Scientists are finding out that the tortoise shell evolved for burrowing purposes not protection.
With the discovery of a fossil that predates others by 5 million years they are learning more about the function of the plastron and carapace. Follow the link below to learn all about it.
2008. It had a fully formed plastron but the carapace was only partially developed. Perhaps the softshell turtle is the last remnant of this prehistoric ancestor.
With the evolution of the turtle shell it became what we see today on most chelonians. They are the only animal that has its ribs fused to the shell. With this change they could no longer breathe expanding the lungs. Below is an article discussing how turtles and tortoises breathe.
Hope you enjoy these articles that offer yet more insight to the wonder that is the turtle!