When thinking of giant animals, dinosaurs might be the first creatures you think of. But before the dinosaurs became extinct, and before the first ever humans existed, there was megafauna. In zoology, this means large animals.当思考巨型动物时，恐龙可能是您想到的第一批生物。 但是在恐龙灭绝之前，在有史以来第一个人类存在之前，有Megafauna。 在动物学中，这意味着大动物。

One of the largest known mammals to have ever walked the Earth is the Paraceratherium – picture a gigantic hornless rhino, and you'll have a rough idea of what they looked like. They lived around 25 million years ago and were most common in areas of the world that are now part of Asia, such as China, India and Kazakhstan. From excavated fossils, these creatures are estimated to have been almost six metres tall – that's the height of two buses on top of each other! 曾经走过地球的最大的已知哺乳动物之一就是游牧 - 想象一个巨大的无角犀牛，您将对它们的外观有一个粗略的了解。 他们居住于大约2500万年前，在现在已经是亚洲的一部分的地区，例如中国，印度和哈萨克斯坦。 从挖掘的化石中，估计这些生物已经高六米，这是两辆公共汽车的高度！ 

And it's not just land enormous creatures lived on – they were in the skies and oceans too. The Argentavis is the largest flying bird to have been discovered. It lived until around six million years ago and had a wingspan of seven metres, over twice the size of the Andean condor, which is one of the largest birds on Earth today. And we can't forget the giant shark that roamed the oceans, megalodon. Imagine swimming in the sea and coming across an 18-metre-long shark! Don't worry. They died out around 2.6 million years ago. 而且，这不仅是土地巨大的生物，而且还在天空和海洋中。 阿根廷是发现的最大的飞鸟。 它一直活到大约600万年前，翼展为7米，是安第斯秃鹰的两倍以上，这是当今地球上最大的鸟类之一。 而且我们不能忘记漫游海洋的巨型鲨鱼，Megalodon。 想象一下在海里游泳，遇到一条18米长的鲨鱼！ 不用担心。 他们大约在260万年前去世。

So, why were pre-historic animals so huge? There are a number of reasons. One is that, in the past, resources were more plentiful, so species were able to grow larger because they could eat more. Scientists also believe that during certain periods of history, such as in the Carboniferous period, there were higher oxygen levels on Earth. At this time, some insects grew to the size of cars because of better oxygen delivery in their tracheal system, the network of tubes that enable insects to breathe. 那么，为什么史前动物如此巨大？ 有很多原因。 一个是，过去的资源更丰富，因此物种能够生长更大，因为它们可以吃得更多。 科学家还认为，在历史的某些时期，例如石炭纪时期，地球上的氧气水平较高。 目前，由于气管系统中的氧气递送更好，该昆虫的大小是汽车的大小，这是使昆虫呼吸的试管网络。 

Can you picture animals being so huge today? 

您能想象动物今天如此巨大吗？