The shells of the earliest proto turtles began as a simple broadening of the ribs, an adaptation researchers say improved the animal's digging abilities but failed to offer added protection. Photo by the Denver Museum of Nature and Science
DENVER, July 15 (UPI) -- No other animal grows armor quite like the turtle. Despite its obvious protective benefits, new research suggests the adaptation was originally adopted for burrowing.
"Why the turtle shell evolved is a very Dr. Seuss-like question and the answer seems pretty obvious -- it was for protection," Tyler Lyson, a paleontologist with the Denver Museum of Nature and Science, said in a news release. "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."
The new research also confirms the significant sacrifices turtles made for their impenetrable shells. The earliest shells began with the broadening of the ribs, which had a significant effect of the turtle's breathing and movement.
As their ribs broadened, turtles became more restricted in their movement. Their stride shrunk. They became the slow waddlers we know today. Thicker ribs also constrict breathing, which further reduced the turtle's aerobic abilities.
"The integral role of ribs in both locomotion and breathing is likely why we don't see much variation in the shape of ribs," explained 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, where they are highly modified to form the majority of the shell."
The latest breakthrough in understanding the evolution of the turtle shell was made possible by a series of discoveries in South Africa. Researchers there uncovered several neatly preserved proto turtle fossils, including a full skeleton with articulated hands and feet. The fossils revealed some of the earliest signs of broadening ribs.
But these broadening ribs -- the evolutionary seeds of the shell -- offered turtles very little extra protection, at first. As researchers recounted in the journal Cell Biology, however, they did offer a "stable base on which to operate a powerful forelimb digging mechanism."
Researchers suggest their improved digging and burrowing abilities likely allowed turtles to begin exploring new aquatic environs -- an adaptability that may have allowed proto turtles, or stem turtles, to survive the Permian/Triassic extinction event.
