Just like the human ribcage or insect exoskeleton, it is no secret that the turtle’s shell is a protective shield. However, new findings suggest that turtle shells didn’t evolve to serve this function; instead they originated as an adaptation for burrowing underground.

An international team of palaeontologists looked at the fossils of turtles from 260 million years ago. These turtles only had partial shells, and evidence suggests that these helped them to burrow underground. The new research was published recently in the journal current biology, and was led by Dr. Tyler Lyson, of the Denver Museum of Nature & Science.

“Why the turtle shell evolved is a very Dr. Seuss-like question and the answer seems pretty obvious — it was for protection,” says Lyson, “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.”

A modified ribcage

“We knew from both the fossil record and observing how the turtle shell develops in modern turtles that one of the first major changes toward a shell was the broadening of the ribs,” explains 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.”

Modification of the rib cage has an effect on breathing and speed in 4-legged animals. Broadened ribs, like the ones in turtles, stiffen the torso. This decreases an animal’s stride, slowing it down, and also effects breathing.

“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 Lyson. However, the ribs of turtles are totally unique.

Turtle shells for digging

Fossils used in the study indicate that the ribs began to broaden to suit a digging lifestyle. Similar to those of modern digging animals, the broad ribs of E. africanus have a stable base on which strong forelimbs can operate for digging. So, the common ancestor of modern turtles had a body plan specialized for digging underground.

The new research was possible thanks to the discovery of a number of partially shelled proto turtles of the species Eunotosaurus africanus, which are 260 million years old. The specimens were found in the Karoo basin area of South Africa. Some were found by members of the research team, Dr. Roger Smith and Dr. Bruce Rubidge, from the University of Witwatersrand in Johannesburg.

The key specimen of the study was discovered by an 8-year-old boy, Kobus Snyman, on his father’s farm in the Western Cape of South Africa. This particular fossil was 15cm long, and not only has a well-preserved skeleton but also fully articulated hands and feet.

Speaking of the find, Lyson said, “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.”

reference

Tyler R. Lyson, Bruce S. Rubidge, Torsten M. Scheyer, Kevin de Queiroz, Emma R. Schachner, Roger M.H. Smith, Jennifer Botha-Brink, G.S. Bever. Fossorial Origin of the Turtle ShellCurrent Biology, 2016; DOI: 10.1016/j.cub.2016.05.020

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