How The Turtle Got Its Shell
Turtles are iconic, instantly recognisable from their strange beaked mouths and enormous protective shells. In fact turtles, and land-dwelling tortoises, have been a feature of our planet’s animal life for over 200 million years. Exactly when and how turtles evolved however, is still hotly debated but now new research is shedding light on this ancient mystery.
Palaeontologists from the University of Witwatersrand, Johannesburg, have discovered that the unique breathing system found in modern turtles was already present in a fossil ‘para-reptile’ which may bridge the gap between reptiles and turtles. They examined thin sections of fossil bones from Eunotosaurus africanus, a small, lizard-like animal from the mid-Permian around 260 million years ago. This species has long been thought to have been ancestral to modern turtles, and possessed unusually wide and flattened ribs.
It is difficult to appreciate just how odd a turtle’s shell really is. It is true that many animals from mammals to dinosaurs have sported protective carapaces over the eons, but turtles are unusual in being fully enclosed by their shells. On their undersides the shells actually incorporates their ribs, which have widened and fused over evolutionary time to form a thick, protective layer called the plastron. In modern turtles this is then attached to the rest of the shell. This arrangement presents problems for the turtles however, in that it is impossible for them to breathe by moving their ribs as mammals and reptiles do. Instead they have had to evolve two unusual ways of drawing breath. In Buccal breathing they take a gulp of air and force it into their lungs using their throat muscles. They also have a muscular ‘sling’ which wraps around their lungs and helps draw air in and out. This second method is the one of particular interest to researchers and was the focus of this new study which has revealed evidence that Eunotosaurus already possessed the unusual and elaborate breathing apparatus seen in modern turtles.
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