The Permian–Triassic extinction event, often called the Great Dying, was a major extinction event that happened near the end of the Permian period and the start of the Triassic period, marking the transition between the Paleozoic and Mesozoic eras. It is the most severe extinction event in Earth's history, 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 largest of the "Big Five" mass extinctions during the Phanerozoic eon. Evidence suggests there were 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. Scientists are still debating whether the extinction on land happened at the same time as the main marine extinction.
Most scientists believe the primary cause was massive volcanic eruptions in the Siberian Traps, which released large amounts of sulfur dioxide and carbon dioxide. These gases 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 added to the ocean and atmosphere. Other possible factors include carbon dioxide released from burning oil and coal deposits, methane emissions from melting methane clathrates, methane produced by new types of microorganisms, stronger and longer El Niño events, and a possible extraterrestrial impact that created the Araguainha crater, releasing methane and damaging the ozone layer, which increased exposure to sunlight.
Dating
Scientists once believed that rock layers from the time of the Permian–Triassic boundary were too few or incomplete to study accurately. However, new methods now allow scientists to date the extinction event with great precision. Using uranium-lead dating of zircon crystals from volcanic ash layers in Meishan, China, researchers created a detailed timeline for the extinction. This timeline helps scientists understand how global environmental changes, shifts in the carbon cycle, mass extinctions, and recovery processes occurred over thousands of years. The first appearance of the conodont fossil Hindeodus parvus is used to mark the Permian–Triassic boundary in rock layers.
The extinction event happened between 251.941 ± 0.037 and 251.880 ± 0.031 million years ago, lasting about 60,000 years. During this time, the ratio of carbon-13 to carbon-12 in Earth’s rocks dropped sharply by 4–7%. This change, called a "carbon isotope excursion," lasted around 500,000 years. However, measuring its exact size is difficult because some rock layers have changed over time.
Evidence also shows that Earth’s temperature rose by about 8°C (14°F), 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. Increased ultraviolet radiation may have caused mutations in plant spores.
Some scientists suggest that the Permian–Triassic boundary is marked by a sudden increase in fungi, which may have fed on the large amounts of dead plants and animals after the extinction. This "fungal spike" has been used to identify the boundary in rocks unsuitable for dating. However, others argue that the spike may not be unique to this event, as it could have occurred multiple times. Some research now supports that the most common fungal spore, Reduviasporonites, may have come from algae, not fungi.
Scientists still debate how long the extinction lasted and when different groups of life disappeared. Some studies suggest the extinction happened in one major event, while others find evidence of two separate waves. For example, in some areas, ostracods and brachiopods went extinct about 670,000 to 1.17 million years apart. In the Bowen Basin of Queensland, repeated periods of stress in marine environments before the end-Permian extinction support the idea that the extinction may have been gradual.
There is also debate about whether land and ocean extinctions happened at the same time. Some evidence from Greenland and South China suggests they occurred together, while other studies show land extinctions began earlier. For example, in the Karoo Basin, land animals declined before marine life. However, in some tropical regions, marine extinctions may have happened before land extinctions.
The timing of the Permian–Triassic extinction is further complicated by the Capitanian extinction, which occurred earlier in the Permian period. Some species thought to have gone extinct at the Permian–Triassic boundary are now believed to have died out during the Capitanian event. This makes it hard to determine whether the end-Permian extinction was a separate event or part of a longer process. Some scientists argue that the Capitanian and Permian–Triassic extinctions may have been a single, long event, while others believe they were distinct. For example, many large marine animals, like dinocephalians and certain foraminifera, disappeared at the end of the Capitanian, long before the Permian–Triassic extinction.
Extinction patterns
Marine invertebrates experienced the most severe losses during the P–Tr extinction. Earlier estimates of 90–96% marine species extinction were incorrect because they mixed up this event with the end-Capitanian mass extinction, which happened 7–10 million years earlier. Scientists found evidence of these losses in samples from south China near the P–Tr boundary. In these samples, 286 out of 329 marine invertebrate genera disappeared in 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 rising atmospheric CO₂ levels. Organisms using hemocyanin or hemoglobin to transport oxygen were more likely to survive than those using hemerythrin or oxygen diffusion. Evidence also shows that species found only in specific regions (endemic species) were more likely to go extinct than those found worldwide (cosmopolitan species). Survival rates did not differ much based on latitude. Organisms in areas less affected by global warming experienced fewer or delayed extinctions.
