danconnolly.co.uk

Shield-Toad

July 19, 2016

TIL that the Dutch and German words for Tortoise (or Turtle) literally translate as Shield-Toad (schildpad ‎and Schildkröte).

Of course, neither are toads, nor (it turns out) did shells arise as a protective adaptation:

For almost a century, biologists argued about how turtles got their shells—a debate almost as slow and plodding as the creatures themselves. Paleontologists mostly argued that the shells evolved from bony scales called osteoderms, which are also responsible for the armor of crocodiles, armadillos, and many dinosaurs. These scales simply expanded to fuse with the ribs and backbone, creating a solid covering. But developmental biologists disagreed. By studying modern turtle embryos, they deduced that the shell evolved from ribs, which broadened out and eventually united.

Everything changed in 2008, when Chinese researchers discovered a 220-million-year-old turtle with a shell that covered just its belly and not its back. They called it Odontochelys semitestacea—literally, the “toothed turtle in a half-shell.” It was as beautiful an intermediate fossil as they could have hoped for. And strikingly, it had no osteoderms at all. It did, however, have very broad ribs. The developmental biologists were right!

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.”

How the Turtle got its shell.