The Great Dying was the worst mass extinction in Earth’s history, occurring roughly 252 million years ago at the boundary between the Permian and Triassic periods. Estimates of marine animal species loss range from 81 to 94 percent, dwarfing every other extinction event in the fossil record, including the one that killed the dinosaurs. The cause, supported by decades of converging evidence, was a massive volcanic episode in what is now Siberia, whose eruptions set off a cascade of environmental disasters that made the planet nearly uninhabitable for millions of years.
How Bad Was It
The scale of the end-Permian extinction is hard to overstate. In the oceans, virtually every major group of organisms was devastated. Bottom-dwelling creatures like brachiopods, corals, bryozoans, and foraminifers were hit hardest, and planktonic communities underwent a fundamental reorganization in which eukaryotic algae were replaced by bacteria and other simple organisms.1PubMed Central. The great catastrophe: causes of the Permo-Triassic marine mass extinction On land, forests vanished, soils eroded on a continental scale, and most vertebrate lineages were wiped out or reduced to a handful of surviving species.
The extinction unfolded in at least two distinct pulses. During the first phase, species richness dropped dramatically but the ecological roles those species filled remained mostly intact. Surviving lineages continued to occupy the same functional niches. The second phase was far more destructive: both the number of species and the number of ecological roles collapsed, with species richness falling by roughly 87 percent and functional diversity nearly halving compared to the first-phase communities.2Current Biology. The stability and collapse of marine ecosystems during the Permian-Triassic mass extinction The first blow thinned the ecosystem; the second gutted its structure entirely.
How Quickly It Happened
For geologists accustomed to thinking in spans of millions of years, the Great Dying was shockingly fast. High-precision uranium-lead dating of volcanic ash layers at the global reference section in Meishan, China, places the main extinction interval between about 251.94 and 251.88 million years ago, a window of roughly 60,000 years.3PubMed Central. High-precision timeline for Earth’s most severe extinction That sounds long in human terms, but in deep time it is essentially instantaneous. Earlier dating work using a different method had pegged the main extinction pulse at about 252.6 million years ago and confirmed it was synchronous with the Siberian volcanic eruptions.4PubMed. Age and timing of the Permian mass extinctions: U/Pb dating of closed-system zircons Either way, the overlap in timing between the eruptions and the extinction is tight enough to be considered cause-and-effect by most researchers.
The Siberian Traps and the Trigger
The primary driver was the eruption of the Siberian Traps, one of the largest volcanic events in Earth’s history. This was not a single volcano but a vast province of lava flows, sills, and intrusions that covered up to five million square kilometers, an area comparable to about half of Europe. The bulk of the eruptions occurred over a period of less than two million years.5Earth and Planetary Science Letters. The timing and extent of the eruption of the Siberian Traps large igneous province: Implications for the end-Permian environmental crisis
But the surface lava flows alone may not have been the deadliest part. Research has identified a critical shift in how the magma was emplaced: early eruptions produced conventional flood lavas on the surface, but the onset of the extinction lines up with a switch to underground sill intrusions, where magma spread laterally through layers of sedimentary rock beneath the surface.6PubMed Central. Initial pulse of Siberian Traps sills as the trigger of the end-Permian mass extinction This distinction matters enormously, because those underground sills baked their way through carbon-rich sedimentary rocks, releasing staggering quantities of greenhouse gases that the surface lavas alone could not have produced.
Why Underground Magma Was More Dangerous Than Surface Lava
When hot magma intrudes into sedimentary basins containing coal, limestone, and organic-rich shales, it essentially cooks those rocks in a process called contact metamorphism. The result is a massive release of carbon dioxide, methane, and other volatiles. Thermal modeling of the sills in the Tunguska Basin, where much of this intrusion occurred, shows that the contact zones around the sills could generate between 52 and 80 tonnes of CO₂ per square meter. Scaled up across the basin, even a tiny fraction of the sill-affected area (around one percent) would have been enough to release a thousand billion tonnes of CO₂.7PubMed Central. Sills and gas generation in the Siberian Traps That is orders of magnitude beyond what the lava itself would have degassed, and it explains why the eruptions had such an outsized environmental impact.
