Toothy heroes in a half-shell

The discovery of three 220 million-year-old turtle fossils in Guizhou province in China has shed new light on the evolution of the turtle shell.

The species has been named Odontochelys semistestacea to reflect two unusual features: unlike extant turtles, it has teeth; and it only has a half-formed upper shell, or carapace, while the lower shell or plastron is fully developed.

The researchers, involving Chinese, Canadian and US groups, hypothesise that the fully developed plastron suggests the earliest turtles were aquatic rather than terrestrial reptiles, living in a shallow marine environment.

The oldest previously known turtle species, Proganochelys quenstedti, from fossils found in Germany, was thought to be terrestrial and had a fully formed carapace. Also a Late Triassic species, these specimens are thought to be between 204 million and 206 million years old.

Study co-author Dr Xiao-chun Wu of the Canadian Museum of Nature in Ottawa said the newly found fossils supported the theory that the shell would have formed from below as extensions of the backbone and ribs, rather than as bony plates from the skin or osteoderms, as others have theorised.

“The new species shows that the plastron evolved before the carapace and that the first step of carapace formation is the ossification of the neural plates coupled with a broadening of the ribs,” the researchers write in today’s issue of Nature.

“This corresponds to early embryonic stages of carapace formation in extant turtles, and shows that the turtle shell is not derived from the fusion of osteoderms.”

“An ancestral turtle from the Late Triassic of southwestern China” by Chun Li et al is published in Nature [doi:10.1038/nature075333].