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Fossils of the oldest-known turtles, unearthed in southwestern China, may help answer an evolutionary enigma—how did the turtle get its shell?
The 220-million-year-old animals did not have full shells, or carapaces, on their backs, researchers found.
But the newfound creatures did sport fully developed plastrons—the flat part of a turtle shell that covers and protects the belly.
The discovery supports the theory that turtle shells formed from the underside—plastron first—and grew bony extensions of ribs and bone formation above backbones that eventually joined to form the classic shell that exists today.
(Related: "Earliest Swimming Turtle Fossils Found—New Species" [November 19, 2008].)
An alternate theory of shell evolution suggests that turtle shells developed from the fusion of bony armor plates in the skin, known as osteoderms, seen in some dinosaurs and some modern-day reptiles, including crocodiles.
But the prehistoric turtle, dubbed Odontochelys semitestacea and described in a recent edition of the journal Nature, has no osteoderms.
"So far there is no direct evidence for the osteoderm theory," said study co-author Chun Li of the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences in Beijing.
"On the contrary, here, in our hands, there is an ideal missing link for turtle evolution. It has no osteoderms on its back, but only ossified neural [central] plates and expanded ribs."
The study also notes that embryonic evidence from modern turtles suggests that their shells begin to form in a similar manner.