The Permian–Triassic extinction event, commonly called the Great Dying, happened at the time when the Permian and Triassic geologic periods ended, marking the end of the Paleozoic era and the start of the Mesozoic era. It is Earth’s most severe extinction event, causing the loss of 57% of biological families, 62% of genera, 81% of marine species, and 70% of land vertebrate species. It also caused the greatest loss of insect species ever recorded. This event is the most extreme of the "Big Five" major extinctions in Earth’s history. Evidence suggests there were one to three separate phases of extinction. The largest marine extinction occurred over 60,000 to 100,000 years around 251.902 million years ago, at the boundary between the Permian and Triassic periods. Scientists debate whether land extinctions happened at the same time as the main marine extinction.

Most scientists agree that the main cause was massive volcanic eruptions in the Siberian Traps, which released large amounts of sulfur dioxide and carbon dioxide. These gases caused oxygen-starved and sulfurous oceans, higher global temperatures, and more acidic oceans. Atmospheric carbon dioxide levels increased from about 400 ppm to 2,500 ppm during this time, with 3,900 to 12,000 gigatonnes of carbon added to the ocean and atmosphere.

Other possible factors include carbon dioxide released from burning oil and coal deposits caused by the eruptions, methane emissions from melting methane clathrates, methane produced by new types of microorganisms fed by minerals from the eruptions, stronger and longer 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 sunlight.

Dating

Previously, scientists believed that rock layers from the Permian–Triassic boundary were too few and incomplete to study in detail. However, new methods now allow researchers to date the extinction with great accuracy. Using uranium-lead (U–Pb) dating of zircon crystals from five volcanic ash layers in Meishan, China, scientists have created a detailed timeline for the extinction. This timeline helps researchers understand how global environmental changes, disruptions in the carbon cycle, mass extinction, and recovery happened over thousands of years. The first appearance of a tiny marine animal called Hindeodus parvus is used to mark the Permian–Triassic boundary.

The extinction 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 sharp drop in the ratio of carbon-13 to carbon-12, known as δ C. This change in carbon isotopes is often used to identify the Permian–Triassic boundary in rocks that cannot be dated using other methods. The drop in carbon-13 levels was 4–7% and lasted about 500,000 years. However, estimating the exact size of this change is difficult because many rock layers near the boundary have been altered over time.

Evidence also shows that Earth’s temperature rose by about 8 °C (14 °F), and carbon dioxide levels reached 2,500 ppm. For comparison, carbon dioxide levels before the Industrial Revolution were 280 ppm, and today they are about 426 ppm. Scientists have also found signs of increased ultraviolet radiation reaching Earth, which may have caused mutations in plant spores.

Some scientists suggest that the Permian–Triassic boundary is linked to a sudden increase in fungi, both in the ocean and on land. This rise in fungi, called a "fungal spike," may have happened because fungi fed on the large number of dead plants and animals after the extinction. This fungal spike has been used to identify the boundary in rocks that cannot be dated using other methods. However, some scientists argue that the fungal spike may not be a unique event, as it might have occurred multiple times after the extinction. Others question whether the most common fungal spore, Reduviasporonites, is actually a type of algae, not a fungus. New chemical evidence supports the idea that Reduviasporonites is a fungal spore, which weakens some earlier criticisms.

There is still uncertainty about how long the overall extinction lasted and when different groups of animals and plants went extinct. Some studies suggest the extinction happened in a single event, while others show evidence of multiple extinction waves. For example, in some areas, two extinction events, called MEH-1 and MEH-2, are recorded. These events had different causes. In other places, the extinction of certain groups, like ostracods and brachiopods, occurred over 670,000 to 1.17 million years. Studies of rock layers in Queensland, Australia, also show repeated periods of stress in marine environments before the end-Permian extinction, supporting the idea that the extinction was a slow process.

Scientists are also debating whether the extinction of land and ocean life happened at the same time or at different times. Some evidence from Greenland shows that both land and ocean life declined together, though plants took longer to show the full effects. Other studies from South China and the Sydney Basin support the idea that land and ocean extinctions happened together. However, other research suggests that land life began to decline 60,000 to 370,000 years before the ocean life declined. Chemical evidence from Norway and dating of rock layers in South Africa also show that land life declined before the ocean extinction. In some tropical regions, ocean life declined before land life.

