Shell-less ancestor of turtles revealed through holes in its head

A 260 million-year-old fossil has been determined to be the oldest ancestor of turtles found to date, revealing something about the reptile's mysterious past.

Illustration of Eunotosaurusand its modern relatives.

Mick Ellison

Back in the olden days, or rather 260 million years ago in the very late Paleozoic Era, turtles looked very different from today. The animal had no shell, but it did have a disc-like torso and wide ribs reminiscent of the turtle's current shape.

Although it has been hypothesised that the animal, called Eunotosaurus africanus, is related to turtles, a new paper published today in the journal Nature claims to have found convincing evidence that Eunotosaurus is indeed a turtle ancestor in a fossil of the animal's skull. In fact, they believe that Eunotosaurus is the earliest known turtle ancestor.

"Eunotosaurus is a critical link connecting modern turtles to their evolutionary past," said study leader Gabe Bever, an assistant professor of anatomy at the NYIT College of Osteopathic Medicine. "This is the fossil for which science has been searching for more than 150 years. You can think of it as a turtle, before turtles had a shell."

An earlier study also led by Bever found the shell could have evolved from the animal's ribs, which were broad and flat like the plates of a turtle shell, while its vertebrae were nearly identical to those found in some later turtles. However, while several skeletons ofEunotosaurushad been studied, there was a lack of research into its skull.

Over four years, Bever and his team used high-res CT scans to analyse several skull fossils housed in museums in South Africa. What they found fits the Eunotosaurus neatly into the turtle family tree.

"Imaging technology gave us the opportunity to take the first look inside the skull of Eunotosaurus, and what we found not only illuminates the close relationship of Eunotosaurusto turtles, but also how turtles are related to other modern reptiles," Bever said.

Specifically, they found a pair of openings behind the eyes that allow the muscles of the jaw to flex when the reptile is chewing. This is called a diapsid skull, and modern lizards, snakes and crocodilians have diapsid skulls. Modern turtles, however, are anapsid, meaning where those openings would be is fully enclosed by bone.

"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," Bever said.

"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."

The reason this wasn't discovered earlier is because, while the skull of the juvenile Eunotosaurus displays clear evidence of the diapsid holes, as it matures, the skull grows in such a way that they become obscured.

This could also explain how modern turtles became anapsid. Over the generations, if that growth became more pronounced, eventually the diapsid skull would evolve into an anapsid one, with the bone closing the holes.

The next step in the research is to study other diapsid reptiles, seeking to expand the turtle family tree.

"The beauty of scientific discoveries is that they tend to reveal more questions than they answer, and there is still much we don't know about the origin of turtles," Bever said.

"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?"