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Fossil turtles solve mystery

Galápagos tortoise from Santa Cruz island (Photo: Markus Lambertz)

Through careful study of modern and early fossil turtles (including terrapins and tortoises), researchers now have a better understanding of how turtles breathe and the evolutionary processes that helped shape their unique breathing apparatus and turtle shell. The findings reported in Nature Communications on 7 November help determine when and how the unique breathing apparatus of turtles evolved. Lead author Dr. Tyler Lyson of the Smithsonian Institution and Denver Museum of Nature and Science says, “Turtles have a bizarre body plan and one of the more puzzling aspects to this body plan is the fact that turtles have locked their ribs up into the iconic turtle shell. No other animal does this and the likely reason why is because ribs play such an important role in breathing in most animals including mammals, birds, crocodylians, and lizards.”

Instead turtles have developed a unique abdominal muscular sling that wraps around their lungs and organs to help them breathe. When and how this mechanism evolved has been unknown. “It seemed pretty clear that the turtle shell and breathing mechanism evolved in tandem, but which happened first? It’s a bit of the chicken or the egg causality dilemma,” says Lyson.

“We studied the anatomy and bone microstructure of the earliest fossil turtle, Eunotosaurus africanus and found that the muscle insertion markers indicating the presence of intercostal muscles, which are critical for breathing in most other animals, were absent, indicating that the modern turtle breathing apparatus was already in place around 260 million years ago” says Dr Jennifer Botha-Brink of the National Museum, Bloemfontein, the South African palaeontologist who analysed the Eunotosaurus thin sections. This animal shares many unique features with modern day turtles, but lacked a shell. A recognizable turtle shell doesn’t appear for another 50 million years. Lyson says “Eunotosaurus bridges the morphological gap between the early reptile body plan and the highly modified body plan of living turtles, making it the Archaeopteryx of turtles.”

The study suggests that early in the evolution of the turtle body plan a gradual increase in body wall rigidity produced a division of function between the ribs and abdominal respiratory muscles. As the ribs broadened and stiffened the torso, they became less effective for breathing which caused the abdominal muscles to become specialized for breathing, which in turn freed up the ribs to eventually – approximately 50 million years later – to become fully integrated into the characteristic turtle shell.

Lyson says he and his colleagues now plan to investigate reasons why the ribs of early turtles starting to broaden in the first place. “Broadened ribs are the first step in the general increase in body wall rigidity of early basal turtles, which ultimately leads to both the evolution of the turtle shell and this unique way of breathing. I plan to study this key aspect to get a better understanding why the ribs started to broaden.”