Among benthic organisms, the extinction event increased background extinction rates, causing the greatest loss for species with naturally high extinction rates (those that change frequently). The extinction rate for marine organisms was 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 (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 became abundant. Groups with high survival rates generally had good control over circulation, complex gas exchange systems, and light calcification. Heavily calcified organisms with simpler breathing systems suffered the most. Among brachiopods, surviving species were typically small and rare compared to their former diverse communities.
Conodonts faced severe losses in both species diversity 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), experienced a major extinction event 10 million years before the main P–Tr event, 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 shapes and structures among ammonoids became more limited as the Permian ended. A few million years into the Triassic, their original range of forms was reoccupied, but the distribution of these traits changed among different groups.
Ostracods faced long-term changes in diversity during the Changhsingian before the P–Tr extinction, with most of them vanishing 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 diversity and saw smaller spicules during the P–Tr extinction. Shallow-water sponges were less affected, showing larger spicules and less loss of physical diversity.
Foraminifera faced a severe drop in diversity. Evidence from South China shows their extinction occurred in two waves. Foraminifera diversity shifted to deeper waters during the P–Tr extinction. About 93% of Permian foraminifera went extinct, including 50% of Textulariina, 92% of Lagenida, 96% of Fusulinida, and 100% of Miliolida. Foraminifera with calcium carbonate shells had a 91% extinction rate. Lagenida may have survived because they had greater environmental tolerance and wider geographic ranges compared to Fusulinida.
Cladodontomorph sharks likely survived by living in deep ocean refuges, as shown by their presence 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. While surviving gastropods were smaller than those that died, it is unclear if the Lilliput effect applied to them. Some gastropod groups, called "Gulliver gastropods," grew larger after the extinction, showing the opposite of the Lilliput effect, known as the Brobdingnag effect.
The Permian had many insect and invertebrate species, 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 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, likely not directly caused by weather-related plant changes. Some insect declines during the P–Tr extinction were due to changes in distribution, not outright extinction.
The geological record of terrestrial plants is limited, relying mostly on pollen and spore studies. Plant changes near the Permian-Triassic boundary vary by location and preservation quality. Plants are generally less affected by mass extinctions, with little impact at the family level. Plant diversity losses were less severe than those of marine animals. A 50% drop in species diversity may be due to taphonomic processes (factors affecting fossil preservation). However, ecosystems changed dramatically, with forests nearly disappearing and dominant plant groups like Cordaites (gymnosperms) and Glossopteris (seed ferns) declining abruptly. The extent of plant extinction remains debated.
The Glossopteris-dominated flora in high-latitude Gondwana collapsed in Australia about 370,000 years before the P–Tr boundary. This decline was less severe in western Gondwana.
Biotic recovery
After the extinction event, the way life is organized today started to change based on the species that survived. In the ocean, the group of ancient sea creatures known as the "Paleozoic evolutionary fauna" became less common, while a new group called the "modern evolutionary fauna" became more dominant. This shift began after the Capitanian mass extinction and reached its peak during the Late Jurassic. Typical sea creatures living on the ocean floor included bivalves, snails, sea urchins, and Malacostraca. Meanwhile, bony fish and marine reptiles became more common in the open ocean. On land, dinosaurs and mammals first appeared during the Triassic period. The change in which species were present was partly because the extinction event affected some groups more than others. For example, brachiopods suffered more than bivalves. Some species that survived the extinction later went extinct without recovering, while others grew in number over time.
A time of widespread species spread began right after the end-Permian extinction. The marine life that appeared after the extinction had few species and was dominated by a small number of disaster species, such as certain bivalves, conodonts, brachiopods, and foraminifera. These groups had low diversity, and the variety of species across different latitudes was very low.
