Artistic rendering of the early proto turtle Eunotosaurus (foreground) burrowing into the banks of a dried up pond to escape the harsh arid environment present 260 million years ago in South Africa. Meanwhile, a herd of Bradysaurus (background) congregates around the remaining muddy water. Credit: Andrey Atuchin
A new study by an international group of paleontologists suggests that the broad ribbed proto shell on the earliest partially shelled fossil turtles was initially an adaptation, for burrowing underground, not for protection. “Why the turtle shell evolved is a very Dr. Seuss-like question and the answer seems pretty obvious – it was for protection,” said Dr. 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.”
“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,” said Dr. Lyson. While distinctly broadened ribs may not seem like a significant modification, it has a serious impact on both breathing and speed in quadrupedal animals. Ribs are used to support the body during locomotion and play a crucial role in ventilating the lungs. Broadened ribs stiffen the torso, which shortens an animals stride length and slows it down, interfering with breathing.
A recent study on the oldest proto turtle, Eunotosaurus (left), suggests the broadening of the ribs in turtles was initially an adaptation for burrowing to escape the extremely arid environment of South Africa 260 million years ago. Later the ribs were incorporated into the modern protective turtle shell as found in Pelusios (right). Credit: Luke Norton
Dr. Lyson said “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.”
A big breakthrough came with the discovery of several specimens of the oldest (260- million-year-old) partially shelled proto turtle, Eunotosaurusafricanus, from the Karoo Basin of South Africa. Several of these specimens were discovered by 2 of the study’s coauthors, Drs. Roger Smith and Bruce Rubidge from the University of Witwatersrand in Johannesburg. But the most important specimen was found by a then 8yo S. African boy on his father’s farm in the Western Cape. This specimen, which is about 15 cm long, comprises a well preserved skeleton together with the fully articulated hands and feet.
http://phys.org/news/2016-07-real-turtles-shells.htmljCp
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