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Extinction event

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Lisa Wolfe
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« on: September 28, 2010, 01:25:29 pm »

Extinction event

An extinction event (also known as: mass extinction; extinction-level event, ELE, or biotic crisis) is a sharp decrease in the diversity and abundance of macroscopic life. They occur when the rate of extinction increases with respect to the rate of speciation. Because the majority of diversity and biomass on earth is microbial, and thus difficult to measure, mass extinctions have little effect on the total diversity and abundance of life, but rather affect the easily observed, biologically complex component of the biosphere.[1]

Over 98% of documented species 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 of their superior fossil record and stratigraphic range compared to 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 as it marks the extinction of nearly all dinosaur species, which were the dominant animal class of the period. In the past 540 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.

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Lisa Wolfe
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« Reply #1 on: September 28, 2010, 01:26:12 pm »



Vertical axis - percentage extinction of genera, horizontal axis - mya (millions of years ago)
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Lisa Wolfe
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« Reply #2 on: September 28, 2010, 01:27:06 pm »

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 Cretaceous, End Triassic, End Permian, Late Devonian, and End Ordovician,[2][3] They were originally identified as outliers to a general trend of decreasing extinction rates during the Phanerozoic,[4] but as more stringent statistical tests have been applied to the accumulating data, the "Big Five" cannot be so clearly defined, but rather appear to represent the largest (or some of the largest) of a relatively smooth continuum of extinction events.[4]
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.
1.   Cretaceous–Tertiary extinction event (End Cretaceous or K-T extinction) - 70 to 65 Ma at the Cretaceous.Maastrichtian-Paleogene.Danian transition interval.[5] About 17% of all families, 50% of all genera[6] and 75% of species went extinct.[2] It ended the reign of dinosaurs and opened the way for mammals and birds 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.
2.   Triassic–Jurassic extinction event (End Triassic) - 205 Ma at the Triassic-Jurassic transition. About 23% of all families and 48% of all genera (20% of marine families and 55% of marine genera) went extinct.[6] Most non-dinosaurian archosaurs, most therapsids, and most of the large amphibians were eliminated, leaving dinosaurs with little terrestrial competition. Non-dinosaurian archosaurs continued to dominate aquatic environments, while non-archosaurian diapsids continued to dominate marine environments. The Temnospondyl lineage of large amphibians also survived until the Cretaceous in Australia (e.g., Koolasuchus).
3.   Permian–Triassic extinction event (End Permian) - 251 Ma at the Permian-Triassic transition. Earth's largest extinction killed 57% of all families and 83% of all genera[6] (53% of marine families, 84% of marine genera, about 96% of all marine species and an estimated 70% of land species) including insects.[7] The evidence of plants is less clear, but new taxa became dominant after the extinction.[8] The "Great Dying" had enormous evolutionary significance: on land, it ended the primacy of mammal-like reptiles. The recovery of vertebrates took 30 million years,[9] but the vacant niches created the opportunity for archosaurs to become ascendant. 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".
4.   Late Devonian extinction - 360-375 Ma near the Devonian-Carboniferous transition. At the end of the Frasnian Age in the later part(s) of the Devonian Period, a prolonged series of extinctions eliminated about 19% of all families, 50% of all genera[6] and 70% of all species.[citation needed] This extinction event lasted perhaps as long as 20 MY, and there is evidence for a series of extinction pulses within this period.
5.   Ordovician–Silurian extinction event (End Ordovician) - 440-450 Ma at the Ordovician-Silurian transition. Two events occurred that killed off 27% of all families and 57% of all genera.[6] Together they 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.
The older the fossil record gets, the more difficult it is to read. This is because:
•   Older fossils are harder to find because they are usually buried at a considerable depth in the rock.
•   Dating older fossils is more difficult.
•   Productive fossil beds are researched more than unproductive ones, therefore leaving certain periods unresearched.
•   Prehistoric environmental disturbances can disturb the deposition process.
•   The preservation of fossils varies on land, but marine fossils tend to be better preserved than their sought after land-based counterparts.[10]
It has been suggested that the apparent variations in marine biodiversity may actually be an artifact, with abundance estimates directly related to quantity of rock available for sampling from different time periods.[11] However, statistical analysis shows that this can only account for 50% of the observed pattern,[citation needed] and other evidence (such as fungal spikes)[clarification needed] 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.[12

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