The Permian–Triassic extinction event, commonly known as the Great Dying, was a major extinction event that 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, causing 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 extreme of the "Big Five" mass extinctions during the Phanerozoic era. Evidence suggests one to three distinct phases of extinction, with the main marine extinction occurring over a 60,000- to 100,000-year period around 251.902 million years ago, at the boundary between the Permian and Triassic periods. Some scientists debate whether the extinctions on land happened at the same time as the main marine extinction event.
Most scientists agree that the primary cause was massive volcanic eruptions in 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 ocean acidification. Atmospheric carbon dioxide levels increased from about 400 parts per million to 2,500 parts per million during this time, with approximately 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 triggered by the eruptions, methane emissions from the breakdown of methane clathrates, methane produced by new types of microorganisms fueled by minerals from the eruptions, stronger and longer El Niño events, and an extraterrestrial impact that created the Araguainha crater. This impact may have caused the release of methane, damaged the ozone layer, and increased exposure to solar radiation.
Dating
Previously, scientists believed that rock layers from the Permian–Triassic boundary were too few and incomplete to study in detail. However, it is now possible to determine the timing of the extinction with great accuracy. Dates from volcanic ash layers in Meishan, China, provide a detailed timeline for the extinction, helping 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 251.941 ± 0.037 and 251.880 ± 0.031 million years ago, lasting about 60,000 years. A sudden drop in the ratio of carbon-13 to carbon-12 in the environment coincided with the extinction. This change, called a "negative carbon isotope excursion," was 4–7% in size and lasted about 500,000 years. However, measuring its exact size is difficult because many rock layers near the boundary have changed over time.
Evidence suggests a global temperature increase of 8 °C (14 °F) and CO₂ levels rising to 2,500 ppm. For comparison, CO₂ levels before the Industrial Revolution were 280 ppm, and today they are about 426 ppm. There is also evidence that more ultraviolet radiation reached Earth, causing mutations in plant spores.
Some scientists think the Permian–Triassic boundary is marked by a sudden increase in fungi, which may have 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 radiometric methods. However, others argue that the fungal spike may not be unique to the boundary, as it might have occurred multiple times after the extinction. Some evidence now supports that the most common fungal spore, Reduviasporonites, may have come from algae rather than fungi.
Scientists are still unsure about the exact duration of the extinction and when different groups of animals and plants disappeared. Some studies suggest the extinction happened in one major event, while others found two separate waves of extinction with different causes. For example, ostracods and brachiopods went extinct about 670,000 to 1.17 million years apart. Studies of rock layers in China and Australia show that marine and land extinctions may have started at the same time or at different times. In some places, plants took longer to show the full effects of the extinction than animals did.
The timing of the Permian–Triassic extinction is also complicated by an earlier event called the Capitanian extinction, which happened just before the Permian–Triassic extinction. Some species that survived the Capitanian extinction may have been affected again during the Permian–Triassic event. Some scientists believe the Capitanian and Permian–Triassic extinctions were part of a single, long event, while others think they were separate. For example, many large animals and marine species died out at the end of the Capitanian period, which may have been a separate extinction event.
Extinction patterns
Marine invertebrates had the most losses during the P–Tr extinction. Earlier estimates of 90–96% marine species extinction were mixed up 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 in the last two sedimentary layers containing Permian conodonts. The drop in diversity was likely caused by a sudden rise in extinctions, not fewer new species forming.
The extinction mainly affected organisms with calcium carbonate skeletons, especially those needing stable CO₂ levels to build them. These organisms were harmed by ocean acidification from high atmospheric CO₂. Organisms using hemoglobin or hemocyanin for oxygen transport were more resistant to extinction than those using hemerythrin or oxygen diffusion. Endemism, or being limited to a specific region, made some species more likely to go extinct. Bivalve species that lived only in one area were more likely to die out than those found worldwide. Survival rates did not differ much by latitude. Organisms in areas less affected by global warming had fewer or delayed extinctions.
Among benthic organisms, the extinction increased background extinction rates, causing the most species loss to groups already prone to high background extinction. Marine extinction rates were extremely high. Bioturbators, organisms that mix sediment, were severely affected, as shown by the loss 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 slowly declined since the P–Tr extinction; the Ceratitida order of ammonites; and crinoids ("sea lilies"), which nearly went extinct but later thrived. Groups with high survival rates usually had active circulation control, complex gas exchange systems, and light calcification. More heavily calcified organisms with simpler breathing systems lost the most diversity. Surviving brachiopods were often small and rare compared to their former diverse communities.