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 diverse and complex group of trace fossils appeared less than a million years after the extinction. In China, the Guiyang biota and the Shanggan fauna also show that life returned quickly in some areas. Differences in recovery speed suggest that the extinction’s effects were not the same everywhere. High-latitude ecosystems may have recovered faster because of higher productivity after the extinction. While species diversity rebounded quickly, it took much longer for ecosystems to return to their previous complexity. Some studies suggest marine ecosystems were still recovering 50 million years after the extinction, even though species diversity had returned in a tenth of that time.
Recovery speed varied based on the type of organism and their lifestyle. Communities on the seafloor remained diverse for a long time, with low diversity until the end of the Early Triassic, about 4 million years after the extinction. Epifaunal benthic organisms (those living on the seafloor) took longer to recover than infaunal benthic organisms (those living within the seafloor). In contrast, nektonic organisms like ammonoids and conodonts recovered quickly, reaching pre-extinction diversity levels within 2 million years.
Recent research suggests recovery was driven by competition between species. Low competition in the Early Triassic slowed recovery, but increased competition later sped it up. Biodiversity growth became a self-reinforcing process during the Spathian and Anisian periods. Slow recovery in seafloor communities, which are dominated by primary consumers, contrasts with faster recovery in nektonic communities, where secondary and tertiary consumers faced more competition. Other explanations suggest recovery was delayed by repeated environmental crises, such as the Smithian-Spathian boundary extinction, and extreme heat in the Early Triassic, which harmed skeletonized species. Some studies link the slow recovery of benthic organisms to widespread anoxia, but high benthic diversity contradicts this. A 2019 study suggests differences in recovery times were due to varying environmental stress in different regions. Recurring environmental problems limited the complexity of marine ecosystems until the Spathian. Recovery communities remained unstable and uneven until the Anisian.
Most marine communities 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 and did not move. During the Mesozoic, only about half of marine animals were sessile, while the rest were free-living. Fossil evidence shows a decline in sessile epifaunal suspension feeders 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 recovery, complex communities outnumbered simple ones by nearly three to one. Increased predation and durophagy (eating hard-shelled prey) led to the Mesozoic Marine Revolution.
Marine vertebrates recovered quickly, showing complex predator-prey interactions with vertebrates at the top of the food web, as shown by coprolites (fossilized feces) from 5 million years after the extinction. Post-extinction hybodont sharks had extremely fast tooth replacement. Ichthyopterygians (a group of marine reptiles) grew rapidly in size after the extinction.
Bivalves quickly returned to many marine environments after the extinction. Before the Permian-Triassic extinction, bivalves were rare but became numerous and diverse in the Triassic, taking over niches previously filled by brachiopods. Bivalves were once thought to have outcompeted brachiopods, but this is not fully supported by evidence.
Hypotheses about cause
Explaining an event that happened 250 million years ago is very difficult. Much of the evidence on land has been worn away by weathering, and evidence on the ocean floor has been completely recycled over time, leaving little to study.
Scientists have still found strong evidence about what caused this event. Some theories suggest sudden, large changes, while others suggest slower, long-term processes. These ideas are similar to those for another major extinction event, but scientists are not sure which theory is correct.
Any explanation must account for why some animals were more affected than others. Organisms with calcium carbonate skeletons were hit hardest. It also took 4 to 6 million years before life began to recover, and when it did, there was very little new biological material formed, even though inorganic carbon was deposited.
The Siberian Traps, a large volcanic area, erupted in a series of huge magma pulses. These eruptions covered about 2 million square kilometers, roughly the size of Saudi Arabia. The timing of these eruptions matches the extinction event. Studies of northern Siberia show that the eruptions happened in large, sudden bursts rather than steady flows.
The Siberian Traps caused a very rapid increase in atmospheric carbon dioxide levels. Scientists estimate that carbon dioxide emissions were five times faster than during a previous extinction event. Before the extinction, carbon dioxide levels were between 500 and 4,000 parts per million (ppm), and they may have risen to about 8,000 ppm afterward. Another study suggests 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 caused extreme global warming, though some evidence suggests a delay of 12,000 to 128,000 years between the start of the eruptions and the warming. This delay might be due to errors in dating.
