It’s intuitively obvious that a hard surface can shield a softer one, absorbing impact energy and preventing the transmission of that energy to the soft tissues underneath. This is a common natural defense strategy and one adopted by organisms as diverse as beetles and bivalves. Different organisms use different materials as their shields, but the underlying pattern of hard protecting soft is consistent. A small number of animals use the reverse of this strategy, however: turtles, alligators and armadillos all have a hard shield with a soft skin outer layer.
Turtle shells are living parts of the animal. The hard bony part of the shell is made of modified ribs coated with a layer of collagen and then a layer of keratin, the same material used to make hair, hooves, nails and scales.
It might seem that this soft outer layer does not contribute much to the defense of the turtle, however it acts like a bumper, diffusing the load felt by the hard structure underneath. The soft skin is particularly useful for protecting against small scale indentation injuries, for example, like those from teeth when a predator attempts to bite the animal. A lot of force confined to a small surface area can cause cracks in the hard shield material, and these cracks can spread, weakening it. By dispersing these highly localized forces over a wider area, the soft layers of collagen and keratin on top reduce the puncture force the hard shield experiences underneath.Edit Summary