From Ed Yong at Atlantic:
Tyler Lyson from the Denver Museum of Nature and Science has devised a fascinating new idea about turtle origins. He thinks that their iconic shells evolved not for defense, but for digging. They anchored the powerful arm strokes needed to shift soil and sand. Before turtles became impregnable walking fortresses, they were professional burrowers.
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. More.
Maybe. An even bigger mystery is how turtles know enough to right each other. How would they have learned that?
See also: Not a Big Bang of turtles TOO? As well as birds and mammals
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