The newly uncovered fossils of a 375 million-year-old organism give scientists a key piece in the scientific narrative involving fin to limb evolution, thereby casting doubt on the popular theory that the hind limbs were developed only after organisms began mobilizing on land.

Scientists say they discovered the well-preserved pelvic fins from a toothy lobe-fined marine organism called a Tiktaalik roseae. According to a statement issued by University of Chicago Medical Center, the organism could grow up to 9-feet long, hunted in shallow freshwater and, “looked like a cross between a fish and a crocodile.”

What makes this organism really interesting is that it’s the best example of a transitional species between fish and the tetrapods who lived on land. The Tiktaalik roseae had both gills and primitive lungs, and also had shoulders, elbows and wrists large and strong enough to support itself 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, PhD, Robert R. Bensley Distinguished Service Professor of Anatomy at the University of Chicago, in a statement. “But it looks like this shift actually began to happen in fish, not in limbed animals.”

The first Tiktaalik roseae fossils, dubbed NUFV 108, were discovered in 2004 at a dig site in Northern Canada and didn’t include the hind limbs of the organism. According to the University of Chicago, the newly-discovered pelvis and hind fin fossils came from the same huge rock sample that contained the first fossil.

The new pelvis and hind fin pieces of Tiktaalik roseae were startlingly comparable to those of early tetrapods, which are the four-limbed vertebrates that dwelled on land.

“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 Edward Daeschler, PhD, 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.”

“The recently-discovered material allows an updated reconstruction of the skeleton of Tiktaalik roseae,” the authors of the study wrote. “With robust pelvic and pectoral fin and girdles, a flattened head, loss of the extrascapular and opercular bones, and expanded ribs, among other characteristics, Tiktaalik was likely a denizen of a continuum of channel, shallow water, and mudflat habitats where appendage-based support would have been advantageous.”

There’s still no way scientists can definitively say whether Tiktaalik used its hind legs like paddles or if it walked on them the way an African lung fish does on the ground underwater.

However, Shubin doesn’t think it matters, saying, “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.”

The exciting fossil findings were disclosed in the Proceedings of the National Academy of Sciences on January 13.