The Permian–Triassic extinction event, commonly known as the Great Dying, occurred at the boundary between the Permian and Triassic geologic periods, marking the end of the Paleozoic era and the start of the Mesozoic era. It is Earth's most severe known extinction event, resulting in the loss of 57% of biological families, 62% of genera, 81% of marine species, and 70% of terrestrial vertebrate species. It also caused the greatest known mass extinction of insects. This event is the most significant of the "Big Five" mass extinctions during the Phanerozoic eon. Evidence suggests one to three distinct phases of extinction, with the most severe marine extinction occurring over a 60,000- to 100,000-year period around 251.902 million years ago. There is debate about whether the extinctions on land happened at the same time as the main marine extinction event.

The primary cause of the extinction is believed to be massive volcanic eruptions that formed the Siberian Traps. These eruptions released large amounts of sulfur dioxide and carbon dioxide, leading to oxygen-starved, sulfurous oceans, higher global temperatures, and acidified oceans. Atmospheric carbon dioxide levels increased from about 400 ppm to 2,500 ppm during this time, with approximately 3,900 to 12,000 gigatonnes of carbon added to the ocean-atmosphere system. Other proposed factors include carbon dioxide emissions from burning oil and coal deposits ignited by the eruptions, methane emissions from the breakdown of methane clathrates, methane emissions from new types of microorganisms fueled by minerals from the eruptions, longer and stronger El Niño events, and a possible extraterrestrial impact that created the Araguainha crater. This impact may have released methane and damaged the ozone layer, increasing exposure to solar radiation.

Dating

Scientists once believed that rock layers from the time of the Permian–Triassic boundary were too few and incomplete to study in detail. However, new methods now allow scientists to date the extinction event with great accuracy. Using special dating techniques on volcanic ash layers from Meishan, China, researchers have created a detailed timeline for the extinction. This timeline helps scientists understand how global environmental changes, carbon cycle disruptions, mass extinctions, and recovery happened over thousands of years. The first appearance of a tiny fossil called Hindeodus parvus is used to mark the Permian–Triassic boundary.

The extinction event occurred between 251.941 ± 0.037 and 251.880 ± 0.031 million years ago, lasting about 60,000 years. During this time, there was a sudden drop in the ratio of carbon-13 to carbon-12 in the environment, a change that helps scientists identify the Permian–Triassic boundary in rocks that cannot be dated using other methods. This carbon change was very large, between 4% and 7%, and lasted about 500,000 years. However, scientists find it hard to measure exactly because some rocks have changed over time.

Evidence suggests that Earth’s temperature rose by 8 degrees Celsius (14 degrees Fahrenheit) and carbon dioxide levels reached 2,500 parts per million. For comparison, carbon dioxide levels before the Industrial Revolution were 280 parts per million, and today they are about 426 parts per million. Scientists also found signs of increased ultraviolet radiation, which may have caused mutations in plant spores.

Some researchers believe the Permian–Triassic boundary is marked by a sudden increase in fungi, which may have grown because of the large number of dead plants and animals. This "fungal spike" has been used to identify rocks near the boundary when other methods are not possible. However, others argue that the fungal spike may not be unique to the Permian–Triassic boundary and that the spores found might not be from fungi at all. Recent studies suggest some of these spores could be from algae instead.

Scientists are still unsure about how long the extinction lasted and when different groups of life went extinct. Some evidence shows the extinction happened in several waves over millions of years, while other studies suggest it was more focused around one main event. In some areas, like Meishan, China, the main extinction seems to have happened quickly, but in other places, like Liangfengya and Shangsi, two separate extinction events were found. Studies also show that different groups of life, such as ostracods and brachiopods, went extinct at different times.

Research on rocks in Queensland, Australia, suggests that marine environments were already stressed before the end-Permian extinction, supporting the idea that the extinction happened gradually. Some studies show that the loss of marine life may have happened before the collapse of entire ecosystems.

