答案是琥珀，树液的化石。

　　We find all kinds of prehistoric insects, plants, and even small reptiles trapped within amber.

　　我们能发现琥珀里面裹着各种史前昆虫、植物甚至小型爬行动物。

　　We also find gas balls.

　　我们还能发现气泡。

　　And so scientists were able to drill into these pieces of amber and extract the oxygen bubbles within them.

　　所以科学家们能钻进这些琥珀把里面的气泡提取出来。

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　　This is how we know what the percentage of oxygen used to be.

　　我们就这样了解到过去空气中的氧气百分比。

　　Higher oxygen levels also allowed plant eaters to be enormous.

　　更高的氧气水平同样让以植物为食者长得巨大。

　　Allosaurus was perfectly suited for taking on big game.

　　异特龙非常适合捕猎大型猎物。

　　When you look at the foot and the leg of Allosaurus, it's certain quite powerful.

　　要是看看异特龙的脚和腿，就知道它们相当有力。

　　I mean, Allosaurus was a large therapods of its time, one of the top predators.

　　我是说，异特龙是当时一种大型兽脚类，顶级掠食者之一。

　　So not only was it built to carry the weight of the animal, but also, was no doubt, used to hold down carcasses and things like that.

　　所以它的身体设计不仅要承载体重，毫无疑问也被用于抓住尸体和之类的东西。

　　We have to remember what was Allosaurus eating.

　　我们别忘了异特龙都吃什么。

　　They were eating some of the biggest animals the earth has ever seen.

　　它们吃的是一些有史以来最大的一些动物。

　　Allosaurus was feeding on sauropods - animals that are multi-times the size of our largest mammals today, the elephant.

　　异特龙捕食蜥脚类恐龙，这些动物的体型是如今最大的哺乳动物大象的好几倍。

　　And they were bringing these animals down.

　　异特龙能击败这些动物。

　　Allosaurus is an example of how all life adapts and changes to the environment.

　　异特龙是所有生命根据环境进行适应和改变的范例。

　　First, you have trees that began to grow to outrageous sizes and it's also when we see the long-necked dinosaurs emerging to take advantage of these tall trees.

　　首先，树木开始长得异常茁壮，也是这个时候我们看到长颈恐龙开始出现来利用这些高大树木。

　　Camarasaurus was the most common and successful dinosaur of that time.

　　圆顶龙是那一时期最普遍、最成功的恐龙。

　　Then Allosaurus had to grow huge because he prey was becoming so huge.

　　之后异特龙也得长得巨大，因为它们的猎物在变得非常庞大。

　　So in a sense you could say Allosaurus grew big because trees grew big.

　　所以在某种程度上可以说异特龙是因为树木变大而变大。

　　This was a good all-purpose generalized animal and I think that's one of the reasons for its success.

　　这真是一种万能的动物，我觉得这是它成功的原因之一。

　　It was very widely distributed and fairly abundant.

　　它的分布范围很广，数量也很多。

　　And that probably was because it was generalized enough that it could adapt to a wide variety of different environments, take a wide variety of prey, and as a result, flourished.

　　很可能是因为是足够全面所以能够适应各种不同的环境，应对个各种各样的猎物，结果，就繁盛了。

　　Its ability to adapt allowed Allosaurus to become one of the most successful predators of the late Jurassic.

　　它的适应能力让异特龙变成了晚侏罗纪时期最成功的掠食者之一。

　　Its long stiff tail gave it excellent balance, allowing it to hunt in the open and in dense forests.

　　又硬又长的尾巴使它拥有出色的平衡能力，在开阔地和密林之中都能狩猎。