While modern-day turtles rely upon their shells as protection from predators, that wasn’t the primary reasons that the reptiles’ bone-based outer casing originally evolved, researchers from the Denver Museum of Nature and Science have revealed in a newly-published study.
Rather, the broad ribbed proto shell found on the earliest partially shelled fossil turtles was an adaptation designed to help the creature burrow underground, not protect itself from becoming lunch for an aggressive carnivore, an international team lead by Denver Museum paleontologist Dr. Tyler Lyson reported in Friday’s edition of the journal Current Biology.
“Why the turtle shell evolved is a very Dr. Seuss-like question and the answer seems pretty obvious – it was for protection,” Dr. Lyson explained in a statement. “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 African environment where these early proto turtles lived.”
These digging adaptations helped facilitate the movement of turtles into aquatic environments early on in the creature’s evolutionary history, and probably played an key role in helping stem turtles survive the Permian/Triassic extinction event, the authors wrote in their study.
Adaptations apparent on newfound proto turtle specimens
Scientists have long been puzzled by the initial evolution of the turtle shell, according to Dr. Lyson. The fossil record and observations of shell development in modern turtles demonstrated that one of the first major physiological changes which takes place is a broadening of the ribs, a change which alters both the breathing and the speed of the reptiles.
“The integral role of ribs in both locomotion and breathing is likely why we don’t see much variation in the shape of ribs,” Dr. Lyson explained. “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.”
He and his colleagues were able to discover new insight about the origin of turtle shells thanks to the discovery of several specimens of a 260-million-year-old, partially shelled proto turtle known as Eunotosaurus africanus, the oldest known creature of its kind, in South Africa’s Karoo Basin. One of those specimens was particularly well-preserved and had fully articulated hands and feet, enabling the researchers to conduct a detailed analysis and draw their conclusions.
The research “indicates the initiation of rib broadening was an adaptive response to fossoriality” or burrowing, Dr. Lyson and his colleagues wrote. Similar to other species with adaptations for digging, these proto turtles had “an intrinsically stable base” that would have helped “operate a powerful forelimb digging mechanism,” as well as other adaptations indicating that its ancestors “possessed a body plan significantly influenced by digging.”
Image credit: Andrey Atuchin