Why does the turtle have a shell?

No other animal has a shell comparable to that of the turtle. Scientists have spent nearly two centuries to try to understand How the turtles obtained this unique structure. In this article which traces the evolution of the turtle, you will finally understand why it has a shell. And no, we are not going to tell you that it is Just to protect yourself Because in fact, The reality is quite different... 🤓

The ancestor of the turtle: the eunotosaurus

Let us plant the decor: it happens 260 million years ago, you are in South Africa, by a river. As you are lucky, you see a small creature appear (the size of one hand) taking its head out of the mud. She looks like a lizard, she has large flanks and stocky legs. You catch it (yes, we consider that you are someone very lively😂), and you find that its ribs are very wide. They seem almost merged among them. This gives the animal a "curved" aspect. It is as if the little creature had a half-pace.

This creature is a Eunotosaurus, and despite his lizard appearance, it is in fact one of the first known turtles. 

Discovered in 1892 it was ignored for almost a century. But that was without counting the research of a man who was going to change everything: Tyler Lyson. This passionate researcher imagined a new fascinating idea On the origins of the turtle shell.

He thinks that their shell has evolved in the first place not to defend yourself, but to dig. Indeed, Eunotosaurus had an ideal anatomy to move in earth and sand. Before turtles became solid “itinerant shells”, they would therefore have been underground reptiles. 😮

The debate on the origin of the shell

For more than a century, biologists discussed the way in which turtles would have inherited their shells. Paleontologists first argued that shells have evolved fromBone scales called osteoderms. These osteoderms are found, especially in tatous, crocodiles and also many old dinosaurs. 

These scales would simply be enlarged over time To merge with the spine, creating a solid coating. However, many development experts were not of this opinion. By studying modern turtles, they deduced that The shell was more like an extension of the ribs, which widen and ended up uniting.

The oldest turtle studied at that time was a creature called Proganochelys. She already had a fully developed shell. Therefore, she has little help in the search for the appearance of shells.
In short, no one agreed and the debate was going in circles ...😓

Discovery of a new fossil: Odontochelys

The debate took a Another turn in 2008, when Chinese researchers have found the 220 million year old turtle fossil. Its shell only covered the belly and not the back. They called it Odontochelys Semitacea. Striking fact, the fossil did not present No osteodema. However, he had very wide ribs Very similar to the Eunotosaurus. The theory of developmental development biologists therefore seemed to be confirmed!😁

The evolution of the turtle shell

Researchers now knew what a turtle that had lived in an intermediate time. But luck did not stop the bus shortly after, a new species nicknamed Papochelys was also found. The latter was dated 240 million years And had no osteodema like Eunotosaurus and Odontochelys. That to contribute to even further strengthen the theory. Thanks to these fossils, scientists were finally able to Reconstitute the evolution of the shell. 🐢

First of all, the lower ribs would have become wider and would have merged with each other to give half a carapace: The Plastron. Then, the upper ribs followed the movement and merged with the spine, creating the shell. Finally, the evolution to continue making your way and the coasts began to develop above the shoulder blades.

But why this evolution?

First of all, it is important to note that if the coasts expand and merge, the ability to breathe is compromised. Second, as a reptile, travel also becomes much slower. Indeed, we already know that the turtle shell prevent the trunk from folding. Therefore, they are not propelled only by their members. This is partly why they are deemed slow. But then what advantage could have drawn the turtle from such an evolution? 🤔

This question has long divided and this is what Lyson said on the subject: 
"When I interact with colleagues during conferences, they automatically told me it was to protect themselves." "But it never made sense to me."

The shells of modern turtles certainly constitute Powerful defenses, but the large ribs of the Eunotosaurs and the Papochelys are far from it. They would not even have covered the heads, neck or back of creatures. If protection had played an important role, it would have been good more logical than evolution follows that of crocodiles and tatous. Turtles would therefore have become osteoderm animals, which would not have not impacted breathing and travel.

An 8 year old child provides the answer

It is an Eunotosaurus specimen, discovered by an 8-year-old South African child, who will end up brought new answers in Lyson.

This eunotosaurus fossil found on the family farm, had a short skull, in the shape of a spike. His front legs were larger and more robust that its rear legs. The shoulder blades and forearms were very developed, ideal for pulling the legs back with extreme strength. In short, the reptile was designed like a “Creuser”. The widening of the coasts finally had a meaning ... 🥳

These same characteristics could also make the animal a very good swimmer. Lyson has discovered that Eunotosaurus has two essential attributes that the tread requires: scratch to break the earth, and robust bone To withstand compression forces. Indeed, when Lyson examined his legs in the cross section, he found that the front pair was loaded with very thick bone, but not the rear legs. Again, all the signs indicated a excavation specialist.

The Eunotosaurus was often found near the ponds and banks of the rivers, but it lived at a time when South Africa was arid. It is likely that the reptile has dug to droughts. In this way it would have fled an unpredictable environment on the surface for an environment more conducive to its survival underground. 

Once turtles have evolved towards this body of "excavators", they could easily have get in the water To prosper as a marine animal. Their ribs would have Expanded even more, thus adding a defensive value.