There is debate about whether the extinction of land and sea life happened at the same time or at different times. Some evidence from Greenland and South China suggests both types of life declined together, while other studies show land life may have started to decline earlier. In some areas, like the Karoo Basin in Africa and the Sunjiagou Formation in China, land life declined before the marine extinction. In tropical regions, some studies suggest marine life declined first.

The timing and causes of the Permian–Triassic extinction are complicated by another mass extinction event called the Capitanian extinction, which happened earlier in the Permian period. Some extinctions once thought to be from the Permian–Triassic boundary may actually belong to the Capitanian event. Scientists are still unsure whether the Capitanian extinction ended before the Permian–Triassic event began or if some species that survived the Capitanian extinction were still affected during the Permian–Triassic event. Some researchers believe the Capitanian and Permian–Triassic extinctions were part of a single, long event, while others argue they were separate. For example, many species, like dinocephalians and Verbeekinidae, went extinct at the end of the Capitanian period, which is earlier than the Permian–Triassic boundary. The impact of this earlier extinction varied by location and group, with brachiopods and corals being especially affected.

Extinction patterns

Marine invertebrates experienced the most severe losses during the P–Tr extinction. Earlier estimates of 90–96% marine species extinction were incorrect because they mixed up data from the end-Capitanian mass extinction, which happened 7–10 million years earlier. Evidence of these losses was found in rock samples from south China near the P–Tr boundary. In these samples, 286 out of 329 marine invertebrate genera disappeared within the last two sedimentary layers containing Permian conodonts. The drop in diversity likely happened because of a sudden increase in extinctions, not a decrease in new species forming.

The extinction mainly affected organisms with calcium carbonate skeletons, especially those that relied on stable CO₂ levels to build them. These organisms were harmed by ocean acidification caused by rising atmospheric CO₂. Organisms using hemocyanin or hemoglobin to carry oxygen were more likely to survive than those using hemerythrin or oxygen diffusion. Endemism, or being limited to a specific region, increased the risk of extinction for certain species. Bivalve species that lived only in one area were more likely to go extinct than those found worldwide. Survival rates of species did not differ much based on latitude. Species that lived in areas less affected by global warming experienced fewer or delayed extinctions.

Among benthic organisms, the extinction event greatly increased background extinction rates, leading to the greatest loss of species among those already prone to high background extinction rates. The extinction rate for marine organisms was extremely high. Bioturbators, which mix sediments, were severely affected, as shown by the disappearance of mixed sediment layers in many marine areas during the end-Permian extinction.

Surviving marine invertebrates included articulate brachiopods (those with a hinge), which had been slowly declining since the P–Tr extinction; the Ceratitida order of ammonites; and crinoids ("sea lilies"), which nearly went extinct but later became abundant. Groups with high survival rates generally had strong control over circulation, complex gas exchange systems, and light calcification. Heavily calcified organisms with simple breathing systems suffered the most. Among brachiopods, surviving species were usually small and rare compared to earlier, more diverse communities.

Conodonts experienced major losses in both types and shapes, though not as severe as during the Capitanian mass extinction.

Ammonoids, which had been declining for 30 million years since the Roadian (middle Permian), faced a selective extinction event 10 million years before the main P–Tr extinction, at the end of the Capitanian stage. This event greatly reduced the variety of ecological roles these organisms filled. Diversity and variety dropped further until the P–Tr boundary, where the extinction was non-selective, suggesting a sudden, catastrophic cause. During the Triassic, diversity increased quickly, but variety remained low. The range of possible shapes and structures among ammonoids became more limited as the Permian progressed. A few million years into the Triassic, the original range of ammonoid forms was reoccupied, but the distribution of these forms among different groups changed.

Ostracods experienced long-term changes in diversity before the P–Tr extinction, with large numbers vanishing suddenly. At least 74% of ostracods died during the P–Tr extinction itself.

Bryozoans had been declining throughout the Late Permian and suffered even greater losses during the P–Tr extinction, becoming the most affected lophophorate group.

Deep-water sponges lost diversity and had smaller spicules during the P–Tr extinction. Shallow-water sponges were less affected, with larger spicules and less loss of shape diversity compared to deep-water sponges.

