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It has been suggested that new, more aggressive fungi, insects and vertebrates evolved, and killed vast numbers of trees.These decomposers themselves suffered heavy losses of species during the extinction, and are not considered a likely cause of the coal gap. It could simply be that all coal forming plants were rendered extinct by the P–Tr extinction, and that it took 10 million years for a new suite of plants to adapt to the moist, acid conditions of peat bogs. On the other hand, abiotic factors (not caused by organisms), such as decreased rainfall or increased input of clastic sediments, may also be to blame. Finally, it is also true that there are very few sediments of any type known from the Early Triassic, and the lack of coal may simply reflect this scarcity.There are several proposed mechanisms for the extinctions; the earlier peak was likely due to gradualistic environmental change, while the later was probably due to a catastrophic event.Possible mechanisms for the latter include large or multiple bolide impact events, increased volcanism, or sudden release of methane hydrates from the sea floor; gradual changes include sea-level change, anoxia, increasing aridity, and a shift in ocean circulation driven by climate change.
It was the Earth's most severe extinction event, with up to 96 percent of all marine species and 70 percent of terrestrial vertebrate species becoming extinct; it is the only known mass extinction of insects.
This event has been described as the "mother of all mass extinctions".
The pattern of extinction is still disputed, as different studies suggest one to three different pulses.
Until 2000, it was thought that rock sequences spanning the Permian–Triassic boundary were too few and contained too many gaps for scientists to determine reliably 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, an duration of 60 ± 48 ka. A large (approximately 0.9%), abrupt global decrease in the ratio of the stable isotope 13C to that of 12C, 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 8 °C (14 °F) rise in temperature, and an increase in CO2 levels by 2000 ppm (by contrast, the concentration immediately before the industrial revolution was 280 ppm.) 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.