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Meet Odontochelys semitestacea. This little creature spends its days splashing in Late Triassic swamps with a host of other reptiles.
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。
Under the surface lies its best defense against attack: a hard shell on its belly.
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。
Odontochelys is an early ancestor of the turtle.
半甲齿龟是海龟们的先祖。
Its half-shelled body illustrates an important point about the modern turtle:
它半边是壳的身体体现了现代海龟重点：
it actually has two shells that develop totally separately while the turtle is still an embryo.
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。
Both are extensions of the animal's skeleton, and together they are made of almost 60 bones.
两者都是动物骨头的延伸，两者总共有近60块骨头组成。
Like other embryos, turtle embryos are made of undifferentiated cells that become specific cell types,
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，
and then organs and tissues, through gene activity and communication between cells.
通过基因活动和细胞间的沟通，然后形成器官和组织。
At first, turtle embryos look very similar to those of other reptiles, birds, and mammals,
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，
except for a bulge of cells called the carapacial ridge.
除了一部分叫做甲脊的细胞。
The ridge expands around the body between the neck and lower back, creating a disc shape.
脊椎在脖子与背部下缘之间不断延伸，形成一个圆盘形。
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.
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。
Instead of curving downwards to make a regular rib cage, the ribs move outwards towards the carapacial ridge.
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。
They then secrete a signaling protein that converts surrounding cells into bone-forming cells.
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。
These fifty bones grow until they meet and connect with sutures.
这50根骨头继续生长，直到彼此相遇并结合在一起。
A ring of bone solidifies the carapace's edges.
一条环状骨将背甲边缘硬化。
The outer layer of skin cells produces the scales, known as scutes, that cover the carapace.
最外层的皮肤细胞长成鳞甲，将背甲覆盖住。
The development of the bottom half of the shell, the plastron, is driven by neural crest cells,
下半部分的龟壳是胸甲，它由神经嵴细胞主导形成，
which can produce a variety of different cell types including neurons, cartilage and bone.
神经嵴细胞可以发育出多种细胞，像神经元、软骨和骨细胞。
A thick shield of these cells spreads across the belly, coming together in regions that produce nine plate-like bones.
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。
Eventually, these connect to the carapace by sutures.
最终与背甲边缘连接起来。
A turtle's shell has obvious advantages for guarding against predators, but the rigid casing also presents some challenges.
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。
As the turtle grows, the sutures between the bones of the carapace and plastron spread.
随着乌龟的成长，背甲和腹甲间的连接处也在延伸。
Most mammals and reptiles rely on a flexible rib cage that expands to allow them to breathe,
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，
but turtles use abdominal muscles attached to the shell instead: one to breathe in, and one to breathe out.
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。
So how did the shell evolve?
那么龟壳如何演变的呢？
Though there are still gaps in the fossil record, the first step seems to have been a thickening of the ribs.
尽管化石记录仍然存在断档，但第一步似乎是肋骨变厚。
The oldest known turtle ancestor, a creature called Eunotosaurus africanus,
已知的最古老的龟祖先，是一种叫做非洲正南龟的生物，
lived 260 million years ago and looked almost nothing like a modern turtle,
它生活在2.6亿年前，看上去几乎不像现代龟，
but it had a set of broad, flat ribs that anchored the muscles of its powerful forearms.
但它有一组固定其强大前臂肌肉的宽而扁平的肋骨。
Eunotosaurus was likely a burrowing creature, digging homes for itself in what's now southern Africa.
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。
Odontochelys semitestacea illustrates another, later step in turtle evolution,
半甲齿龟则说明了乌龟进化中后来的又一步，
with thick ribs like Eunotosaurus plus a belly plate for protection.
它有和正南龟一样的厚肋骨和一块有保护作用的腹板。
Our first fossil evidence of the full shell characteristic of modern turtles is about 210 million years old,
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，
and belongs to a species called Proganochelys quenstedti, whose ribs had fused.
化石是一种原颚龟物种，其肋骨已融合在一起。
Proganochelys could move between water and land.
原颚龟可以在水陆上行走。
Unlike modern turtles, it couldn't retract its head into its shell, but had defensive spines on its neck.
与现代海龟不同，它无法将头缩回壳中，但脖子上有防御刺。
Modern turtle shells are almost as diverse as the turtles themselves.
现代龟的壳几乎和龟本身一样繁多。
Sea turtles have flatter, lighter shells for streamlined gliding through the water.
更扁更轻的龟壳方便海龟在水中滑行。
Land-dwelling tortoises, meanwhile, have domed shells that can slip free of predators' jaws
而陆龟具有圆顶状的外壳，方便其从掠食者的颚中滑脱，
and help them turn right-side up if they fall on their backs.
当它们仰面摔倒时，圆壳也能帮它们翻身。
Leatherback and softshell turtles have shells without the ring of bone around the edge of the carapace or the tough scutes covering it,
棱皮龟和软壳龟的甲壳边缘没有骨环，也无硬鳞甲覆盖，
making it easier for them to squeeze into tight spaces.
使它们更容易挤入狭窄的空间。
