Researchers gained a better understanding of the evolutionary process that helped develop tortoises' unique breathing apparatuses and shells. 

The findings could provide insight into how tortoises breathe and when and how they use their unique breathing apparatus, the University of the Witwatersrand reported. 

"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. 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," said lead author Tyler Lyson of Wits University's Evolutionary Studies Institute, the Smithsonian Institution and the Denver Museum of Nature and Science. 

Tortoises have developed a unique abdominal muscle string that wraps around the lungs and organs and helps them breathe. In the past, it has been a mystery when and how this one-of-a-kind mechanism developed. 

"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. 

Researchers discovered this breathing apparatus was present in the earliest tortoise fossils called Eunotosaurus africanus. The ancient ancestor lived in South Africa 260 million years ago and lacked a shell. The finding helps bridge the gap between the early body plan of tortoises and what they have developed into today. 

The study suggests that, over time, the tortoise's body slowly increased in rigidity and a division of function between the ribs and abdominal respiratory muscles developed. As this occurred, the rib cage became less adept at breathing, which forced the abdominal muscles to overcompensate. 

"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. We plan to study this key aspect to get a better understanding why the ribs started to broaden," Lyson said. 

The findings were published Nov. 7 in the journal Nature Communications.