box turtle

Happy World Turtle day: How the turtle got its shell

New evidence pushes back the origin of the turtle’s shell by about 40 million years, linking it to a 260-million-year-old fossil reptile from South Africa.

The connection to Eunotosaurus strengthens the fossil record and bolsters an existing theory about shell development while providing new details about its precise evolutionary pathway, researchers say.

“Now we’ve got an intermediate shell, a transitional form that bridges the gap between turtles and other reptiles and helps explain how the turtle shell evolved,” says Tyler Lyson, a curatorial affiliate of the Peabody Museum of Natural History at Yale University and a Smithsonian postdoctoral researcher. “Eunotosaurus is an early offshoot of the lineage that gave rise to modern turtles—it’s an early stem turtle.”

Molecular divergence studies indicate turtles split from a stem ancestor about 270 million years ago. But until 2008, the oldest turtle fossils, dating to about 210 million years ago, already had fully developed shells, leaving shell evolution a mystery.

That year a 220-million-year-old turtle-like fossil found in China showed a fully developed belly shell and broadened ribs, but not a full shell.

This demonstrated that the shell did not form through the accretion of ossified scales—as in all other animals with shells, such as armadillos, various lizards, and placodonts—but rather through the broadening of the ribs and vertebrae.

Published in Current Biology, Lyson’s analysis of more than 45 Eunotosaurus fossils—including new specimens and some of the most complete yet—reinforces and expands this view, showing some of the same features in an older animal and establishing a new proto-turtle.

The Eunotosaurus specimens have features found exclusively in turtles and in Eunotosaurus, researchers report—including nine broadened ribs, nine elongated trunk vertebrae, ribs that are T-shaped in cross-section, and paired abdominal ribs lacking lateral and medial elements. These elements emerge in this order in modern turtle embryos. And like turtles, Eunotosaurus had no muscles between its ribs.

“The first thing we see in a developing turtle embryo is the broadening of its ribs, followed by the broadening of its vertebrae, and finally by the acquisition of the osteoderms along the perimeter of the shell,” Lyson says. “Eunotosaurus fits neatly into this model.”

A histological analysis—the first of this kind for Eunotosaurus—confirmed other anatomical similarities.

“The consilience between the fossil and developmental data indicate that first the ribs broadened, found in Eunotosaurus, then the neural spines of the vertebrae broadened, found in later stem turtles, and finally osteoderms on the perimeter of the shell, found in most living turtles, formed, and everything sutured together to form the modern-day turtle shell.”

Support for the research came from Yale Peabody Museum, the National Science Foundation, the Smithsonian Institution, and the Swiss National Science Foundation.

Source: Yale University