Wits University researchers reveal how the tortoise got its shell

11 November 2014

Ever wondered how tortoises got their shells and how these shy little creatures manage to breathe when they withdraw inside their protective shield? Well, a University of Witwatersrand researcher and his colleagues seem to have come up with an answer to these intriguing questions.

In a paper entitled “Origin of the unique ventilator apparatus of turtles’ published in the scientific journal, Nature Communications, Dr Tyler Lyson from Wits University’s Evolutionary Studies Institute says through careful study of modern and early fossil tortoise, researchers now have better understanding how these creatures developed their breathing apparatus.

“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,’ says Lyson.

Tortoises have developed a unique abdominal muscular sling that wraps around their lungs and organs to help them breathe. But when and how this mechanism developed in unknown.

“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 says.

By studying the anatomy and thin sections (also known as histology), Lyson and his colleagues discovered that the modern tortoise breathing apparatus was already in place in the earliest fossil tortoise, an animal known as Eunotosaurus africanus.

Eunotosaurus africanus lived in South Africa 260-million years ago and shares many unique features with modern day tortoises. However, Eunotosaurus africanus lacked a shell.

Lyson says a recognisable tortoise shell does not appear for another 50-million years and Eunotosaurus africanus bridges the morphological gap between early body plan and the highly modified body plan of living tortoises, making it the Archaeopteryx of turtles.

Sometimes referred to by its German name Urvogel, Archaeopteryx, is a genus of early bird that is transitional between feathered dinosaurs and modern birds.

Co-author of the paper and Director of the Evolutionary Studies Institute at Wits University, Professor Bruce Rubidge, says named in 1892, Eunotosaurus is one of the earliest tortoise ancestors and is known from early rocks near Beaufort West, in the Western Cape.

“There are some 50 specimen 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 suggest that there are more ancestral forms of tortoises still to be discovered in the Karoo,’ Rubidge says.

The study reveals important information about how tortoises have evolved over the years. Lyson and his fellow researchers found 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 – approximately 50-million years later – to become fully integrated into the characteristic tortoise shell,’ says Lyson.

SAinfo reporter