New light has been shed on the origins of turtles by the discovery of a 260-million-year-old fossil species found in South Africa's Karoo basin.
The extinct reptile, named "Eunotosaurus africanus", is being touted by researchers as the earliest known branch of the turtle tree of life. "Eunotosaurus is a critical link connecting modern turtles to their evolutionary past," said lead researcher Gaberiel Bever from New York Institute of Technology (NYIT).
"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," Bever said.
While Eunotosaurus lacks the iconic turtle shell, its extremely wide ribs and distinctively circular torso are the first indications that this fossil represents an important clue in a long unsolved mystery - the origin of turtles.
One of the study's key findings is that the skull of Eunotosaurus 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 from Eunotosaurus rejects 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," Bever explained.
"That expectation remained unfulfilled for a long time, but we can now draw the well-supported and satisfying conclusion that Eunotosaurus is the diapsid turtle," he added.
The skull of Eunotosaurus grows in such a way that its diapsid nature is obvious in juveniles but almost completely obscured in adults, researchers 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," Bever said. The findings were published in the journal, Nature