28 juillet 2016

L'évolution de la carapace des tortues

Une nouvelle étude postule que l'évolution de la carapace des tortues n'a pas été initialement motivé par un besoin de protection:

(...) First, the lower ribs became wider and fused with each other to give half a shell—the plastron. Then, the upper ribs followed suit and merged with the spine, creating the carapace. (This means that, contrary to cartoons, you can’t pull a turtle out of its shell.) Eventually, through an intricate bit of evolutionary origami, the ribs started growing over the shoulder blades, rather than sitting below them as in you, me, and most other land-living vertebrates.

That takes care of how the shell evolved. “For me, the next question was: Why?” says Lyson. “And there are two huge reasons why not.”

(...) So, to explain why the turtle shell evolved, you need to explain why they first started widening their ribs, despite the substantial costs of doing so. Lyson came up with a fresh answer by studying a large number of Eunotosaurus fossils. One specimen, which had been recently discovered by an eight-year-old South African boy, was especially important because it preserved the animal’s entire body, including its hands and feet.

Lyson noticed that Eunotosaurus had many distinctive features. It had a short, spade-shaped skull. Its hands were larger and sturdier than its feet. Its shoulder blades and forearms had large attachment points for especially buff triceps, all the better for pulling its arms back with extreme force. In short, it was built like a digger. Now the ribs made sense: they would have anchored the front legs as they scooped away at earth. They were excavation innovations.

(...) Once turtles had their digging bodies—wider ribs and powerful front limbs—they could easily have taken to water, where many modern species still thrive. And they could expanded their ribs even further, adding defensive value. “Ribs are pretty boring,” says Lyson. “From snakes to whales, they’re pretty much the same because they’re so integrated with breathing. But once ribs were freed from that constraint, they could be selected for a shell.”

That would have been useful because the same adaptations that made early turtles good diggers also made them slow. “The selective pressure to develop protective structures may have come from the slower gait that resulted from the broader ribs,” says Judy Cebra-Thomas from Millersville University.

The turtle’s shell, then, is a wonderful example of exaptation—the evolutionary process where a trait evolves for one function and is then co-opted to serve another. They began as digging platforms and then became suits of armor. Feathers are another example. They now help birds to fly, but they probably originated as ways of keeping warm or signaling to mates and rivals.

“A change in a structure of the body can only provide a selective advantage based on its current abilities, not potential future ones,” says Cebra-Thomas. “That’s very important, and not just for understanding the evolution of turtles.”

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Pappochelys rosinae: l'ancêtre des tortues

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