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It sounds like the title to a Rudyard Kipling tale: how the turtle got its shell. But it’s actually a question that has puzzled scientists. After all, no other animal, living or extinct, has a similarly constructed bony shield surrounding its body. Scientists had thought that, over evolutionary time, small bony plates fused with the animal’s skin. But a new study published July 10th in the journal Science offers a different pathway.
In most animals the shoulder blades lie outside the ribs. Not so with turtles. And there’s no intermediate evolutionary form in which the shoulder blades lie beneath ribs. So researchers in Japan compared chicken, mouse and Chinese soft-shelled turtle embryos at different stages of development. They show that initially the embryos develop along the same pathways. But the turtle takes a turn.
As it develops, part of its body folds in on itself. Shoulder blades get folded within the ribs. The ribs stay connected, but new connections also develop between bone and muscles. Then the shell starts to develop as the ribs fuse together and encase the shoulder blades. Not as droll perhaps as a “Just So” story. But more fascinating for being true.
—Cynthia Graber