Foraminifera faced a severe drop in diversity. Evidence from South China shows their extinction had two phases. Foraminifera biodiversity hotspots shifted to deeper waters during the P–Tr extinction. About 93% of late Permian foraminifera went extinct, including 50% of Textulariina, 92% of Lagenida, 96% of Fusulinida, and 100% of Miliolida. Foraminifera with calcium carbonate shells had a 91% extinction rate. Lagenida may have survived because they tolerated a wider range of environments and were more widely distributed than Fusulinida.

Cladodontomorph sharks likely survived by living in deep ocean refuges, as shown by their discovery in deep, outer shelf environments during the Early Cretaceous. Ichthyosaurs, which evolved just before the P–Tr extinction, also survived.

The Lilliput effect, where species shrink during and after mass extinctions, was observed near the P–Tr boundary in foraminifera, brachiopods, bivalves, and ostracods. While surviving gastropods were smaller than those that died, it is unclear if the Lilliput effect applied to all gastropods. Some gastropod groups, called "Gulliver gastropods," grew larger after the extinction, showing the opposite of the Lilliput effect, called the Brobdingnag effect.

The Permian had high diversity in insects and other invertebrates, including the largest insects ever. The end-Permian extinction was the largest known insect mass extinction, with eight to nine insect orders going extinct and ten others losing diversity. Palaeodictyopteroids (insects with piercing mouthparts) declined during the mid-Permian, possibly linked to changes in plant life. The greatest decline happened in the Late Permian and was likely not directly caused by weather-related plant changes. Some insect declines during the P–Tr extinction were due to changes in their geographic distribution rather than outright extinction.

The geological record of terrestrial plants is limited, mostly based on pollen and spore studies. Plant changes across the P–Tr boundary vary by location and preservation quality. Plants are generally less affected by mass extinctions, with little impact at the family level. Plant diversity losses were less severe than marine animal losses. A 50% reduction in species diversity may be due to taphonomic processes (factors affecting fossil preservation). However, ecosystems changed dramatically, with forests nearly disappearing and dominant plant groups like Cordaites (gymnosperms) and Glossopteris (seed ferns) declining abruptly. The extent of plant extinction remains debated.

The Glossopteris-dominated plant communities in high-latitude Gondwana collapsed in Australia about 370,000 years before the P–Tr boundary. This collapse happened earlier in western Gondwana.

Biotic recovery

After the Permian-Triassic mass extinction, the structure of Earth's ecosystems changed significantly. In the ocean, groups of animals known as the "Paleozoic evolutionary fauna" declined, while the "modern evolutionary fauna" became more common. This shift, which began after an earlier extinction event and reached its peak in the Late Jurassic, was marked by the Permian-Triassic extinction. Typical marine life included bivalves, snails, sea urchins, and Malacostraca, while bony fish and marine reptiles became more widespread in open ocean areas. On land, dinosaurs and mammals first appeared during the Triassic period. The change in which species survived was partly due to the extinction event affecting some groups more than others, such as brachiopods being more severely impacted than bivalves. Recovery also varied: some species that survived eventually went extinct without diversifying again, while others, like bivalves, became dominant over time.

Immediately after the end-Permian extinction, marine life was dominated by a few species known as "disaster taxa," such as bivalves like Claraia and Unionites, conodonts like Clarkina and Hindeodus, and foraminifera like Rectocornuspira kalhori. These ecosystems had low diversity and showed little variation in species distribution across different latitudes.

Scientists disagree about how quickly life recovered after the extinction. Some say it took 10 million years until the Middle Triassic, while others point to evidence from areas like Bear Lake County in Idaho, where marine life rebounded in about 1.3 million years. Similar findings in Italy, China, and Oman suggest that recovery varied by region. Some areas may have recovered faster due to higher productivity or less severe environmental stress. While taxonomic diversity (the number of species) returned quickly, ecological diversity (how species interacted in ecosystems) took much longer to recover, with some studies showing marine ecosystems still adjusting 50 million years after the extinction.

