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An extinction event (also known as: '''mass extinction'''; '''extinction-level event''', ELE) is a sharp decrease in the number of Species in a relatively short period of time. Mass extinctions affect most major Taxonomic groups present at the time — Birds , Mammal s, Reptile s, Amphibian s, Fish , Invertebrate s and other simpler life forms. They may be caused by one or both of:
Over 90% of species that ever lived are extinct, but extinction occurs at an uneven rate. Based on the Fossil Record , the background rate of Extinction s on Earth is about two to five Taxonomic Families of marine Invertebrate s and Vertebrate s every million years.1 Marine fossils are mostly used to measure extinction rates because they are more plentiful and cover a longer time span than fossils of land organisms. going extinct at any given time, as reconstructed from the Fossil Record . (Graph not meant to include recent epoch of Holocene Extinction Event )]] Since life began on Earth, several major mass extinctions have significantly exceeded the background extinction rate. The most recent, the K-T Extinction Event , occurred 65 million years ago, and has attracted more attention than all others because it killed the Dinosaur s. In the past 550 million years there have been five major events when over 50% of animal species died. There probably were mass extinctions in the Archean and Proterozoic Eons , but before the Phanerozoic there were no animals with hard body parts to leave a significant fossil record. Estimates of the number of major mass extinctions in the last 540 million years range from as few as five to more than twenty. These differences stem mostly from the threshold chosen for describing an extinction event as "major", and the data chosen to measure past diversity. MAJOR EXTINCTION EVENTS The classical "Big Five" mass extinctions identified by , Late Devonian , End Permian , End Triassic , and End Cretaceous . These and a selection of other extinction events are outlined below. The articles about individual mass extinctions describe their effects in more detail and discuss theories about their causes. # 488 million years (MY) ago — a series of mass extinctions at the Cambrian - Ordovician transition (the Cambrian-Ordovician Extinction Events ) eliminated many Brachiopod s and Conodont s and severely reduced the number of Trilobite species. # 444 MY ago — at the Ordovician - Silurian transition two Ordovician-Silurian Extinction Events occurred, and together are ranked by many scientists as the second largest of the five major extinctions in Earth's history in terms of percentage of Genera that went extinct. # 360 MY ago — near the Devonian - Carboniferous transition (the Late Devonian Extinction ) a prolonged series of extinctions eliminated about 70% of all species. This was not a sudden event — it lasted perhaps as long as 20 MY, and there is evidence for a series of extinction pulses within this period. # 251 MY ago — at the and created the opportunity for Archosaur s and then Dinosaur s to become the dominant land vertebrates; in the seas the percentage of animals that were Sessile dropped from 67% to 50%. The whole late Permian was a difficult time for at least marine life — even before the "Great Dying", there was level of extinction large enough to be included in the "Big Five". # 200 MY ago — at the Triassic - Jurassic transition (the Triassic-Jurassic Extinction Event ) about 20% of all marine families as well as most non-dinosaurian Archosaur s, most Therapsids , and the last of the large Amphibia ns were eliminated. # 65 MY ago — at the Cretaceous - Paleogene transition (the K/T or Cretaceous-Tertiary Extinction Event ) about 50% of all species became extinct. It has great significance for humans because it ended the reign of Dinosaur s and opened the way for Mammal s to become the dominant land vertebrates. In the seas it reduced the percentage of Sessile animals to about 33%. The K/T extinction was rather uneven — some groups of organisms became extinct, some suffered heavy losses and some appear to have got off relatively lightly.
