A fossil of a reptile that lived in South Africa around 260 million years ago shows that the ancestors of turtles had a flexible ribcage and breathed like humans by alternately expanding and contracting the lungs and thorax, a research has found.
Present-day turtles breathe with the aid of a muscle sling attached to the shell, which contracts and relaxes to aerate the lungs.
"Instead of a rigid plastron and shell like modern turtles, Eunotosaurus africanus had extremely broad, partly overlapping T-shaped ribs," said Torsten Scheyer from the University of Zurich.
However, in due course, the development of a solid shell on the back and belly rendered this kind of respiratory process impossible.
Eunotosaurus evidently not only had reduced back muscles but already possessed a muscle sling that aided respiration, noted the study.
"The small fossil reptile, thus, provides the explanation as to how the vital adaptation of the breathing apparatus could come about in turtle evolution," Scheyer added.
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Eunotosaurus constitutes a morphological link between the body plan of early reptiles and the highly modified body blueprint of the turtles that exist today.
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," the authors concluded in a paper that appeared in the journal Nature Communications.