Turtle Evolution

As a Floridian that has long been enamored by gopher tortoise shells I found this article interesting.

We think of turtles as having a shell as protection from predators. However the author of a recent paper says the original purpose of the shell was to dig. The first part was to fuse lower ribs to form was the bony front plate. It became a great anchor spot for large digging muscles. Later upper ribs formed to create the carapace (shell) to allow even larger muscles.

And what do you know. Very strong limbs are nifty if you move to an aquatic environment. Efficient swimmers that don’t take the form of a fish really need strong limbs.

So turtle evolution went something like this: A group of digging lizards gradually developed a compressed but very strong physique to specialize in digging. Part of that specialization was a bony front plate and later a bony back shell that supported large leg muscles. These bony plates afforded predator protection that allowed turtles to fill other terrestrial niches. One or more small groups of the terrestrial turtles ventured into the water where the heavily muscled limbs developed into swimming limbs.

Which is something that is misunderstood about evolution. Many times a feature can fill several purposes over the course of time. The turtle shell allowed anchoring of heavier muscles to allow digging. Then it became a way to protect against predators. For aquatic turtles it also allows storage of air for buoyancy. And there are probably many other purposes that are poorly understood.

Just remember next time you see a the half moon shape gopher tortoise burrow. He’s still using the shell for the original purpose. The rest are recent gopher tortoise wannabes.