The Permian–Triassic extinction event, commonly known as the Great Dying, happened near the end of the Permian period and the beginning of the Triassic period, marking the transition between the Paleozoic and Mesozoic eras. It is Earth's most severe known extinction event, with 57% of biological families, 62% of genera, 81% of marine species, and 70% of terrestrial vertebrate species disappearing. It also caused the largest known mass extinction of insects. This event is the most extreme of the "Big Five" mass extinctions during the Phanerozoic era. Evidence suggests one to three separate phases of extinction, with the most significant marine extinction occurring over 60,000 to 100,000 years around 251.902 million years ago. Some debate exists about whether land extinctions happened at the same time as the main marine extinction event.

The leading scientific explanation for the extinction is the massive volcanic eruptions that formed the Siberian Traps, which released large amounts of sulfur dioxide and carbon dioxide. These emissions led 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 3,900 to 12,000 gigatonnes of carbon entering the ocean-atmosphere system. Other possible factors include carbon dioxide released from burning oil and coal deposits, methane emissions from methane clathrates, methane produced by new microorganisms fed by minerals from eruptions, stronger and longer El Niño events, and an extraterrestrial impact that created the Araguainha crater, releasing methane and damaging the ozone layer, which increased exposure to solar radiation.

Dating

Previously, scientists 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 determine the exact timing of the extinction with great precision. Dates from volcanic ash layers in Meishan, China, help create a detailed timeline for the extinction. This timeline helps scientists explore how 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 occurred between about 251.941 million years ago and 251.880 million years ago, lasting roughly 60,000 years. A sudden drop in the ratio of carbon-13 to carbon-12 in rocks from this time is linked to the extinction. This change, called a "negative carbon isotope excursion," was 4–7% in size and lasted about 500,000 years. However, scientists find it difficult to measure the exact size because rock layers from this time have changed over time.

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

Some scientists believe that the Permian–Triassic boundary is marked by a sudden increase in fungi, both in the ocean and on land. This increase 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 are hard to date. However, others argue that the fungal spike may not be a global event and that the spores found might not be fungi at all. Some newer studies support a fungal origin for these spores, which weakens earlier criticisms.

Scientists are still unsure about the exact timing and duration of the extinction. Some evidence suggests the extinction happened in several waves over millions of years, while other studies show a single major event. In some areas, like Meishan, China, the extinction appears to have been concentrated in one peak. In other areas, like Liangfengya and Shangsi, two extinction waves were found, each with different causes. Studies also show that different groups of animals and plants became extinct at different times. For example, ostracods and brachiopods became extinct about 670,000 to 1.17 million years apart.

Analysis of rocks from the Bowen Basin in Queensland suggests that marine environments faced repeated stress before the end-Permian extinction, supporting the idea that the extinction was gradual. Some studies also suggest that the decline in marine life may have happened before the collapse of marine ecosystems.

Scientists debate whether the extinction of land and sea life happened at the same time or at different times. Some evidence from Greenland shows that land and sea life declined together, but plants took longer to show the full effects. Other studies in South China and the Sydney Basin support a simultaneous extinction of land and sea life. However, other research suggests that land life began to decline 60,000 to 370,000 years before the sea life extinction. Some studies also show that land plants declined before the major drop in carbon levels during the sea life extinction.

The timing and causes of the Permian–Triassic extinction are complicated by the Capitanian extinction, which occurred earlier in the Permian period. Some extinctions once thought to happen at the Permian–Triassic boundary are now linked to the Capitanian event. It is unclear whether species that survived earlier extinctions were fully recovered before the Permian–Triassic extinction. Some scientists argue that the Capitanian and Permian–Triassic extinctions were part of a single, long event, while others believe they were separate. One theory suggests two major extinction waves, 9.4 million years apart, with the final wave killing about 80% of marine life. Another wave, during the Capitanian, led to the extinction of many species, such as dinocephalian animals and certain types of foraminifera. The effects of this earlier extinction varied by location and group, with brachiopods and corals suffering severe losses.

Extinction patterns

Marine invertebrates had the most losses during the P–Tr extinction, although earlier estimates of 90–96% marine species extinction were mixed with the end-Capitanian mass extinction, which happened 7–10 million years earlier. Evidence of these losses was found in samples from south China near the P–Tr boundary. Here, 286 out of 329 marine invertebrate genera disappeared within the final two sedimentary layers containing Permian conodonts. The drop in diversity was likely caused by 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 hemocyanin or hemoglobin to transport oxygen were more resistant to extinction than those using hemerythrin or oxygen diffusion. Evidence suggests that being endemic (limited to a specific region) increased a taxon’s risk of extinction. Bivalve species that were endemic were more likely to go extinct than those found worldwide. Survival rates of taxa did not differ much by latitude. Organisms in areas less affected by global warming had fewer or delayed extinctions.

