"If plastrons developed first, they may point to a marine lifestyle in which turtle bellies needed protection from predators," Li noted.
"We are not sure if the water [was] marine or [from other water bodies], so we presumed … that the animal inhabited marginal areas of the sea or deltas," he said.
(See a photo of the largest sea turtle in today's oceans.)
Exciting Discovery
Scientists have waited a long time for a find like Odontochelys, Li said: The previously oldest known turtles featured fully formed shells.
"The new specimens are a very exciting discovery," agreed Robert Reisz of the University of Toronto, who was not involved in the research.
But Reisz suggests an alternate evolutionary interpretation for the intriguing fossils.
"Their argument is valid," he said of Li and colleagues.
"But we argue that it's equally possible that this could already be a [shell] reduction in an earlier turtle that we haven't found. Lots of marine turtles actually reduce their shell once they get into the water."
"Hopefully we'll find more," Reisz added. "We're closing the gap, but there is still a big morphological gap between this turtle and its non-turtle ancestors."
Odontochelys also boasts another feature seen in no other turtles so far—teeth, Reisz added.
"Basically if you look at all the turtles we know, other than this one, they all have a beak rather than teeth," he said.
"Turtles come from reptile ancestors with teeth so we expected this, but it's still a great thing to find."
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