【英语时差8,16】海龟是怎么长壳的？

（单词翻译）

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 dinosaurs¹, 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 fully² 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 reptiles³, 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 embryos⁴, they found that shells grow out from the backbone⁵ and ribs⁶ 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 unearthed⁷ 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 marine⁸ reptiles, indicating it was aquatic⁹. It had a plastron or lower shell. But the upper shell, called a carapace¹⁰, 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 defense¹¹ against ocean predators¹². On land, the belly¹³ 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.