Published online 26 November 2008 | Nature | doi:10.1038/news.2008.1260

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How the turtle got its shell

Chinese fossil forces palaeontologists to rethink turtle origins.

Artists impression of Odontochelys semitestaceaAn artist's impression of Odontochelys semitestacea.M. DONNELLY

The discovery, in China, of the oldest known turtle fossil has turned palaeontologists' understanding of the species' origin and ecology on its head.

The fossil of Odontochelys semitestacea was found in sediments deposited in the Nanpanjiang Trough Basin and dates back 220 million years — around 14 million years older than previous fossils found in Germany.

The find suggests that turtles evolved in a marine environment. The oldest turtle fossils previously found are thought to be from land-based animals, which led scientists to conclude that turtles evolved in a terrestrial environment.

Those fossil reptiles had similarly formed shells to those of existing animals, offering palaeontologists few clues to how the turtle's distinctive shell evolved. The Chinese specimen has a fully developed plastron — the flat ventral part of the shell — but the carapace, the dorsal part, is absent. The team says that this suggests that the two parts of the shell evolved separately, with the plastron developing first — an idea that contradicts the prevailing hypothesis that the shell was formed by osteoderms (bony deposits) fusing together. The team's findings are published in Nature1.

Missing link

One of the study's authors, Chun Li, from the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences, told Nature News, "[The fossil] could be seen as the missing link of turtle evolution. We found how the turtle shell formed. It is not derived from a fusion of osteoderms."

"It is the first fossil evidence to show that the turtle may have originated from a water rather than land environment," he says.

But Robert Reisz and Jason Head, palaeontologists at the University of Toronto in Mississauga, Canada, disagree with the team's interpretation of the shell's origin2. Reisz and Head propose that the absence of the carapace is an adaptation to living in a marine environment.

"This is a very exciting discovery," says Reisz, speaking on the Nature podcast.

Turtle fossil.The O. semitestacea fossil, viewed from above.Institute of Vertebrate Palaeontology and Palaeanthropology, Beijing

"The paper suggests there is a disconnect between the evolution of two parts of the shell. This is an interesting idea but we disagree. Because what you see in turtles that live in marine environments today is that their shell is reduced. So another interpretation of the fossil could be that this form of the Chinese fossil is actually a specialised condition, so that this shell may actually be in the processing of reducing," he adds.

Embryonic clue

Walter Joyce, a palaeontologist, who will take up a position at the University of Tübingen, Germany, in the new year, is equally excited by the new find but agrees that questions remain over the origins of the shell.

"The specimen does not answer the question of whether this is the basal condition or not," he says. "This is unbelievable material. This specimen is a blessing and a curse because it throws everything back up in the air again."

But another of the study's authors, Olivier Rieppel, of the department of geology at the Field Museum in Chicago, Illinois, says the view that the shell is reducing is wrong. He says the shell formation seen in the fossil more closely resembles that of embryonic turtle shells than that of extant aquatic turtles. This supports his team's interpretation that the two parts of the shell evolved separately, says Rieppel.

"If you look at aquatic turtles alive today that have reduced carapace, none match the patterns seen in the fossil. Our fossil looks like the embryonic pattern," he says. 

  • References

    1. Li, C., Wu, X.-C., Rieppel, O., Wang, L.-T. & Zhao, L.-J. Nature 456, 497–501 (2008).
    2. Reisz, R. R. & Head, J. J. Nature 456, 450–451 (2008).

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