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The Permian–Triassic (P–Tr) extinction event, colloquially known as the Great Dying or the Great Permian Extinction, occurred about 252 Ma (million years) ago, forming the boundary between the Permian and Triassic geologic periods, as well as the Paleozoic and Mesozoic eras. It is the Earth's most severe known extinction event, with up to 96% of all marine species and 70% of terrestrial vertebrate species becoming extinct. It is the only known mass extinction of insects.〔 Some 57% of all families and 83% of all genera became extinct. Because so much biodiversity was lost, the recovery of life on Earth took significantly longer than after any other extinction event,〔 possibly up to 10 million years. There is evidence for between one to three distinct pulses, or phases, of extinction.〔 There are several proposed mechanisms for the extinctions; the earlier phase was probably due to gradual environmental change, while the latter phase has been argued to be due to a catastrophic event. Suggested mechanisms for the latter include one or more large bolide impact events, massive volcanism, coal or gas fires and explosions from the Siberian Traps,〔 〕 and a runaway greenhouse effect triggered by sudden release of methane from the sea floor due to methane clathrate dissociation or methane-producing microbes known as methanogens;〔(''Ancient whodunit may be solved: The microbes did it!'', MIT News Office March 31, 2014 )〕 possible contributing gradual changes include sea-level change, increasing anoxia, increasing aridity, and a shift in ocean circulation driven by climate change. ==Dating the extinction== Until 2000, it was thought that rock sequences spanning the Permian–Triassic boundary were too few and contained too many gaps for scientists to reliably determine its details. Based on high-precision U-Pb zircon dates for five volcanic ash beds from the Global Stratotype Section and Point for the Permian-Triassic boundary at Meishan, China, define an age model for the extinction and allow exploration of the links between global environmental perturbation, carbon cycle disruption, mass extinction, and recovery at millennial timescales. The extinction occurred between 251.941 ± 0.037 and 251.880 ± 0.031 Ma, a duration of 60 ± 48 ka. A large (approximately 0.9%), abrupt global decrease in the ratio of the stable isotope to that of , coincides with this extinction,〔〔 〕 and is sometimes used to identify the Permian–Triassic boundary in rocks that are unsuitable for radiometric dating. Further evidence for environmental change around the P–Tr boundary suggests an rise in temperature,〔 and an increase in levels by (by contrast, the concentration immediately before the industrial revolution was .) There is also evidence of increased ultraviolet radiation reaching the earth causing the mutation of plant spores.〔 It has been suggested that the Permian–Triassic boundary is associated with a sharp increase in the abundance of marine and terrestrial fungi, caused by the sharp increase in the amount of dead plants and animals fed upon by the fungi.〔 〕 For a while this "fungal spike" was used by some paleontologists to identify the Permian–Triassic boundary in rocks that are unsuitable for radiometric dating or lack suitable index fossils, but even the proposers of the fungal spike hypothesis pointed out that "fungal spikes" may have been a repeating phenomenon created by the post-extinction ecosystem in the earliest Triassic.〔 The very idea of a fungal spike has been criticized on several grounds, including that: ''Reduviasporonites'', the most common supposed fungal spore, was actually a fossilized alga;〔 the spike did not appear worldwide; and in many places it did not fall on the Permian–Triassic boundary.〔 The algae, which were misidentified as fungal spores, may even represent a transition to a lake-dominated Triassic world rather than an earliest Triassic zone of death and decay in some terrestrial fossil beds. Newer chemical evidence agrees better with a fungal origin for ''Reduviasporonites'', diluting these critiques. Uncertainty exists regarding the duration of the overall extinction and about the timing and duration of various groups' extinctions within the greater process. Some evidence suggests that there were multiple extinction pulses〔 or that the extinction was spread out over a few million years, with a sharp peak in the last million years of the Permian.〔 〕〔 〕 Statistical analyses of some highly fossiliferous strata in Meishan, Zhejiang Province in South China, suggest that the main extinction was clustered around one peak.〔 Recent research shows that different groups became extinct at different times; for example, while difficult to date absolutely, ostracod and brachiopod extinctions were separated by 670 to 1170 thousand years. In a well-preserved sequence in east Greenland, the decline of animals is concentrated in a period 10 to long, with plants taking several hundred thousand additional years to show the full impact of the event. An older theory, still supported in some recent papers,〔 is that there were two major extinction pulses 9.4 million years apart, separated by a period of extinctions well above the background level, and that the final extinction killed off only about 80% of marine species alive at that time while the other losses occurred during the first pulse or the interval between pulses. According to this theory one of these extinction pulses occurred at the end of the Guadalupian epoch of the Permian.〔 For example, all but one of the surviving dinocephalian genera died out at the end of the Guadalupian, as did the Verbeekinidae, a family of large-size fusuline foraminifera. The impact of the end-Guadalupian extinction on marine organisms appears to have varied between locations and between taxonomic groups—brachiopods and corals had severe losses. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Permian–Triassic extinction event」の詳細全文を読む スポンサード リンク
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