The turtle fossil, dubbed Odontochelys semistestacea, is the oldest ever found. It was unearthed last year in China's Guizhou province and described this week in the journal Nature. It predates the previous record-holder, the terrestrial turtle Proganochelys, by at least 10 million years.
Odontochelys didn't look exactly like a modern turtle. Unlike modern turtles, which have beaked mouths and no teeth, Odontochelys had a long, pointed snout lined with teeth. And although the shell on its belly, called the plastron, was fully formed, the shell on its back, called the carapace, was not.
Scientists have long debated how turtle shells -- unique in nature -- might have evolved.
"Lots of animals develop armor, but turtles have done it in a very distinctive way," says paleontologist Robert Reisz of the University of Toronto, an expert on turtles who was not involved in this study.
One school of thought held that turtle shells evolved from bony plates on the skin that fused together to form the shell -- some modern reptiles including crocodiles have these bony plates, and some dinosaurs had them as well. Another theory was that the turtle shell developed from below, as an extension of the backbone and ribs.
The new discovery seems to support the second hypothesis, as Odontochelys shows no signs of bony skin plates, but did have broad ribs and a belly shell that extended from the backbone.
The find also adds information to another long-standing debate -- whether turtles evolved in water or on land.
The fact that this early turtle had only a belly shell suggests that water is the more likely answer, says study co-author Olivier Rieppel, of the Department of Geology at the Field Museum in Chicago.
"Why would you armor the belly of something that sits on the ground?" Rieppel says. "If you have a plastron it must be for protection from attacks from below -- in the water."
Not all scientists are convinced, however, that the new find answers the question of where turtles evolved.
Reisz, of the University of Toronto, calls the fossils "a great discovery" and "a significant scientific find."
But in an article accompanying the Nature study, he suggests another explanation for their origin. He suggests that even older, terrestrial turtles might have evolved a back shell, which Odontochelys did when it moved into the water.
"We often see in marine animals there is a tendency to reduce the shell," Reisz says. "So this could represent an adaptation."
Eugene Gaffney, a paleontologist at the American Museum of Natural History and the world expert on the now second-oldest turtle, Proganochelys, also cautions that there are likely other, perhaps even older turtle fossils out there.
"Turtles aren't the easiest thing to find," he says. "So what you're getting is a little glimpse, a snapshot of what is obviously a much bigger picture."