This 2013 video says about itself:
Animal Armageddon The Great Dying – Episode 5
The Permian-Triassic extinction event, informally known as the Great Dying, was an extinction event that occurred 252 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.
Researchers have variously suggested that there were from one to three distinct pulses, or phases, of extinction. There are several proposed mechanisms for the extinctions; the earlier phase was likely 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 large or multiple impact events, increased volcanism, coal/gas fires and explosions from the Siberian Traps, and sudden release of methane from the sea floor; gradual changes include sea-level change, increasing aridity, and a shift in ocean circulation driven by climate change.
From the Geological Society of America:
15 April 2015
Boulder, Colo., USA – Since the Cambrian Explosion, ecosystems have suffered repeated mass extinctions, with the “Big 5″ crises being the most prominent. Twenty years ago, a sixth major extinction was recognized in the Middle Permian (262 million years ago) of China, when paleontologists teased apart losses from the “Great Dying” at the end of the period. Until now, this Capitanian extinction was known only from equatorial settings, and its status as a global crisis was controversial.
David P.G. Bond and colleagues provide the first evidence for severe Middle Permian losses amongst brachiopods in northern paleolatitudes (Spitsbergen). Their study shows that the Boreal crisis coincided with an intensification of marine oxygen depletion, implicating this killer in the extinction scenario.
The widespread loss of carbonates across the Boreal Realm also suggests a role for acidification. The new data cements the Middle Permian crisis’s status as a true “mass extinction.” Thus the “Big 5″ extinctions should now be considered the “Big 6.”
An abrupt extinction in the Middle Permian (Capitanian) of the Boreal Realm (Spitsbergen) and its link to anoxia and acidification: David P.G. Bond et al., University of Hull, Hull, UK. Published online ahead of print on 14 Apr. 2015; http://dx.doi.org/10.1130/B31216.1. This article is OPEN ACCESS (available for free online).