Tortoises, because of their solid shells on the back and belly, can't breathe like most animals do, by alternately contracting and expanding the thorax and ribs and the lungs inside. Instead they rely on muscle power, scientists say.

Unable to utilize the more common respiratory process, they depend on a sling of muscles attached to their shells, which wraps around the lungs and contracts and expands to fill and then empty them.

"Tortoises have a bizarre body plan and one of the more puzzling aspects to this body plan is the fact that tortoises have locked their ribs up into the iconic tortoise shell," says study leader Tyler Lyson of the Smithsonian Institution. "No other animal does this and the likely reason is that ribs play such an important role in breathing in most animals including mammals, birds, crocodilians, and lizards."

Now, an international group of researchers says it's identified when tortoises turned from expandable thorax to a muscle sling for breathing, and when tortoises lost their flexible ribcage.

The turning point is exhibited in a fossil of Eunotosaurus africanus, a reptile living in what is now South Africa 260 million years ago, they report in the journal Nature Communications.

While not yet sporting a hard, rigid shell, Eunotosaurus possessed ribs that were broad, T-shaped, and which partly overlapped.

"However, these already heavily restricted the freedom of movement of the ribcage," explains study participant Torsten Scheyer from the Paleontological Institute of the University of Zurich.

Eunotosaurus was in the process of losing its back muscles that would have been involved in the breathing procedure, the researchers say, but had already evolved the muscle sling that would aid in respiration.

"Eunotosaurus constitutes a morphological link between the body plan of early reptiles and the highly modified body blueprint of the tortoises that exist today," says Scheyer.

That body modification did not happen overnight, the researchers stress.

"Based on what we know today, solid shells did not appear in fossil stem tortoises until 50 million years after Eunotosaurus," says Lyson.

To compare respiration processes in a number of animal groups, the researchers looked at the rib plates of tortoise shells, known as costals, and the ribs of various extinct and living vertebrate groups including dinosaurs, crocodiles and mammals.

The steadily increasing rigidity of the body wall as tortoises evolved meant the ribs were becoming less involved in respiration while the muscles increasingly assumed their the role, they found.

"The ribs became thus free and later completely integrated in the tortoise's shell," says Scheyer.