EVOLUTIONARY IMPORTANCE Mass extinctions have sometimes accelerated the evolution of Life On Earth . When dominance of particular ecological niches passes from one group of organisms to another, it is rarely because the new dominant group is "superior" to the old and usually because an extinction event eliminates the old dominant group and makes way for the new one.5 For example Mammaliformes ("almost mammals") and then Mammal s existed throughout the reign of the Dinosaur s, but could not compete for the large terrestrial vertebrate niches which dinosaurs monopolized. The End-Cretaceous mass extinction removed the non-avian dinosaurs and made it possible for mammals to expand into the large terrestrial vertebrate niches. On the other hand, many groups which survive mass extinctions do not recover in numbers or diversity, and many of these go into long-term decline, and these are often referred to as " Dead Clades Walking ".6 So analysing extinctions in terms of "what died and what survived" often fails to tell the full story. APPARENT DECREASING FREQUENCY The diagram at the top of this page appears to show that:
The idea that mass extinctions are becoming less frequent is rather speculative - from a Statistical point of view a sample of about 10 extinction events is too small to be a reliable sign of any actual trend. But the average and background rates of extinction are based on hundreds of samples over a period of 550M years, so the apparent decrease in these rates is statistically significant and needs to be explained. Both of these phenomena could be explained by one or more explanations.7 Reasonably complete Fossil s are very rare, most extinct organisms are represented only by partial fossils, and complete fossils are rarest in the oldest rocks. So paleontologists have mistakenly assigned parts of the same organism to different Genera which were often defined solely to accommodate these finds (an example is the story of '' Anomalocaris ''). The risk of this mistake is higher for older fossils because these are often unlike parts of any living organism. Many of the "superfluous" genera are represented by fragments which are not found again and the "superfluous" genera appear to become extinct very quickly. Martin (1994, 1996) has argued that the oceans have become more hospitable to life over the last 500M years and less vulnerable to mass extinctions: Dissolved Oxygen became more widespread and penetrated to greater depths; the development of life on land reduced the run-off of nutrients and hence the risk of Eutrophication and Anoxic Event s; and marine ecosystems became more diversified so that Food Chain s were less likely to be disrupted.89 CAUSES There is still debate about the causes of all mass extinctions before the Holocene . Looking for the causes of particular mass extinctions A good theory for a particular mass extinction should: (i) explain all of the losses, not just focus on a few groups (such as dinosaurs); (ii) explain why particular groups of organisms died out and why others survived; (iii) provide killing mechanisms which are strong enough to cause a mass extinction but not a total extinction; (iv) be based on events or processes that can be shown to have happened, not just inferred from the extinction. It may be necessary to consider combinations of causes. For example the marine aspect of the End-Cretaceous extinction appears to have been caused by several processes which partially overlapped in time and may have had different levels of significance in different parts of the world.10 Arens and West (2006) proposed a "press / pulse" model in which mass extinctions generally require two types of cause: long-term pressure on the eco-system ("press") and a sudden catastrophe ("pulse") towards the end of the period of pressure.Arens, N.C. and West, I.D. (2006). "Press/Pulse: A General Theory of Mass Extinction?"" 'GSA Conference paper' Abstract Their statistical analysis of marine extinction rates throughout the Phanerozoic suggested that neither long-term pressure alone nor a catastrophe alone was sufficient to cause a significant increase in the extinction rate. Most widely-supported explanations Macleod (2001)11 summarised the relationship between mass extinctions and events which are most often cited as causes of mass extinctions, using data from Courtillot ''et al'' (1996),Courtillot, V., Jaeger, J-J., Yang, Z., Féraud, G., Hofmann, C. (1996). "The influence of continental flood basalts on mass extinctions: where do we stand?" in Ryder, G., Fastovsky, D., and Gartner, S, eds. "The Cretaceous-Tertiary event and other catastrophes in earth history". ''The Geological Society of America'', Special Paper 307, 513-525. Hallam (1992)Hallam, A. (1992). "Phanerozoic sea-level changes". New York; ''Columbia University Press''. and Grieve ''et al'' (1996)Grieve, R., Rupert, J., Smith, J., Therriault, A. (1996). "The record of terrestrial impact cratering". ''GSA Today'' 5: 193-195:
vol. 53 issues 1-2 pp 1-33
