WASHINGTON, Jan. 13 -- An analysis of the 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, reveals that the evolution of hind legs actually began as enhanced hind fins, U.S. researchers said Monday.

The discovery, published online in the U.S. journal Proceedings of the National Academy of Sciences, challenged an existing theory that large, mobile hind appendages were developed only after vertebrates lived on land.

Neil Shubin, professor of the University of Chicago, and colleagues examined five specimens of Tiktaalik roseae remains recovered in 2004, 2006, 2008, and 2013 in northern Canada. The creature represents the best-known transitional species between fish and land-dwelling tetrapods.

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 (about 2.7 meters) as it hunted in shallow freshwater environments.

It had gills, scales and fins, but also had tetrapod-like features such as a mobile neck, robust ribcage and primitive lungs. Its large forefins had shoulders, elbows and partial wrists, which allowed it to support itself on ground.

The researchers studied the rear portion of Tiktaalik, which contained the pelves as well as partial pelvic fin material. The fossils included a complete pelvis, making a direct comparison of the front and rear appendages of a single animal possible.

They found that the pelvis was comparable to those of some early tetrapods and that the pelvic girdle was nearly identical in size to its shoulder girdle, a tetrapod-like characteristic. "This is an amazing pelvis ... which is very different from anything that we knew of in the lineage leading up to limbed vertebrates," said co-author 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... They appear to have used the fin in a way that's more suggestive of the way a limb gets used."

Tiktaalik pelves were still clearly fish-like, with primitive features such as an undivided skeletal configuration, as opposed to the three-part pelvic girdle of early tetrapods, the researchers said.

However, the expanded size, mobility and robusticity of the pelvic girdle, hip joint and fin of Tiktaalik made a wide range of motor behaviors possible, they added.

"It's reasonable to suppose with those big fin rays that Tiktaalik used its hind fins to swim like a paddle," Shubin said. "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 added.