All’s well that ends up in a tortoise’s iconic shell

PROFESSOR Bruce Rubidge digs rocks. But not just any old rocks. Fossils gleaned from the Karoo that document the early evolution of tortoises, dinosaurs and mammals.

Rubidge, of the School for Geosciences at Wits, notes how the Karoo is the only place on Earth “where such an extended fossil record of early evolution of reptilian life is preserved in a single basin”.

And it’s these remarkable fossils recovered from the Karoo that have helped Rubidge and his colleague, lead author Dr Tyler Lyson, of Wits’ evolutionary studies institute, to unravel one of the more bizarre aspects of tortoise anatomy: just why their ribs are embedded in their shell.

Their research has shown that the modern tortoise breathing apparatus was already in place in the earliest fossil tortoise, an animal called Eunotosaurus africanus, which lived in South Africa 260 million years ago.

Yesterday, their findings were published in a paper, Origin of the unique ventilatory apparatus of turtles, in the scientific journal, Nature Communications.

Fascinated by the “bizarre body plan” of tortoises, 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 which caused the abdominal muscles to become specialised for breathing, which in turn freed up the ribs to eventually become fully integrated into the characteristic tortoise shell.”

They noted how one of the more puzzling aspects to this body plan is 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 an important role in breathing in most animals. Tortoises have developed a unique abdominal muscular sling that wraps around their lungs and organs and helps them breathe. But how this evolved is a mystery.”