時差N小時:海龜是怎么長殼的?

We may never know if the chicken or the egg camefirst, but scientists have discovered a fossil thatsheds light on how turtles got their shells. Since thetime of the dinosaurs, turtles have looked much thesame as they do today, complete with boney shells.But scientists know turtles didn't just appear on theEarth fully formed.

可能我們永遠不會知道先有雞還是先有蛋，但是科學(xué)家們發(fā)現(xiàn)的一個海龜化石揭示了它們是如何長出龜殼的?自恐龍時代起，海龜?shù)耐饷簿透F(xiàn)在相差無幾，一樣背著骨組織外殼。但是科學(xué)家們知道原始海龜?shù)凝敋げ]有發(fā)育完整。

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

海龜?shù)淖嫦戎幸欢ㄓ兄婚L出部分龜殼，或者根本沒有龜殼的海龜。那么龜殼是如何長出來的?遠古時代的恐龍和現(xiàn)在的爬行動物(比如說鱷魚)都長有叫做“皮內(nèi)成骨”的骨組織皮膚板塊。因此研究者認為龜殼是從這種皮內(nèi)成骨進化而來。問題是當(dāng)他們研究海龜胚胎時發(fā)現(xiàn)，龜殼從脊椎骨和肋骨里長出，并沒有參差不齊的骨化鱗片。

The evolution of the shell remained a mystery until a 220- million-year-old fossil wasunearthed 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 foundwith other marine reptiles, indicating it was aquatic. It had a plastron or lower shell.

直到中國貴州省出土了一個2.2億年前的海龜化石，龜殼進化的迷霧才被揭開。這個化石被命名為半甲齒龜。與現(xiàn)代的陸地海龜不同的是，這種長約16英尺的龜類與其他海洋爬行動物一同被發(fā)現(xiàn)，這表明它是水生動物。它長有龜板。

But the upper shell, called a carapace, 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 asto why a turtle has a full-body shell. Scientists think the shell evolved as a defense againstocean predators. On land, the belly is protected by the ground, but in an aquaticenvironment, turtles can be attacked from above or below. After the plastron evolved, thecarapace developed, giving turtles extra protection.

但是背部卻沒有殼(即沒有甲殼)，只有增厚的脊椎骨和肋骨。半甲齒龜不僅回答了龜殼起源的問題，還為海龜為什么長出完整龜殼提供了線索?？茖W(xué)家們認為海龜張出硬殼是為了防御海里的獵手。在陸地上有土地來保護肚皮，但是在水生環(huán)境里，海龜容易腹背受敵。龜板進化之后，甲殼也開始進化，這給了海龜更多的保護。