Among benthic (bottom-dwelling) organisms, the extinction event increased background extinction rates, causing the greatest loss to taxa with high background extinction rates (which also had high turnover). The extinction rate for marine organisms was catastrophic. Bioturbators (organisms that mix sediments) were severely affected, as shown by the loss of the sedimentary mixed layer in many marine environments 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 and diverse. Groups with high survival rates generally had active circulation control, complex gas exchange systems, and light calcification. More heavily calcified organisms with simpler breathing structures suffered the most species loss. Among brachiopods, surviving species were typically small and rare compared to their former diverse communities.

Conodonts faced severe taxonomic and morphological diversity loss, though not as severe as during the Capitanian mass extinction.

Ammonoids, which had been declining for 30 million years since the Roadian (middle Permian), experienced a selective extinction pulse 10 million years before the main event, at the end of the Capitanian stage. This early extinction greatly reduced ecological diversity, or the variety of different ecological roles. Diversity and disparity (range of forms) fell further until the P–Tr boundary, where the extinction was non-selective, suggesting a catastrophic cause. During the Triassic, diversity rose quickly, but disparity remained low. Ammonoid shapes and structures became more restricted as the Permian progressed. A few million years into the Triassic, the original range of ammonoid forms was reoccupied, but the traits were shared differently among groups.

Ostracods experienced long-term diversity changes during the Changhsingian before the PTME, when most of them suddenly disappeared. At least 74% of ostracods died during the PTME itself.

Bryozoans had been declining throughout the Late Permian before suffering even greater losses during the PTME, making them the most affected lophophorate group.

Deep-water sponges lost significant diversity and saw a decrease in spicule size during the PTME. Shallow-water sponges were less affected, showing increased spicule size and lower morphological diversity loss compared to deep-water sponges.

Foraminifera faced a severe diversity bottleneck. Evidence from South China shows the foraminiferal extinction had two pulses. Foraminiferal biodiversity hotspots shifted to deeper waters during the PTME. Approximately 93% of latest Permian foraminifera went extinct, including 50% of the Textulariina clade, 92% of Lagenida, 96% of Fusulinida, and 100% of Miliolida. Foraminifera with calcareous shells had a 91% extinction rate. Lagenides may have survived due to greater environmental tolerance and geographic spread compared to fusulinoidean fusulinides.

Cladodontomorph sharks likely survived by living in deep-ocean refugia, as suggested by the discovery of Early Cretaceous cladodontomorphs in deep, outer shelf environments. Ichthyosaurs, which evolved just before the PTME, also survived the event.

The Lilliput effect, where species shrink during and after mass extinctions, was observed across the Permian-Triassic boundary in foraminifera, brachiopods, bivalves, and ostracods. While surviving gastropods were smaller than those that did not survive, it is debated whether the Lilliput effect applied to gastropods. Some gastropod groups, called "Gulliver gastropods," grew larger after the extinction, showing the opposite trend, known as 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 mass extinction for insects, with eight to nine insect orders going extinct and ten others losing diversity. Palaeodictyopteroids (insects with piercing and sucking mouthparts) began declining in the mid-Permian, linked to changes in plant life. The greatest decline occurred in the Late Permian and was likely not directly caused by weather-related plant changes. Some declines in insect diversity during the PTME were due to biogeographic shifts rather than outright extinctions.

The geological record of terrestrial plants is limited, relying mostly on pollen and spore studies. Floral 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. Floral 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 (gymnosperms) and Glossopteris (seed ferns) declining sharply. The extent of plant extinction remains debated.

The Glossopteris-dominated flora of high-latitude Gondwana collapsed in Australia about 370,000 years before the Permian-Triassic boundary. This collapse occurred earlier in western Gondwana.

Biotic recovery

After the extinction event, the way ecosystems were organized changed as surviving species adapted. In the ocean, ancient animal groups declined, while newer groups became more common. The Permian-Triassic extinction was a major turning point in this change, which started after a previous extinction and continued until the Late Jurassic. Typical sea creatures 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. The change in species was partly because the extinction event affected some groups more than others, like brachiopods compared to bivalves. However, recovery also varied: some species went extinct later without returning to their former diversity, while others became dominant over time.

