It's not just armour

Turtles have shells to protect themselves, right? Now, Tyler Lyson and his team have found out that they actually use it for something else; digging.

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A turtle shell is made out of the turtle’s ribs. The ribs have merged together to become one solid thing, instead of being separate bones, like a human’s. The shell is covered with keratin, the same stuff our hair and nails are made from. The keratin protects the turtle against parasites. And, although they can hide in their shells when there’s danger, turtles can never exit it, since the shell is literally part of their body. We can’t just crawl out of our ribcage either. You can also find out to what species a turtle belongs by just looking at their shell. Sea turtles have a flat shell for example, because it’s easier to swim with, while land turtles have a more dome-shaped shell. Tyler Lyson tries to find out how turtles got their shells in the first place.




That thing is useless!

The problem is, when turtles formed shells for the first time, around 260 million years ago, the shells didn’t really give much protection to the turtles. And on top of that, the new shell also restricted the turtle’s movement and breathing. So it wasn’t a thing that was really beneficial to the turtle. But why did it evolve then in the first place? Tyler Lyson has an answer. He thinks the first turtles used it for digging. According to his theory, the shell provides a starting point of the arm muscles that a turtle needs for digging. And later on, the shell became much thicker and started being actually useful as a form of protection. While the first shells were much too thin to provide that. Lyson got this idea after studying Gopher turtles, which also have strong muscles for digging connected to their shells. He found out that the first turtles’ and Gopher turtles’ shells have many things in common, and with that in mind he made this theory.

Digging the water
We can’t, of course, be sure of this. We can’t just go back in time and study the first turtles. What we can do is study its fossils and compare those to land turtles that live today. Tyler Lyson and his team did that, and they found that the bone structure of both turtles is quite similar, which supports Lysons theory. Also, the strong digging arms of land turtles also came in conveniently later, when they went into the water. The digging arms enabled turtles to swim around, and sea turtles evolved. All in all, Lysons theory is basically proven. So the next time you see a turtle digging; it’s just using its shell.

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Sources:

http://sci-hub.bz/10.1016/j.cub.2016.05.020

http://sci-hub.bz/10.1016/j.cub.2013.05.003

https://en.wikipedia.org/wiki/Eunotosaurus

https://en.wikipedia.org/wiki/Turtle

https://nl.wikipedia.org/wiki/Schildpadden#Schild

https://upload.wikimedia.org/wikipedia/commons/b/bc/Eunotosaurus_africanus.jpg

http://animalia-life.com/data_images/turtle/turtle1.jpg