【英语时差8,16】海龟是怎么长壳的？（在线收听）

We may never know if the chicken or the egg came first, but scientists have discovered a fossil that sheds light on how turtles got their shells. Since the time of the dinosaurs, turtles have looked much the same as they do today, complete with boney shells. But scientists know turtles didn't just appear on the Earth fully formed. There must have been an ancestor with a partial shell or no shell at all. So how did the shell get there? Some dinosaurs and present day reptiles, like crocodiles, have boney skin plates called osteoderms. So researchers thought turtle shells might have evolved from these plates. The trouble was, when they studied turtle embryos, they found that shells grow out from the backbone and ribs and don't appear to be over-grown osteoderms. The evolution of the shell remained a mystery until a 220- million-year-old fossil was unearthed in Ghizhou Province, China. The fossil was named Odontochelys semitestacea, or half-shelled turtle with teeth. Unlike modern land turtles, this sixteen-inch-long species was found with other marine reptiles, indicating it was aquatic. It had a plastron or lower shell. But the upper shell, called a carapace, was not there, only a thickened backbone and ribs. Odontochelys not only answered some questions about the shell's origin, it also gave a clue as to why a turtle has a full-body shell. Scientists think the shell evolved as a defense against ocean predators. On land, the belly is protected by the ground, but in an aquatic environment, turtles can be attacked from above or below. After the plastron evolved, the carapace developed, giving turtles extra protection.