The most commonly-suggested causes of mass extinctions are listed below Flood Basalt events The formation of aerosols which inhibited photosynthesis and thus caused Food Chain s to collapse both on land and at sea; emitted sulfur oxides which were precipitated as Acid Rain and poisoned many organisms, contributing further to the collapse of food chains; emitted Carbon Dioxide and thus caused Sustained Global Warming once the dust and Particulate aerosols dissipated. Flood basalt events occur as pulses of activity punctuated by dormant periods. As a result they are likely to cause the climate to oscillate between cooling and warming, but with an overall trend towards warming as the carbon dioxide they emit can stay in the atmosphere for hundreds of years. Various scientists have suggested that massive volcanism caused or contributed to the End-Cretaceous , End-Permian , End Triassic and End Jurassic extinctions. Sea-level falls These are often clearly marked by world-wide sequences of contemporaneous sediments which show all or part of a transition from sea-bed to tidal zone to beach to dry land - and where there is no evidence that the rocks in the relevant areas were raised by geological processes such as Orogeny . Sea-level falls could reduce the continental shelf area (the most productive part of the oceans) sufficiently to cause a marine mass extinction, and could disrupt weather patterns enough to cause extinctions on land. But sea-level falls are very probably the result of other events, such as sustained global cooling or the sinking of the Mid-ocean Ridges . Sea-level falls are associated with most of the mass extinctions, including all of the "Big Five" — End-Ordovician , Late Devonian , End-Permian , End-Triassic , and End-Cretaceous . Impact Event s The impact of a sufficiently large asteroid or comet could have caused Food Chain s to collapse both on land and at sea by producing dust and Particulate aerosols and thus inhibiting photosynthesis. If it hit Sulfur -rich rocks, it could also have emitted sulfur oxides which were precipitated as Acid Rain and poisoned many organisms — contributing further to the collapse of food chains. Some scientists have suggested that impacts could also have caused Megatsunami s and / or global Forest Fire s, but these ideas are now regarded as exaggerations. Only the End-Cretaceous Extinction is associated with strong evidence of such an impact, but that impact is easily the largest for which there is strong evidence. Sustained global cooling Sustained global cooling could kill many Polar and Temperate species and force others to migrate towards the Equator ; reduce the area available for Tropical species; often make the Earth's climate more arid on average, mainly by locking up more of the planet's water in ice and snow. The Glaciation cycles of the current Ice Age are believed to have had only a very mild impact on biodiversity, so the mere existence of a significant cooling is not sufficient on its own to explain a mass extinction. It has been suggested that global cooling caused or contributed to the End-Ordovician , Permian-Triassic , Late Devonian extinctions, and possibly others. Sustained global cooling is distinguished from the temporary climatic effects of flood basalt events or impacts. Sustained global warming This would have the opposite effects: expand the area available for Tropical species; kill Temperate species or force them to migrate towards the Poles (or perish); possibly cause severe extinctions of polar species; often make the Earth's climate wetter on average, mainly by melting ice and snow and thus increasing the volume of the Water Cycle . It might also cause anoxic events in the oceans (see below). The most dramatic example of sustained warming is the Paleocene-Eocene Thermal Maximum , which was associated with one of the smaller mass extinctions. Clathrate Gun Hypothesis Clathrates are composites in which a lattice of one substance forms a cage round another. Methane Clathrates (in which water molecules are the cage) form on Continental Shelves . These clathrates are likely to break up rapidly and release the methane if the temperature rises quickly or the pressure on them drops quickly — for example in response to sudden Global Warming or a sudden drop in sea level or even Earthquake s. Methane is a much more powerful Greenhouse gas than carbon dioxide, so a methane eruption ("clathrate gun") could cause rapid global warming or make it much more severe if the eruption was itself caused by global warming. The most likely signature of such a methane eruption would be a sudden decrease in the Ratio Of Carbon-13 To Carbon-12 in sediments, since methane clathrates are low in carbon-13; but the change would have to be very large, as other events can also reduce the percentage of carbon-13.12 It has been suggested that "clathrate gun" methane eruptions were involved in the End-Permian Extinction ("the Great Dying") and in the Paleocene-Eocene Thermal Maximum , which was associated with one of the smaller mass extinctions. Anoxic events Anoxic Event s are situations in which the upper and even the middle layers of the ocean become deficient or totally lacking in oxygen. Their causes are complex and controversial, but all known instances are associated with severe and sustained global warming, mostly caused by massive sustained volcanism. It has been suggested that anoxic events caused or contributed to the Late Devonian , Permian-Triassic and Triassic-Jurassic extinctions. On the other hand, there are widespread black shale beds from the mid-Cretaceous which indicate anoxic events but are not associated with mass extinctions. Hydrogen sulfide emissions from the seas Kump, Pavlov and Arthur (2005) have proposed that during the loops in the catastrophic release of Hydrogen Sulphide proposed by Kump, Pavlov and Arthur (2005).Kump, L.R., Pavlov, A., and Arthur, M.A. (2005). "Massive release of hydrogen sulfide to the surface ocean and atmosphere during intervals of oceanic anoxia". ''Geology'' v. 33, p.397–400. Abstract . Summarised by Ward (2006).Ward, P.D. (2006). " Impact from the Deep ". ''Scientific American'' October 2006. Oceanic overturn
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