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Extinction event

From Academic Kids

An extinction event (also extinction-level event, ELE) is a period in time when a large number of species die out. The normal background rate of extinctions is about two to five taxonomic families of marine invertebrates and vertebrates every million years. Since life began on Earth, this background extinction rate has been punctuated during the Phanerozoic Eon with six major extinction events. Extinction events also occurred during the Proterozoic and Archaean Eons, but these earlier events are less well documented.

Contents

Extinction events

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  1. 488 million years ago — a series of mass extinctions at the Cambrian-Ordovician transition (the Cambrian-Ordovician extinction events) eliminated many brachiopods and conodonts and severely reduced the number of trilobite species.
  2. 444 million years ago — at the Ordovician-Silurian transition 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, then another occurred between 500 thousand and a million years later when sea levels rose rapidly. It has been suggested that a gamma ray burst may have triggered this extinction.[1] (http://www.guardian.co.uk/uk_news/story/0,3604,1456594,00.html)
  3. 360 million years ago — at the Devonian-Carboniferous transition (the Late Devonian extinction) about 70% of all species were eliminated. This was not a sudden event; evidence suggests that the extinctions took place over a period of some three million years.
  4. 251 million years ago — at the Permian-Triassic transition (the Permian-Triassic extinction event) about 95% of all marine species went extinct. This catastrophe was Earth's worst mass extinction, killing 53% of marine families, 84% of marine genera, and an estimated 70% of land species (including plants, insects, and vertebrate animals.)
  5. 200 million years ago — at the Triassic-Jurassic transition (the Triassic-Jurassic extinction event) about 20% of all marine families as well as most non-dinosaurian archosaurs, most therapsids, and the last of the large amphibians were eliminated.
  6. 65 million years ago — at the Cretaceous-Paleogene transition (the Cretaceous-Tertiary extinction event) about 50% of all species got killed (including the non-avian dinosaurs) which is widely thought to have been the result of an asteroid or comet impact event.
  7. Present day — the Holocene extinction event. Most biologists believe that humans are currently causing another extinction event. E.O. Wilson of Harvard University, in his book The Future of Life, estimates that at current rates of human destruction of the biosphere one-half of all species of life will be extinct in 100 years. A survey by the American Museum of Natural History in 1998 found that the vast majority of biologists agreed with Wilson's assessment, and numerous confirmatory studies in the years since then – led by the IUCN's annual "Red List" of threatened species – have now produced a scientific consensus on the subject.[2] (http://www.well.com/user/davidu/extinction.html)

Extinction event refers to extinction of species, not all life. Although many life forms may become extinct, the usual connotation is that the "event" is at most a transition in dominant life forms. For example, the Cretaceous-Tertiary extinction event promoted the domination of spores and swamp life for a period almost directly after the event. A complete extinction of all known life forms may be possible, but no such event has ever been discovered.

The Big Five are the Ordovician-Silurian extinction events, the Late Devonian extinction, the Permian-Triassic extinction event, the Triassic-Jurassic extinction event and the Cretaceous-Tertiary extinction event.

Postulated extinction cycles

It has been suggested by Raup and Sepkoski that there is a cycle of extinctions, with a mass extinction occurring every 26 to 30 million years. It is difficult to date fossils accurately enough to produce a reliable result, but most studies of this hypothetical cycle suggest that another mass extinction would be due in little less than 10 million years.

One theory, for which no real evidence exists, suggested that the extinction cycle is caused by the orbit of a hypothetical companion star dubbed Nemesis that periodically disturbs the Oort cloud, sending storms of large asteroids and comets towards the Solar System every 26 million years. Another similar theory suggests that the Solar System's oscillations through the plane of the galaxy results in periods of comet showers.

The collision of a large asteroid, or other impact event, with the earth is one of several hypothetical scenarios put forward in recent years that scientists believe may cause or trigger an extinction event (another is global nuclear warfare). The 1998 motion picture Deep Impact referenced the term ELE extensively in relation to a predicted comet collision with earth.

The most recent theory, based solely upon computer modelling at present, is that our solar system periodically passes through large, dense interstellar dust clouds that could result in one of two scenarios. In the first, a reduction in the amount of sunlight the Earth receives would diminish global heating while simultaneously allowing existing heat to escape. This would result in a snowball type of glaciation, an event known to have occurred multiple times in the past. In the second scenario, the cloud would disturb the Earth's ozone layer and allow increased penetration of UV light to the surface.

See also

References

External links

es:Extincin masiva fr:Extinction massive ja:大量絶滅 zh:生物集群灭绝 he:הכחדה המונית pt:Extino em massa sv:Massutdenden

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