Conodonts suffered major losses in both species and physical traits, though not as badly as during the Capitanian mass extinction.
Ammonoids, which had been declining for 30 million years since the Roadian (middle Permian), faced a major extinction 10 million years before the main event, at the end of the Capitanian stage. This early extinction greatly reduced the variety of ecological roles. Diversity and variety dropped further until the P–Tr boundary, where the extinction was not selective, suggesting a catastrophic cause. During the Triassic, diversity increased quickly, but variety stayed low. Ammonoid shapes became more limited as the Permian ended. A few million years into the Triassic, their original range of forms was reoccupied, but the traits were shared differently among groups.
Ostracods had long-term diversity issues during the Changhsingian before the P–Tr extinction, when most of them vanished suddenly. At least 74% of ostracods died during the P–Tr extinction itself.
Bryozoans had been declining throughout the Late Permian before suffering even greater losses during the P–Tr extinction, making them the most affected lophophorate group.
Deep-water sponges lost much diversity and had smaller spicules during the P–Tr extinction. Shallow-water sponges were less affected, with larger spicules and less loss of physical diversity.
Foraminifera faced a severe drop in diversity. Evidence from South China shows their extinction had two stages. Foraminiferal diversity 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. Lagenides may have survived due to their greater environmental tolerance and wider geographic range compared to Fusulinida.
Cladodontomorph sharks likely survived by living in deep ocean refuges, as shown by Early Cretaceous fossils found in deep, outer shelf environments. Ichthyosaurs, which evolved just before the P–Tr extinction, also survived.
The Lilliput effect, where species shrink during and after mass extinctions, was seen across the P–Tr boundary in foraminifera, brachiopods, bivalves, and ostracods. Though surviving gastropods were smaller, it is unclear if the Lilliput effect applied to them. Some gastropod groups, called "Gulliver gastropods," grew larger, showing the opposite effect, called the Brobdingnag effect.
The Permian had many insect and invertebrate species, including the largest insects ever. The end-Permian extinction was the largest known for insects, with eight to nine orders going extinct and ten others losing diversity. Palaeodictyopteroids (insects with piercing mouthparts) declined during the mid-Permian, linked to changes in plant life. The worst decline happened in the Late Permian, likely not caused directly by weather-related plant changes. Some insect declines were due to geographic shifts, not full extinctions.
Terrestrial plant records are limited, relying mostly on pollen and spore studies. Plant changes across the P–Tr boundary vary by location and preservation. Plants are less affected by mass extinctions, with impacts "insignificant" at the family level. Plant diversity loss was less severe than in marine animals. A 50% drop in species diversity may be due to fossilization processes. However, ecosystems changed greatly, with forests nearly disappearing. Dominant plant groups like Cordaites (gymnosperms) and Glossopteris (seed ferns) declined sharply. The extent of plant extinction remains debated.
The Glossopteris-dominated forests of high-latitude Gondwana collapsed in Australia about 370,000 years before the P–Tr boundary. This collapse was less clear in western Gondwana.
Biotic recovery
After the extinction event, the way ecosystems are structured today came from the species that survived. In the ocean, older groups of marine life, like certain types of shell-covered animals, decreased in number, while newer groups, such as clams, snails, sea urchins, and crustaceans, became more common. This shift started after a major extinction event and became more noticeable during the Late Jurassic period. On land, dinosaurs and mammals began to appear during the Triassic period. The change in the types of species present was partly because some groups, like brachiopods, were more affected by the extinction than others, like clams. However, recovery also varied among species. Some species that survived eventually went extinct without growing in numbers again, while others became dominant over time, like clams.
