Evolution of the Turtle Shell: Scientists Learn Turtles Lived in Shells Earlier than Predicted

Ever wondered how a turtle gets its shell? They are one of the most unique animals as they form a shell on the outside of their bodies through a fusion of modified ribs, vertebrae and shoulder girdle bones. However, scientists have been left dumbfounded for more than two centuries as to how and when did the turtle's shell originated.

In a new paper published Thursday in the journal Current Biology, a Smithsonian scientist and colleagues suggest that the origins of the turtle shell started 40 million years earlier than previously believed.

"It was very contentious," said Tyler Lyson, a paleontologist who recently received his doctoral degree at Yale and is now a fellow at the Smithsonian. "For the past 200 years, there's been a lot of ink spilled on the question."

A 220 million-year-old turtle fossil was found in 2008 in China, called the Odontochelys semitestacea. This fossil allowed scientists to learn that the turtle had a fully developed plastron, which is the belly portion of the shell. However, these early fossils only had a partial carapace made up of distinctively broadened ribs and vertebrae on its back.

Later in South Africa, a newly discovered specimen called the Eunotosaurus africanus was shown to be 40 million years older than the specimen found in China. Researchers detailed study of Eunotosaurus indicated it uniquely shared many features only found in turtles, such as no intercostal muscles that run in between the ribs, paired belly ribs and a specialized mode of rib development, which indicates that Eunotosaurus represents one of the first species to form the evolutionary branch of turtles.

"Eunotosaurus neatly fills an approximately 30-55-million year gap in the turtle fossil record," said Tyler Lyson, a Peter Buck Postdoctoral Fellow at the Smithsonian's National Museum of Natural History. "There are several anatomical and developmental features that indicate Eunotosaurus is an early representative of the turtle lineage; however, its morphology is intermediate between the specialized shell found in modern turtles and primitive features found in other vertebrates. As such, Eunotosaurus helps bridge the morphological gap between turtles and other reptiles."