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 known 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 in Earth's history. This event is the most severe of the "Big Five" major extinctions that occurred during the Phanerozoic eon. Evidence suggests there were one to three separate phases of extinction. The largest marine extinction happened over a 60,000- to 100,000-year period around 251.902 million years ago, marking the boundary between the Permian and Triassic periods. Some scientists debate whether the extinction on land happened at the same time as the main marine extinction.
Scientists believe the main cause of the extinction was massive volcanic eruptions in an area called the Siberian Traps. These eruptions released large amounts of sulfur dioxide and carbon dioxide into the atmosphere. This 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 3,900 to 12,000 gigatonnes of carbon entering the ocean and atmosphere.
Other possible causes include carbon dioxide released from burning oil and coal deposits caused by the eruptions, methane gas released from ice-like structures called methane clathrates, methane produced by new types of microorganisms that fed on minerals from the eruptions, stronger and longer El Niño weather patterns, and an impact from an object from space that created the Araguainha crater. This impact may have released methane and damaged the ozone layer, increasing exposure to sunlight.
Dating
Scientists used to believe 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 time of the extinction with great accuracy. Uranium-lead dating of zircon crystals from five volcanic ash layers at the Meishan Global Stratotype Section and Point in China has created a detailed timeline for the extinction. This timeline helps scientists study how changes in the environment, the carbon cycle, and life on Earth were connected 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 Earth’s rocks, called a negative carbon isotope excursion, happened during this time. This change is often used to identify the Permian–Triassic boundary in rocks that cannot be dated using other methods. The carbon change was between 4% and 7% and lasted about 500,000 years. However, scientists struggle to measure it exactly because changes in sediment layers over time have altered the evidence.
Evidence suggests Earth’s temperature rose by about 8°C (14°F), and carbon dioxide levels reached 2,500 ppm. For comparison, carbon dioxide levels were 280 ppm before the Industrial Revolution and are now about 426 ppm. Scientists also found signs of increased ultraviolet radiation, which may have caused mutations in plant spores.
Some scientists believe the Permian–Triassic boundary is marked by a sudden increase in fungi, which may have grown because of the large amount of dead plants and animals available to feed on. This "fungal spike" has been used to identify the boundary in rocks that cannot be dated or lack clear fossils. However, others argue that the fungal spike may not have been unique to this time, as similar patterns could have occurred in the early Triassic. Some research now supports that the most common fungal spore, Reduviasporonites, may have come from algae rather than fungi.
Scientists are still unsure how long the overall extinction lasted or how long it took for different groups of animals and plants to go extinct. Some evidence suggests the extinction happened in several waves over millions of years, while other studies show a single major event. Research from China’s Meishan area suggests the main extinction happened quickly, but other studies from different locations show two separate extinction events with different causes. For example, ostracod and brachiopod extinctions may have occurred up to 1.17 million years apart. Studies of rock layers in Australia suggest repeated environmental stress in the oceans before the extinction, supporting the idea that the extinction was gradual.
It is unclear whether the extinction of land and ocean life happened at the same time or at different times. Some evidence from Greenland shows both types of life declined together, but plants took longer to be affected. Other studies from China and Australia suggest land and ocean extinctions happened at the same time. However, other research shows land life may have started declining up to 370,000 years before the ocean life declined.
The timing and causes of the Permian–Triassic extinction are complicated by earlier extinctions, such as the Capitanian extinction, which occurred just before the Permian–Triassic event. Some scientists believe the Capitanian extinction may have ended before the Permian–Triassic event, but it is unclear if species that survived earlier extinctions had recovered enough to be considered separate from the Permian–Triassic event. Some theories suggest the extinction was a single long event, while others argue it had two major waves 9.4 million years apart. For example, many species of large animals and marine life disappeared during the Capitanian extinction, which may have been less severe than the later Permian–Triassic event.
Extinction patterns
During the Permian-Triassic extinction (P–Tr), marine invertebrates experienced the most severe losses. Earlier estimates of 90–96% marine species extinction were incorrect because they mixed up the P–Tr event with a similar extinction that happened 7–10 million years earlier, called the end-Capitanian mass extinction. Evidence from south China shows that 286 of 329 marine invertebrate genera disappeared in the last two sedimentary layers of the Permian period. This drop in diversity likely happened because of a sudden rise in extinctions, not because fewer new species formed.
