Fossil discovery upends thinking on turtle origins

In this week’s edition of Nature, an international team of scientists describes the most primitive species of turtle discovered to date. Unearthed from marine deposits of the Late Triassic period in southwestern China, the remarkably well preserved 220-million-year-old fossil “documents an intermediate step in the evolution of the shell and associated structures,” the scientists say.

Chun Li of the Chinese Academy of Sciences, Beijing, and colleagues report that this previously unknown species, called Odontochelys semitestacea, has expanded dorsal ribs but no shell, or carapace, on its back. It does have a fully developed plastron — the flat belly part of a turtle’s shell — that suggests to the scientists that the first step in carapace formation is the neural plates becoming bone in tandem with a broadening of the ribs. This, Li and colleagues say, corresponds with the early development of carapace formation seen in young turtles today and shows that the turtle shell is not derived solely from a fusion of bony plates in the skin. The scientists also note that this primitive turtle was likely to have inhabited marginal areas of the sea or river deltas.

In a related News and Views article, Robert Reisz and Jason Head of the University of Toronto, Ontario, Canada, note that aspects of turtle origin are debated vigorously among vertebrate paleontologists. They then add to the debate by suggesting that Odontochelys did have a carapace but some of its components did not become bone. Thus, they argue that rather than being primitive, the shell morphology of Odontochelys is a specialized adaptation.

Whether Odontochelys represents turtle shells evolving in aquatic environments, as Li and colleagues contend, or it represents an early move by turtles from terrestrial to marine environments, as Reisz and Head believe, the discovery will alter current thinking on turtle origins and the evolution of this reptile’s exceptional body plan.