Immediately after the end-Permian extinction, a period of widespread species spread began. Marine life after the extinction was mostly made up of very few types of species, dominated by disaster species such as certain clams, 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 after the extinction. Some say it took about 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 in Idaho and Nevada showed that a marine ecosystem in the Early Triassic recovered much faster, taking about 1.3 million years. In Italy, complex fossilized tracks and burrows appeared less than a million years after the extinction. In China, the Guiyang biota and the Shanggan fauna show that life was thriving in some areas just a million years after the extinction. Other sites, like the Wangmo biota in Guizhou and a gastropod group in Oman, also show signs of recovery. These differences suggest that the extinction's effects varied by region, with some areas recovering faster due to better environmental conditions. High-latitude ecosystems may have recovered faster because of higher productivity after the extinction. While the number of species increased quickly, the variety of ecological roles took much longer to return to pre-extinction levels. One study found that marine ecosystems were still recovering 50 million years after the extinction, even though species diversity had rebounded much sooner.
The speed and timing of recovery also depended on the type of organism and its lifestyle. Communities on the seafloor 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 within the sediment. This slow recovery contrasts with the quick recovery of free-swimming organisms like ammonoids, which had more diversity than before two million years after the extinction, and conodonts, which diversified rapidly in the Early Triassic.
Recent research suggests that recovery speed was influenced by how much competition existed between species, which affects how quickly new species form. The slow recovery in the Early Triassic can be explained by low competition due to fewer species, while the faster recovery in the Anisian period was due to increased competition as niches filled up. This created a cycle where more biodiversity led to even more biodiversity. In seafloor communities, where many species are primary consumers, low competition meant slower diversification. In contrast, free-swimming predators and secondary consumers faced high competition, leading to faster diversification. Other explanations suggest that recovery was delayed because harsh conditions returned repeatedly during the Early Triassic, causing further extinctions like the Smithian-Spathian boundary event. Extremely hot climates during this time may have harmed ocean life, especially species sensitive to high carbon dioxide levels. The slower recovery of seafloor organisms was once thought to be due to widespread oxygen-poor conditions, but the presence of many benthic species contradicts this. A 2019 study found that differences in recovery times among ecosystems were due to varying environmental stress after the extinction, with areas facing long-term stress recovering more slowly. These repeated environmental problems limited the complexity of marine ecosystems until the Spathian period. Recovery ecosystems remained uneven and unstable into the Anisian, making them vulnerable to further stress.
Most marine communities were fully recovered by the Middle Triassic, but global marine diversity did not reach pre-extinction levels until the Middle Jurassic, about 75 million years after the extinction.
Before the extinction, about two-thirds of marine animals were attached to the seafloor and did not move. During the Mesozoic era, only about half of marine animals were attached, while the rest were free-moving. Fossil records show a decrease in the number of attached, filter-feeding animals like brachiopods and sea lilies and an increase in more complex, mobile species like snails, sea urchins, and crabs. Before the Permian extinction, both simple and complex marine ecosystems were equally common. After the extinction, complex ecosystems became much more common than simple ones, with complex communities outnumbering simple ones by about three to one. Increased predation and the ability to crush hard shells led to the Mesozoic Marine Revolution.
Marine vertebrates recovered quickly, showing complex predator-prey relationships, with vertebrates at the top of the food chain, as shown by fossilized feces from 5 million years after the extinction. Some fish species, like hybodonts, replaced their teeth very quickly after the extinction. Ichthyopterygians, a group of marine reptiles, grew rapidly in size after the extinction.
Clams quickly returned to many marine environments after the extinction. Before the Permian-Triassic extinction, clams were rare, but they became numerous and diverse in the Triassic, taking over roles previously held by brachiopods. It was once thought that clams outcompeted brachiopods, but this idea has been questioned.
Hypotheses about cause
Explaining events from 250 million years ago is very hard because much of the evidence on land has been worn away or buried deep underground. The ocean floor spreads and is recycled over 200 million years, so it leaves little useful information about the past.
Scientists have still found strong evidence about what caused the event. Some theories suggest sudden, large changes, while others suggest slower processes. These ideas are similar to those for the Cretaceous–Paleogene extinction event, but scientists are not as sure about the causes here.
Any explanation must account for why certain animals were most affected, especially those with calcium carbonate skeletons. The event lasted 4 to 6 million years before life began to recover. Also, even though inorganic carbonates were deposited after the event, there was very little biological mineralization.
The flood basalt eruptions that formed the Siberian Traps were among the largest volcanic events in Earth’s history. They covered an area about the size of Saudi Arabia, or 2,000,000 square kilometers. These eruptions happened in a few large bursts of magma, not in steady flows. The timing of these eruptions matches the extinction event.
