Ancient reptiles without shells linked to modern turtles.

DENVER – Sept. 14 – A research team that included paleontologist Tyler Lyson of the Denver Museum of Nature & Science has determined that a 260-million-year-old fossil species found in South Africa’s Karoo Basin provides a long-awaited glimpse into the murky origins of turtles.

Named Eunotosaurus africanus, this ancient reptile is described as the earliest known branch of the turtle tree of life.

In a new study published in “Nature”, Lyson and colleagues from the New York Institute of Technology, Yale University and the University of Chicago focus their attention on the skull of Eunotosaurus. Their findings indicated that the complex anatomy of the head houses convincing evidence of the important role played by Eunotosaurus in the deep history of turtle evolution.

“While Eunotosaurus lacks the iconic turtle shell, our previous studies indicated it had the early beginnings of the modern shell, including extremely wide ribs, the same number of ribs, and a distinctively circular torso which suggested this fossil represents an important clue in a long unsolved mystery: the origin of turtles,” said Lyson, curator of paleontology at the Museum and a coauthor of the study. “A detailed analysis of the skull was what was missing from our previous studies.”

“Eunotosaurus is a critical link connecting modern turtles to their evolutionary past,” said Gabriel Bever, an assistant professor of anatomy at the NYIT College of Osteopathic Medicine and lead author of the study. “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.”

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.

Researchers used CT data to carefully analyze the skull of Eunotosaurus and found that it shares several features with other living reptiles, indicating it is a diapsid turtle.

In linking turtles to their diapsid ancestry, the skull of Eunotosaurus also reveals how the evidence of that ancestry became obscured during later stages of turtle evolution.

“A new juvenile specimen of Eunotosaurus clearly has openings in its skull, while adults do not,” Lyson explained. “The openings appear to close as the animal gets older, making it a cryptic diapsid.”

Although the new study represents a major step toward understanding the reptile tree of life, Lyson emphasized that it will not be the final chapter in the science of turtle origins.

“There is still a lot of work to be done regarding the origin of turtles,” Lyson said. “Our team is currently working on the ecological conditions that led to the evolution of the turtle’s shell and anapsid skull.”

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