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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 families of marine invertebrates and vertebrates every million years. Since life began on Earth, this background extinction rate has been punctuated by seven major extinction events.- 500 million years ago a series of mass extinctions at the Cambrian- Ordovician boundary (the Cambrian-Ordovician extinction events ) eliminated many brachiopods and conodonts and severely reduced the number of trilobite species.
- 440 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 levelFor considerations of sea level change, in particular rise associated with possible global warming, see sea level rise''. Mean sea level (MSL) is the average height of the sea, with reference to a suitable reference surface. Defining the reference level ,s decreased, causing the first die-off, then another occurred between 500 thousand and a million years later when sea levels rose rapidly.
- 365 million years ago in the transition from the Devonian period to the CarboniferousThe Carboniferous is a major division of the geologic timescale that extends from the end of the Devonian period (about 280 million years before the present (BP)) to the beginning of the Permian period (about 340 million Years BP). As with most older geol 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.
- 252 million years ago, in the Permian-Triassic extinction eventThe Permian-Triassic extinction event was an extinction event that occurred approximately 252 million years ago (mya), forming the boundary of the Permian and Triassic periods. It was the Earth's most severe extinction event, with about 90 percent of all, about 95% of all marine species went extinct. This catastrophe was Earth's worst mass extinction, killing 53% of marine families, 84% of marine generaSee genus (mathematics) for the use of the term in mathematics. See genus (music) for the use of the term in music. In biology, a genus (plural genera is a grouping in the classification of living organisms having one or more related and morphologically s, and an estimated 70% of land species (including plants, insects, and vertebrate animals.)
- 195 million years ago, the Triassic-Jurassic extinction eventThe Triassic-Jurassic extinction event is one of the major extinction events of the Phanerozoic eon, profoundly affecting life on land and in the oceans. 20% of all marine families and most non-dinosaurian archosaurs, most therapsids, and the last of the eliminated about 20% of all marine families as well as most non-dinosaurian archosaurArchosaurs : Animalia : Chordata : Vertebrata Infraphylum: Tetrapoda Microphylum: Amniota Archosauria subgroups Archosauria Dinosauria Crocodilia ( crocodiles and alligators) Aves ( birds) Archosaurs (Greek: "ruling reptiles") are a group of diapsid reptis, most therapsidsTherapsida : Eukaryota : Animalia : Chordata : Vertebrata Infraphylum: Tetrapoda Microphylum: Amniota : Synapsida (unknown taxon): Therapsida Therapsida is an animal taxon within Synapsida. Traditionally, synapsids have been referred to as reptiles. Howev, and the last of the large amphibians.
- 65 million years ago, the Cretaceous-Tertiary extinction event killed about 50% of all species, including the dinosaurs.
- 20 thousand years ago through today, humans are causing another extinction event. Hunting and overfishing have already caused extinctions and population collapses of many large land animals and fish species. Industrial development is causing habit destruction and climate changes which are bringing about the extinction of many animals and plants throughout the world.
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. There is abundant evidence 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 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.
An even more recent theory, which is still being evaluated, is that periodic large scale vulcanism along continental rifts may include eruption events named verneshots which launch gigatonnes of rock into sub-orbital trajectories. The consequent impacts are expected to have very similar effects to asteroid impacts. This theory explains the periodicity of extinction events as well as the apparently coincidental occurrence of large-scale impacts and vulcanism for at least three of the extinction events without relying on coincidence in the way that the asteroid impact theory does.
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 collision of a large asteroid with the earth is one of several hypothetical scenarios put forward in recent years that scientists believe may cause or trigger an ELE (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.
