Extinction event Totally Explained

Everything about Mass Extinction totally explained

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, mammals, reptiles, amphibians, fish, invertebrates and other simpler life forms. They may be caused by one or both of:

extinction of an unusually large number of species in a short period.
a sharp drop in the rate of speciation.

Over 99% of species that ever lived are now extinct, but extinction occurs at an uneven rate. Based on the fossil record, the background rate of extinctions on Earth is about two to five taxonomic families of marine invertebrates and vertebrates every million years. Marine fossils are mostly used to measure extinction rates because they're more plentiful and cover a longer time span than fossils of land organisms.
Since life began on earth, several major mass extinctions have significantly exceeded the background extinction rate. The most recent, the Cretaceous–Tertiary extinction event, occurred 65 million years ago, and has attracted more attention than all others because it killed the dinosaurs. 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 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 Jack Sepkoski and David M. Raup in their 1982 paper are widely agreed upon as some of the most significant: End Ordovician, Late Devonian, End Permian, End Triassic, and End Cretaceous. Some paleontologists, however, question whether the available data support a comparison with mass extinctions in the past.

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 dinosaurs and opened the way for mammals 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 been only minimally affected.

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 archosaurs, most therapsids, and the last of the large amphibians were eliminated.

251 MY ago — at the Permian-Triassic transition, Earth's largest extinction (the P/Tr or Permian-Triassic extinction event) killed 53% of marine families, 84% of marine genera, about 96% of all marine species and an estimated 70% of land species (including plants, insects, and vertebrate animals). The "Great Dying" had enormous evolutionary significance: on land it ended the dominance of mammal-like reptiles and created the opportunity for archosaurs and then dinosaurs 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".

360 MY ago — near the Devonian-Carboniferous transition (the Late Devonian extinction) a prolonged series of extinctions eliminated about 70% of all species. This wasn't a sudden event — it lasted perhaps as long as 20 MY, and there's evidence for a series of extinction pulses within this period.

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.

488 million years (MY) 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. It has been suggested that the apparent variations in marine biodiversity may actually be an artefact, with abundance estimates directly related to quantity of rock available for sampling from different time periods. However, statistical analysis shows that this can only account for 50% of the observed pattern, and other evidence (such as fungal spikes) provides reassurance that most widely accepted extinction events are indeed real. A quantification of the rock exposure of Western Europe does indicate that many of the minor events for which a biological explanation has been sought are most readily explained by sampling bias.

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's 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.
For example mammaliformes ("almost mammals") and then mammals existed throughout the reign of the dinosaurs, but couldn't 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 don't recover in numbers or diversity, and many of these go into long-term decline, and these are often referred to as "Dead Clades Walking". 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 gaps between mass extinctions are becoming longer.

The average and background rates of extinction are decreasing. 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 in one or more ways:

Reasonably complete fossils 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 events; and marine ecosystems became more diversified so that food chains were less likely to be disrupted.

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.
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. 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) 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), Hallam (1992) and Grieve et al (1996):

Flood basalt events: 11 occurrences, all associated with significant extinctions But Wignall (2001) concluded that only 5 of the major extinctions coincided with flood Basalt eruptions and that the main phase of extinctions started before the eruptions.

Sea-level falls: 12, of which 7 were associated with significant extinctions.
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 events 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 Permian-Triassic extinction event the warming also upset the oceanic balance between photosynthesising plankton and deep-water sulfate-reducing bacteria, causing massive emissions of hydrogen sulfide which poisoned life on both land and sea and severely weakened the ozone layer, exposing much of the life that still remained to fatal levels of UV radiation.

Oceanic overturn

Oceanic overturn is a disruption of thermo-haline circulation which lets surface water (which is more saline than deep water because of evaporation) sink straight down, bringing anoxic deep water to the surface and therefore killing most of the oxygen-breathing organisms which inhabit the surface and middle depths. It may occur either at the beginning or the end of a glaciation, although an overturn at the start of a glaciation is more dangerous because the preceding warm period will have created a larger volume of anoxic water.
Unlike other oceanic catastrophes such as regressions (sea-level falls) and anoxic events, overturns don't leave easily-identified "signatures" in rocks and are theoretical consequences of researchers' conclusions about other climatic and marine events.
It has been suggested that oceanic overturn caused or contributed to the late Devonian and Permian-Triassic extinctions.

