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.
Causes for mass extinction
Some of the hypotheses for the causes of mass extinction events are:.
1 Impact events. The impact of a sufficiently large asteroid or comet could create large tsunamis, global forest fires, and reduction of incoming sunlight due to large amounts of dust and smoke in the atmosphere. Taken together, it is not surprising that these and other related effects from an impact event might be sufficiently severe as to disrupt the global ecosystem and cause extinctions. Only for the End Cretaceous extinction (about 65 mya) is there strong evidence of such an impact. Circumstantial evidence of such events is also given for the End Ordovician extinction (about 444 mya), End Permian extinction (about 251 mya), End Jurassic extinction (about 145 mya), and End Eocene extinction (about 40 mya).
2 Climate change. Rapid transitions in climate may be capable of stressing the environment to the point of extinction. However, it is worth observing that the recent cycles of ice ages are believed to have had only very mild impacts on biodiversity. Extinctions suggested to have this cause include: End Ordovician (about 444 mya), End Permian (about 251 mya), and Late Devonian (about 360 mya).
3 Volcanism. The formation of large igneous provinces through the outflow of up to millions of cubic kilometers of lava in a short duration is likely to poison the atmosphere and oceans in a way that may cause extinctions. This cause has been proposed for the End Cretaceous extinction (about 65 mya), End Permian extinction (about 251 mya), End Triassic extinction (about 200 mya), and End Jurassic extinction (about 145 mya).
4 Gamma ray burst. A nearby gamma ray burst (less than 6,000 light years distance) could destroy the ozone layer and sufficiently irradiate the surface of the Earth to kill organisms living there. From statistical arguments, approximately 1 gamma ray burst would be expected to occur in close proximity to Earth in the last 540 million years. This has been suggested as a possible explanation for the End Ordovician extinction (about 444 mya). However, a recent study by leading gamma ray burst researchers says that gamma ray bursts are not possible in metal rich galaxies like our own (Stanek et al. 2006).
5 Plate tectonics. The opening and closing of seaways and land bridges may play a role in extinction events as previously isolated populations are brought into contact and new dynamics are established in the ecosystem. This is most frequently discussed in relation to the End Permian extinction (about 251 mya).
Other hypotheses, such as the spread of a new disease or simple competition following an especially successful biological innovation are also considered. However, it is often thought that the major mass extinctions in Earth's history are too sudden and too extensive to have resulted solely from biological events.