地质史上共有五次大灭绝（Mass Extinction， or die-off），其中两次和恐龙息息相关。据维基百科，所谓大灭绝，就是整科、整目甚至整纲的生物在很短的时间内彻底消失或仅有极少数存留下来。参考历史上的朝代更替，我们很容易就可以把它和改朝换代的决定性战役作类比，经此一役，会出现新的领袖、新的政局、新的经济。

那么同样的，大灭绝往往也是两个地质时期的节点，为新物种的诞生创造条件。恐龙就是这样一种在大灭绝后出现，又在大灭绝中消亡的典型。

首先，明确恐龙生活的年代——中生代。我们看到图片中的中轴部分，Paleozoic（古生代），Mesozoic（中生代），Cenozoic（新生代），整个绿色的中生代都是恐龙主宰地球的时期，长达一亿三千多万年，而已知最早的猿人出现在60-80万年前。这是什么概念？

四舍五入就是你和王思聪名下存款的差距啊各位。

然后说说中生代开始和结束的标志，分别是第三次和第五次大灭绝。

第三次大灭绝距今2.5亿年，即Permian-Triassic extinction（二叠纪-三叠纪大灭绝），是五次物种大灭绝事件中最严重的一次，造成了98%的海洋生物以及96%的陆地生物都在50万年内忽然消失，大量的生物“蒸发”殆尽，整个海洋、陆地的生命迹象都基本消失。以此为界限，中生代三叠纪开始，恐龙出现。

经过三叠纪（初现）——侏罗纪（鼎盛）——白垩纪（尾声），距今6500万年的白垩纪末期发生了第五次大灭绝，即图片中的“big，big extinction”，托福阅读中惯称其为“K-T boundary”，T”指的是第三纪（the Tertiary period），“K”是德语中白垩纪（Kreidezeit） 的首字母。

在TPO42-2中阐述如下：

Dinosaurs rapidly became extinct about 65 million years ago as part of a mass extinction known as the K-T event, because it is associated with a geological signature known as the K-T boundary, usually a thin band of sedimentation found in various parts of the world (K is the traditional abbreviation for the Cretaceous, derived from the German name Kreidezeit).

第五次生物灭绝中，恐龙很遗憾地被团灭了，而它的后代鸟类存活下来且繁衍至今。

而关于灭绝的几种假说，需要大家自己从补充材料中寻找啦。

补充阅读（节选）：

Scientists have narrowed down several of the most likely causes of mass extinction. Flood basalt events (volcano eruptions), asteroid collisions, and sea level falls are the most likely causes of mass extinctions, though several other known events may also contribute. These include global warming, global cooling, methane eruptions and anoxic events–when the earth's oceans lose their oxygen.

Both volcano eruptions and asteroid collisions would eject tons of debris into the atmosphere, darkening the skies for at least months on end. Starved of sunlight, plants and plant-eating creatures would quickly die. Space rocks and volcanoes could also unleash toxic and heat-trapping gases that—once the dust settled—enable runaway global warming.

An extraterrestrial impact is most closely linked to the Cretaceous-Paleogene extinction event, one of the five largest in the history of the world, and the most recent. A huge crater off Mexico's Yucatán Peninsula is dated to about 65 million years ago, coinciding with the extinction. Global warming fueled by volcanic eruptions at the Deccan Flats in India may also have aggravated the event. Dinosaurs, as well as about half of all species on the planet, went extinct.