Life reconstruction of the early proto-turtle Eunotosaurus (foreground) burrowing into the sediments of a dried-out lake to escape the harsh environmental conditions in South Africa 260 million years ago (Illustration: Andrey Atuchin).
In today's turtles the shell has a key protective function. The animals can withdraw into it and protect themselves against predators. No other group of vertebrates has modified its physique to such an extent to develop an impenetrable protective structure. "For a long time, paleontologists and developmental biologists puzzled over the origin of the tortoise shell," explains Torsten Scheyer, a paleontologist at the University of Zurich. Based on fossils and observations of the development of present-day turtle shells, the first more significant changes towards a shell are known to have been the broadening of the ribs. The ribs are required to support the body while moving and play a key role in ventilating the lungs. Heavily broadened ribs result in a stiffened, less mobile torso, which shortens the length of an animal's steps; it moves more slowly and its breathing is affected.
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