An additional amplifier may have involved methane-producing microbes. Volcanic eruptions released large amounts of nickel into the environment, and nickel is a key nutrient for methanogens. Sediment records in South China show a sharp spike in nickel concentrations right at the extinction horizon, and researchers have proposed that this nickel pulse removed a growth limitation on methane-producing archaea, triggering a burst of biogenic methane that piled even more greenhouse gas into the atmosphere.8PubMed Central. Methanogenic burst in the end-Permian carbon cycle The proposed source of this nickel was the enormous Norilsk ore deposits, which formed in the magmatic conduits that fed the Siberian eruptions.9PubMed Central. Role of degassing of the Noril’sk nickel deposits in the Permian-Triassic mass extinction event
A Planet That Became Hostile in Every Way at Once
The carbon and methane pouring into the atmosphere did not kill life through a single mechanism. Instead, they set off a chain reaction of overlapping environmental crises, each reinforcing the others.
The most immediate effect was extreme global warming. Oxygen isotope records from conodont fossils in South China document a drop in isotope ratios that translates to roughly 8°C of warming in tropical surface waters during the latest Permian.10Geology. Climate warming in the latest Permian and the Permian–Triassic mass extinction For context, that is several times greater than the warming from modern climate change, compressed into a geologically brief window.
As the oceans warmed, they lost dissolved oxygen. Warmer water holds less oxygen to begin with, and the temperature rise also intensified ocean stratification, meaning surface waters stopped mixing downward effectively. The result was widespread anoxia, stretching from deep waters into shallower zones. In many areas, conditions went beyond merely oxygen-free: they became euxinic, meaning the water column was actively poisoned by dissolved hydrogen sulfide. Biomarker evidence from sites in Western Australia and South China shows that these toxic, sulfide-rich conditions reached all the way into the sunlit upper ocean, where green sulfur bacteria thrived in place of normal photosynthetic life.11PubMed. Photic zone euxinia during the Permian-triassic superanoxic event Hydrogen sulfide in shallow waters would have been directly lethal to most marine animals.
At the same time, the flood of CO₂ into the oceans lowered seawater pH. Boron isotope records from fossil brachiopod shells reveal a substantial decline in ocean pH coinciding with the onset of the mass extinction, directly linked to a large pulse of carbon degassing from the Siberian sill intrusions.12Nature Geoscience. Permian–Triassic mass extinction pulses driven by major marine carbon cycle perturbations Ocean acidification attacks shell-building organisms directly, dissolving their carbonate structures and making it metabolically expensive to grow new ones.
The Ozone Layer Collapsed Too
The damage was not confined to the oceans. On land, one of the most insidious effects was the destruction of the ozone layer. The Siberian eruptions released massive quantities of organohalogens, chlorine- and bromine-bearing compounds generated by contact metamorphism of salt-bearing sedimentary rocks. These chemicals rose into the stratosphere and catalyzed ozone destruction, stripping away the shield that protects life from ultraviolet-B radiation.13PubMed Central. Environmental mutagenesis during the end-Permian ecological crisis
Direct evidence for this comes from the fossil record itself. Spikes in ultraviolet-absorbing compounds preserved in end-Permian sediments coincide with mercury concentration spikes and carbon isotope shifts, tying volcanic eruptions, carbon cycle disruption, and ozone loss together in a tight temporal package.14PubMed Central. Dying in the Sun: Direct evidence for elevated UV-B radiation at the end-Permian mass extinction Experiments on living conifers exposed to the UV-B levels inferred for the end-Permian show a fivefold increase in pollen malformation. The trees survived but were effectively sterilized, unable to reproduce. Pulsed ozone weakening could have repeatedly collapsed terrestrial food webs without needing to kill plants outright, simply by preventing them from reproducing.15PubMed Central. UV-B-induced forest sterility: Implications of ozone shield failure in Earth’s largest extinction
Forests Burned, Soils Washed Away, Rivers Changed Shape
