时差N小时 海龟是怎么长壳的?

We may never know if the chicken or the egg came first, but scientists have discovered a fossil that sheds light on how turtles got their shells. Since the time of the dinosaurs¹, turtles have looked much the same as they do today, complete with boney shells. But scientists know turtles didn't just appear on the Earth fully² formed.

可能我们永远不会知道先有鸡还是先有蛋，但是科学家们发现的一个海龟化石揭示了它们是如何长出龟壳的？自恐龙时代起，海龟的外貌就跟现在相差无几，一样背着骨组织外壳。但是科学家们知道原始海龟的龟壳并没有发育完整。

There must have been an ancestor with a partial shell or no shell at all. So how did the shell get there? Some dinosaurs and present day reptiles³, like crocodiles, have boney skin plates called osteoderms. So researchers thought turtle shells might have evolved from these plates. The trouble was, when they studied turtle embryos⁴, they found that shells grow out from the backbone⁵ and ribs⁶ and don't appear to be over-grown osteoderms.

海龟的祖先中一定有只长出部分龟壳，或者根本没有龟壳的海龟。那么龟壳是如何长出来的？远古时代的恐龙和现在的爬行动物（比如说鳄鱼）都长有叫做“皮内成骨”的骨组织皮肤板块。因此研究者认为龟壳是从这种皮内成骨进化而来。问题是当他们研究海龟胚胎时发现，龟壳从脊椎骨和肋骨里长出，并没有参差不齐的骨化鳞片。

The evolution of the shell remained a mystery until a 220- million-year-old fossil was unearthed⁷ in Ghizhou Province, China. The fossil was named Odontochelys semitestacea, or half-shelled turtle with teeth. Unlike modern land turtles, this sixteen-inch-long species was found with other marine⁸ reptiles, indicating it was aquatic⁹. It had a plastron or lower shell.

直到中国贵州省出土了一个2.2亿年前的海龟化石，龟壳进化的迷雾才被揭开。这个化石被命名为半甲齿龟。与现代的陆地海龟不同的是，这种长约16英尺的龟类与其他海洋爬行动物一同被发现，这表明它是水生动物。它长有龟板。

But the upper shell, called a carapace¹⁰, was not there, only a thickened backbone and ribs. Odontochelys not only answered some questions about the shell's origin, it also gave a clue as to why a turtle has a full-body shell. Scientists think the shell evolved as a defense¹¹ against ocean predators¹². On land, the belly¹³ is protected by the ground, but in an aquatic environment, turtles can be attacked from above or below. After the plastron evolved, the carapace developed, giving turtles extra protection.

但是背部却没有壳（即没有甲壳），只有增厚的脊椎骨和肋骨。半甲齿龟不仅回答了龟壳起源的问题，还为海龟为什么长出完整龟壳提供了线索。科学家们认为海龟张出硬壳是为了防御海里的猎手。在陆地上有土地来保护肚皮，但是在水生环境里，海龟容易腹背受敌。龟板进化之后，甲壳也开始进化，这给了海龟更多的保护。

1 dinosaurs   

n.恐龙( dinosaur的名词复数 )；守旧落伍的人，过时落后的东西

参考例句：

• The brontosaurus was one of the largest of all dinosaurs. 雷龙是所有恐龙中最大的一种。 来自《简明英汉词典》
• Dinosaurs have been extinct for millions of years. 恐龙绝种已有几百万年了。 来自《简明英汉词典》

2 fully   

adv.完全地，全部地，彻底地；充分地

参考例句：

• The doctor asked me to breathe in,then to breathe out fully.医生让我先吸气，然后全部呼出。
• They soon became fully integrated into the local community.他们很快就完全融入了当地人的圈子。

3 reptiles   

n.爬行动物，爬虫( reptile的名词复数 )

参考例句：

• Snakes and crocodiles are both reptiles. 蛇和鳄鱼都是爬行动物。 来自《简明英汉词典》
• Birds, reptiles and insects come from eggs. 鸟类、爬虫及昆虫是卵生的。 来自《现代汉英综合大词典》

4 embryos   

n.晶胚；胚，胚胎( embryo的名词复数 )

参考例句：

• Somatic cells of angiosperms enter a regenerative phase and behave like embryos. 被子植物体细胞进入一个生殖阶段，而且其行为象胚。 来自辞典例句
• Evolution can explain why human embryos look like gilled fishes. 进化论能够解释为什么人类的胚胎看起来象除去了内脏的鱼一样。 来自辞典例句

5 backbone   

n.脊骨，脊柱，骨干；刚毅，骨气

参考例句：

• The Chinese people have backbone.中国人民有骨气。
• The backbone is an articulate structure.脊椎骨是一种关节相连的结构。

6 ribs   

n.肋骨( rib的名词复数 )；（船或屋顶等的）肋拱；肋骨状的东西；（织物的）凸条花纹

参考例句：

• He suffered cracked ribs and bruising. 他断了肋骨还有挫伤。
• Make a small incision below the ribs. 在肋骨下方切开一个小口。

7 unearthed   

出土的（考古）

参考例句：

• Many unearthed cultural relics are set forth in the exhibition hall. 展览馆里陈列着许多出土文物。
• Some utensils were in a state of decay when they were unearthed. 有些器皿在出土时已经残破。

8 marine   

adj.海的；海生的；航海的；海事的；n.水兵

参考例句：

• Marine creatures are those which live in the sea. 海洋生物是生存在海里的生物。
• When the war broke out,he volunteered for the Marine Corps.战争爆发时，他自愿参加了海军陆战队。

9 aquatic   

adj.水生的，水栖的

参考例句：

• Aquatic sports include swimming and rowing.水上运动包括游泳和划船。
• We visited an aquatic city in Italy.我们在意大利访问过一个水上城市。

10 carapace   

n.（蟹或龟的）甲壳

参考例句：

• The tortoise pulled its head into his carapace.乌龟把头缩进它的壳里。
• He tickled gently at its glossy carapace,but the stubborn beetle would not budge.他轻轻地搔着甲虫光滑的壳，但这只固执的甲虫就是不动。

11 defense   

n.防御，保卫；[pl.]防务工事；辩护，答辩

参考例句：

• The accused has the right to defense.被告人有权获得辩护。
• The war has impacted the area with military and defense workers.战争使那个地区挤满了军队和防御工程人员。

12 predators   

n.食肉动物( predator的名词复数 )；奴役他人者（尤指在财务或性关系方面）

参考例句：

• birds and their earthbound predators 鸟和地面上捕食它们的动物
• The eyes of predators are highly sensitive to the slightest movement. 捕食性动物的眼睛能感觉到最细小的动静。 来自《简明英汉词典》

13 belly   

n.肚子，腹部；(像肚子一样)鼓起的部分，膛

参考例句：

• The boss has a large belly.老板大腹便便。
• His eyes are bigger than his belly.他眼馋肚饱。