Soon after the end-Permian extinction, a widespread event occurred. Marine life after the extinction had few species and was dominated by a few disaster species, such as certain bivalves, conodonts, brachiopods, and foraminifera. These groups had low diversity, and the variety of species across different latitudes was minimal.

Scientists disagree about how quickly life recovered. Some say it took 10 million years until the Middle Triassic because the extinction was so severe. However, studies in areas like Bear Lake County, Idaho, and nearby regions showed faster recovery in some local marine ecosystems during the Early Triassic, with life rebounding in about 1.3 million years. In Italy, complex ecosystems appeared less than a million years after the extinction. Similar findings in China and Oman suggest that life recovered quickly in some areas. Differences in recovery rates indicate that the extinction’s impact varied by region, possibly due to environmental conditions. High-latitude ecosystems may have recovered faster because of increased productivity. While species diversity rebounded quickly, it took much longer for ecosystems to return to their pre-extinction complexity, with some studies showing marine recovery still ongoing 50 million years later.

Recovery speed also depended on species type and lifestyle. Seafloor communities remained less diverse until the end of the Early Triassic, about 4 million years after the extinction. Animals living on the seafloor took longer to recover than those living in the sediment. In contrast, free-swimming species like ammonoids and conodonts recovered quickly, surpassing pre-extinction levels within 2 million years.

Recent research suggests recovery was driven by competition between species, which affects how quickly new species form. Slow recovery in the Early Triassic may have been due to low competition because of fewer species, while faster recovery in the Anisian stage was caused by increased competition as species filled available niches. This created a cycle where greater biodiversity led to more competition and further diversification. Other factors, like repeated environmental crises and extreme heat, may have delayed recovery, especially for vulnerable species.

By the Middle Triassic, most marine communities had recovered fully. However, global marine diversity reached pre-extinction levels only by the Middle Jurassic, about 75 million years after the extinction.

Before the extinction, about two-thirds of marine animals were attached to the seafloor. During the Mesozoic, only about half of marine animals were attached, with the rest being mobile. Fossil records show fewer attached filter feeders like brachiopods and sea lilies, and more mobile species like snails, sea urchins, and crabs. Before the Permian extinction, simple and complex ecosystems were equally common. After recovery, complex ecosystems became far more common, leading to the Mesozoic Marine Revolution, driven by increased predation and specialized feeding.

Marine vertebrates recovered quickly, showing complex predator-prey relationships. Fossils from 5 million years after the extinction show vertebrates at the top of the food chain. Some fish species, like hybodonts, had extremely fast tooth replacement. Ichthyopterygians grew rapidly after the extinction.

Bivalves quickly returned to many marine environments after the extinction. Before the Permian-Triassic extinction, bivalves were rare but became common and diverse in the Triassic, taking over roles previously filled by brachiopods. While bivalves were once thought to have outcompeted brachiopods, this idea remains under study.

Hypotheses about cause

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

Despite these challenges, scientists have collected a lot of evidence about what caused this event. Different theories have been proposed, including both sudden, large-scale events and slower, long-term changes, similar to those thought to have caused the Cretaceous–Paleogene extinction event, although there is less agreement about this event.

• The sudden events include one or more large meteorite impacts, increased volcanic activity, and the sudden release of methane from the ocean floor. This could happen through the breakdown of methane ice deposits or the activity of methane-producing microbes.
• The slow changes include changes in sea level, increasing oxygen shortages in the water, and increasing dryness on land.

Any theory about the cause must explain why this event affected organisms with calcium carbonate skeletons the most. It must also explain why it took 4 to 6 million years before life started to recover, and why there was very little formation of biological minerals, even though inorganic carbonates were deposited once recovery began.

