IMNH, ISU Researchers Upend Understanding of Turtle Origins

Published in the pages of Current Biology, a new paper co-authored by Xavier Jenkins, an ISU doctoral alum who’s 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 associate professor in ISU’s Department of Biological Sciences, makes the case for where exactly the shelled animals fit in the planet’s evolutionary past. Studies of turtle genetics have indicated that turtles belong with the archosaurs, a group of animals that includes modern-day crocodiles and birds whose ancestors emerged around 250 million years ago. However, the anatomical features found in turtles today and in ancient turtle fossils have long placed them among other reptiles. 

“Historically, turtles have such weird anatomy that it was very difficult to figure out where they fit on the tree of life exactly,” explains Peecook. "They've turned their spine and ribs into a shell, rearranging how their shoulders and limbs work, and greatly modified their skulls."

Peecook, Jenkins, and their 13 collaborators from across the globe homed in on Eunotosaurus africanus. These animals lived 260 million years ago and were considered among the earliest of the turtle ancestors. 

“Eunotosaurus was a 30-centimeter-long burrowing reptile with a compact, shortened body,” said Jenkins. “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."

Looking at the fossilized braincases–the top and back parts of the skull that protect the brain–of Eunotosaurus specimens using specialized CT scans and X-rays, they found that many features of Eunotosaurus are shared with other earlier reptiles rather than archosaurs, and that early fossil turtles do look like archosaurs. This study provides the first fossil-backed support for the studies of turtle genetics that say turtles belong with the 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,” said Peecook.

“I'm honored to have led this project and worked alongside so many exceptional people, especially our collaborators in South Africa,” said Jenkins. “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."

“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?’” said Peecook. “Not anymore.”

Additional IMNH and ISU-affiliated co-authors are Timothy Gomes, Idaho Virtualization Laboratory manager at IMNH; Gary McGaughey, a doctoral candidate at Idaho State University; and Cy Marchant, an undergraduate student researcher at ISU.

For more information on the Idaho Museum of Natural History, visit isu.edu/imnh. 

More on ISU’s Department of Biological Sciences can be found at isu.edu/biology.