Canadian Museum of Nature

Canadian Museum of Nature

November 26, 2008 13:00 ET

Canadian Museum of Nature: Discovery of Oldest-Known Turtle Fossil Helps Resolve Mystery of Shell Development

OTTAWA, ONTARIO--(Marketwire - Nov. 26, 2008) - Scientists in Canada, China and the United States have discovered and described the world's oldest known fossilized turtle, and with it evidence to crack a long-standing mystery of evolution - how the turtle got its shell.

Dr. Xiao-chun Wu, a palaeontologist with the Canadian Museum of Nature in Ottawa, worked with colleagues to describe the estimated 220-million-year-old turtle, unearthed in China's Guizhou province in 2007. Three adult specimens, remarkably intact, have been found and all have characteristics never before seen in turtles - the presence of teeth, and an incomplete upper shell that offers important clues to understanding the shell's development. With this in mind, the species has been dubbed Odontochelys semistestacea (literally meaning toothed, half-shelled turtle).

"Since the 1800s, there have been many hypotheses about the origin of the turtle shell," explains Dr. Wu. "Now we have these fossils of the earliest known turtle. They support 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 as others have theorized," explains Dr. Wu.

The report appears in the Nov. 27 issue of the scientific journal Nature. Dr. Wu is co-author of the study along with Chun Li of the Institute of Vertebrate Palaeontology, Chinese Academy of Sciences in Beijing (where the fossils reside), Dr. Olivier Rieppel of the Field Museum in Chicago, Li-Ting Wang of the Geological Survey of Guizhou Province and Li-Jun Zhao of the Zhejiang Museum of Nature History. Chun Li (Dr. Wu's student) discovered the fossils with local farmers.

Since the time of the dinosaurs, turtles have appeared pretty much as they do today, with an intact protective shell consisting of an upper part (carapace) and a lower part (plastron). However, there has been little conclusive evidence to support competing theories about the shell's development, partly because no one has found intermediate forms in the fossil record.

One theory holds that the turtle shell develops through a series of composite steps. The shell is thought to grow from bony plates on the skin (osteoderms). These broaden in size to form a type of dermal "armour" that then fuses to the underlying ribs and backbone to form a protective shell. Modern reptiles such as crocodiles have these bony plates, as did some dinosaurs such as ankylosaurs.

However, there is no physical evidence in the fossil record of this process taking place. Before the discovery of Odontochelys, the oldest known turtle taxon was Proganochelys, which was discovered in Germany and dates to about 210 million years ago. But because it has a fully formed shell, scientists cannot conclusively state how it developed.

The other theory, now substantiated by the discovery of Odontochelys, argues that the plastron formed first, followed by an outgrowth and broadening of the ribs and the backbone to form the upper carapace. This process mirrors what is seen in modern-day turtle embryos and hatchlings, where the ribs and backbone expand and grow outwards, then widen to form the upper shell. "With Odontochelys, we now have clear fossil evidence of this process emerging in an adult," explains Dr. Wu.

The team also concludes that this earliest known of all turtles was an aquatic animal and not land-based as suggested by other turtle fossils. The presence of the lower plastron is one clue: it would protect the animal from predators below while swimming. The scientists also found other marine reptiles and invertebrates from the same rocks of the same time period in the locality. The aquatic way of life employed by this earliest turtle implies that turtles, as a group, may have originated from water.

The Canadian Museum of Nature is a federal Crown corporation and Canada's national museum of natural history and natural sciences. It promotes awareness of Canada's natural heritage through permanent and travelling exhibitions, public education programmes, active scientific research, and the maintenance of a 10-million-specimen collection.

EDS NOTE: Hi-res images of Odontochelys are available upon request. For more information, or to arrange interviews with Dr. Wu, contact Dan Smythe.

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