For decades, turtles have stumped scientists trying to place the shelled reptiles on the evolutionary tree. Now, a new study published in Current Biology argues the mystery is finally closer to being solved, using fossil evidence to back up what genetics has long suggested: Turtles belong with archosaurs, the group that includes modern crocodiles and birds.
The paper was co-authored by Xavier Jenkins, an Idaho State University doctoral alum now serving as a postdoctoral fellow at the American Museum of Natural History, and Brandon Peecook, associate curator of paleontology at the Idaho Museum of Natural History and an associate professor in ISU’s Department of Biological Sciences.
“Historically, turtles have such weird anatomy that it was very difficult to figure out where they fit on the tree of life exactly,” Peecook said. “They’ve turned their spine and ribs into a shell, rearranging how their shoulders and limbs work, and greatly modified their skulls.”
Previous studies of turtle genetics have indicated turtles belong with archosaurs, whose ancestors emerged around 250 million years ago. But anatomical features in living turtles and ancient turtle fossils have long placed them among other reptiles, creating a long-running conflict between genetic and fossil-based interpretations.
To address that, Peecook, Jenkins and 13 collaborators from around the world focused on Eunotosaurus africanus, an animal that lived 260 million years ago and has been considered among the earliest turtle ancestors.
“Eunotosaurus was a 30-centimeter-long burrowing reptile with a compact, shortened body,” Jenkins said. “Unlike most reptiles of its time, it possessed only 10 expanded trunk ribs, giving it a distinctive short, robust appearance that would be somewhat reminiscent of a shell-less turtle.”
Researchers examined fossilized braincases of Eunotosaurus specimens — the top and back parts of the skull that protect the brain — using specialized CT scans and X-rays. They found many Eunotosaurus features were shared with other earlier reptiles rather than archosaurs, while early fossil turtles showed archosaur-like traits. The findings provide fossil-backed support for genetic studies that place turtles with archosaurs.
“You can see how it has been confusing: ancient animals like Eunotosaurus had turtle-like features like wide ribs, but once we could see inside, there were so many features that were nothing like living reptile groups, including details of the brain and huge teeth on the roof of the mouth,” Peecook said.
Jenkins said the project began early in his career and became defining work. “I’m honored to have led this project and worked alongside so many exceptional people, especially our collaborators in South Africa,” he said. “This was one of my first projects as a doctoral student at Idaho State University, and this project ended up determining my research path and ultimately my whole academic career! It’s incredibly rewarding to see our work finally published, and I’m proud to be a Bengal.”
Peecook said the study answers a question he once used to highlight how difficult turtle evolution has been to pin down. “I used to tell my students that humans have built space stations, developed the internet, decoded their genome, and drastically changed the surface of the planet, but we’ve still not been able to answer: ‘Where do turtles fit into the tree of life?’” he said. “Not anymore.”
You can read more about the research project by clicking here.