The most obvious feature of any tortoise is the shell. This is the tortoises primary defence mechanism against would-be predators. The shell has remained almost unaltered by two hundred million years of evolution. The shell is basically an extension of the rib cage, which unlike most vertebrates is housed on the “outside” rather than inside the body.
The shell is made up of two halves, the underneath known as the plastron and the top known as the carapace. Both parts are fused together at the sides by a “bridge”.
The whole shell of the tortoise is made up of numerous small bones which are covered by separate plates of keratin called scutes. As a tortoise grows, extra layers of keratin are added underneath the existing layer, causing “growth rings”. Contrary to popular belief, a tortoise cannot be accurately aged by counting these rings. However they can tell us approximately how many spurts of growth the tortoise has had, thus we could also gauge what type of seasonal changes the tortoise has in its natural environment. Abundant vegetation means more food, which relates to more growth. Sparse vegetation due to extreme climatic conditions would mean little food, leading to little or no keratin growth.
Very old tortoises often have extremely worn scutes, giving their shells an alost completely smooth appearance.