Extinction event
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 six 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 levels 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 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.
- 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.)
- 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.
- 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 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.
See also