The Siberian Traps caused one of the fastest increases in atmospheric carbon dioxide levels in Earth’s history. Scientists estimate that carbon dioxide emissions during this time were five times faster than during a previous mass extinction. One study suggests carbon dioxide levels rose from 500 to 4,000 ppm before the extinction to about 8,000 ppm afterward. Another study estimates carbon dioxide levels were 400 ppm before the event, rising to 2,500 ppm. This would have caused extreme global warming. Some evidence shows a delay of 12,000 to 128,000 years between the rise in carbon dioxide and global warming.
Before the extinction, global temperatures were about 18.2°C. They rose to as high as 35°C, with this extreme heat lasting up to 500,000 years. In the southern part of the supercontinent Gondwana, temperatures increased by 10–14°C. In South China, ocean surface temperatures rose by about 8°C. In present-day Iran, tropical ocean temperatures increased from 27–33°C to over 35°C. These high temperatures caused stronger El Niño events, making climate changes more unpredictable.
High carbon dioxide levels lasted for a long time. The position of the supercontinent Pangaea made it harder for Earth’s systems to remove carbon from the atmosphere. A 2020 study used a model to show how volcanic carbon dioxide emissions led to the extinction. Evidence also suggests that volcanic activity burned underground fossil fuels, as shown by spikes in mercury and carbon isotopes. Scientists found evidence of large amounts of isotopically light zinc from the Siberian Traps, linking volcanism to the extinction.
The Siberian Traps had features that made them more dangerous. The region had high levels of halogens, which destroy the ozone layer. Evidence shows that up to 70% of these halogens were released into the atmosphere. This would have caused acid rain and blocked sunlight, harming plants and ocean life. Volcanic eruptions also released sulfur, creating dust and acid aerosols that caused short-term global cooling. However, these cooling events were too brief to be the main cause of the extinction.
The eruptions may have also caused acid rain, which could have killed plants and marine life with calcium carbonate shells. While flood basalts usually produce smooth lava, 20% of the Siberian Traps eruptions sent pyroclastic ash high into the atmosphere. This ash increased short-term cooling, but once it settled, carbon dioxide levels rose, leading to long-term warming.
The Siberian Traps were located over layers of ancient carbonate, evaporite, and coal deposits. When these rocks were heated by volcanic activity, they released large amounts of greenhouse gases and toxic chemicals. This likely made the extinction more severe. The way the eruptions changed from flood basalt to sills (horizontal rock layers) released more trapped carbon, linking this to the extinction.
A 2011 study found evidence that volcanic activity ignited massive coal deposits, possibly releasing over 3 trillion tons of carbon. Ash deposits near the Buchanan Lake Formation suggest that toxic elements were released into water bodies. However, some scientists argue that these ashes may have come from wildfires unrelated to coal burning. A 2013 study estimated that the Siberian Traps released large amounts of carbon dioxide, carbon monoxide, hydrogen sulfide, and sulfur dioxide.
The way the Siberian Traps spread as sills prolonged their warming effects, contributing to the long-lasting changes in Earth’s climate.
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 three involve rapid increases in carbon dioxide levels. Although current greenhouse gas emissions are more than ten times faster than those during the PTME, the timing and pattern of carbon release during the PTME are not well understood. Scientists believe that carbon was likely released in short, sudden bursts rather than continuously over time. Within these bursts, the rate of carbon release may have been similar to today’s human-caused emissions.
Today’s oceans, like those during the PTME, are experiencing lower pH levels and reduced oxygen, 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 struggles to form shells and skeletons—may occur, causing serious harm to modern ocean ecosystems. 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, scientists warn that reducing carbon dioxide emissions is urgently needed to avoid a disaster like the PTME.
As during the PTME, today’s oceans are changing rapidly, with falling pH and oxygen levels. This is highlighted by geologist Lee Kump:
The Permian-Triassic mass extinction shows the dangers of fast carbon dioxide emissions. Volcanic activity during the PTME released huge amounts of CO₂, causing ocean acidification, loss of oxygen, and widespread ecosystem collapse. Today, human activities are causing similar changes, but even faster. Geological records show that once these critical thresholds are crossed, the long-term effects on ecosystems can last millions of years.
If carbon dioxide levels keep rising, another biocalcification crisis—like the one seen in the fossil record—could occur, leading to severe damage to modern marine ecosystems.