Turtles have shells due to embryo origami.

This is neat! From National Geographic:

Turtles develop their upper shells thanks to a unique feat of "origami" with their muscles and bones that occurs while the reptiles are still in their eggs, a new study has found.

In most other animals with backbones, the shoulder blade lies outside the ribs, explained study team leader Shigeru Kuratani of Japan's RIKEN Center for Developmental Biology.

But in turtles, the ribs grow over the shoulder blades and fuse to form the upper shell.

To figure out when exactly turtles start to develop differently, Kuratani and his team examined various stages of embryos of Chinese soft-shelled turtles, chickens, and mice.

Initially, turtle embryos develop like the birds and mammals do, the team found.

But as turtles mature, their bodies undergo an unusual folding process during which certain portions of their skeletons and muscles tuck in on themselves.

This folding causes some bones and muscles to connect in ways that they don't in other animals.