Studying the timing and causes of the Permian–Triassic extinction is complicated by the Capitanian extinction, which occurred just before the Permian–Triassic event. Some extinctions once thought to happen at the Permian–Triassic boundary are now believed to have occurred during the Capitanian period. It is unclear whether species that survived earlier extinctions had recovered enough to be affected again during the Permian–Triassic event. Some scientists argue that the environmental changes during the Capitanian and Permian–Triassic periods form one long extinction event, while others believe they are separate. One theory suggests that two major extinction events occurred 9.4 million years apart, with the final event killing about 80% of marine species. Earlier extinctions, like the end of the Guadalupian epoch, also caused significant losses, especially for brachiopods and corals.

Extinction patterns

Marine invertebrates experienced the most severe losses during the P–Tr extinction. Earlier estimates of 90–96% marine species extinction were based on confusion with the end-Capitanian mass extinction, which occurred 7–10 million years earlier. Evidence of these losses was found in rock layers from south China near the P–Tr boundary. In these layers, 286 of 329 marine invertebrate genera disappeared in the final two sedimentary layers containing Permian conodont fossils. The drop in diversity was likely due to a sudden rise 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 vulnerable to ocean acidification caused by increased atmospheric CO₂. Organisms using haemocyanin or haemoglobin for oxygen transport were more resistant to extinction than those using hemerythrin or oxygen diffusion. Endemism, or being found only in a specific area, was a strong risk factor for extinction. Bivalve species limited to one region were more likely to go extinct than those found worldwide. Survival rates of species did not vary much by latitude. Organisms in areas less affected by global warming suffered fewer or delayed extinctions.

Among benthic organisms, the extinction event greatly increased background extinction rates, causing the most severe losses for species already prone to extinction (those with high turnover rates). Marine extinction rates were extremely high. Bioturbators, which mix sediment, were severely affected, as shown by the loss of mixed sediment layers in many marine environments during the end-Permian extinction.

Surviving marine invertebrates included articulate brachiopods (with hinged shells), which had been declining since the P–Tr extinction; the Ceratitida order of ammonites; and crinoids ("sea lilies"), which nearly went extinct but later recovered. Groups with high survival rates generally had efficient circulation, complex gas exchange systems, and light calcification. More heavily calcified species with simpler 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 species variety and physical forms, though not as severely as during the Capitanian mass extinction.

Ammonoids, which had been declining for 30 million years since the Roadian stage, suffered a major extinction event 10 million years before the P–Tr extinction, at the end of the Capitanian stage. This earlier extinction 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 rapidly, but variety remained low. The range of possible ammonoid shapes and structures became more limited as the Permian progressed. By the early Triassic, the original range of ammonoid forms was reoccupied, but the distribution of these traits among groups changed.

Ostracods faced long-term disruptions in diversity during the Changhsingian period before the P–Tr extinction, when most of them suddenly disappeared. 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 group among lophophorates.

Deep-water sponges lost much of their diversity and had smaller spicules during the P–Tr extinction. Shallow-water sponges were less affected, showing larger spicules and less loss of physical diversity.

Foraminifera experienced a severe drop in diversity. Evidence from South China shows their extinction occurred in two phases. Foraminifera biodiversity hotspots shifted to deeper waters during the P–Tr extinction. About 93% of Permian foraminifera became extinct, including 50% of the Textulariina group, 92% of the Lagenida, 96% of the Fusulinida, and 100% of the Miliolida. Foraminifera with calcium carbonate shells had a 91% extinction rate. Lagenida may have survived because they had greater environmental tolerance and wider geographic ranges compared to Fusulinida.

Cladodontomorph sharks likely survived by living in deep-ocean refuges, as suggested 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 Permian-Triassic boundary in foraminifera, brachiopods, bivalves, and ostracods. Surviving gastropods were smaller than those that did not survive, though some "Gulliver gastropods" grew larger, 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 event affecting insect diversity, with eight to nine insect orders going extinct and ten others losing much of their diversity. Palaeodictyopteroids (insects with piercing mouthparts) declined during the mid-Permian, possibly due 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 geographic distribution rather than outright extinction.