Organisms with calcium carbonate skeletons, especially those relying on stable CO₂ levels to build them, were most affected. These species were vulnerable to ocean acidification caused by high atmospheric CO₂ levels. Organisms using hemocyanin or hemoglobin for oxygen transport were more likely to survive than those using hemerythrin or oxygen diffusion. Species that lived only in specific areas (endemic species) were more likely to go extinct than those found in many regions (cosmopolitan species). Survival rates did not differ much between regions. Organisms in areas less affected by global warming experienced less severe or delayed extinctions.
The extinction increased background extinction rates for benthic (bottom-dwelling) organisms, causing the greatest losses among species already prone to extinction. Marine organisms faced a very severe extinction event. Bioturbators, which mix sediments, were severely affected, as shown by the loss of mixed sediment layers in many marine environments.
Surviving marine invertebrates included articulate brachiopods (with hinged shells), which had declined since the P–Tr event; the Ceratitida order of ammonites; and crinoids ("sea lilies"), which nearly went extinct but later thrived. Groups with high survival rates had efficient circulation systems, complex gas exchange, and light calcification. Heavily calcified species with simple 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 extinction. Ammonoids, which had declined for 30 million years, faced a major extinction 10 million years before the P–Tr event. This early extinction reduced the variety of ecological roles they played. Diversity and variety dropped further until the P–Tr event, which caused a non-selective extinction, likely due to a sudden disaster. In the Triassic period, diversity increased quickly, but variety remained low. Ammonoid shapes became more limited as the Permian ended. By the early Triassic, their range of forms was restored, but different groups shared these traits differently.
Ostracods faced long-term diversity changes before the P–Tr event, with most species vanishing suddenly. At least 74% of ostracods died during the P–Tr event. Bryozoans, which had already declined in the Late Permian, suffered even greater losses during the P–Tr event, becoming the most affected group among lophophorates.
Deep-water sponges lost diversity and had smaller spicules (skeletal structures) during the P–Tr event. Shallow-water sponges were less affected, with larger spicules and less loss of diversity. Foraminifera faced a severe drop in diversity. Evidence from South China shows two waves of extinction. Foraminifera diversity hotspots shifted to deeper waters. About 93% of Permian foraminifera went extinct, including 100% of Miliolida, 96% of Fusulinida, 92% of Lagenida, and 50% of Textulariina. Calcified foraminifera had a 91% extinction rate. Lagenida may have survived because they could live in more environments and had a wider geographic range than Fusulinida.
Cladodontomorph sharks likely survived in deep-ocean refuges, as shown by their presence in deep-water areas in the Early Cretaceous. Ichthyosaurs, which evolved just before the P–Tr event, also survived.
The "Lilliput effect," where species shrink during and after mass extinctions, was seen in foraminifera, brachiopods, bivalves, and ostracods. Some surviving gastropods were smaller, but others, called "Gulliver gastropods," grew larger, showing the opposite of the Lilliput effect, called the Brobdingnag effect.
Insects and other invertebrates had high diversity in the Permian, including the largest insects ever. The end-Permian event was the largest known mass extinction for insects, with eight to nine orders going extinct and ten others losing diversity. Palaeodictyopteroids (insects with piercing mouthparts) declined in the mid-Permian, possibly due to changes in plant life. The greatest decline happened in the Late Permian and was not directly caused by weather-related plant changes. Some insect declines were due to changes in geographic distribution, not outright extinction.
Terrestrial plant records are limited, relying mostly on pollen and spores. Plant changes across the P–Tr boundary vary by location and preservation. Plants are less affected by mass extinctions, with little impact at the family level. Plant diversity losses were less severe than marine losses. A 50% reduction in species diversity may be due to preservation 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 is debated.
The Glossopteris-dominated forests in high-latitude Gondwana collapsed in Australia 370,000 years before the P–Tr boundary. This decline was less severe 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 sea animals, called the "Paleozoic evolutionary fauna," became less common, while newer groups, called the "modern evolutionary fauna," became more dominant. This change started after a major extinction event called the Capitanian mass extinction and reached its peak during the Late Jurassic. Typical sea creatures in the ocean floor included clams, 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 big change in the types of species was partly because some species were more affected by the extinction than others, such as brachiopods being hit harder than clams. However, recovery also varied: some species that survived eventually went extinct without diversifying again, while others became dominant over time, like clams.
