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 customary 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 tortoiseshell,” 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. This is a reptile living in what is now South Africa 260 million years ago, reports the journal Nature Communications.
While not yet sporting a solid, stiff 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 but had already evolved the muscle sling that would aid in respiration, the researchers say.
“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.
This animal lived in South Africa 260 million years ago and shares many unique features with modern-day tortoises, but lacked a shell. A recognizable tortoiseshell does not appear for another 50 million years.
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 they are the role, they found.
“The ribs became thus free and later completely integrated into the tortoise’s shell,” says Scheyer.
“Named in 1892, Eunotosaurus is one of the earliest tortoise ancestors and is known from early rocks near Beaufort West,” said Professor Bruce Rubidge, Director of the Evolutionary Studies Institute at Wits University and co-author of the paper. “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-called costals, of turtle shells and the ribs of various fossil and living vertebrate groups, including mammals, crocodiles, and even dinosaurs.
“There are some 50 specimens of Eunotosaurus. The rocks of the Karoo are remarkable in the diversity of fossils of early tortoises they have produced.
The fact that we find Eunotosaurus at the base of the Karoo succession strongly suggests that there are more ancestral forms of tortoises still to be discovered in the Karoo,” says one of the scientists, Dr. Rubidge.
The study suggests that early in the evolution of the tortoise 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. This caused the abdominal muscles to become specialized for breathing, which in turn freed up the ribs to eventually become fully integrated into the characteristic tortoiseshell about 50 Million years later.
Lyson and his colleagues now plan to investigate the reasons why the ribs of early tortoises starting to broaden in the first place. “Broadened ribs are the first step in the general increase in body wall rigidity of early basal tortoises.
This ultimately leads to both the evolution of the tortoiseshell and this unique way of breathing. We plan to study this key aspect to get a better understanding of why the ribs started to broaden.”
Turtle shells are unique in the animal kingdom. In order to be able to breathe in this inflexible casing, tortoises have a muscle sling that is attached to the shell to ventilate the lung.
Today’s turtles breathe with the aid of a muscle sling attached to the shell, which contracts and relaxes to aerate the lungs.