This little creature spends its days splashing in Late Triassic swamps

來見見半甲齒龜。

with a host of other reptiles.

這小生物會整天在 晚三叠世的沼澤中玩水，

Under the surface lies its best defense against attack:

和其他爬蟲類一起玩。

a hard shell on its belly.

在水面之下，有著 抵禦攻擊的最佳防護：

Odontochelys is an early ancestor of the turtle.

腹部的硬殼。

Its half-shelled body illustrates an important point about the modern turtle:

半甲齒龜是烏龜的早期祖先。

it actually has two shells that develop totally separately

牠的身體上有半甲，可以說明 現代烏龜的一個重要特點：

while the turtle is still an embryo.

烏龜其實有兩個殼，

Both are extensions of the animal's skeleton,

在胚胎就是完全分開發展的。

and together they are made of almost 60 bones.

兩個殼都是從該動物的 骨骼延伸出來的，

Like other embryos,

加起來，它們一共由 近六十塊骨頭組成。

turtle embryos are made of undifferentiated cells

和其他胚胎一樣，

that become specific cell types,

烏龜胚胎是由未分化的細胞構成，

and then organs and tissues,

它們會變成特定的細胞類型，

through gene activity and communication between cells.

接著透過基因活動 以及細胞之間的溝通，

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

變成器官和組織。

birds, and mammals,

一開始，烏龜胚胎 看起來和其他爬蟲類、

except for a bulge of cells called the carapacial ridge.

鳥類、哺乳類的胚胎很相似，

The ridge expands around the body between the neck and lower back,

差別只有一塊細胞 凸起，稱為背甲脊。

creating a disc shape.

背甲脊沿著身體擴張， 從脖子一直延伸到下背，

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

創造出一個圓盤型。

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

它引導了龜殼上半部的形成，

Instead of curving downwards to make a regular rib cage,

上半部稱為背甲，形成方式 可能是吸引會變成肋骨的細胞。

the ribs move outwards towards the carapacial ridge.

這些肋骨並沒有向下彎曲 形成一般的胸廓，

They then secrete a signaling protein

反而朝背甲脊的方向向外移動。

that converts surrounding cells into bone-forming cells.

它們能夠分泌一種訊號蛋白質，

These fifty bones grow until they meet and connect with sutures.

將周圍的細胞轉換為 形成骨頭的細胞。

A ring of bone solidifies the carapace's edges.

這五十塊骨頭不斷成長， 直到相碰、相接，形成縫合線。

The outer layer of skin cells produces the scales, known as scutes,

一圈骨頭讓背甲的邊緣 變得更堅固。

that cover the carapace.

皮膚細胞的外層會產生鱗片， 即一般所知的鱗甲，

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

將背甲覆蓋住。

is driven by neural crest cells,

龜殼的下半部的發展， 也就是胸甲的發育，

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

是由神經脊細胞所驅動，

cartilage and bone.

這些細胞能產生各種細胞類型，

A thick shield of these cells spreads across the belly,

包括神經元、軟骨，和骨頭。

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

這些細胞所形成的厚盾 散佈在整個腹部上，

Eventually, these connect to the carapace by sutures.

集結起來，在一些部位 產生出九塊板狀的骨頭。

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

最後，透過縫合線， 它們會和背甲連結起來。

but the rigid casing also presents some challenges.

龜殼很明顯有利於 抵抗捕食性動物，

As the turtle grows,

但堅硬的外殼也造成了一些困難。

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

隨著烏龜長大，

Most mammals and reptiles rely on a flexible rib cage

背甲和胸甲的骨頭 之間的縫合線會伸展開。

that expands to allow them to breathe,

大部分的哺乳類和爬蟲類

but turtles use abdominal muscles attached to the shell instead:

仰賴擴張有彈性的胸廓， 讓牠們能夠呼吸，

one to breathe in, and one to breathe out.

但烏龜卻是用連結在 龜殼上的腹部肌肉：

So how did the shell evolve?

一組肌肉拉動肺部吸氣， 另一組肌肉擠壓肺部吐氣。

Though there are still gaps in the fossil record,

所以，龜殼怎麼演進？

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

雖然在化石記錄中 還有些空白未知之處，

The oldest known turtle ancestor,

第一步似乎是肋骨變粗。

a creature called Eunotosaurus africanus,

我們所知最古老的烏龜祖先

lived 260 million years ago and looked almost nothing like a modern turtle,

是一種叫做正南龜的生物，

but it had a set of broad, flat ribs

牠們生活在兩億六千萬年前，

that anchored the muscles of its powerful forearms.

看起來一點也不像現代的烏龜，

Eunotosaurus was likely a burrowing creature,

但牠們有一組寬廣、平坦的肋骨，

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

用來固定其強壯前臂的肌肉。

Odontochelys semitestacea illustrates another, later step in turtle evolution,

正南龜很可能是會挖地洞的生物，

with thick ribs like Eunotosaurus plus a belly plate for protection.

在現今的南非，自己挖洞造窩。

Our first fossil evidence of the full shell characteristic of modern turtles

半甲齒龜則代表了烏龜 演化過程中較後段的另一步，

is about 210 million years old,

牠們有正南龜的粗肋骨， 再加上腹部的保護板塊。

and belongs to a species called Proganochelys quenstedti,

我們所能找到具有現代龜殼 所有特性的化石證據當中，

whose ribs had fused.

最早的來自大約兩億一千萬年前，

Proganochelys could move between water and land.

隸屬於一種叫做原顎龜的物種，

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

牠們的肋骨是接在一起的。

but had defensive spines on its neck.

原顎龜可以在水中及陸地上移動。

Modern turtle shells are almost as diverse as the turtles themselves.

和現代烏龜不同， 牠們無法把頭縮到龜殼中，

Sea turtles have flatter, lighter shells for streamlined gliding through the water.

但在脖子上有防禦用的刺。

Land-dwelling tortoises, meanwhile,

現代烏龜的殼，幾乎 和烏龜本身一樣多樣化。

have domed shells that can slip free of predators' jaws

海龜的殼比較平、比較輕， 流線造型讓牠們能在水中滑行。

and help them turn right-side up if they fall on their backs.

至於生活在陸地上的陸龜，

Leatherback and softshell turtles

則有半球形的殼，被捕食性 動物咬住時還可以滑出來，

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

且如果翻過來的時候， 可以協助牠們翻身回來。

or the tough scutes covering it,

稜龜和軟殼龜

making it easier for them to squeeze into tight spaces.

背甲的邊緣沒有一圈骨頭，