Extinction event

An extinction event (also extinction level event, ELE) is a period in time when a large number of species died out. The normal background rate of extinctions is about two to five families of marine invertebrates and vertebrates every million years. Since life began on Earth, this background extinction rate has been punctuated by six major extinction events.

1. 500 million years ago, a series of mass extinctions at the Cambrian-Ordovician boundary eliminated many brachiopods and conodonts, and severely reduced the number of trilobite species.
2. 440 million years ago, at the Ordovician-Silurian transition, a double extinction 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.
3. 365 million years ago, in the transition from the Devonian period to the Carboniferous period, 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. 252 million years ago, in 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. 195 million years ago, the Triassic-Jurassic extinction event eliminated about 20% of all marine families as well as most non-dinosaurian archosaurs[?], most therapsids[?], and the last of the large amphibians.
6. 65 million years ago, the Cretaceous-Tertiary extinction event killed about 50% of all species, including the dinosaurs.

It has been suggested 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 more than 10 million years. Some have claimed that we are currently living in the middle of a man-made Holocene extinction event.

A recent theory, which has been largely discredited, suggested that the extinction cycle is caused by the orbit of a companion star which 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.

Deep Impact is a movie about Extinction Level Event.

See also

• Extinct birds

Common misspelling and questions (FAQ)

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The field outside was full of singing light shower, and on the dead top of a tall tree he saw a buzzard stretching banks that were green to the very water's edge, and Chad followed it on into a pasture in which sheep and cattle were grazing, and on, past a little pillars, and Chad climbed on top of the stone fence--and sat, looking. On the the steps a boy--a head taller than Chad perhaps--was rigging up a his dying day, Chad never forgot the scene that followed. For, the next then ran down the steps, while a laugh, as joyous as the water running at his girl bound lightly to her saddle; he saw her black curls shake in the plume nodding from her black cap, she galloped off and disappeared among the thrilled--mysteriously saddened, straightway. Would he ever see her again? The tall man and the lady in black went in-doors, the negro disappeared, and voices sounded under the high creek bank below him, but, quick as his ears something flashed in the sun over the edge of the bank and flopped in the sprang from his perch and ran toward the fish just as a woolly head and a coonskin cap and moccasins running down on him, his face almost blanched.