The Earth has been dominated by multi-cellular life forms (animals, plants etc.) for a little over half a billion years, but life has not had it easy throughout all of this time, it has been hit by a number of mass extinction events, during each of which a large proportion of the life-forms existent went suddenly extinct. The best known of these is the extinction event at the end of the Cretaceous, which included the extinction of the non-avian dinosaurs and other large Mesozoic animals, but the biggest extinction event occurred at the end of the Permian, when roughly 96% of all marine and 70% of all terrestrial life was wiped out. The Permian Extinction is sometimes known as the Great Extinction or the Great Dying.
During the Permian Extinction marine invertebrate groups such as the trilobites, eurypterids (water scorpions) and corals (modern corals are not closely related to Palaeozoic corals) became extinct, as did fish groups such as the acanthodians. Terrestrial life was also badly hit, as well as wiping out many species of vertebrates the Permian Extinction also badly effected insects and plants, groups not effected to any significant extent by any other extinction event.
Trilobites dominated Earth's seas for 250 million years, but were wiped out at the end of the Permian.
Exactly how long the extinction event lasted has been a source of debate, with estimates ranging from over 5 million years to under 10 thousand. How long the extinction lasted is important when trying to determine the cause of the extinction. The most probable cause is generally considered to be environmental degradation due to gasses released by the Siberian Traps flood basalts, the largest known volcanic episode in the Earth's history, which covered an area the size of modern Europe in molten lava. This would have released vast amounts of CO₂ into the atmosphere, with a catastrophic outcome for the Earth's climate. Geochemists have been able to determine that the concentration of CO₂ in the atmosphere at the end of the Permian rose by about 2000 parts per million (ppm), which certainly fits in well with this theory (modern CO₂ levels have risen by slightly over 100 ppm since the industrial revolution, causing great alarm among climatologists). This works well as a model if the extinction and the volcanic outburst were both rapid, but if either took place over a longer period then it does not tie up so well; a longer period of eruptions would not cause the CO₂ levels to rise so sharply, and a longer period of extinction could not be explained by an abrupt event.
Map showing the extent of the Siberian Traps flood basalts (striped area), pyroclastic material (dotted area), intrusive magmas (i.e. magma that spreads within strata bellow the surface, areas inside the solid lines) and total extent of the Siberian Platform (within dashed lines).
The 17 November edition of the journal Science contains a paper by a team lead by Shen Shuzong of the Nanjing Institute of Geology and Palaeontology in which they describe the results of detailed studies of nine sites in southern China at which there are well preserved sequences of rocks spanning the end of the Permian and the beginning of the Triassic. Such sequences are extremely rare, which is why the subject has remained the centre of controversy for so long, so that the sites examined in this study are a significant proportion of the total available data on the subject (only two of these sites have been studied previously).
In addition to the fossil data recovered from the sites the scientists were able to study layers of volcanic ash within the rocks, from which they were able to obtain dates based upon the rate at which uranium decays into lead. Lead does not form the same minerals as uranium when it cools from molten magma, so any lead present in uranium minerals must be there due to the decay of uranium since the minerals formed. Uranium decays at a known rate, so the amount of lead in the minerals can be used to precisely date the ash layers. This enabled the scientists to obtain dates for the fossil beds calibrated to under 100 000 years, remarkable on deposits a little over 250 million years old.
Using this data the team concluded that the end Permian extinction took place over less than 200 000 years, and probably less than 100 000 years; the peak of the extinction event was 252 280 000 years ago. This puts the extinction within a few tens of thousands of years of the eruption of the Siberian Traps flood basalts, supporting the theory that these were the primary cause of the extinction event.
See also The End of the Cretaceous, The dangers of a modern Laki-style eruption in Iceland, Eruptions on Mount Nyamuragira in the Virunga National Park and Volcanoes on Sciency Thoughts YouTube.