There's not much doubt about what purpose a turtle shell serves. However, the purpose it initially evolved to serve may surprise you.
A group of paleontologists has shed new light on the evolutionary origins of the turtle shell, and their findings suggest that protection was not its original function.
"Why the turtle shell evolved is a very Dr. Seuss-like question and the answer seems pretty obvious--it was for protection...But just like the bird feather did not initially evolve for flight, the earliest beginnings of the turtle shell was not for protection but rather for digging underground to escape the harsh South Africal environment where these early proto turtles lived," says paleontologist Dr. Tyler Lyson from the Denver Museum of Nature & Science.
Science has long pondered the early evolution of the turtle shell. What scientists did know, thanks to the fossil record and knowledge of the modern turtle shell's development, was that a broadening of the ribs was one of the first significant anatomical steps toward the evolution of the structure.
Although broadened ribs might not seem noteworthy, quite the opposite is true. Rib structure significantly impacts breathing and speed in four-legged animals. Ribs are a crucial support mechanism for the body, and also help ventilate the lungs. Broadened ribs make for a stiffer torso, which results in a shorter stride, slower locomotion, and changed breathing.
According to Dr. Lyson, the key role that ribs play in movement and respiration is why they look so similar across so many species.
"Ribs are generally pretty boring bones. The ribs of whales, snakes, dinosaurs, humans, and pretty much all other animals look the same. Turtles are the one exception, where they are highly modified to form the majority of the shell."
So how did the team figure out that the earliest turtle shells were burrowing tools and not protective covers? Well, they had some help from a couple of 260-million-year-old friends. Two of the study's co-authors, Drs. Roger Smith and Bruce Rubridge from the University of Witwaterstrand in Johannesburg, discovered fossilized specimens of Eunotosaurus, a partially shelled proto turtle in South Africa's Karoo Basin. These, along with an invaluable specimen of the same animal discovered by an eight-year-old South African boy, made the whole study possible.
The team's study of these specimens revealed that broadened ribs alone did not provide much actual protection. The ribs did, however, indicate an adaptive response to the proto turtle's living environment. The broad ribs of the Euntosaurus provided a solid foundation for forelimb digging. In other words, the body plan of the proto turtle, and by extension, today's turtles, is a body plan that facilitates digging. The shell as a protective structure was a later adaptation.
If you're interested in learning more about this research on turtle shell evolution, you can read the study here.