Recovery speed also depended on the type of organism. Seafloor communities remained diverse for a long time, while free-swimming species like ammonoids and conodonts recovered faster. By the Anisian period, increased competition for resources led to faster diversification. However, some areas experienced repeated environmental stress, such as extreme heat, which delayed recovery.

Before the extinction, about two-thirds of marine animals were attached to the seafloor. After the extinction, fewer marine animals were sessile, with more species becoming mobile, such as snails and crabs. This shift, along with increased predation, led to the Mesozoic Marine Revolution.

Marine vertebrates, like fish, recovered quickly, showing complex predator-prey relationships. Bivalves, which were rare before the extinction, became widespread in the Triassic, taking over roles previously held by brachiopods. While some theories suggest bivalves outcompeted brachiopods, this is still debated.

Global marine diversity reached pre-extinction levels no earlier than the Middle Jurassic, about 75 million years after the extinction.

Hypotheses about cause

Explaining an event that happened 250 million years ago is very difficult. Much of the evidence on land has been worn away by weathering or is buried deep underground. Meanwhile, the ocean floor spreads apart and is recycled over 200 million years, leaving no useful clues beneath the sea.

Scientists have still found a lot of evidence about what caused the event. Many different explanations have been suggested. These include both sudden, large events and slow, gradual changes, similar to those thought to have caused the Cretaceous–Paleogene extinction, but with less agreement among scientists today.

• The sudden events include one or more large meteor impacts, increased volcanic activity, and the sudden release of methane from the seafloor. This methane could be released either by the melting of methane ice deposits or by the activity of certain microbes that produce methane.
• The slow changes include changes in sea level, an increase in low oxygen conditions in the ocean, and an increase in dryness on land.

Any explanation for the event must account for several key facts. These include why the event affected organisms with calcium carbonate skeletons most severely, why it took 4 to 6 million years before life began to recover, and why biological minerals were not formed in large amounts even though inorganic carbonates were deposited.

The flood basalt eruptions that formed the Siberian Traps were among the largest volcanic events in Earth's history. These eruptions covered an area of more than 2,000,000 square kilometers (770,000 square miles), about the size of Saudi Arabia, and caused a major environmental impact. The timing of these eruptions matches closely with the extinction event. Studies of the Norilsk and Maymecha-Kotuy regions in northern Siberia show that volcanic activity happened in a few large bursts of magma, not in a steady flow.

The Siberian Traps caused one of the fastest increases in atmospheric carbon dioxide levels in Earth's history. The rate of carbon dioxide emissions was estimated to be five times faster than during the previous major extinction event, the Capitanian mass extinction. Some estimates suggest that carbon dioxide levels increased from between 500 and 4,000 parts per million before the extinction to about 8,000 parts per million after. Another study estimated that carbon dioxide levels were around 400 parts per million before the extinction, rising to 2,500 parts per million. This added about 3,900 to 12,000 gigatonnes of carbon to the ocean and atmosphere. The extreme rise in temperature that followed may have been delayed by as much as 12,000 to 128,000 years, though this delay could also be due to errors in dating. During the time before the extinction, global average surface temperatures were about 18.2 degrees Celsius, but they rose as high as 35 degrees Celsius, with this extreme heat lasting up to 500,000 years. In the high southern latitudes of Gondwana, temperatures increased by about 10 to 14 degrees Celsius. In South China, surface water temperatures in the low latitudes rose by about 8 degrees Celsius. In present-day Iran, tropical sea surface temperatures were between 27 and 33 degrees Celsius during the Changhsingian period but jumped to over 35 degrees Celsius during the Permian-Triassic Mass Extinction Event. These high temperatures led to stronger El Nino events, increasing short-term climate changes.

