A newly discovered fossil from China has shed light on how the turtle's shell evolved.
The 220 million-year-old find, described in Nature journal, shows that the turtle's breast plate developed earlier than the rest of its shell.
The breast plate of this fossil was an extension of its ribs, but only hardened skin covered its back.
Researchers say the breast plate may have protected it while swimming.
The turtle fossil, found near Guangling in south-west China, is thought to be the ancestor of all modern turtles, although it differs markedly; it has teeth rather than a bony plate, the shell only covers its underside and it has a long tail.
The fossil find helps to answer key questions about the evolution of turtles, Dr Xiao-Chun Wu from the Canadian Museum of Nature was one of the first to examine the fossil.
"Since the 1800s, there have been many hypotheses about the origin of the turtle shell," explained 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 theorised," Dr Wu explained.
The researchers say this idea is supported by evidence from the way modern turtle embryos develop. The breast plate grows before the shell covering their backs.
The fossilised turtle ancestor, which has been named Odontochelys semitestacea, meaning half-shelled turtle with teeth, probably inhabited the river deltas or coastal shallows of China's Nanpanjiang trough basin - the area where the fossil was unearthed.
Researchers say the development of the shell to first protect the underside points to a mainly aquatic lifestyle.
Dr Olivier Rieppel from Chicago's Field Museum also examined the fossil.
"This strongly suggests Odontochelys was a water dweller whose swimming exposed its underside to predators. Reptiles living on the land have their bellies close to the ground with little exposure to danger," he said.
The researchers say further evidence to support the idea that this species lived mainly in water comes from the structure and proportions of the fossil's forelimbs, which closely resemble those of modern turtles that live in similar conditions.