Turtle shells are unique in the animal kingdom. In order to be able to breathe in this inflexible casing, tortoises have a muscle sling which is attached to the shell to ventilate the lung. A team of researchers including paleontologist Torsten Scheyer from the University of Zurich can now reveal that the turtle's ancestor Eunotosaurus africanus already breathed with the aid of such a sling -- even though it did not yet have a solid shell. The muscle sling was thus the anatomical prerequisite for the development of the rigid turtle shell.
The present-day extinct ancestors of turtles had a flexible ribcage and breathed, like us, by alternately expanding and contracting the lungs and thorax. The development of a solid shell on the back and belly, however, rendered this kind of respiratory process impossible. Today's turtles breathe with the aid of a muscle sling attached to the shell, which contracts and relaxes to aerate the lungs. An international team of researchers from North American, African and European institutes and museums have now discovered the origin of this muscle sling: in Eunotosaurus africanus, a fossil reptile which lived in South Africa during the Middle Permian around 260 million years ago, as the study just published in Nature Communications reveals.
Instead of a rigid plastron and shell like modern turtles, Eunotosaurus merely had extremely broad, partly overlapping T-shaped ribs. "However, these already heavily restricted the freedom of movement of the ribcage"explains Torsten Scheyer from the Paleontological Institute and Museum of the University of Zurich, who is involved in the study. Judging by the internal and external bone structures of the ribs, Eunotosaurus evidently only had reduced back muscles, but already possessed a muscle sling that aided respiration. "The small fossil reptile thus provides the explanation as to how the vital adaptation of the breathing apparatus could come about in turtle evolution," says the UZH paleontologist.
Muscle loop enables shell development
"Eunotosaurus constitutes a morphological link between the body plan of early reptiles and the highly modified body blueprint of the turtles that exist today," explains Scheyer. The scientists studied the rib plates, so-calledcostals, of turtle shells and the ribs of various fossil and living vertebrate groups, including mammals, crocodiles and even dinosaurs. Head of the study Tyler Lyson from the Smithsonian Institution in Washington D.C. and the Denver Museum of Nature and Science, Colorado, adds that, "Based on what we know today, solid shells did not appear in fossil stem turtles until 50 million years after Eunotosaurus."
The study shows that the steady increase of rigidity of the body wall triggered a separation of the rib and abdominal respiratory muscle functions: The increasing broadening and hardening of the body caused the ribs to become less involved in the respiratory process while the muscles increasingly took over the role. "The ribs became thus free and later completely integrated in the turtle's shell," says Scheyer.
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