A new study by an international group of scientists on the earliest partially shelled fossil turtles suggests that the turtle’s broad ribbed proto shell evolved as an adaption for burrowing underground, rather than for protection.
The lead author of the study, Dr. Tyler Lyson of the Denver Museum of Nature and Science says, “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 – we now have early relatives of birds such as tyrannosaur dinosaurs with feathers that definitely were not flying.”
Dr. Lyson suggests that the earliest beginnings of the turtle shell was for digging underground to escape the harsh South African environment where these early proto-turtles lived. And this early evolution of turtles has long been a mystery to scientists.
It all has to do with the ribs. From studying early fossil records, and observing turtles today, one of the first major changes toward a shell was in the broadening of the ribs. Dr. Lyson says that while this may not seem to be a significant modification, it does have an impact on both breathing and speed of four-legged animals.
We know that ribs are used for support and as an aid in respiration. But distinctly broadened ribs make the torso stiff, shortening the animal’s stride, slowing it down and also making breathing more difficult. “The integral role of ribs in both locomotion and breathing is likely why we don’t see much variation in the shape of ribs,” says Dr. Lyson.
“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,” says Dr. Lyson, “They are highly modified to form the majority of the shell.”
The discovery of the proto-turtle fossils
Several fossil specimens of the oldest (260 million years old) partially shelled proto-turtle, Eunotosaurus africanus were discovered in the Karoo Basin of South Africa, including several specimens discovered by co-authors, Drs. Roger Smith and Bruce Rubidge from the University of Witwatersrand, in Johannesburg.
But the prize fossil and most complete specimen was found by then eight-year-old Kobus Snyman while walking on his father’s farm in the Western Cape of South Africa. The specimen proved to be the missing piece of the puzzle.
The specimen is about 15 centimeters in length and in very good condition. The skeleton of Eunotosaurus africanus has fully articulated feet and hands. Professor Rubidge from the University of Witwatersrand in South Africa made it clear how important Snyman’s contribution was to the study.
“I want to thank Kobus Snyman and shake his hand because without Kobus both finding the specimen and taking it to his local museum, the Fransie Pienaar Museum in Prince Albert, this study would not have been possible,” he said.
The study, “Fossorial Origin of the Turtle Shell,” includes authors from the United States, South Africa, and Switzerland, and was published on July 14, 2016, in the journal Current Biology.
