Paleontologists have determined that Eunotosaurus africanus – a 260 million year old fossil reptile from the Karoo Basin of South Africa and a relative of modern turtles – provides a long awaited glimpse into the origins of these magnificent reptiles.
While Eunotosaurus africanus lacks the iconic turtle shell, its wide ribs and distinctively circular torso are the first indications that this ancient animal represents an important clue in a long unsolved mystery: the origin of turtles.
In a study published in the journal Nature, the scientists focus their attention on the skull of Eunotosaurus africanus. Their findings indicated that the complex anatomy of the head houses convincing evidence of the important role played by this species in the history of turtle evolution.
“Our previous studies showed that Eunotosaurus africanus possessed structures that likely represent the first steps in the evolution of the turtle shell, but what those studies lacked was a detailed analysis of the skull,” said co-author Dr Tyler Lyson of the Denver Museum of Science and Nature.
Using high-resolution computed tomography, the scientists digitally dissected the bones and internal structures of multiple Eunotosaurus skulls, all of which are housed in South African museums. They then incorporated the observations into a new analysis of the reptile tree of life.
One of the study’s findings is that the skull of Eunotosaurus africanus has a pair of openings set behind the eyes that allowed the jaw muscles to lengthen and flex during chewing. Known as the diapsid condition, this pair of openings is also found in lizards, snakes, crocodilians, and birds. The skull of modern turtles is anapsid (without openings), with the chamber housing the jaw muscles fully enclosed by bone.
The anapsid-diapsid distinction strongly influenced the long-held notion that turtles are the remnants of an ancient reptile lineage and not closely related to modern lizards, crocodiles, and birds. The new data reject this hypothesis. “If turtles are closely related to the other living reptiles then we would expect the fossil record to produce early turtle relatives with diapsid skulls,” said lead author Dr Gaberiel Bever from the New York Institute of Technology.
“That expectation remained unfulfilled for a long time, but with some help from technology and a lot of hard work on our part, we can now draw the well-supported and satisfying conclusion that Eunotosaurus africanus is the diapsid turtle that earlier studies predicted would be discovered.” In linking turtles to their diapsid ancestry, the skull of Eunotosaurus africanus also reveals how the evidence of that ancestry became obscured during later stages of turtle evolution. “The skull of Eunotosaurus africanus grows in such a way that its diapsid nature is obvious in juveniles but almost completely obscured in adults,” Dr Bever said. “If that same growth trajectory was accelerated in subsequent generations, then the original diapsid skull of the turtle ancestor would eventually be replaced by an anapsid skull, which is what we find in modern turtles.”