"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.
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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."
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Sues said that now, "protection seems to be the primary role of the shell. Like other parts of the skeleton, the bony shell also provides calcium resources on which the animal can draw -- for example, for getting calcium for eggshells."
So shells have served lots of different functions, past and present, in turtles and their ancestral proto-turtles.
"I think turtles have kept their shell because it works," Lyson said. "Very few other animals possess such a protective feature. Very few animals prey on turtles (humans excluded) as it is so difficult to extract the meat from the shell."
"Turtles found a very successful body plan at 210 million years ago in the late Triassic, and have largely remained unchanged ever since."