The geological record of terrestrial plants is limited, relying mostly on pollen and spore studies. Plant changes across the Permian-Triassic boundary vary by location and preservation quality. Plants are generally less affected by mass extinctions, with impacts at the family level being minimal. Plant diversity losses were less severe than those of marine animals. A 50% reduction in species diversity may be due to taphonomic processes (fossilization biases). However, ecosystems changed dramatically, with forests nearly disappearing and dominant plant groups like Cordaites (g

Biotic recovery

After the mass extinction event, the structure of Earth's ecosystems changed as surviving species adapted and new ones emerged. In the oceans, the "Paleozoic evolutionary fauna" declined, while the "modern evolutionary fauna" became more common. The Permian-Triassic extinction marked a major shift in ocean life that began after earlier extinctions and continued into the Late Jurassic. Typical ocean floor species included bivalves, snails, sea urchins, and crustaceans, while bony fish and marine reptiles became more common in open ocean areas. On land, dinosaurs and mammals appeared during the Triassic period. The changes in species diversity were partly due to the extinction event’s uneven impact, which affected some groups, like brachiopods, more than others, such as bivalves. Recovery also varied: some species went extinct years after the event without recovering, while others became dominant over time.

Immediately after the end-Permian extinction, marine life was dominated by a few "disaster taxa," such as certain bivalves, conodonts, brachiopods, and foraminifera. These ecosystems had low species 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, but studies of specific areas, like Bear Lake County in Idaho and sites in China and Oman, show evidence of faster recovery in some regions. For example, marine life in the Paris biota rebounded in about 1.3 million years, and complex ecosystems in Italy and China appeared less than a million years after the extinction. These differences suggest that environmental conditions varied by region, with some areas recovering faster due to higher productivity or less severe stress. While taxonomic diversity returned quickly, it took much longer for ecosystems to regain their full complexity.

Recovery speeds also depended on species type and lifestyle. Seafloor communities remained diverse for a long time, with epifaunal (surface-dwelling) species recovering slower than infaunal (burrowing) species. In contrast, mobile species like ammonoids and conodonts recovered rapidly, exceeding pre-extinction diversity within 2 million years. Recent research suggests recovery was driven by competition between species, which influenced how quickly new niches were filled. Low competition in the Early Triassic slowed recovery, but increased competition later sped it up. Other factors, like repeated environmental stress and extreme heat, may have delayed recovery, especially for vulnerable species like skeletonized organisms.

By the Middle Triassic, most marine communities had recovered, but global marine diversity did not reach pre-extinction levels until the Middle Jurassic, about 75 million years later.

Before the extinction, about two-thirds of marine animals were sessile (attached to the seafloor). During the Mesozoic era, only half of marine animals were sessile, with more mobile species like snails, sea urchins, and crabs becoming common. After the extinction, complex ecosystems, which included more predators and specialized species, outnumbered simple ecosystems by about three to one. This shift, called the Mesozoic Marine Revolution, was linked to increased predation and the evolution of durophagy (the ability to crush hard-shelled prey).

Marine vertebrates, such as fish, recovered quickly, showing complex predator-prey relationships. Fossil evidence from 5 million years after the extinction shows vertebrates at the top of the food chain. Some fish species, like hybodonts, had extremely fast tooth replacement, and ichthyopterygians (a group of ancient marine reptiles) grew rapidly in size.

Bivalves expanded into many marine environments after the extinction. Before the Permian-Triassic extinction, bivalves were rare, but they became numerous and diverse in the Triassic, taking over roles previously filled by brachiopods. While bivalves were once thought to have outcompeted brachiopods, this idea remains debated.

Hypotheses about cause

Explaining an event that happened 250 million years ago is very challenging. Much of the evidence on land has been worn away or buried deep underground. Meanwhile, the seafloor spreads apart and is recycled over 200 million years, leaving little useful information beneath the ocean.

Scientists have still collected a lot of evidence about what caused the event, and several possible explanations have been suggested. These explanations include both sudden, large events and slower, long-term 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 impacts from space objects, increased volcanic activity, and the sudden release of methane from the ocean floor. This methane could have come from the breakdown of methane hydrates or from microbes breaking down organic material.
• The slower changes include changes in sea level, increasing oxygen shortages in the ocean, and increasing dryness on land.

Any theory about the cause must explain why the event affected certain organisms more than others. It especially harmed those with calcium carbonate skeletons. It also must explain why it took 4 to 6 million years before life began to recover and why biological minerals were not formed as much as expected once recovery started, 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 more than 2,000,000 square kilometers (770,000 square miles), which is about the size of Saudi Arabia. These eruptions happened at the same time as the extinction event. Studies in the Norilsk and Maymecha-Kotuy areas of northern Siberia show that the eruptions happened in a few large bursts of magma, not in regular flows.

