New Data Regarding The Origin Of Turtles

Eunotosaurus africanus, the earliest known turtle.

Turtles are one of the most unusual groups of reptiles. They have a marine lifestyle and aspects of their anatomy are flat and wide, unlike the majority of other vertebrate groups, from their paddle-limbs to the wide, fused ribs which form the basis of their shell.

Key fossil discoveries show a chain of forms connecting modern day turtles with their more lizard-like ancestors. The first link is Eunotosaurus africanus, a 260 million year old species from the Karoo Basin in South Africa. While it lacked a shell, its wide, flat body displays the hallmarks of its evolutionary future.

'Our previous studies showed that Eunotosaurus possessed structures that likely represent the first steps in the evolution of the turtle shell' said Tyler Lyson from the Denver Museum of Science and Nature, 'but what those studies lacked was a detailed analysis of the skull.' A team, led by Gaberiel Bever from the New York Institute of Technology, conducted high resolution tomographic scans of multiple Eunotosaurus specimens. 'Imaging technology gave us the opportunity to take the first look inside the skull of Eunotosaurus,' said Bever, 'and what we found not only illuminates the close relationship of Eunotosaurus to turtles, but also how turtles are related to other modern reptiles.'

A body fossil of Eunotosaurus africanus

Reptiles can be classified according to the number of openings in their skulls. Diapsid reptiles, namely snakes, lizards, squamates and dinosaurs, have two openings in their skulls while anapsids, the turtles and tortoises, have none. In diapsid reptiles these openings, called temporal fenestrae, sit behind the eyes and allow the jaw muscles to lengthen and flex. In anapsids, the muscles are fully enclosed by bone. The anapsid-diapsid distinction strongly influenced the popular hypothesis that turtles are the descendants of an ancient reptile lineage and not closely related to modern lizards. Yet the new data from Eunotosaurus shows otherwise.

'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 Bever. '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 is the diapsid turtle that earlier studies predicted would be discovered.' In juvenile members of the species, the diapsid temporal fenestrae were evident. During development into the mature adult form, however, these were lost, masking the evolutionary affiliations of Eunotosaurus.

'The beauty of scientific discoveries is that they tend to reveal more questions than they answer' said Bever, 'and there is still much we don't know about the origin of turtles. Which of the other diapsid groups form their closest cousin? What were the ecological conditions that led to the evolution of the turtle's shell and anapsid skull? And how much of the deep history of turtle evolution can be discovered by studying the genes and developmental pathway of modern turtles?' Future studies will likely confirm Bever's words. Equally, however, the discoveries made may still be surprising.