How fish fins evolved on the way to land-ready limbs

Fossil cast of a fin from a juvenile Sauripterus taylori, a late Devonian fish with primitive features of tetrapods. (Credit: Matt Wood)

Fossilized fish from the late Devonian period, roughly 375 million years ago, are shedding light on the evolution of fins as they began to transition into limbs fit for walking on land.

The study uses CT scanning to examine the shape and structure of fin rays still encased in surrounding rock. The imaging tools allowed the researchers to construct digital 3D models of the entire fin of the fishapod Tiktaalik roseae and its relatives in the fossil record for the first time. They could then use these models to infer how the fins worked and changed as they evolved into limbs.

Much of the research on fins during this key transitional stage focuses on the large, distinct bones and pieces of cartilage that correspond to those of our upper arm, forearm, wrist, and digits. Known as the “endoskeleton,” researchers trace how these bones changed to become recognizable arms, legs, and fingers in tetrapods, or four-legged creatures.

A gif of a 3D model of the CT scan of the fin skeleton rotating against a black background
A CT scan of Tiktaalik’s fin skeleton, showing its dorsal rays (yellow) and ventral rays (cyan). (Credit: Tom Stewart)

The delicate rays and spines of a fish’s fins form a second, no less important “dermal” skeleton, which was also undergoing evolutionary changes in this period. Researchers often overlook these pieces because they can fall apart when the animals are fossilized or because fossil preparators remove them intentionally to reveal the larger bones of the endoskeleton. Dermal rays form most of the surface area of many fish fins but were completely lost in the earliest creatures with limbs.

“We’re trying to understand the general trends and evolution of the dermal skeleton before all those other changes happened and fully-fledged limbs evolved,” says Thomas Stewart, a postdoctoral researcher at the University of Chicago who led the new study. “If you want to understand how animals were evolving to use their fins in this part of history, this is an important data set.”

Stewart and his colleagues worked with three late Devonian fishes with primitive features of tetrapods: Sauripterus taylori, Eusthenopteron foordi, and Tiktaalik roseae, which a team led by paleontologist Neil Shubin, the senior author of the new study, discovered in 2006. Sauripterus and Eusthenopteron were believed to have been fully aquatic and used their pectoral fins for swimming, although they may have been able to prop themselves up on the bottom of lakes and streams.

Tiktaalik may have been able to support most of its weight with its fins and perhaps even used them to venture out of the water for short trips across shallows and mudflats.

Tiktaalik seen from above looks a bit like alligator with a fish tail. The fishapod has 4 fins and a long finned tail. It's covered in leopard like dark spots
Tiktaalik roseae, a 375-million-year-old “fishapod” has features of both fish and four-legged tetrapods. (Credit: Flick Ford)

“By seeing the entire fin of Tiktaalik we gain a clearer picture of how it propped itself up and moved about. The fin had a kind of palm that could lie flush against the muddy bottoms of rivers and streams,” Shubin says.

The researchers scanned specimens of these fossils while still encased in rock. Using imaging software, they then reconstructed 3D models that allowed them to move, rotate, and visualize the dermal skeleton as if it were completely extracted from the surrounding material.

The models showed that the fin rays of these animals were simplified, and the overall size of the fin web was smaller than that of their fishier predecessors. Surprisingly, they also saw that the top and bottom of the fins were becoming asymmetric. Pairs of bones actually form fin rays. In Eusthenopteron, for example, the dorsal, or top, fin ray was slightly larger and longer than the ventral, or bottom one. Tiktaalik’s dorsal rays were several times larger than its ventral rays, suggesting that it had muscles that extended on the underside of its fins, like the fleshy base of the palm, to help support its weight.

“This provides further information that allows us to understand how an animal like Tiktaalik was using its fins in this transition,” Stewart says. “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.”

Stewart and his colleagues also compared the dermal skeletons of living fish like sturgeon and lungfish to understand the patterns they were seeing in the fossils. They saw some of the same asymmetrical differences between the top and bottom of the fins, suggesting that those changes played a larger role in the evolution of fishes.

“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.”

The study appears in the Proceedings of the National Academy of Sciences.

Support for the study came from the Brinson Foundation, the Academy of Natural Sciences, the University of Chicago Biological Sciences Division, and the National Science Foundation. Additional authors are from the University of Wisconsin-Parkside and Drexel University.

Source: University of Chicago