The Permian–Triassic (P–Tr) extinction event, informally known as the Great Dying, was an extinction event that occurred 251.4 million years ago, forming the boundary between the Permian and Triassic geologic periods.
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.
It has been suggested that new, more aggressive fungi, insects and vertebrates evolved, and killed vast numbers of trees.
These cyclical flora shifts occurred a few times over the course of the extinction period and afterwards.
These fluctuations of the dominant flora between woody and herbaceous taxa indicate chronic environmental stress resulting in a loss of most large woodland plant species.
The caloneurodeans, monurans, paleodictyopteroids, protelytropterans, and protodonates became extinct by the end of the Permian.
In well-documented Late Triassic deposits, fossils overwhelmingly consist of modern fossil insect groups.
Marine invertebrates suffered the greatest losses during the P–Tr extinction.
Evidence of this was found in samples from south China sections at the P–Tr boundary.
The geological record of terrestrial plants is sparse and based mostly on pollen and spore studies.
Interestingly, plants are relatively immune to mass extinction, with the impact of all the major mass extinctions "insignificant" at a family level. Even the reduction observed in species diversity (of 50%) may be mostly due to taphonomic processes. However, a massive rearrangement of ecosystems does occur, with plant abundances and distributions changing profoundly and all the forests virtually disappearing; the Palaeozoic flora scarcely survived this extinction. At the P–Tr boundary, the dominant floral groups changed, with many groups of land plants entering abrupt decline, such as Cordaites (gymnosperms) and Glossopteris (seed ferns). Dominant gymnosperm genera were replaced post-boundary by lycophytes—extant lycophytes are recolonizers of disturbed areas. Palynological or pollen studies from East Greenland of sedimentary rock strata laid down during the extinction period indicate dense gymnosperm woodlands before the event.
The greatest decline occurred in the Late Permian and was probably not directly caused by weather-related floral transitions. Most fossil insect groups found after the Permian–Triassic boundary differ significantly from those that lived prior to the P–Tr extinction.
With the exception of the Glosselytrodea, Miomoptera, and Protorthoptera, Paleozoic insect groups have not been discovered in deposits dating to after the P–Tr boundary.
The groups with the highest survival rates generally had active control of circulation, elaborate gas exchange mechanisms, and light calcification; more heavily calcified organisms with simpler breathing apparatus were the worst hit. In the case of the brachiopods at least, surviving taxa were generally small, rare members of a diverse community. The ammonoids, which had been in a long-term decline for the 30 million years since the Roadian (middle Permian), suffered a selective extinction pulse 10 million years before the main event, at the end of the Capitanian stage.