可能我们永远不会知道先有鸡还是先有蛋，但是科学家们发现的一个海龟化石揭示了它们是如何长出龟壳的?自恐龙时代起，海龟的外貌就跟现在相差无几，一样背着骨组织外壳。但是科学家们知道原始海龟的龟壳并没有发育完整。
There must have been an ancestor with a partial shell or no shell at all. So how did the shellget there? Some dinosaurs and present day reptiles, like crocodiles, have boney skin platescalled 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 thebackbone and ribs and don't appear to be over-grown osteoderms.
海龟的祖先中一定有只长出部分龟壳，或者根本没有龟壳的海龟。那么龟壳是如何长出来的?远古时代的恐龙和现在的爬行动物(比如说鳄鱼)都长有叫做“皮内成骨”的骨组织皮肤板块。因此研究者认为龟壳是从这种皮内成骨进化而来。问题是当他们研究海龟胚胎时发现，龟壳从脊椎骨和肋骨里长出，并没有参差不齐的骨化鳞片。
The evolution of the shell remained a mystery until a 220- million-year-old fossil wasunearthed 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 foundwith other marine reptiles, indicating it was aquatic. It had a plastron or lower shell.
直到中国贵州省出土了一个2.2亿年前的海龟化石，龟壳进化的迷雾才被揭开。这个化石被命名为半甲齿龟。与现代的陆地海龟不同的是，这种长约16英尺的龟类与其他海洋爬行动物一同被发现，这表明它是水生动物。它长有龟板。
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 asto why a turtle has a full-body shell. Scientists think the shell evolved as a defense againstocean predators. On land, the belly is protected by the ground, but in an aquaticenvironment, turtles can be attacked from above or below. After the plastron evolved, thecarapace developed, giving turtles extra protection.
但是背部却没有壳(即没有甲壳)，只有增厚的脊椎骨和肋骨。半甲齿龟不仅回答了龟壳起源的问题，还为海龟为什么长出完整龟壳提供了线索。科学家们认为海龟张出硬壳是为了防御海里的猎手。在陆地上有土地来保护肚皮，但是在水生环境里，海龟容易腹背受敌。龟板进化之后，甲壳也开始进化，这给了海龟更多的保护。
