TURTLE SHELLS: HOME SWEET ARMOUR

Turtle shells are different from the body armour or armoured shells we see adorning dinosaurs like the ankylosaurs. 

Ankylosaurs were blessed with huge plates of bone embedded into their skin that acted as a natural shield against predators. Crocodilians have these same bony plates, or osteoderms, embedded in their skin to give them extra protection. 

We find similar body armour on armadillos. Yet, armadillos, crocodiles and ankylosaurs each evolved body armour that differs significantly from that found in turtles. 

Remarkably, the carapace we see in fully grown turtles is formed from different parts of their skeletons. And, once fully formed, turtle shell fully integrates with the backbone and ribs, growing over the animal in a domed carapace — both protection and home sweet home. 

When we look to the oldest known members of the turtle lineage, Proterochersis and Proganochelys, found as fossils in 210 million-year-old outcrops in present-day Germany and Poland. Like the turtles we find today, these stem-turtles already had fully formed shells — special bony or cartilaginous shell that originates in their ribs. It is a useful adaptation to help deter predators as their soft interior makes for a tasty snack. 

Though I have never eaten turtle (and never will), it was a common and sought after meat for turtle soup. Years ago, I read of Charles Darwin craving it after trying it for the first time on his trip in 1831 aboard the HMS Beagle. It seems Charlie like to taste every exotic new species he had the opportunity to try.

Turtle armour is made of dermal bone and endochondral bones from their vertebrae and rib cage. It is fundamentally different from the armour seen on our other vertebrate friends and the design creates some unique features in turtles. 

Because turtle ribs fuse together with some of their vertebrae, they have to pump air in and out of the lungs with their leg muscles. 

Another unusual feature in turtles is their limb girdles, pectoral and pelvic, which have come to lie within their rib cage, a feature that allows some turtles to pull their limbs inside the shell for protection. 

Sea turtles didn't develop this behaviour or ability and do not retract into their shells like other turtles.

Armadillos have armour formed by plates of dermal bone covered in relatively small, overlapping epidermal scales called scutes, composed of bone with a covering of horn. In crocodiles, their exoskeletons form their armour, similar to ankylosaurs. A bit of genius design, really. It is made of protective dermal and epidermal components that begin as rete Malpighii: a single layer of short, cylindrical cells that lose their nuclei over time as they transform into a horny layer.

Depending on the species and age of the turtle, turtles eat all kinds of food including seagrass, seaweed, crabs, jellyfish, and shrimp,. That tasty diet shows up in the composition of their armour as they have oodles of great nutrients to work with. The lovely example you see here is from the Oxford Museum collections.