Permian–Triassic extinction event
The Permian/Triassic extinction event (P/Tr for short) was the largest extinction event in the Phanerozoic eon. 96% of all marine species and 70% of terrestrial vertebrates became extinct. It is the only known extinction event of insects. It ended the Palaeozoic era, and began the Mesozoic era.
The event forms the boundary between the Permian and Triassic periods, at about 252 million years ago. Because so much biodiversity was lost, the recovery of life on earth took much longer than after other extinction events. This event has been described as the "mother of all mass extinctions". The pattern of extinction is still unclear, as different studies suggest one to three different pulses.
There are several proposed mechanisms for the extinctions. These include: large or multiple meteorite impacts, increased volcanism, sudden release of methane hydrates from the sea floor. Gradual changes include sea level change, oceanic anoxic events, increasing aridity, and a shift in ocean circulation driven by climate change.
One thing is probably significant. One of the largest ever flood basalt eruptions took place across the P/Tr junction. These eruptions, from 251 to 250 million years ago, produced the Siberian Traps, a huge volcanic province in Siberia. This would certainly have made the world's climate much worse, and is now thought to be the main cause of this great extinction.
Another factor is that global sea levels were at an historic low point at the time.
- International Stratigraphic Chart. 
- Jin Y.G. et al. (2000). "Pattern of marine mass extinction near the Permian–Triassic boundary in South China". Science 289 (5478): 432–436. doi:10.1126/science.289.5478.432. PMID 10903200.
- Benton M J (2005). When life nearly died: the greatest mass extinction of all time. Thames & Hudson. ISBN 978-0500285732.
- Sole R.V. and Newman M. 2002. Extinctions and biodiversity in the fossil record: volume two, The earth system: biological and ecological dimensions of global environment change pp297-391, Encyclopedia of global environmental change Wilely.
- Erwin DH (1993). The great Paleozoic crisis; life and death in the Permian. Columbia University Press. ISBN 0231074670.
- Yin H, Zhang K, Tong J, Yang Z, Wu S. The Global Stratotype Section and Point (GSSP) of the Permian-Triassic Boundary. Episodes 24 (2): 102–114.
- Yin HF, Sweets WC, Yang ZY, Dickins JM. 1992. Permo-Triassic events in the eastern Tethys. Cambridge Univ. Press, Cambridge.
- Sahney, Sarda and Benton, Michael J. 2008. Recovery from the most profound mass extinction of all time. Proceedings of the Royal Society series B, 275, 759-765. 
- Erwin D.H 2006. Extinction: how life on Earth nearly ended 250 million years ago. Princeton, NJ: Princeton University Press