First Known Turtle Had Shell Shortcomings
By Jeanna Bryner, Senior Writer
posted: 26 November 2008
A half-shell turtle species that swam in China’s coastal waters 220 million years ago is the oldest turtle known to date, a new analysis of fossils reveals.
The turtle had a belly shell, but its back was basically bare of armor.
Last week, a team of scientists had reported the discovery of the oldest aquatic turtle, dating back 164 million years. That was a short-lived title. The new half-shell aquatic turtle, studied by Chun Li of the Chinese Academy of Sciences in Beijing and his colleagues and dubbed Odontochelys semistestacea, swam around even longer ago.
Li and his team looked at remains that included two skulls and other skeletal bones unearthed in China’s Guizhou province in 2007. The analysis, detailed in the Nov. 27 issue of the journal Nature, suggests that today’s turtles must have originated from an aquatic-turtle ancestor. The results also provide support for a theory of how turtle shells evolved.
Shells start at belly
The specimens studied by Li showed many signs of being from primitive turtles. For instance, the researchers found Odontochelys had an elongated, pointed snout. Most modern turtles have short snouts. In addition, the roof of its mouth, along with the upper and lower jaws, were equipped with teeth, which the researchers say is a primitive feature for turtles whose mugs are now tipped with beaks but contain no teeth.
The fact that the turtle had a partial shell (only covering its belly) sheds light on an intermediate stage of shell evolution that scientists hadn’t seen. Before the discovery of Odontochelys, the oldest known turtle species (aquatic or land-based) was the terrestrial turtle Proganochelys, which lived about 210 million years ago. But this turtle had a fully formed shell, providing little evidence as to how the shell evolved.
One idea has been that the turtle shell evolved from bony plates on the skin that broadened and fused together to form the turtle’s armor. The entire structure would then fuse to the underlying ribs and backbone. (Modern reptiles, such as crocodiles, have these bony plates, as did some dinosaurs, such as ankylosaurs.)