Immediately after the end-Permian extinction, a time of widespread species spread began. The marine life that followed the extinction had few species and was dominated by a few types of disaster species, such as clams like Claraia, Unionites, Eumorphotis, and Promyalina, conodonts like Clarkina and Hindeodus, brachiopods like Lingularia, and foraminifera like Earlandia and Rectocornuspira kalhori (sometimes called Ammodiscus). These groups had low diversity, and their variety across different latitudes was very small.
Scientists disagree about how quickly life recovered after the extinction. Some say it took 10 million years until the Middle Triassic because the extinction was so severe. However, studies in Bear Lake County, Idaho, and nearby areas showed that a marine ecosystem in the Early Triassic recovered quickly, taking about 1.3 million years. In Italy, a complex group of trace fossils appeared less than a million years after the extinction. In China, the Guiyang biota and Shanggan fauna show that life was already thriving in some areas just a million years after the extinction. Similarly, fossil groups in Guizhou and Oman also show life returning quickly. Differences in recovery speed across regions suggest that the extinction's effects were not the same everywhere, with some areas recovering faster due to less environmental stress. High latitude ecosystems may have recovered faster because of higher productivity after the extinction. Some studies say that while the number of species increased quickly, it took much longer for ecosystems to return to their pre-extinction complexity, with marine recovery still ongoing 50 million years after the extinction during the latest Triassic.
Recovery speed also varied by group and lifestyle. Seafloor communities had low diversity until the end of the Early Triassic, about 4 million years after the extinction. Surface-dwelling sea creatures took longer to recover than those living in the seafloor. This slow recovery contrasts with the quick recovery of floating animals like ammonoids, which had more species than before two million years after the extinction, and conodonts, which diversified rapidly in the Early Triassic.
Recent research suggests recovery was driven by competition 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, called niche crowding. This created a cycle where more diversity led to even more diversity. In seafloor communities dominated by primary consumers, low competition meant slower diversification, while in floating communities with secondary and tertiary consumers, high competition led to faster diversification. Some scientists say recovery was delayed because harsh conditions returned repeatedly during the Early Triassic, causing more extinctions like the Smithian-Spathian boundary extinction. Extremely hot weather during the Early Triassic may have slowed recovery, especially for species vulnerable to high carbon dioxide levels. The slow recovery of seafloor species was once blamed on oxygen-poor waters, but the presence of many seafloor species contradicts this. A 2019 study said differences in recovery times were due to local environmental stress after the extinction, with areas facing ongoing stress recovering more slowly. Repeated environmental problems in the Early Triassic limited the complexity of marine ecosystems until the Spathian. Recovery ecosystems remained unstable and uneven into the Anisian, making them vulnerable to stress.
Most marine communities were fully recovered by the Middle Triassic, but global marine diversity reached pre-extinction levels no earlier than 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, while the rest were free-living. Fossil records show fewer attached filter-feeding animals like brachiopods and sea lilies, and more mobile species like snails, sea urchins, and crabs. Before the Permian extinction, simple and complex marine ecosystems were equally common. After recovery, complex ecosystems outnumbered simple ones by about three to one, and increased predation and shell-crushing behavior led to the Mesozoic Marine Revolution.
Marine vertebrates recovered quickly, showing complex predator-prey relationships, with top predators like fish, as shown by 5-million-year-old fossil feces. Fish called hybodonts had extremely fast tooth replacement after the extinction. Ichthyopterygians, a group of fish, grew rapidly in size after the extinction.
Clams quickly returned to many marine environments after the extinction. Clams were rare before the Permian-Triassic extinction but became common and diverse in the Triassic, taking over niches once filled by brachiopods. Clams were once thought to outcompete brachiopods, but this idea is now debated.
Hypotheses about cause
Explaining events that happened 250 million years ago is very difficult because much of the evidence on land has been worn away by weathering or buried deep underground. Meanwhile, the ocean floor spreads apart over time and is completely recycled every 200 million years, leaving no useful clues beneath the sea.
Scientists have still found strong evidence about what caused the event. Some theories suggest sudden, large changes, while others propose slower, long-term processes. These ideas are similar to those used to explain a different mass extinction event, but scientists are not as certain about the causes of this one.
Any explanation for the event must address three main facts: First, it severely affected organisms with calcium carbonate skeletons. Second, it took 4 to 6 million years before life began to recover. Third, even though large amounts of inorganic carbon were deposited in the environment after the event, biological minerals formed very slowly.
