Turtle Shells Probably Didn’t Evolve For Protection

July 18, 2016 | Erica Tennenhouse

Artist's concept of an ancient ancestor of modern turtles
Photo credit: Courtesy of Tyler Lyson

Turtle ancestors were skilled diggers.

The function of a trait can shift dramatically over the course of evolution. For example, bird feathers are believed to have originally been used not for flight, but for keeping warm or courtship displays.

New research in the journal Current Biology suggests that although turtle shells currently function as a highly effective shield, protecting all of the vital organs and often the head, they did not start out that way.

The evidence comes from oldest known turtle ancestor, Eunotosaurus africanus, latin for “toothed turtle in a half-shell.” Fossil specimen of this extinct species from South Africa indicate that the ribs were becoming wide and flat in a likely step towards a primitive turtle shell.

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But rather than providing protection for turtle ancestors, these broadened ribs would have actually posed some rather serious problems. Broad ribs stiffen the torso, limiting stride length, and ultimately slowing the animal down. They can also interfere with breathing.

"The integral role of ribs in both locomotion and breathing is likely why we don't see much variation in the shape of ribs," said study lead author Tyler Lyson, from the Denver Museum of Nature & Science, in a press release.

"Ribs are generally pretty boring bones. The ribs of whales, snakes, dinosaurs, humans, and pretty much all other animals look the same. Turtles are the one exception, where they are highly modified to form the majority of the shell."

Modern turtle compared with a fossil of E. africanus

The broad ribs of Euntosaurus africanus (left) evolved for burrowing but later developed into the shell of the modern turtle (right)Courtesy of Tyler Lyson

To understand why these modified turtle ribs evolved in the first place, Lyson and his team examined an E. africanus specimen comprising an entire preserved body that was recently discovered by an 8-year-old boy in South Africa

The specimen’s notable features included a spade-shaped skull, hands larger than its feet, and shoulder blades and forearm bearing large muscle attachment points — all hallmark traits of a digger.

According to the researchers, their discoveries suggest that the broad ribs would have served as a “stable base on which to operate a powerful forelimb digging mechanism.” It appears that the basis of the shell was initially an adaptation for burrowing underground, and not for protection.

The digging bodies of turtle ancestors would have enabled them to move into the water with ease, which is where many turtle species still live today.

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