These extremely high levels of carbon dioxide lasted for a long time. The position of the supercontinent Pangaea at that time made the inorganic carbon cycle very inefficient at removing carbon from the atmosphere. In a 2020 study, scientists used a biogeochemical model to show how the greenhouse effect affected the ocean and concluded that the mass extinction was linked to volcanic carbon dioxide emissions. Evidence also suggests that volcanic activity burned underground fossil fuel deposits, as shown by the presence of certain chemical markers. Another study found that the amount of a certain type of zinc increased twentyfold during the Permian-Triassic Mass Extinction Event, indicating that volcanism was happening at the same time.

The Siberian Traps eruptions had special features that made them even more dangerous. The region was rich in halogens, which are very harmful to the ozone layer. Evidence shows that up to 70% of these halogens were released into the atmosphere. About 18 teratonnes of hydrochloric acid were emitted, along with sulfur-rich gases that formed dust clouds and acid rain. These would have blocked sunlight and disrupted photosynthesis on land and in the ocean, causing food chains to collapse. These sulfur emissions also caused brief but severe global cooling, which led to a drop in sea levels. However, these cold events were too short to be the main cause of the extinction.

The eruptions may have also caused acid rain as the sulfur particles washed out of the atmosphere. This could have killed land plants and organisms with calcium carbonate shells, such as mollusks and plankton. Flood basalts usually produce lava that flows easily and does not throw debris into the atmosphere. However, 20% of the material from the Siberian Traps was pyroclastic ash, which was thrown high into the atmosphere, increasing the short-term cooling effect. Once this ash had settled, the excess carbon dioxide would have remained, leading to continued global warming.

Burning of hydrocarbon deposits may have made the extinction worse. The Siberian Traps are located over thick layers of ancient carbonate and evaporite deposits, as well as coal-bearing rocks. When these rocks are heated, such as by volcanic activity, they can release large amounts of greenhouse and toxic gases. The unique location of the Siberian Traps over these deposits likely explains the severity of the extinction. The basalt lava erupted or intruded into these rocks, which would have released large amounts of carbon dioxide, leading to stronger global warming after the dust and aerosols settled. The change in the eruptions from flood basalt to a form that created large underground layers of magma, releasing even more trapped hydrocarbons, coincides with the main start of the extinction and is linked to a major drop in carbon isotope levels. The moderate temperature of the magma from the Siberian Traps helped release large amounts of carbon dioxide by heating the evaporites and carbonates.

The release of methane from coal was accompanied by the burning of coal and the release of coal ash. A 2011 study led by Stephen E. Grasby found evidence that volcanic activity caused massive coal beds to ignite, possibly releasing more than 3 trillion tons of carbon. They found ash deposits in deep rock layers near what is now the Buchanan Lake Formation. Grasby said, "Coal ash dispersed by the explosive Siberian Trap eruption would be expected to have an associated release of toxic elements in impacted water bodies where fly ash slurries developed. … Mafic megascale eruptions are long-lived events that would allow significant build-up of global ash clouds." However, some researchers suggest that these ash deposits may have actually come from wildfires rather than volcanic activity.

Comparison to present global warming

The Permian-Triassic Mass Extinction (PTME) has been compared to today’s human-caused global warming and the Holocene extinction because all involve the fast release of carbon dioxide. Although the current rate of greenhouse gas emissions is much higher than during the PTME, the timing and pattern of carbon release during the PTME are not well understood. Scientists believe carbon was released in short, intense bursts rather than continuously over time. These bursts likely occurred at rates similar to today’s human-caused emissions.

Today’s oceans, like those during the PTME, are experiencing lower pH and oxygen levels, which strengthens the connection between the two events. If carbon dioxide levels continue to rise, another event similar to the PTME’s biocalcification crisis—where marine life with calcium-based shells and skeletons is severely affected—may occur. Changes in how plants and insects interact during the PTME have also been studied as possible signs of future ecological changes. Because of these similarities, geologists warn that reducing carbon dioxide emissions is urgent to avoid a disaster like the PTME.

Just as during the PTME, today’s oceans are changing rapidly, with falling pH and oxygen levels. This link is highlighted by geologist Lee Kump:

If carbon dioxide levels keep rising, it could cause another biocalcification crisis, similar to what appears in the fossil record. This would have very harmful effects on modern marine ecosystems.