The latest report by an international paleontology research team showed that the eunotosaurus living in South Africa 280 million years ago is the ancestor of modern tortoises. They survived the mass extinction from Permian to Triassic. At that time, in order to avoid the harsh natural environment, they worked hard to dig into the ground. At the same time, to ensure that their digging movement of forelimbs was strong enough, their body needed a stable support, which resulted in the continuous widening of their ribs. It can be concluded that the tortoise has a shell to adapt to the environment. And the shell is not for protection, but for digging holes underground.
The statement 'The forelimbs of the eunotosaurus are strong enough, so they do not need the protection of the tortoise shell' should be added as a prerequisite for the rationality of the above conclusion.