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龟壳如何进化两次? – 译学馆
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龟壳如何进化两次?

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

Meet Odontochelys semitestacea.
认识一下半甲齿龟
This little creature spends its days splashing in Late Triassic swamps
这种小生物和一群其他爬行动物一起
with a host of other reptiles.
整日在三叠纪晚期的沼泽中嬉戏
Under the surface lies its best defense against attack:
身体之下隐藏着它抵御攻击的最佳防御:
a hard shell on its belly.
位于腹部的坚硬甲壳
Odontochelys is an early ancestor of the turtle.
齿龟属是龟类的早期祖先
Its half-shelled body illustrates an important point about the modern turtle:
其半甲的身体体现了现代龟类的一个重要特点:
it actually has two shells that develop totally separately
龟类胚胎的两片龟甲
while the turtle is still an embryo.
完全独立生长
Both are extensions of the animal’s skeleton,
都是骨骼的延伸
and together they are made of almost 60 bones.
共由60块骨头组成
Like other embryos,
和其他胚胎一样
turtle embryos are made of undifferentiated cells
龟类胚胎是由未分化的细胞组成
that become specific cell types,
这些细胞通过基因活动和细胞间的交流
and then organs and tissues,
逐渐分化
through gene activity and communication between cells.
成为器官和组织
At first, turtle embryos look very similar
龟类胚胎的最初形态与
to those of other reptiles, birds, and mammals,
其他爬行动物 鸟类和哺乳动物非常相似
except for a bulge of cells called the carapacial ridge.
差别只有一团叫做背甲脊的凸起细胞
The ridge expands around the body between the neck and lower back,
背脊在颈部和下背部之间向身体四周扩展
creating a disc shape.
形成一个圆盘
It guides the formation of the upper part of the turtle’s shell, called the carapace,
大概通过聚集形成肋骨的细胞
likely by attracting the cells that will become ribs.
引导龟壳上部背甲的形成
Instead of curving downwards to make a regular rib cage,
肋骨并非向下弯曲形成规则的胸腔
the ribs move outwards towards the carapacial ridge.
而是朝着背甲脊向外移动
They then secrete a signaling protein
之后分泌信号蛋白
that converts surrounding cells into bone-forming cells.
将周围的细胞转化为成骨细胞
These fifty bones grow until they meet and connect with sutures.
这50块骨头持续生长 直至骨缝相连
A ring of bone solidifies the carapace’s edges.
骨头围成一圈使背甲更为稳固
The outer layer of skin cells produces the scales, known as scutes,
外层的皮肤细胞产生鳞片 即鳞甲
that cover the carapace.
覆盖背甲
The development of the bottom half of the shell, the plastron,
甲壳下半部分 即胸甲
is driven by neural crest cells,
由神经嵴细胞驱动
which can produce a variety of different cell types
这些细胞会产生不同种类的细胞
including neurons, cartilage and bone.
包括神经元 软骨和骨骼
A thick shield of these cells spreads across the belly,
这些细胞组成的厚屏障在腹部扩展开
coming together in regions that produce nine plate-like bones.
在产生九块板状的骨头的地方聚合
Eventually, these connect to the carapace by sutures.
最终 通过骨缝连接到背甲上
A turtle’s shell has obvious advantages for guarding against predators,
对于抵御捕食者 龟壳有明显优势
but the rigid casing also presents some challenges.
但坚硬的外壳也带来了一些挑战
As the turtle grows,
随着龟类不断成长
the sutures between the bones of the carapace and plastron spread.
背甲和胸甲处的骨缝不断扩大
Most mammals and reptiles rely on
很多哺乳动物和爬行动物都依靠
a flexible rib cage that expands to allow them to breathe,
灵活扩张的胸腔自由呼吸
but turtles use abdominal muscles attached to the shell instead:
但是龟类却用壳上的腹肌来代替:
one to breathe in, and one to breathe out.
一块腹肌用来吸气 一块用来呼气
So how did the shell evolve?
那龟壳是怎样进化的呢?
Though there are still gaps in the fossil record,
虽然化石记录中仍有空白
the first step seems to be a thickening of the ribs.
但第一阶段似乎便是肋骨的增厚
The oldest known turtle ancestor,
已知最古老的龟类祖先
a creature called Eunotosaurus africanus,
是一种叫做正南龟的生物
lived 260 million years ago and looked almost nothing like a modern turtle,
生活在2.6亿年前 与现代龟类长相迥异
but it had a set of broad, flat ribs
但它的肋骨宽而平
that anchored the muscles of its powerful forearms.
将前臂的肌肉牢牢固定住
Eunotosaurus was likely a burrowing creature,
正南龟很可能是一种穴居生物
digging homes for itself in what’s now southern Africa.
在今天的南非地区挖洞安家
Odontochelys semitestacea illustrates another, later step in turtle evolution,
半甲齿龟展现了龟类的另一种后期进化阶段
with thick ribs like Eunotosaurus plus a belly plate for protection.
粗壮的肋骨与正南龟类似 而且有腹部有板甲保护
Our first fossil evidence of the full shell characteristic of modern turtles
我们第一个证明现代龟类全壳特征的化石
is about 210 million years old,
距今已有2.1亿年
and belongs to a species called Proganochelys quenstedti,
来自叫做原腭龟的物种
whose ribs had fused.
其肋骨已经合为一体
Proganochelys could move between water and land.
原腭龟可以水陆两栖
Unlike modern turtles, it couldn’t retract its head into its shell,
与现代龟类不同 它不能将头缩回壳中
but had defensive spines on its neck.
但颈部有刺可以防御
Modern turtle shells are almost as diverse as the turtles themselves.
现代龟类的壳几乎随着物种不同而相异
Sea turtles have flatter, lighter shells for streamlined gliding through the water.
海龟壳更加轻而平 有利于水中滑行
Land-dwelling tortoises, meanwhile,
同时 陆龟的
have domed shells that can slip free of predators’ jaws
半球形壳有利于其摆脱捕食
and help them turn right-side up if they fall on their backs.
帮助其仰倒在地上时向右翻身
Leatherback and softshell turtles
棱皮龟和软壳龟的
have shells without the ring of bone around the edge of the carapace
背甲没有一圈骨头环绕
or the tough scutes covering it,
也没有鳞甲覆盖
making it easier for them to squeeze into tight spaces.
方便其挤进狭小空间
Now you know how the turtles got their shells,
现在你已经了解了龟壳的进化历程
but what about the jellyfishes’ sting,
那水母的刺
or the electric fish is electricity?
还有电鱼的电又是怎样进化的呢?
Keep learning with these lessons.
请在本系列课程中继续学习

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龟壳的进化历程

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