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

譯者: Lilian Chiu 審譯者: Carol Wang

Meet Odontochelys semitestacea.

來見見半甲齒龜。

This little creature spends its days splashing in Late Triassic swamps

這小生物會整天在 晚三叠世的沼澤中玩水，

with a host of other reptiles.

和其他爬蟲類一起玩。

Under the surface lies its best defense against attack:

在水面之下，有著 抵禦攻擊的最佳防護：

a hard shell on its belly.

腹部的硬殼。

Odontochelys is an early ancestor of the turtle.

半甲齒龜是烏龜的早期祖先。

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

牠的身體上有半甲，可以說明 現代烏龜的一個重要特點：

it actually has two shells that develop totally separately

烏龜其實有兩個殼，

在胚胎就是完全分開發展的。

while the turtle is still an embryo.

Both are extensions of the animal’s skeleton,

兩個殼都是從該動物的 骨骼延伸出來的，

and together they are made of almost 60 bones.

加起來，它們一共由 近六十塊骨頭組成。

Like other embryos,

和其他胚胎一樣，

turtle embryos are made of undifferentiated cells

烏龜胚胎是由未分化的細胞構成，

that become specific cell types,

它們會變成特定的細胞類型，

and then organs and tissues,

接著透過基因活動 以及細胞之間的溝通，

through gene activity and communication between cells.

變成器官和組織。

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

一開始，烏龜胚胎 看起來和其他爬蟲類、

鳥類、哺乳類的胚胎很相似，

birds, and mammals,

except for a bulge of cells called the carapacial ridge.

差別只有一塊細胞 凸起，稱為背甲脊。

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

背甲脊沿著身體擴張， 從脖子一直延伸到下背，

creating a disc shape.

創造出一個圓盤型。

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

它引導了龜殼上半部的形成，

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

上半部稱為背甲，形成方式 可能是吸引會變成肋骨的細胞。

Instead of curving downwards to make a regular rib cage,

這些肋骨並沒有向下彎曲 形成一般的胸廓，

the ribs move outwards towards the carapacial ridge.

反而朝背甲脊的方向向外移動。

They then secrete a signaling protein

它們能夠分泌一種訊號蛋白質，

that converts surrounding cells into bone-forming cells.

將周圍的細胞轉換為 形成骨頭的細胞。

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

這五十塊骨頭不斷成長， 直到相碰、相接，形成縫合線。

A ring of bone solidifies the carapace’s edges.

一圈骨頭讓背甲的邊緣 變得更堅固。

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

皮膚細胞的外層會產生鱗片， 即一般所知的鱗甲，

that cover the carapace.

將背甲覆蓋住。

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

龜殼的下半部的發展， 也就是胸甲的發育，

is driven by neural crest cells,

是由神經脊細胞所驅動，

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

這些細胞能產生各種細胞類型，

包括神經元、軟骨，和骨頭。

cartilage and bone.

A thick shield of these cells spreads across the belly,

這些細胞所形成的厚盾 散佈在整個腹部上，

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

集結起來，在一些部位 產生出九塊板狀的骨頭。

Eventually, these connect to the carapace by sutures.

最後，透過縫合線， 它們會和背甲連結起來。

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

龜殼很明顯有利於 抵抗捕食性動物，

but the rigid casing also presents some challenges.

但堅硬的外殼也造成了一些困難。

As the turtle grows,

隨著烏龜長大，

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

背甲和胸甲的骨頭 之間的縫合線會伸展開。

Most mammals and reptiles rely on a flexible rib cage

大部分的哺乳類和爬蟲類

仰賴擴張有彈性的胸廓， 讓牠們能夠呼吸，

that expands to allow them to breathe,

but turtles use abdominal muscles attached to the shell instead:

但烏龜卻是用連結在 龜殼上的腹部肌肉：

one to breathe in, and one to breathe out.

一組肌肉拉動肺部吸氣， 另一組肌肉擠壓肺部吐氣。

So how did the shell evolve?

所以，龜殼怎麼演進？

Though there are still gaps in the fossil record,

雖然在化石記錄中 還有些空白未知之處，

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

第一步似乎是肋骨變粗。

The oldest known turtle ancestor,

我們所知最古老的烏龜祖先

a creature called Eunotosaurus africanus,

是一種叫做正南龜的生物，

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

牠們生活在兩億六千萬年前，

看起來一點也不像現代的烏龜，

but it had a set of broad, flat ribs

但牠們有一組寬廣、平坦的肋骨，

that anchored the muscles of its powerful forearms.

用來固定其強壯前臂的肌肉。

Eunotosaurus was likely a burrowing creature,

正南龜很可能是會挖地洞的生物，

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

在現今的南非，自己挖洞造窩。

Odontochelys semitestacea illustrates another, later step in turtle evolution,

半甲齒龜則代表了烏龜 演化過程中較後段的另一步，

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

牠們有正南龜的粗肋骨， 再加上腹部的保護板塊。

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

我們所能找到具有現代龜殼 所有特性的化石證據當中，

is about 210 million years old,

最早的來自大約兩億一千萬年前，

and belongs to a species called Proganochelys quenstedti,

隸屬於一種叫做原顎龜的物種，

whose ribs had fused.

牠們的肋骨是接在一起的。

Proganochelys could move between water and land.

原顎龜可以在水中及陸地上移動。

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

和現代烏龜不同， 牠們無法把頭縮到龜殼中，

but had defensive spines on its neck.

但在脖子上有防禦用的刺。

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

現代烏龜的殼，幾乎 和烏龜本身一樣多樣化。

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

海龜的殼比較平、比較輕， 流線造型讓牠們能在水中滑行。

Land-dwelling tortoises, meanwhile,

至於生活在陸地上的陸龜，

have domed shells that can slip free of predators’ jaws

則有半球形的殼，被捕食性 動物咬住時還可以滑出來，

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

且如果翻過來的時候， 可以協助牠們翻身回來。

Leatherback and softshell turtles

稜龜和軟殼龜

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

背甲的邊緣沒有一圈骨頭，

or the tough scutes covering it,

也沒有牢固的鱗甲覆蓋在其上方，

making it easier for them to squeeze into tight spaces.

這麼一來，牠們便 很容易擠進狹小的空間。