现代龟壳几乎和龟本身一样品类繁多。海龟的壳更平坦、更轻一些，便于它们在水中滑行；陆龟具有圆顶状的外壳，方便其从掠食者的颚中滑脱。棱皮龟的壳边缘没有骨环。那么龟壳是如何演变的呢？朱迪·塞布拉-托马斯探究了乌龟的祖先。
Meet Odontochelys semitestacea. This little creature spends its days splashing in Late Triassic swamps with a host of other reptiles. Under the surface lies its best defense against attack: a hard shell on its belly. 文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/3007.html
来认识一下半甲齿龟吧。这种小生物与其它爬行动物一起，游荡在三叠纪的沼泽中。在这表面之下，是它应对攻击的最好防御：肚子上一层坚硬的壳。文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/3007.html
Odontochelys is an early ancestor of the turtle. Its half-shelled body illustrates an important point about the modern turtle: it actually has two shells that develop totally separately while the turtle is still an embryo. Both are extensions of the animal’s skeleton, and together they are made of almost 60 bones. 文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/3007.html
半甲齿龟是海龟们的先祖，它半边是壳的身体体现了现代海龟重点：它实际上有两个分别生长的壳，海龟还是胚胎的时候，就分别发育了。两者都是动物骨头的延伸，两者总共有近60块骨头组成。文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/3007.html
Like other embryos, turtle embryos are made of undifferentiated cells that become specific cell types, and then organs and tissues, through gene activity and communication between cells. 文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/3007.html
和其它动物胚胎一样，海龟的胚胎是由同样的细胞组成，这些细胞逐渐分化，通过基因活动和细胞间的沟通，然后形成器官和组织。文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/3007.html
At first, turtle embryos look very similar to those of other reptiles, birds, and mammals, except for a bulge of cells called the carapacial ridge. The ridge expands around the body between the neck and lower back, creating a disc shape. It guides the formation of the upper part of the turtle’s shell, called the carapace, likely by attracting the cells that will become ribs. Instead of curving downwards to make a regular rib cage, the ribs move outwards towards the carapacial ridge. They then secrete a signaling protein that converts surrounding cells into bone-forming cells. These fifty bones grow until they meet and connect with sutures. A ring of bone solidifies the carapace’s edges. The outer layer of skin cells produces the scales, known as scutes, that cover the carapace. 文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/3007.html
起初，海龟胚胎看上去与其它爬行动物、鸟类和哺乳动物十分相似，除了一部分叫做甲脊的细胞。脊椎在脖子与背部下缘之间不断延伸，形成一个圆盘形。它主导了称为背甲的海龟之背壳的形成，可能脊椎通过吸引肋骨细胞逐渐结合在一起而形成。肋骨没有向下弯曲长成正常胸廓，而是沿着甲脊方向向外长。它们然后分泌一种信号蛋白质，该蛋白质将周围的细胞变为可形成骨头的细胞，这50根骨头继续生长，直到彼此相遇并结合在一起，一条环状骨将背甲边缘硬化。最外层的皮肤细胞长成鳞甲，将背甲覆盖住。文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/3007.html
The development of the bottom half of the shell, the plastron, is driven by neural crest cells, which can produce a variety of different cell types including neurons, cartilage and bone. A thick shield of these cells spreads across the belly, coming together in regions that produce nine plate-like bones. Eventually, these connect to the carapace by sutures. 文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/3007.html
下半部分的龟壳是胸甲，它由神经嵴细胞主导形成，神经嵴细胞可以发育出多种细胞，像神经元、软骨和骨细胞。厚厚的一层这样的细胞扩展到整个腹部，形成九块板状骨并聚在一起，并最终与背甲边缘连接起来。文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/3007.html
A turtle’s shell has obvious advantages for guarding against predators, but the rigid casing also presents some challenges. As the turtle grows, the sutures between the bones of the carapace and plastron spread. Most mammals and reptiles rely on a flexible rib cage that expands to allow them to breathe, but turtles use abdominal muscles attached to the shell instead: one to breathe in, and one to breathe out. 文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/3007.html
龟壳具有明显的防掠食者的优势，但坚固的外壳也带来了一些挑战。随着乌龟的成长，背甲和腹甲间的连接处也在延伸。多数哺乳动物和爬行动物都仰赖一个能扩张的弹性胸廓呼吸，而乌龟则用附在壳上的腹肌来呼吸：一组腹肌帮助吸气，另一组则帮助呼气。文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/3007.html
So how did the shell evolve? Though there are still gaps in the fossil record, the first step seems to have been a thickening of the ribs. The oldest known turtle ancestor, a creature called Eunotosaurus africanus, lived 260 million years ago and looked almost nothing like a modern turtle, but it had a set of broad, flat ribs that anchored the muscles of its powerful forearms. Eunotosaurus was likely a burrowing creature, digging homes for itself in what’s now southern Africa. 文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/3007.html
那么龟壳如何演变的呢？尽管化石记录仍然存在断档，但第一步似乎是肋骨变厚。已知的最古老的龟祖先，是一种叫做非洲正南龟的生物，它生活在2.6亿年前，看上去几乎不像现代龟，但它有一组固定其强大前臂肌肉的宽而扁平的肋骨。正南龟可能是穴居动物，在如今南部非洲的位置为自己挖掘栖身之所。文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/3007.html
Odontochelys semitestacea illustrates another, later step in turtle evolution, with thick ribs like Eunotosaurus plus a belly plate for protection. Our first fossil evidence of the full shell characteristic of modern turtles is about 210 million years old, and belongs to a species called Proganochelys quenstedti, whose ribs had fused. Proganochelys could move between water and land. Unlike modern turtles, it couldn’t retract its head into its shell, but had defensive spines on its neck. 文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/3007.html
半甲齿龟则说明了乌龟进化中后来的又一步，它有和正南龟一样的厚肋骨和一块有保护作用的腹板。我们的第一个化石证据表明，现代海龟具有完整外壳特征大约有2.1亿年的历史，化石是一种原颚龟物种，其肋骨已融合在一起。原颚龟可在水陆上行走，与现代海龟不同，它无法将头缩回壳中，但脖子上有防御刺。文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/3007.html
Modern turtle shells are almost as diverse as the turtles themselves. Sea turtles have flatter, lighter shells for streamlined gliding through the water. Land-dwelling tortoises, meanwhile, have domed shells that can slip free of predators’ jaws and help them turn right-side up if they fall on their backs. Leatherback and softshell turtles have shells without the ring of bone around the edge of the carapace or the tough scutes covering it, making it easier for them to squeeze into tight spaces. 文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/3007.html
现代龟的壳几乎和龟本身一样繁多，更扁更轻的龟壳方便海龟在水中滑行。而陆龟具有圆顶状的外壳，方便其从掠食者的颚中滑脱，当它们仰面摔倒时，圆壳也能帮它们翻身。棱皮龟和软壳龟的甲壳边缘没有骨环，也无硬鳞甲覆盖，使它们更容易挤入狭窄的空间。文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/3007.html
