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). Many explanations have been proposed for why dinosaurs became extinct. For example, some have blamed dinosaur extinction on the development of flowering plants, which were supposedly more difficult to digest and could have caused constipation or indigestion—except that flowering plants first evolved in the Early Cretaceous, about 60 million years before the dinosaurs died out. In fact, several scientists have suggested that the duckbill dinosaurs and horned dinosaurs, with their complex battery of grinding teeth, evolved to exploit this new resource of rapidly growing flowering plants.Others have blamed extinction on competition from the mammals, which allegedly ate all the dinosaur eggs—except that mammals and dinosaurs appeared at the same time in the Late Triassic, about 190 million years ago, and there is no reason to believe that mammals suddenly acquired a taste for dinosaur eggs after 120 million years of coexistence. Some explanations (such as the one stating that dinosaurs all died of diseases) fail because there is no way to scientifically test them, and they cannot move beyond the realm of speculation and guesswork.

This focus on explaining dinosaur extinction misses an important point: the extinction at the end of the Cretaceous was a global event that killed off organisms up and down the food chain. It wiped out many kinds of plankton in the ocean and many marine organisms that lived on the plankton at the base of the food chain. These included a variety of clams and snails, and especially the ammonites, a group of shelled squidlike creatures that dominated the Mesozoic seas and had survived many previous mass extinctions. The K-T event marked the end of the marine reptiles, such as the mosasaurs and the plesiosaurs, which were the largest creatures that had ever lived in the seas and which ruled the seas long before whales evolved. On land, there was also a crisis among the land plants, in addition to the disappearance of dinosaurs. So any event that can explain the destruction of the base of the food chain (plankton in the ocean, plants on land) can better explain what happened to organisms at the top of the food chain, such as the dinosaurs. By contrast, any explanation that focuses strictly on the dinosaurs completely misses the point. The Cretaceous extinctions were a global phenomenon, and dinosaurs were just a part of a bigger picture.

According to one theory, the Age of Dinosaurs ended suddenly 65 million years ago when a giant rock from space plummeted to Earth. Estimated to be ten to fifteen kilometers in diameter, this bolide (either a comet or an asteroid) was traveling at cosmic speeds of 20-70 kilometers per second, or 45,000-156,000 miles per hour. Such a huge mass traveling at such tremendous speeds carries an enormous amount of energy. When the bolide struck this energy was released and generated a huge shock wave that leveled everything for thousands of kilometers around the impact and caused most of the landscape to burst into flames. The bolide struck an area of the Yucatan Peninsula of Mexico known as Chicxulub, excavating a crater 15-20 kilometers deep and at least 170 kilometers in diameter. The impact displaced huge volumes of seawater, causing much flood damage in the Caribbean. Meanwhile, the bolide itself excavated 100 cubic kilometers of rock and debris from the site, which rose to an altitude of 100 kilometers. Most of it fell back immediately, but some of it remained as dust in the atmosphere for months. This material, along with the smoke from the fires, shrouded Earth, creating a form of nuclear winter. According to computerized climate models, global temperatures fell to near the freezing point, photosynthesis halted, and most plants on land and in the sea died. With the bottom of the food chain destroyed, dinosaurs could not survive.

大约在6500万年前，恐龙迅速地灭绝，成为K-T大规模灭绝事件的一部分。之所以称作K-T是因为它与地质学上的标签K-T边界有关。K-T边界通常是一个薄薄的沉积带，在世界各处都有存在（K在惯例上是白垩纪Cretaceous的缩写，源自德国名字kreidezeit）。关于恐龙为何绝种有很多解释。例如，有些将恐龙的灭绝归咎于开花植物的发展，这种植物据称更难消化，可引起便秘和消化不良——但是开花植物最初是在早白垩世纪进化的，也就是在恐龙灭绝前约60万年就出现了。实际上，一些科学家提出，鸭嘴恐龙和有角恐龙已经进化出一口复杂的磨牙，能够消化吸收这些迅速增长的开花植物。也有科学家将恐龙的灭绝归咎于其他哺乳动物的竞争，据说是它们吃掉了所有的恐龙蛋——但是，哺乳动物和恐龙都生活在在约190万年前的晚三叠世，没理由说哺乳动物在和恐龙共同生活了120万年之后，忽然爱吃恐龙蛋了。还有些解释（比如有一种解释认为恐龙死于疾病）也说不通，因为有没有办法进行科学验证，而且这些解释也无非都是臆想和猜测。 对恐龙灭绝的解释都错过了一个重要信息：白垩纪末的灭绝是一个全球性的事件，这次事件杀死的生物来自整个食物链。有许多海洋中的浮游生物都灭绝了，而在食物链的底端以这些浮游生物为食的许多海洋生物也都灭绝了。这些生物包括各种蛤蜊和蜗牛，尤其是菊石，一种类似鱿鱼的带壳生物，它曾经在中生代海洋中大规模存在，并且在以前的很多大规模灭绝中幸存了下来。K-T事件标志着海洋爬行动物（如沧龙类与蛇颈龙，曾经生活在海洋，并且早在鲸鱼出现之前统治海洋很久的最大生物）的结束。在陆地上，除了恐龙灭绝以外，陆地植物也面临着生存危机。因此，任何能解释食物链底端生物（海洋中的浮游生物、陆地上的植物）毁灭的原因，都能更好地解释发生在食物链顶端的生物，如恐龙的灭绝。相比之下，那些只关注恐龙本身的解释就没有解释到点子上，白垩纪灭绝是一个全球性的现象，恐龙的灭绝只是其中的一部分。 根据某一个理论，恐龙时代在6500万年前突然结束，是因为当时一个巨大的星体从太空撞到了地球上。这颗火流星（彗星或小行星）的直径估计在10000~15000米左右，以20~70千米每秒或45000~156000英里每小时的速度旋转。这个体积庞大、速度飞快的星体带着大量的能量。当星体撞击地球时，这些能量被释放，产生了巨大的冲击波，受到影响的数千公里的土地瞬间夷为平地，大部分景观突然起火。星体击中了墨西哥尤卡坦半岛的希克苏鲁伯地区，凿出一个直径15~20千米、深度至少有170千米的大坑。撞击造成大量海水移位，在加勒比海地区引起了很多洪灾。与此同时，火球自身也在撞击地凿出了100立方公里的岩石和碎片，使得当地的高度增加了100千米。大部分的碎片立刻就掉下来了，但有一些变成了灰尘仍然在大气中漂浮了数月之久。这些灰尘与火灾产生的烟雾一起笼罩着地球，创造出一种“核冬天”的感觉。根据计算机模拟的气候模型，全球气温下降到接近冰点，光合作用停止，陆地和海洋中的大部分植物都死掉了。随着食物链的底端被破坏，恐龙也就无法生存了。