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Mass extinction is the extinction of a large number of species of organisms on Earth within a short period of time. Research of fossils has proven that mass extinctions have occurred at certain times in the geological history. The cause of the extinctions is not definitely determined in all cases.
Simplified diagram of major mass extinctions
Around the Ordovician-Silurian boundary nearly 50% of all marine species disappeared. Include many types of brachiopods, trilobites and graptolites. Possible causes are the Hirnantien glaciation, rise in CO2 levels, and radiation from space by a gamma-ray burst.
In the Famennian, Upper Devonian an estimated 75% of all marine species became extinct, including many species of brachiopods, trilobites and ammonites. Rugose corals became extinct completely. Organisms on land were much less affected. Possible causes of extinction are a meteorite impact, decrease the CO2 content because of the emergence of land plants, and the occurrence of ice ages.
During the mass extinction at the Permian-Triassic boundary about 95% of all marine life and 70% of the land animals became extinct. The last trilobites became extinct too. Possible causes are a meteorite impact, forming Pangea, radiation from space in the form of a gamma-ray burst, a period of increased volcanic activity, or a highly elevated levels of methane in the atmosphere due to the release of gas hydrates.
At the Triassic-Jurassic boundary 20 % of the families in the sea became extinct. On land, many therapsids, amphibians and all Archosauria died. Possible causes include a period of high volcanic activity by the break-up of the continent Pangea, and a high increased level of methane in the atmosphere by the release of gas hydrates.
At the K-Pg boundary all dinosaurs and ammonites went extinct due to a large meteorite impact in Yucatan, Mexico. Many species of corals, sea urchins and bivalves also did not survived this event.
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