but also had tetrapod-like characteristics such as a mobile neck, rib cage and lungs.
The species also had large forefins, with shoulders, elbows and partial wrists.
However, analysis of fossils recovered from a dig site in northern Canada in 2004 had revealed a fuller picture of the hind quarters of the creature for the first time.
Scientists found a pelvic girdle, a prominent ball and socket hip joint and a highly mobile femur which extended 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 one of the report's co-authors, Edward Daeschler, associate curator of Vertebrate Zoology at the Academy of Natural Sciences of Drexel University.
"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."
