Dr Tyler Lyson of the University of the Witwatersrand has published a paper on the origin of the tortoise's shell, and how it affected their respiratory systems.
Lyson is the lead author of "Origin of the unique ventilatory apparatus of turtles", published in Nature Communications scientific journal on Friday.
"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," Lyson said in a statement.
He is affiliated to Wits's evolutionary studies institute, the Smithsonian Institution, and the Denver Museum of Nature and Science.
Director of the Wits institute, Professor Bruce Rubidge, was Lyson's co-author on the paper.
In most animals, the ribs play an important role in breathing and so evolutionary scientists have been puzzled about how the tortoise's shell developed and the ribs' role in breathing was replaced.
"It seemed pretty clear that the tortoise shell and breathing mechanism evolved in tandem, but which happened first? It's a bit of the chicken or the egg causality dilemma," Lyson said.
He and his colleagues showed that the modern tortoise breathing apparatus was already present in the earliest fossil tortoise, an animal known as Eunotosaurus africanus.
It lived in South Africa 260 million years ago and did not have a shell.
A recognisable tortoise shell appeared only 50m years later.
The research suggested that in early tortoise evolution, the body wall grew rigid, producing a division of function between the ribs and abdominal respiratory muscles.
"Broadened ribs are the first step in the general increase in body wall rigidity of early basal tortoises, which ultimately leads to both the evolution of the tortoise shell and this unique way of breathing," Lyson said.
Next the researchers wanted to study why the tortoise's ribs started to broaden.
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