Japanese scientists have learned that a turtle starts developing its shell while it is still an embryo in what they describe as the folding of its upper body wall on itself.

The team of scientists from the Riken Center for Developmental Biology in Kobe, Japan said in their report appearing in the journal Science that a turtle embryo looks like that of a chick and mice with the muscles and skeleton in similar position until the change in its body plan occurs.

The folding of the body wall forces the turtle's ribs outward and the deep layer of the reptile's skin begins thickening or form into a carapacial disc. The ribs stops growing and the shoulder blade in held in place to eventually become encased in the future shell.

Aside from lending insights on how the turtle shell came to be, the scientist's findings helped define the evolutionary stage of a 220-year-old turtle fossil found in China last year.

Shigeru Kuratani, one of the authors of the study, told BBC that the fossil showed a turtle's body not yet encapsulating the shoulder blade indicating that the ancient animal is in the intermediate part of its evolutionary stage.