第五次生物大灭绝，又称白垩纪大灭绝或恐龙大灭绝。

　　白垩纪(Cretaceous Period，Cretaceous)是中生代最后的一纪，始于距今1.37亿年，结束于距今6500万年，其间经历了7000万年。无论是无机界还是有机界在白垩纪都经历了重要变革。位于侏罗纪之下、新生界之上。白垩纪是中生代地球表面受淹没程度最大的时期，在此期间北半球广泛沉积了白垩层，1822年比利时学者J.奥马利达鲁瓦将其命名为白垩系。白垩层是一种极细而纯的粉状灰岩，是生物成因的海洋沉积，主要由一种叫做颗石藻的钙质超微化石和浮游有孔虫化石构成。

　　在五次大灭绝中，这次大灭绝事件最为著名，因长达14000万年之久的恐龙时代在此终结， 海洋中的菊石类也一同消失。其最大贡献在于消灭了地球上处于霸主地位的恐龙及其同类，并为哺乳动物及人类的最后登场提供了契机。这次灾难来自于地外空间和火山喷发，在白垩纪末期发生的一次或多次陨星雨造成了全球生态系统的崩溃。撞击使大量的气体和灰尘进入大气层，以至于阳光不能穿透，全球温度急剧下降，黑云遮蔽地球长达数年(零点几至几个百万年)之久，植物不能从阳光中获得能量，海洋中的藻类和成片的森林逐渐死亡，食物链的基础环节被破坏，大批的动物因饥饿而死，其中就是恐龙。

Cretaceous-Tertiary extinction

　　Badlands near Drumheller, Alberta where erosion has exposed the KT boundary.

　　The Cretaceous-Tertiary extinction event was a period of massive extinction of species that occurred about 65.5 million years ago. It corresponds to the end of the Cretaceous period and the beginning of the Tertiary period.

　　The duration of this extinction event, like many others, is unknown. Many forms of life perished, encompassing approximately 50 percent of all plant and animal families, including the non-avian dinosaurs. Barnosky et al. (2011) and dos Reis et al. (2014) place the species lost at 76 percent. Many possible causes of the mass extinctions have been proposed. The most widely accepted current theory is that an object from space produced an impact event on Earth.

　　The extinction event is also known as the K-T extinction event and its geological signature is the KT boundary. ("K" is the traditional abbreviation for the Cretaceous period, named from the Latin for chalk, creta, which in German is kreide and in Greek is kreta. "K" is used to avoid confusion with the Carboniferous period, abbreviated as "C." "T" is the abbreviation for Tertiary a long-standing geological name for the period following the Cretaceous that has, in some scientific circles, been supplanted by the alternate name “Paleogene.")

　　A broad range of organisms became extinct at the end of the Cretaceous, the most conspicuous being the dinosaurs. While dinosaur diversity appears to have declined in the last ten million years of the Cretaceous, at least in North America, many species are known from the Hell Creek, Lance Formation, and Scollard Formation, including six or seven families of theropods (the "lizard-hipped" dinosaurs that were also carniverous) and a similar number of Ornithischian ("bird-hipped") dinosaurs. Birds were the sole survivors among Dinosauria, but they also suffered heavy losses. A number of diverse groups became extinct, including Enantiornithes (primitive birds) and Hesperornithiformes (toothed and perhaps flightless diving birds). The last of the pterosaurs (flying reptiles that occurred in a great range of sizes) also vanished. Mammals suffered as well, with marsupials and multituberculates (rodent-like, tree-dwelling mammals) experiencing heavy losses; placentals were less affected. The great sea reptiles of the Cretaceous, the mosasaurs and plesiosaurs, also fell victim to extinction. Among mollusks, the ammonites, a diverse group of coiled cephalopods, were exterminated, as were the specialized rudist and inoceramid clams. Freshwater mussels and snails also suffered heavy losses in North America. As much as 57 percent of the plant species in North America may have become extinct as well. Much less is known about how the K-T event affected the rest of the world, due to the absence of good fossil records spanning the K-T boundary. It should be emphasized that the survival of a group does not mean that the group was unaffected: a species may be 99 percent annihilated, yet still manage to survive.

　　Darkness from an impact-generated dust cloud (Alvarez et al. 1980), one of the main theories for the extinction, would have resulted in reduction of photosynthesis both on land and in the oceans. On land, preferential survival may be closely tied to animals that were not in food chains directly dependent on plants. Dinosaurs, both herbivores and carnivores, were in plant-eating food chains. Mammals of the Late Cretaceous are not considered to have been herbivores. Many mammals fed on insects, larvae, worms, snails and so forth, which in turn fed on dead plant matter. During the crisis when green plants would have disappeared, mammals could have survived because they lived in "detritus-based" food chains. In stream communities, few groups of animals became extinct. Stream communities tend to be less reliant on food from living plants and are more dependent on detritus that washes in from land. The stream communities may also have been buffered from extinction by their reliance on detritus-based food chains. Similar, but more complex patterns have been found in the oceans. For example, animals living in the water column are almost entirely dependent on primary production from living phytoplankton. Many animals living on or in the ocean floor feed on detritus, or at least can switch to detritus feeding. Extinction was more severe among those animals living in the water column than among animals living on or in the sea floor.

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