The Siberian Traps, a massive volcanic area, erupted in one of the largest volcanic events in Earth’s history. These eruptions covered about 2 million square kilometers (roughly the size of Saudi Arabia) with lava. The timing of these eruptions matches the mass extinction event. Studies of the Siberian region show that the volcanic activity happened in sudden, large bursts of magma rather than steady flows.
The eruptions caused a very rapid increase in atmospheric carbon dioxide levels. Scientists estimate that carbon dioxide emissions from the Siberian Traps were five times faster than those from a previous mass extinction event. Some studies suggest carbon dioxide levels rose from 500 to 4,000 parts per million before the event to about 8,000 parts per million afterward. Another study estimated 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 caused extreme global warming, though some evidence suggests a delay of 12,000 to 128,000 years between the rise in carbon dioxide and warming.
Before the extinction, global average surface temperatures were about 18.2°C. These temperatures rose to as high as 35°C and remained extremely hot for up to 500,000 years. In regions near the South Pole, temperatures increased by about 10–14°C. In South China, ocean surface temperatures rose by about 8°C. In present-day Iran, tropical sea temperatures jumped from 27–33°C to over 35°C. These high temperatures likely caused stronger El Niño events, increasing short-term climate changes.
The high levels of carbon dioxide lasted for a long time. At that time, the supercontinent Pangaea was positioned in a way that made it harder for the Earth’s natural systems to remove carbon from the atmosphere. A 2020 study used a model to show how volcanic carbon dioxide emissions caused the extinction. Evidence also suggests that volcanic activity burned underground fossil fuel deposits, as shown by spikes in certain chemicals like coronene and mercury. Other studies found large amounts of isotopically light zinc from the Siberian Traps, linking the eruptions to the extinction event.
The Siberian Traps had special features that made them especially dangerous. The region contained high levels of halogens, which can destroy the ozone layer. Evidence suggests up to 70% of these halogens were released into the atmosphere. This would have caused acid rain, blocked sunlight, and disrupted photosynthesis, harming plants and marine life. Volcanic eruptions also released sulfur-rich gases, creating dust clouds and acid aerosols that temporarily cooled the planet. However, these cooling effects were short-lived and unlikely to have caused the extinction.
The eruptions may have also released large amounts of carbon dioxide from underground coal and carbonate deposits. These deposits were heated by volcanic activity, releasing greenhouse gases and toxic fumes. The unique location of the Siberian Traps over these deposits likely worsened the extinction. The type of volcanic activity changed, releasing more trapped carbon and causing a major drop in carbon levels.
A 2011 study found evidence that volcanic eruptions ignited massive coal deposits, possibly releasing over 3 trillion tons of carbon. Ash deposits in deep rock layers suggest that toxic elements were released into water bodies. However, some scientists argue that these ash layers might have come from wildfires unrelated to the eruptions. A 2013 study estimated the total emissions from the Siberian Traps as 8.5 × 10 Tg CO₂, 4.4 × 10 Tg CO, 7.0 × 10 Tg H₂S, and 6.8 × 10 Tg SO₂.
The way the Siberian Traps erupted, with large amounts of magma intruding into rock layers, prolonged the warming effects of the event.
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 the fast release of carbon dioxide. Although the current rate of greenhouse gas emissions is much higher than the rate during the PTME, scientists believe the PTME’s carbon release likely occurred in short bursts over a few time periods, not continuously. The speed of carbon release during these bursts may have been similar to today’s human-caused emissions.
Today’s oceans, like those during the PTME, are experiencing lower pH levels and less oxygen, which strengthens the connection between the two events. Scientists warn that if carbon dioxide levels keep rising, another event similar to the PTME’s biocalcification crisis—where marine life with calcium-based shells and skeletons is severely affected—could happen again. Changes in how plants and insects interact during the PTME are also being studied as possible signs of future ecological changes.
Geologists emphasize that the PTME shows the dangers of rapid carbon dioxide emissions. During the PTME, volcanic activity released large amounts of carbon dioxide, causing ocean acidification, low oxygen levels, and major ecological damage. Today, human activities are causing similar changes, but even faster. The geological record shows that once ecosystems reach critical tipping points, the effects can last for millions of years. If carbon dioxide levels continue to rise, another biocalcification crisis, like the one seen in the fossil record, could harm modern marine ecosystems.