Turtles developed shells as a tool for burrowing underground to escape harsh climatic conditions, a study has found, contradicting the traditional belief that they used their shells for their protection.
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The study was conducted on new fossil material, a 15cm long specimen of the 260-million-year-old, partially shelled, proto turtle or stem turtle, Eunotosaurus Africanus from the Karoo Basin of South Africa, which indicated that the initiation of rib broadening was an adaptive response to fossoriality.
Numerous fossorial animal -- one that is adapted to digging and life underground such as the badger -- correlates are expressed throughout Eunotosaurus' skeleton.
These stem turtles indicate that the shell did not evolve for protection, rather adaptation related to digging was the initial impetus in the origin of the shell.
"The earliest beginnings of the turtle shell was not for protection but rather for digging underground to escape the harsh South African environment where these early proto turtles lived," said lead author Tyler Lyson, Paleontologist Denver Museum of Nature & Science in Colorado, US.
The adaptations related to fossoriality likely facilitated movement of stem turtles into aquatic environments early in the groups' evolutionary history, and this ecology may have played an important role in stem turtles surviving the Permian/Triassic extinction event that occurred about 252-million-years ago, said the paper published in the journal Current Biology.
Further, the developmental and fossil data showed that one of the first steps toward the shelled body plan was broadening of the ribs.
The distinctly broadened ribs -- that play a crucial role in ventilating the lungs and are used to support the body during locomotion -- has a serious impact on both breathing and speed in these quadrupedal animals.
These broadened ribs stiffen the torso, which shortens an animals stride length and slows it down, interfering with breathing.
"We knew from both the fossil record and observations how the turtle shell develops into modern turtles that one of the first major changes toward a shell was the broadening of the ribs," Lyson added.
The broad ribs of Eunotosaurus provide an intrinsically stable base on which to operate a powerful forelimb digging mechanism.
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Most of these features are widely distributed along the turtle stem and into the crown clade, indicating the common ancestor of Eunotosaurus and modern turtles possessed a body plan significantly influenced by digging, the researchers concluded.