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, an area about the size of Saudi Arabia, and caused a major environmental impact. The timing of the Siberian Traps eruptions matches closely with the extinction event. Studies of the Norilsk and Maymecha-Kotuy regions in northern Siberia suggest that the 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 is estimated to be five times faster than during the previous catastrophic Capitanian mass extinction, which was caused by the Emeishan Traps. One estimate suggests that carbon dioxide levels increased from between 500 and 4,000 parts per million before the extinction to about 8,000 parts per million afterward. Another study estimated that carbon dioxide levels were about 400 parts per million before the extinction, and then rose to 2,500 parts per million. This would have added between 3,900 and 12,000 gigatonnes of carbon to the ocean and atmosphere. This would have caused extreme global warming, although some evidence suggests there was a delay of 12,000 to 128,000 years between the rise in carbon dioxide and the warming. This delay might be due to errors in the dating of fossils. Before the extinction, the average global surface temperature was about 18.2 degrees Celsius, but it increased to as high as 35 degrees Celsius, with this extreme heat lasting up to 500,000 years. In the high southern latitudes of Gondwana, temperatures rose by about 10 to 14 degrees Celsius. In South China, oxygen isotope shifts in conodont apatite suggest that surface water temperatures in the tropics increased 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. The increased temperatures also caused stronger El Niño events, leading to more 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 less effective at removing carbon from the atmosphere. In a 2020 study, scientists used a biogeochemical model to reconstruct the mechanisms that led to the extinction event. They showed the effects of the greenhouse effect on the marine environment 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 the presence of coronene and mercury spikes that match widespread mercury anomalies and the rise in isotopically light carbon. Te/Th values increased twentyfold during the Permian-Triassic Mass Extinction, indicating that this was linked to extreme volcanism. A major influx of isotopically light zinc from the Siberian Traps has also been recorded, confirming that volcanism occurred at the same time as the Permian-Triassic Mass Extinction.

The Siberian Traps eruptions had some unusual features that made them even more dangerous. The lithosphere in Siberia is rich in halogens, which are very harmful to the ozone layer. Evidence from xenoliths in the subcontinental lithosphere suggests that up to 70% of the halogen content was released into the atmosphere. Around 18 teratonnes of hydrochloric acid were emitted, along with sulfur-rich gases that caused dust clouds and acid aerosols. These would have blocked sunlight and disrupted photosynthesis on land and in the ocean's photic zone, causing food chains to collapse. These volcanic eruptions also caused brief but severe global cooling, which led to a drop in sea levels. However, these cold periods were too short to be a major cause of the extinction.

The eruptions may have also caused acid rain as the aerosols 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 fluid, low-viscosity lava that does not throw debris into the atmosphere. However, about 20% of the output from the Siberian Traps eruptions was pyroclastic ash that was thrown high into the atmosphere, increasing the short-term cooling effect. Once this ash had washed out of the atmosphere, the excess carbon dioxide would have remained, leading to continued global warming.

The burning of hydrocarbon deposits may have made the extinction even worse. The Siberian Traps are located over thick layers of carbonate and evaporite deposits from the Early-Mid Paleozoic and Carboniferous-Permian periods, as well as coal-bearing rocks. When heated by igneous intrusions, these rocks may have released large amounts of greenhouse and toxic gases. The unique position of the Siberian Traps over these deposits likely explains the severity of the extinction. Basalt lava erupted or intruded into carbonate rocks and sediments that were forming large coal beds, releasing large amounts of carbon dioxide and causing stronger global warming after the dust and aerosols settled. The change in the eruptions from flood basalt to sill-dominated emplacement, which released even more trapped hydrocarbon deposits, coincides with the main start of the extinction and is linked to a major negative δC excursion. The moderate temperature of the Siberian Traps magma optimized the large release of carbon dioxide by heating evaporites and carbonates.

The release of methane from coal was accompanied by the explosive burning of coal and the discharge of coal fly 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: "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." Grasby said, "In addition to these volcanoes causing fires through coal, the ash it spewed was highly toxic and was released in the land and water, potentially contributing to the worst extinction event in Earth history." However, some researchers suggest that these supposed fly ashes were actually the result of wildfires.

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 rapid increases in carbon dioxide levels. Although current greenhouse gas emissions are much faster than those during the PTME, scientists are unsure about the exact timing of greenhouse gas release during the PTME. It is likely that these emissions occurred in short, intense bursts rather than continuously over time. Within these bursts, the rate of carbon release may have been similar to modern human-caused emissions.

Today’s oceans, like those during the PTME, are experiencing lower pH levels and less oxygen, which strengthens the connection between these two events. If carbon dioxide levels continue to rise, scientists predict another event similar to the PTME, where marine life with shells, such as corals and shellfish, could face severe challenges. 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 urgently needed to avoid a disaster like the PTME.

As during the PTME, today’s oceans are undergoing major changes, including drops in pH and oxygen levels. This link is highlighted by geologist Lee Kump:

If carbon dioxide levels keep rising, it could lead to another crisis for marine life with shells, as seen in the fossil record. This would cause serious harm to modern ocean ecosystems.