Turtles Evolved Shells for Digging, Not Protection

Artistic rendering of the early turtle ancestor Eunotosaurus (foreground) burrowing with a herd of Bradysaurus nearby.

The age-old question, "Why do turtles have shells?" would seem to have an obvious answer, protection, yet new research finds that turtles first evolved shells to help with digging underground to escape hot, dry environments.

The findings, published in the journal Current Biology, add to the growing list of key anatomical features that evolved to solve one problem but were then applied to another function. The features, known as "exaptations," include everything from feathers to the gas bladder of early fish, which evolved to become lungs in terrestrial animals.

The new study focused on a 260-million-year-old fossil of the oldest "proto turtle," or early turtle ancestor, which in this case is called Eunotosaurus africanus. Its remains were found by then 8-year-old Kobus Snyman, who spotted it on his father's farm in the Karoo region of South Africa.

"The weather was extremely arid in the Karoo Basin of South Africa 260 million years ago," lead author Tyler Lyson said. "A common mechanism for dealing with arid environments, among both extant and extinct animals, is to burrow underground. Burrows provide a more moderate and stable environment."

Lyson, who is the curator of vertebrate paleontology at the Denver Museum of Nature & Science, and his colleagues studied the fossil found by Snyman, as well as other Eunotosaurus specimens unearthed by two of the study's co-authors, Roger Smith and Bruce Rubidge from the University of Witwatersrand.

They determined that the animal's broad ribs, which over time replaced soft tissue with bone, provided a stable base from which Eunotosaurus could operate its front leg digging.

"A stable base is needed to counteract the force of the digging mechanism, in this case the forelimbs," Lyson explained. "Digging animals thus have adaptations to dig burrows (large hands, large claws, strong forelimbs, etc.) and adaptations to deal with counteracting the digging mechanism force."

The modification of the ribs that eventually led to the shell resulted in other dramatic changes, since the ribs and nearby muscles are involved in both breathing and locomotion. In fact, some early animals could not breathe and run at the same time. To this day, animals that retain such an early breathing mechanism, such as lizards, must hold their breath as they run.

In turtles, as the ribs broadened over millions of years, they became less effective at helping to ventilate the lungs and more associated with locomotion, according to the researchers. The nearby hypoxial muscles, on the other hand, took on a purely respiratory role over time.

"Such a division of function allows turtles to breathe and walk simultaneously and helps them deal with the constraint of having a dual function for both the ribs and muscles," Lyson said.

The extreme changes turned out to be extremely fortuitous for turtles. When the Permian-Triassic mass extinction occurred 252 million years ago, burrowing likely helped to save them, Lyson believes.

When the Triassic-Jurassic extinction event happened much later, 66 million years ago, turtles had evolved to become largely aquatic, with their water environment helping to buffer them from the extinction that killed off dinosaurs (save for some birds) and many other animals. The ability to dig well helped the turtles live on both land and in water.

Hans-Dieter Sues, curator and department chairman of vertebrate paleontology at the Smithsonian Museum of Natural History, told Discovery News that shells helped to protect the vital organs of water-dwelling early turtles. He also believes that shells "may have initially helped with buoyancy control by making the animal heavier."

Read more at Discovery News