How turtle shells evolved... twice - Judy Cebra Thomas

翻译人员: Carol Wang 校对人员: David Tang

Meet Odontochelys semitestacea.

来认识一下半甲齿龟吧。

This little creature spends its days splashing in Late Triassic swamps

这种小生物与其它爬行动物一起，

with a host of other reptiles.

游荡在三叠纪的沼泽中。

Under the surface lies its best defense against attack:

在这表面之下， 是它应对攻击的最好防御：

a hard shell on its belly.

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

它实际上有两个分别生长的壳，

while the turtle is still an embryo.

海龟还是胚胎的时候，就分别发育了。

Both are extensions of the animal’s skeleton,

两者都是动物骨头的延伸，

and together they are made of almost 60 bones.

两者总共有近 60 块骨头组成。

Like other embryos,

和其它动物胚胎一样，

turtle embryos are made of undifferentiated cells

海龟的胚胎是由同样的细胞组成，

that become specific cell types,

这些细胞逐渐分化，

and then organs and tissues,

通过基因活动和细胞间的沟通，

through gene activity and communication between cells.

然后形成器官和组织。

At first, turtle embryos look very similar to those of other reptiles,

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，

birds, and mammals,

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,

脊椎在脖子与背部下缘之间不断延伸，

creating a disc shape.

形成一个圆盘形。

It guides the formation of the upper part of the turtle’s shell,

它主导了称为背甲的 海龟之背壳的形成，

called the carapace, likely by attracting the cells that will become ribs.

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,

肋骨没有向下弯曲 长成正常胸廓，

the ribs move outwards towards the carapacial ridge.

而是沿着甲脊方向向外长。

They then secrete a signaling protein

它们然后分泌一种信号蛋白质，

that converts surrounding cells into bone-forming cells.

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,

最外层的皮肤细胞长成鳞甲，

that cover the carapace.

将背甲覆盖住。

The development of the bottom half of the shell, the plastron,

下半部分的龟壳是胸甲，

is driven by neural crest cells,

它由神经嵴细胞主导形成，

which can produce a variety of different cell types including neurons,

神经嵴细胞可以发育出多种细胞，

像神经元、软骨和骨细胞。

cartilage and bone.

A thick shield of these cells spreads across the belly,

厚厚的一层这样的细胞 扩展到整个腹部，

coming together in regions that produce nine plate-like bones.

形成九块板状骨并聚在一起，

Eventually, these connect to the carapace by sutures.

并最终与背甲边缘连接起来。

A turtle’s shell has obvious advantages for guarding against predators,

龟壳具有明显的防掠食者的优势，

but the rigid casing also presents some challenges.

但坚固的外壳也带来了一些挑战。

As the turtle grows,

随着乌龟的成长，

the sutures between the bones of the carapace and plastron spread.

背甲和腹甲间的连接处也在延伸。

Most mammals and reptiles rely on a flexible rib cage

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，

that expands to allow them to breathe,

but turtles use abdominal muscles attached to the shell instead:

而乌龟则用附在壳上的腹肌来呼吸：

one to breathe in, and one to breathe out.

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,

尽管化石记录仍然存在断档，

the first step seems to have been a thickening of the ribs.

但第一步似乎是肋骨变厚。

The oldest known turtle ancestor,

已知的最古老的龟祖先，

a creature called Eunotosaurus africanus,

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

但它有一组固定其强大前臂肌肉的

that anchored the muscles of its powerful forearms.

宽而扁平的肋骨。

Eunotosaurus was likely a burrowing creature,

正南龟可能是穴居动物，

digging homes for itself in what’s now southern Africa.

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

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，

is about 210 million years old,

and belongs to a species called Proganochelys quenstedti,

化石是一种原颚龟物种，

whose ribs had fused.

其肋骨已融合在一起。

Proganochelys could move between water and land.

原颚龟可在水陆上行走，

Unlike modern turtles, it couldn’t retract its head into its shell,

与现代海龟不同， 它无法将头缩回壳中，

but had defensive spines on its neck.

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,

而陆龟具有圆顶状的外壳，

have domed shells that can slip free of predators’ jaws

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

棱皮龟和软壳龟的甲壳边缘

have shells without the ring of bone around the edge of the carapace

没有骨环，

or the tough scutes covering it,

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。