Animation: The Evolution of the Turtle Shell

Posted on May 31, 2013

Eunotosaurus africanus skeleton next to turtle shell

This animation shows the evolution of the turtle shell based on developmental and fossil data. The animation shows fossils and drawings of turtle ancestors, including Eunotosaurus and Odontochelys. The animation is based on the work of Dr. Tyler Lyson, currently at the Smithsonian Institution. Take a look:

The oldest known fossil turtle dates back about 210 million years. This turtle already had fully formed shell. It was not until 2008 when the 220 million-year-old fossil remains of an early turtle species, Odontochelys semitestacea, was discovered in China that scientists got a clue as to the development of the turtle. Odontochelys semitestacea had a fully developed plastron (the belly portion of a turtle shell), but only a partial carapace made up of broadened ribs and vertebrae on its back.

Scientists then turned to a newly discovered specimen of Eunotosaurus africanus, a South African species 40 million years older than O. semitestacea that also had distinctively broadened ribs. A skeleton of Eunotosaurus africanus is pictured above. Study of Eunotosaurus indicated it shared many features only found in turtles, such as no intercostal muscles between the ribs, paired belly ribs and a specialized mode of rib development. The scientists say this indicates that Eunotosaurus represents one of the first species to form the evolutionary branch of turtles.

Tyler Lyson, a Peter Buck Postdoctoral Fellow at the Smithsonian’s National Museum of Natural History, said in a Smithsonian release, "Eunotosaurus neatly fills an approximately 30–55-million-year gap in the turtle fossil record. 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."

The research was published here in Current Biology.

Photo: Tyler Lyson/Smithsonian