What makes a turtle a turtle? There are a few other things that differentiate turtles from their reptile relatives, but, really, it’s all about the shell.
A turtle’s shell is made up of expanded ribs that have fused together into bone plates. The ribs give the shell its shape and are fused into the bone of the carapace, or top shell. The carapace is a kind of bone called dermal bone because it is derived from the skin. It is not odd to have dermal bone; the human skull is also dermal bone. Turtles just have an exceptionally large amount of dermal bone. Bone mass makes up almost 40% of a turtle’s weight, compared to less than 15% for some crocodiles, a turtle’s closest cousins.
The shell provides protection for the turtle’s heart, lungs, and other organs which is much better than that offered by your ribcage. By retracting their heads and limbs into their shells, turtles can protect those, too. The box turtle’s hinged plastron, or bottom shell, allows the shell to close completely.
The shell is not like a suit of armor over the turtle but, rather, a living part of the turtle. The turtle’s shell has nerves and blood vessels, and it grows and heals like any other broken bone. It can heal so well, in fact, that for many cracks we only need to realign the pieces and hold them in place until the turtle’s body takes care of the rest.
Unfortunately, there are instructions on the internet for repairing a turtle’s shell with glue or epoxy. Do not glue a turtle’s shell back together! If glue gets between the pieces of shell, they will not be able to heal back together. Without restored blood flow, some pieces of the shell may die. Please contact a veterinarian or wildlife rehabilitator if you find a turtle with a broken shell.
You can learn more about turtle shells and see the shells of our educational ambassadors up close in our educational outreach programs.