Fish fossils show how fins became limbs

Before early marine species could make the transition to land, they had to develop tools for getting around out of the water. They needed limbs.

Now, thanks to the discovery and study of an ancient fish fossil, scientists are beginning to understand how fins became limbs.

Using CT scans, paleontologists at the University of Chicago created digital 3D models of the fin of the fishapod species Tiktaalik roseae. Researchers described their efforts in a new paper published this week in the journal PNAS.

The study of the evolution of limbs during the Devonian period, some 375 million years ago, has mostly focused on the development of upper arm, forearm, wrist and digits -- knowledge gleaned from the study of ancient endoskeletons, the structure formed by bones and cartilage.

The latest study focuses on fishapod's dermal skeleton, formed by the rays and spines that form the species' fins. Because the dermal skeleton is more fragile, it's less often found intact among ancient fish fossils.

By studying the dermal skeleton of Tiktaalik roseae, scientists were able to gain new insights into the ways early fishapod species began experimenting with limb-like appendages and new types of locomotion -- experiments that predate the emergence of full-fledged limbs and life on dry land.

"By seeing the entire fin of Tiktaalik we gain a clearer picture of how it propped itself up and moved about," paleontologist Neil Shubin, senior author of the new study, said in a news release. "The fin had a kind of palm that could lie flush against the muddy bottoms of rivers and streams."

By creating 3D models of the ancient fin rays, researchers were able to see that these structures were smaller than those of the species' more fish-like predecessors. The fins had also become asymmetrical and developed palm-like muscles along their undersides.

"This provides further information that allows us to understand how an animal like Tiktaalik was using its fins in this transition," said Thomas Stewart, a postdoctoral researcher at the University of Chicago. "Animals went from swimming freely and using their fins to control the flow of water around them, to becoming adapted to pushing off against the surface at the bottom of the water."

Researchers found the fin patterns of Tiktaalik matched those found in the fins of sturgeon and lungfish, including asymmetry of the top and bottom fins, which suggests these evolutionary experiments did more than set the stage for the transition to dry land.

"That gives us more confidence and another data set to say these patterns are real, widespread and important for fishes, not just in the fossil record as it relates to the fin-to-limb transition, but the function of fins broadly," Stewart said.

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