Turtle shells are some of the most unique armor platings in the natural world, and after a careful combing of the turtle fossil record, researchers have unearthed more clues about the shell's origins through the study of the 260-million-year-old ancestor, Eunotosaurus.
"The turtle shell is a complex structure whose initial transformations started over 260 million years ago in the Permian period," says Yale University's Tyler Lyson. "Like other complex structures, the shell evolved over millions of years and was gradually modified into its present-day shape."
Until 2008, the oldest complete turtle shell fossils dated back around 215 million years. Recently, however, a Chinese fossil from 220 million years ago was found with a full belly piece and a partial back plate. The Eunotosaurus fossil shell is even more rudimentary - being only nine broadened ribs.
Turtle shells, which are made up of approximately 50 bones, are the only ones in the animal kingdom that form from ribs and vertebrae fusing. Most other animals simply have bony plates on the outside of their internal structures. Because turtle shells reappropriate the rib structure away from supporting the respiratory system, turtles have had to develop a muscular hammock to house their vitals.
"The reason, I think, that more animals don't form a shell via the broadening and eventually suturing together of the ribs is that the ribs of mammals and lizards are used to help ventilate the lungs," Lyson says. "If you incorporate your ribs into a protective shell, then you have to find a new way to breathe!"
"It is clear that this novel lung ventilation mechanism evolved in tandem with the origin of the turtle shell," he explained.
You can read the full published study in the journal Current Biology.
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