A nearby nova, supernova or gamma ray burst

A nearby gamma ray burst (less than 6000 light years away) could sufficiently irradiate the surface of Earth to kill organisms living there and destroy the ozone layer in the process. From statistical arguments, approximately 1 gamma ray burst would be expected to occur close to Earth in the last 540 million years. A proposal that a supernova or gamma ray burst had caused a mass extinction would also have to be backed up by astronomical evidence of such an explosion at the right place and time.
It has been suggested that a supernova or gamma ray burst caused the End-Ordovician extinction.

Continental drift

Movement of the continents into some configurations can cause or contribute to extinctions in several ways: by initiating or ending ice ages; by changing ocean and wind currents and thus altering climate; by opening seaways or land bridges which expose previously isolated species to competition for which they're poorly-adapted (for example the extinction of most American marsupials after the creation of a land bridge between North and South America). Occasionally continental drift creates a super-continent which includes the vast majority of Earth's land area, which in addition to the effects listed above is likely to reduce the total area of continental shelf (the most species-rich part of the ocean) and produce a vast, arid continental interior which may have extreme seasonal variations.
It is widely thought that the creation of the super-continent Pangaea contributed to the End-Permian mass extinction. Pangaea was almost fully formed at the transition from mid-Permian to late-Permian, and the "Marine genus diversity" diagram at the top of this article shows a level of extinction starting at that time which might have qualified for inclusion in the "Big Five" if it were not overshadowed by the "Great Dying" at the end of the Permian.

Plate tectonics

Plate tectonics is the mechanism which drives many of the possible causes of mass extinctions, especially volcanism and continental drift. So it's implicated in many extinctions, but in each case it's necessary to specify which manifestations of plate tectonics were involved.

Other hypotheses

Many other hypotheses have been proposed, such as the spread of a new disease or simple out-competition following an especially successful biological innovation. But all have been rejected, usually for one of the following reasons: they require events or processes for which there's no evidence; they assume mechanisms which are contrary to the available evidence; they're based on other theories which have been rejected or superseded.

Postulated extinction cycles

It has been suggested by several sources that biodiversity and/or extinction events may be influenced by cyclic processes. The best-known hypothesis of extinction events by a cyclic process is the 26M to 30M year cycle in extinctions proposed by Raup and Sepkoski (1986). More recently, Rohde and Muller (2005) have suggested that biodiversity fluctuates primarily on 62 ± 3 million year cycles.
It is difficult to evaluate the validity of such claims except through reduction to statistical arguments about how plausible or implausible it's for the observed data to exhibit a particular pattern, as the causes of most extinction events are still too uncertain to attribute to them any specific cause let alone a recurring one. Much early work in this area also suffered from the poor accuracy of geological dating, where errors often exceed 10M years. However, improvements in radiometric dating have reduced the scale of uncertainty to at most 4M years — theoretically adequate for studying these processes.
While the statistics alone have been judged as sufficiently compelling to warrant publication, it's important to consider processes that might be responsible for a cyclic pattern of extinctions and future work may focus on trying to find evidence of such processes.

Hypothetical companion star to the sun

The physicist Richard A. Muller has produced a number of speculative hypotheses for the regularity of mass extinctions. One is that the extinction cycle could be 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.

Galactic plane oscillations

Muller has also speculated the periodicity of mass extinctions may be related to the solar system's oscillation through the plane of our Milky Way galaxy as it rotates around the galactic centre, with a number of possible hypothesized effects including gravitationally-induced comet showers or periods of intense radiation as the solar system hits the galactic shock wave.

Passage through galactic spiral arms

It has also been suggested that extinction events correlate to the passage of the solar system through the spiral arms of the Milky Way. We pass through all four arms every 700 million years, and there's dubious evidence to suggest a cyclicity of extra-terrestrial activity back to 2 billion years ago.

Geological instabilities

Other hypotheses are that geological instabilities allow heat to periodically build up deep in the Earth, which is then released through mantle plumes, periods of major volcanism and active plate tectonics.

Impending Mass Extinction

Paleontologist Peter Ward offers evidence in his April 2007 book, Under a Green Sky, that all but one of the major extinction events in history have been brought on by climate change — the same global warming that occurs today. Reviewer Doug Brown goes further, stating (External Link

) "This Is How The World Ends". Scientists at the Universities of York and Leeds also warn that the (External Link

) fossil record supports evidence of impending mass extinction], according to a ScienceDaily release, Oct. 24, 2007.

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