Ancient pelvic fin fossils may reveal how back legs developed.
The bones came from a 375 million-year-old Tiktaalik roseae, which is believed to be a transitional species between "fish and the first legged animals," a University of Chicago Medical Center news release reported.
The finding suggests that "mobile hind appendages" started out as "enhanced" hind fins and that back legs only popped up after these creatures had transitioned to 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," Neil Shubin, PhD, Robert R. Bensley Distinguished Service Professor of Anatomy at the University of Chicago and corresponding author of the study, said. "But it looks like this shift actually began to happen in fish, not in limbed animals."
The Tiktaalik roseae is the best example of a transitional species between land and sea that scientists have seen. The ancient creature boasted sharp teeth and a flat head; it was believed to have looked like a mix between a crocodile and shark that could grow up to nine feet long.
The creature had "gills, scales and fins," as well as a ribcage, movable neck, and even "primitive" lungs; these features are closer to tetrapod than fish. The Tiktaalik roseae is also believed to have had elbows and writs which allowed it to support itself on land.
What made this animal really stand out was its pelvis. The pelvic girdle was about the same size as the shoulder girdle, it also contained a ball-in-socket joint that may have connected to a femur. There were also crests on the hips which suggested strong muscles were attached.
"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," co-author Edward Daeschler, PhD, Associate Curator of Vertebrate Zoology at the Academy of Natural Sciences of Drexel University said. "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 fossil's pelves were still relatively fish-like, but the extraordinary pelvis would have made a variety of movements possible.
"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," he said.
