“The turtle shell is a complex structure whose initial transformations started over 260 million years ago in the Permian period,” said lead author Dr. Tyler Lyson of Yale University and the Smithsonian Institution, who traveled to the Karoo Basin in South Africa to examine specimens in the field and in museums with Dr. Gaberiel Bever, an anatomy professor at the New York Institute of Technology College of Osteopathic Medicine. “Like other complex structures, the shell evolved over millions of years and was gradually modified into its present-day shape.”
The turtle shell isn’t really just one thing — it is made up of approximately 50 bones. Turtles are the only animals that form a shell through the fusion of ribs and vertebrae. In all other animals, shells are formed from bony scales on the surface; they don’t stick their bones on the outsides of their bodies.
“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 said. “If you incorporate your ribs into a protective shell, then you have to find a new way to breathe.” Turtles have done just that, with the help of a muscular sling.