Rock-solid evidence for the origins of birds
胃里的石头，如何揭示鸟类起源？
柔软的内脏难以保存，胃中的石头却能留下消化方式的线索。文章由现代动物的比较走向恐龙，再提出食物研磨位置与飞行之间的联系。精读时既要拆开嵌套长句，也要分清观察、类比和演化解释。
原文来源：The Economist
学习目标
- 画出“石头形状—胃的作用—身体重心”的推理链
- 读懂夹有插入语的多层关系从句
- 区分圆润胃石所支持的推断与尚未证明的因果
- 解释标题和结尾围绕石头、胃展开的双关
中英对照阅读
HOW DO YOU know what a dinosaur ate? Guts rot away after death, so the details of dinosaur digestion remain obscure. The best indicators are teeth. Sharp, pointed teeth probably ripped chunks of flesh off prey for swallowing whole. Robust grinding surfaces suggest lots of chewing was involved. Yet many dinosaurs belonging to a group called the theropods, which gave rise to birds, lost their teeth altogether and evolved beaks. This has made it hard to work out what they ate and how they digested it.
怎样知道一只恐龙吃过什么？ 内脏在死亡后腐烂消失，因此恐龙消化过程的细节仍不清楚。 最好的线索是牙齿。 尖锐的牙齿很可能把肉块从猎物身上撕下来，整块吞下。 坚固的研磨面表明，进食时涉及大量咀嚼。 然而，许多属于兽脚类这一类群的恐龙完全失去了牙齿，演化出了喙；鸟类正是由兽脚类演化而来。 这让人很难弄清它们吃什么，以及如何消化食物。
There is, however, a second clue. Lots of dinosaurs had stones, known as gastroliths, in their stomachs. These are assumed to have assisted with digestion, and thus have their own tales to tell. And modern birds and crocodilians (which are both, like dinosaurs, members of a larger evolutionary group called the archosaurs) often have gastroliths, too.
不过，还有第二条线索。 许多恐龙胃里有被称作胃石的石头。 人们推测这些石头曾辅助消化，因此它们也有自己的故事可讲。 现代鸟类和鳄类也常有胃石；它们与恐龙一样，都属于更大的演化类群——主龙类。
Takasaki Ryuji at Okayama University of Science, in Japan, decided to investigate. In a study published in Paleobiology, he and his colleagues examined the gastroliths of 46 modern archosaur species (42 birds and four crocodilians) and compared them with those from 16 species of dinosaur.
日本冈山理科大学的Takasaki Ryuji决定展开调查。 在发表于《古生物学》的一项研究中，他和同事检查了46种现代主龙类动物——42种鸟类和4种鳄类——的胃石，并将它们与16种恐龙的胃石比较。
The birds included herbivores, omnivores and carnivores. The crocodilians were all carnivorous. The shapes of the stones varied. As might be expected, if an animal had a highly muscular stomach (ducks and geese), its gastroliths tended to have been worn into the sorts of rounded shapes displayed by beach pebbles. Weaker stomachs (seabirds and crocodiles) contained more angular stones.
研究中的鸟类包括植食、杂食和肉食动物。 鳄类则全是肉食动物。 石头形状各不相同。 正如人们或许会预料的，如果一种动物的胃部肌肉很发达，例如鸭和鹅，其胃石往往已被磨成类似海滩卵石的圆润形状。 力量较弱的胃，例如海鸟和鳄的胃，则含有棱角更多的石头。
Looking at the dinosaurs, Dr Takasaki and his colleagues found that ornithischians (think Triceratops), a group which their teeth suggest were herbivores, retained angular gastroliths. These creatures, this implies, relied mostly on their jaws to grind up their food, leaving their stomachs to apply merely the finishing touches.
再看恐龙，Takasaki博士和同事发现，鸟臀类——可以想想三角龙——保留着棱角分明的胃石；牙齿表明，这个类群的动物是植食性的。 这意味着，这些动物主要依靠颌部磨碎食物，只让胃完成最后一点加工。
Long-necked sauropods (think Diplodocus) also had angular gastroliths. That is more of a puzzle, for these animals had pencil-like teeth using which, it is assumed, they stripped the leaves off tree branches that less cervically elongated animals were unable to reach. The explanation, Dr Takasaki suspects, is that like those of modern ungulates, sauropods’ stomachs depended mainly on fermentation to break food down.
长颈的蜥脚类——可以想想梁龙——也有棱角分明的胃石。 这就更令人费解了，因为这些动物的牙齿像铅笔一样；据推测，它们用这种牙齿从那些颈部较短的动物够不到的树枝上捋下叶子。 Takasaki博士推测，解释可能在于：蜥脚类的胃像现代有蹄类动物的胃一样，主要依赖发酵来分解食物。
Stones in the guts of theropods tell yet another story. Tarbosaurus, a hypercarnivore similar to Tyrannosaurus, and similarly endowed with sharp, pointed teeth, had angular gastroliths. However, theropods with beaks (of which science recognises at least four groups) had rounded ones. Indeed, Dr Takasaki saw a clear relation between tooth loss and stone roundness, as the work of grinding up food was transferred from jaws to stomach.
兽脚类肚子里的石头又讲述了另一番故事。 特暴龙是一种与暴龙相似的高度肉食性动物，同样长着尖利的牙齿，其胃石也有明显棱角。 然而，有喙的兽脚类——科学界至少识别出四个这样的类群——却有圆润的胃石。 事实上，Takasaki博士观察到牙齿丧失与石头圆润程度之间存在清晰联系：研磨食物的工作从颌部转移到了胃中。
That shift may have helped make possible the evolutionary success of birds. Teeth are heavy, as are the jaw muscles those teeth require to do their job. This puts a lot of weight near an animal’s front. Shifting the job of grinding, and the muscles and hard surfaces involved, to the stomach centralises an animal’s centre of gravity in a way that would make it easier for it to rise from the ground.
这种转移可能帮助促成了鸟类在演化上的成功。 牙齿很重，让牙齿发挥作用所需要的颌部肌肉也很重。 这让动物身体前端承受了很大重量。 把研磨工作以及相关肌肉和坚硬表面移到胃部，会让动物的重心更集中于身体中部，从而更容易离地飞起。
Curiously, it is clear from the fossil record that the very earliest birds of all did not have beaks. But the fact that so many of their theropod relatives did, and that birds themselves evolved them at least twice, suggests the transition was easy. It may thus be that part of the secret to taking successfully to the skies was, quite literally, having the stomach for it.
有趣的是，化石记录清楚表明，最早的鸟类并没有喙。 但鸟类的许多兽脚类亲缘动物有喙，而且鸟类自身至少两次演化出了喙；这些事实表明，这种转变并不难。 因此，成功飞上天空的部分秘诀，可能真的就在于拥有能胜任这份工作的胃。