Before this the oldest know turtle ancestor was Proganochelys found in Germany. However, this new species sheds much more light on turtle evolution. These two differ in "in presence of teeth on premaxilla, maxilla and dentary; relatively long preorbital skull; distinct transverse process on pterygoid; absence of fully formed carapace; no acromial process on scapula; dorsal ribs articulating at midline of centrum; free sacral ribs; free caudal transverse processes; presence of long tail; four (rather than three) phalanges in digits III and IV of manus and pes; absence of osteoderms and tail-club." (Nature 456, 497-501 ) The most surprising feature here the absence of the carapace or top shell.
Since Odontochelys semitestacea spent its time swimming in aquatic environments, it is more likely that the bottom shell (Plastron) developed first, to protect from predators attacking from beneath. It has only a partially formed shell on top. Li believes this proves that the plastron developed before the carapace. The image of the skeleton above shows the lack of a top shell. More than likely, when turtles started walking on land their carapaces developed better because the ground was also protecting their undersides. However, some researchers believe that Odontochelys semitestacea lost its top shell and that an even older ancestor had the presence of both the carapace and plastron.
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