Washington, July 10 (ANI): It is due to a unique feat of "origami" with their muscles and bones that turtles develop their upper shells, according to a new study.

Shigeru Kuratani, of Japan's RIKEN Center for Developmental Biology, has found that this process takes place while the reptiles are still in their eggs.

He points out that in most other animals with backbones, the shoulder blade lies outside the ribs. However, in turtles, the ribs grow over the shoulder blades and fuse to form the upper shell, says the lead researcher.

With an eye on determining when exactly turtles start to develop differently, the research team examined various stages of embryos of Chinese soft-shelled turtles, chickens, and mice.

They observed that turtle embryos initially develop like the birds and mammals do, but as the reptiles mature, their bodies undergo an unusual folding process during which certain portions of their skeletons and muscles tuck in on themselves.

According to them, this folding causes some bones and muscles to connect in ways that they don't in other animals, and, consequently, the turtle's shoulder blades slip beneath its rib cage, and its rib bones grow out to the sides instead of curving downward.

The researchers say that, over time, the flat and splayed-out ribs harden to form the turtle's upper shell.

They insist that their findings also cast light on turtle evolution.

The team compared modern turtle embryos to the fossil structure of a turtle ancestor, and found that the animals go through a stage of development when their bodies resemble those of their ancient relatives.

Kuratani and his colleagues, however, admit that the steps involved in the formation of a turtle's bottom shell are less well understood.

"The (belly) shell is an enigmatic structure," National Geographic News quoted Kuratani as saying.

A research article on this study has been published in the journal Science. (ANI)