五次地球性集体大灭绝

　　现在公认的全球性集群大灭绝有5次，即奥陶纪、晚泥盆纪、二叠纪、三叠纪及白垩纪大灭绝。据研究，在地球上多细胞生物的六亿年进化历史中，曾经有四十亿种动植物在地球上生活过。而现在生存的生物仅仅只有几百万种，绝大多数在地球上生活过的生物都灭绝了。

Extinction events

　　The classical "Big Five" mass extinctions identified by Raup and Sepkoski (1982) are widely agreed upon as some of the most significant: (1) End Ordovician (Ordovician-Silurian extinction), (2) Late Devonian (Late Devonian extinction), (3) End Permian (Permian-Triassic extinction), (4) End Triassic (Triassic-Jurassic extinction), and (5) End Cretaceous (Cretaceous-Tertiary extinction). (See geologic time scale for an overview of these time periods.)

　　These and a pair of other extinction events acting as "book ends" for the Big Five are highlighted below：

　　1 End Ordovician extinction (about 444 million years ago). Two Ordovician-Silurian extinction events occurred, probably as the result of a period of glaciation. Marine habitats changed drastically as sea levels decreased causing the first die-off, and then, when sea levels rose rapidly between five hundred thousand to a million years later, a second great die-off occurred. On hypothesis is that a gamma ray burst may have triggered this extinction (Jha 2005).

　　2 Late Devonian extinction (about 360 million years ago). Near the Devonian-Carboniferous transition a prolonged series of extinctions led to the elimination of about 70 percent of all species. This was not a sudden event, with the period of decline lasting perhaps as long as 20 million years. However, there is evidence for a series of extinction pulses within this period.

　　3 End Permian extinction (about 251 million years ago). At the Permian-Triassic transition (the Permian-Triassic extinction event) about 95 percent of all marine species went extinct. This catastrophe was Earth's worst mass extinction, killing 53 percent of marine families, 84 percent of marine genera, and an estimated 70 percent of land species (including plants, insects, and vertebrate animals.)

　　4 End Triassic extinction (about 200 million years ago). At the time of the Triassic-Jurassic transition, about 20 percent of all marine families as well as most non-dinosaurian archosaurs, most therapsids (except the order from which mammals descended), and the last of the large amphibians were eliminated.

　　5 End Cretaceous extinction (about 65 million years ago). At the time of the Cretaceous-Paleogene transition (the Cretaceous-Tertiary extinction event) about 50 percent of all species became extinct (including all non-avian dinosaurs). This extinction is widely believed to have resulted from an asteroid or comet impact event, although there is not a consensus on this theory.

　　6 End Cambrian extinction (about 488 million years ago). A series of mass extinctions at the Cambrian-Ordovician transition eliminated many brachiopods and conodonts (a group of small eel-like vertebrates characterized by multiple pairs of bony toothplates) and severely reduced the number of trilobite species.

　　7 Holocene extinction (Present day). A 1998 survey by the American Museum of Natural History found that 70 percent of biologists view the present era as part of a mass extinction event, the Holocene extinction event. The extinction of many megafauna near the end of the most recent ice age is also sometimes considered a part of the Holocene extinction event.

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