The Siberian Traps caused one of the fastest increases in atmospheric carbon dioxide levels in Earth's history. Scientists estimate that carbon dioxide emissions were five times faster than during a previous extinction event. One study suggests that carbon dioxide levels increased from between 500 and 4,000 ppm before the extinction to about 8,000 ppm after. Another study estimates that carbon dioxide levels rose from 400 ppm to 2,500 ppm, with 3,900 to 12,000 gigatonnes of carbon added to the ocean and atmosphere. This would have led to extreme temperature increases, although some evidence suggests there was a delay of 12,000 to 128,000 years between the rise in carbon dioxide and global warming. This delay might also be due to errors in the dating of fossils. Before the extinction, global average surface temperatures were about 18.2 °C, but they increased to as high as 35 °C, with this extreme heat lasting up to 500,000 years. In Gondwana’s southern regions, temperatures increased by about 10–14 °C. In South China, ocean temperatures near the equator increased by about 8 °C. In present-day Iran, tropical ocean temperatures were between 27 and 33 °C before the extinction but jumped to over 35 °C during the event. The increased temperatures also led to stronger El Niño events, increasing short-term climate changes.

These very high levels of carbon dioxide lasted for a long time. At that time, the supercontinent Pangaea was positioned in a way that made the natural carbon cycle inefficient at removing carbon from the atmosphere. In a 2020 study, scientists used a model to show how the greenhouse effect affected the ocean and concluded that the mass extinction was caused by volcanic carbon dioxide emissions. Evidence also suggests that volcanic activity burned underground fossil fuel deposits, as shown by spikes in certain chemicals found in rocks. These findings support the idea that the Siberian Traps were active at the same time as the extinction.

The eruptions of the Siberian Traps had some unusual features that made them even more dangerous. The area had a lot of halogens, which can destroy the ozone layer. Evidence shows that about 70% of these halogens were released into the atmosphere. This would have released a large amount of hydrochloric acid and sulfur-rich gases, causing dust clouds and acid rain. These effects would have blocked sunlight, reducing photosynthesis on land and in the ocean, leading to the collapse of food chains. These sulfur emissions also caused short periods of global cooling, but these cold periods were too brief to be the main cause of the extinction.

The eruptions may also have caused acid rain, which could have killed plants on land and marine organisms with calcium carbonate shells. Flood basalts usually produce lava that flows easily and does not throw debris into the atmosphere. However, about 20% of the material from the Siberian Traps was pyroclastic ash that reached high into the atmosphere, increasing the short-term cooling effect. Once this ash settled, the excess carbon dioxide would have remained, causing continued global warming.

Burning of hydrocarbon deposits may have made the extinction worse. The Siberian Traps were located above thick layers of old carbonate and evaporite deposits, as well as coal deposits. When heated by volcanic activity, these rocks may have released large amounts of greenhouse and toxic gases. The location of the Siberian Traps over these deposits likely explains the severity of the extinction. The lava from the eruptions may have heated these deposits, releasing more carbon dioxide and increasing global warming. The change in the way the eruptions occurred, from spreading lava to forming thick layers of rock, may have released even more trapped hydrocarbons, which coincides with the start of the extinction and is linked to a major drop in carbon levels.

The release of methane from coal was accompanied by the burning of coal and the release of coal ash. A 2011 study by Stephen E. Grasby found evidence that volcanic activity caused massive coal deposits to catch fire, possibly releasing more than 3 trillion tons of carbon. They found ash deposits in deep rock layers near the Buchanan Lake Formation and said that the explosive eruptions would have released toxic elements into water bodies. Grasby also said that the ash from the eruptions was highly toxic and may have contributed to the worst extinction in Earth's history. However, some researchers suggest that these ash deposits may have come from wildfires instead.

Comparison to present global warming

The PTME has been compared to today’s human-caused global warming and the Holocene extinction because all three involve the quick release of carbon dioxide. Although current greenhouse gas emissions are much higher than those during the PTME, the timing and pattern of carbon release during the PTME are not fully understood. Scientists believe that carbon was likely released in short bursts over a few key periods, rather than continuously. The rate of carbon release during these periods may have been 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. Scientists predict that if carbon dioxide levels continue to rise, another biocalcification event—similar to one recorded in the fossil record—could harm modern marine ecosystems. Changes in plant-insect interactions during the PTME are also being studied as possible signs of future ecological changes.

Geologists warn that the similarities between the PTME and current conditions highlight the urgent need to reduce carbon dioxide emissions to avoid a disaster like the PTME.

As during the PTME, today’s oceans are undergoing major changes, including falling pH and oxygen levels. This connection is emphasized by geologist Lee Kump:

"If CO2 levels keep increasing, another biocalcification crisis could occur, similar to what happened in the past. This would have serious consequences for modern marine ecosystems."