Before the extinction, global average surface temperatures were about 18.2°C. These temperatures rose to as high as 35°C and stayed this way for up to 500,000 years. In the southern part of the supercontinent Gondwana, temperatures increased by about 10–14°C. In South China, water temperatures near the equator rose by about 8°C. In present-day Iran, tropical ocean temperatures jumped from 27–33°C to over 35°C during the extinction event. Higher temperatures also led to stronger El Niño events, making short-term climate changes more extreme.
High carbon dioxide levels lasted for a long time. The arrangement of the supercontinent Pangaea at the time made it harder for the Earth to remove carbon from the atmosphere. A 2020 study used a scientific 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 mercury and carbon. These findings support the idea that volcanism was closely linked to the extinction.
The Siberian Traps had special features that made them more dangerous. The area was rich in halogens, which can destroy the ozone layer. Studies show that up to 70% of these halogens were released into the atmosphere. This would have caused large amounts of hydrochloric acid and sulfur-rich gases to be released, forming dust clouds and acid rain. These effects would have blocked sunlight, harming plants and ocean life. The sulfur also caused short periods of global cooling, but these were too brief to explain the extinction.
The eruptions may have caused acid rain, which could have killed land plants and marine organisms with calcium carbonate shells. Flood basalts usually produce lava that flows easily and doesn’t send debris into the air. However, 20% of the Siberian Traps eruptions produced pyroclastic ash that rose high into the atmosphere, increasing short-term cooling. Once this ash settled, carbon dioxide levels would have risen again, causing uncontrolled global warming.
Burning of hydrocarbon deposits may have worsened the extinction. The Siberian Traps were located above layers of ancient carbonate, evaporite, and coal deposits. When these rocks were heated by volcanic activity, they released large amounts of greenhouse and toxic gases. The unique position of the Siberian Traps over these deposits likely made the extinction more severe. The eruptions changed from producing lava flows to forming thick layers of rock, releasing even more trapped hydrocarbons. This change coincided with the start of the extinction and a major drop in carbon levels.
The eruptions also released methane from coal deposits, which burned explosively, creating coal ash. A 2011 study found evidence that volcanic activity caused coal beds to ignite, possibly releasing over 3 trillion tons of carbon. Ash deposits near what is now the Buchanan Lake Formation suggest that volcanic eruptions spread toxic materials into water and land. However, some scientists argue that these ash deposits may have come from wildfires unrelated to volcanic activity. A 2013 study estimated that the Siberian Traps released 8.5 × 10 Tg of carbon dioxide, 4.4 × 10 Tg of carbon monoxide, 7.0 × 10 Tg of hydrogen sulfide, and 6.8 × 10 Tg of sulfur dioxide.
The way the Siberian Traps erupted, forming thick layers of rock, prolonged their warming effects.
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 the current rate of greenhouse gas emissions is much higher than the rate during the PTME, the timing and pattern of carbon release during the PTME are not fully understood. Scientists believe the carbon was released in short bursts over a few key periods, not continuously. Within these periods, the rate of carbon release may have been similar to today’s human-caused emissions.
Today’s oceans are experiencing similar changes to those during the PTME, including lower pH levels (ocean acidification) and reduced oxygen levels. These similarities have led scientists to compare modern ecological conditions with those of the PTME. If carbon dioxide levels continue to rise, another event similar to the biocalcification crisis seen in the fossil record could occur, which would harm marine life that depends on calcium carbonate. Changes in how plants and insects interact during the PTME have also been studied as possible signs of future ecological changes.
Geologists warn that the similarities between the PTME and today’s situation highlight the urgent need to reduce carbon dioxide emissions to avoid a disaster similar to 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:
The Permian-Triassic mass extinction shows the dangers of rapid carbon dioxide emissions. During the PTME, volcanic activity released huge amounts of CO₂, causing ocean acidification, low oxygen levels, and widespread loss of life. Today, human activities are causing similar changes, but even faster. The geological record shows that once these tipping points are reached, the chain of effects on ecosystems can last for millions of years.
If carbon dioxide levels continue to rise, another biocalcification crisis may occur, as seen in the fossil record. This would have severe consequences for modern marine ecosystems.