The loss of forests during the Great Dying was not gradual. Evidence from terrestrial sedimentary sequences shows that typical Permian coal-forming forest communities disappeared abruptly, replaced by a meter-thick layer of mudstone containing little organic matter, mainly fragments of degraded wood, charcoal, and fungal spores. Researchers describe this as a “dead zone,” the signature of catastrophic vegetation die-off at high southern latitudes.16Earth and Planetary Science Letters. End-Permian (252 Mya) deforestation, wildfires and flooding—An ancient biotic crisis with lessons for the present Geochemical records from other sites confirm that the crisis unfolded in stages: first a significant decline in terrestrial biomass, then intensified soil erosion, the disappearance of coal seams, a spike in charcoal reflecting widespread wildfires, and sharp shifts in carbon isotopes.17Earth and Planetary Science Letters. Stepwise deforestation during the Permian-Triassic boundary crisis linked to rising temperatures
Without rooted vegetation to hold landscapes together, soil erosion became catastrophic. Organic geochemical analyses of marine sediments from northeastern Italy reveal a massive influx of land-derived soil material into the ocean at the boundary, linking the terrestrial collapse directly to marine ecosystem disruption.18Geology. Catastrophic soil erosion during the end-Permian biotic crisis The physical landscape itself was transformed. In South Africa’s Karoo Basin, rivers shifted rapidly from meandering to braided systems, the kind of change that happens when there are no longer plant roots stabilizing river banks. The same pattern appears in other Permian-Triassic boundary sections around the world, suggesting a global event.19PubMed. Altered river morphology in south africa related to the permian-triassic extinction
What About an Asteroid Impact
Given that an asteroid strike ended the age of dinosaurs 66 million years ago, the question naturally arises: could a similar impact have caused the Great Dying? Some early evidence seemed suggestive. Researchers found fullerene molecules in boundary sediments containing trapped helium and argon with isotope ratios matching those of carbonaceous chondrites, a type of meteorite. That finding pointed toward an impact event accompanying the extinction.20PubMed. Impact event at the Permian-Triassic boundary: evidence from extraterrestrial noble gases in fullerenes
However, a critical evaluation of the impact hypothesis found that numerous claims of impact evidence had generally been rejected, proven difficult to reproduce, or remained unconfirmed. While some features of the extinction, like its speed and the coincidence of terrestrial and marine losses, are consistent with an impact, the tight link to Siberian volcanism and the complex, multi-stage nature of the carbon isotope shift argue against a single-impact explanation.21PubMed. Impact at the Permo-Triassic boundary: a critical evaluation Today, the dominant view is that the Siberian Traps were the primary cause. An impact may have occurred, but if so, it was not the main driver.
Why Some Groups Died and Others Survived
The extinction was not random. Which organisms survived and which perished depended in part on their physiology, particularly how efficiently they could transport oxygen through their bodies. Under the hot, oxygen-poor, acidifying ocean conditions of the end-Permian, animals with high-capacity oxygen-carrying blood proteins like hemoglobin and hemocyanin had a critical advantage. They could ramp up oxygen binding and increase circulation to keep their tissues supplied even as the water around them grew depleted.
Animals relying on hemerythrin, a less efficient oxygen-carrying protein found in brachiopods and bryozoans, or on simple diffusion of oxygen through their body walls, fared far worse. These groups experienced significantly higher extinction rates and greater body-size reductions.22The Innovation. Respiratory protein-driven selectivity during the Permian-Triassic mass extinction This physiological filter explains a pattern that had puzzled paleontologists for decades: why brachiopods and crinoids, which had dominated Permian oceans, were preferentially killed, while bivalves and gastropods, which carry hemocyanin, survived and went on to dominate afterward. Temperature-dependent oxygen loss is the only kill mechanism shown to explain the magnitude, geographic pattern, and taxonomic selectivity of the extinction.23PubMed Central. Differences in physiological tolerance to global warming caused the Permian-Triassic transition between the Paleozoic and Modern faunas
The Aftermath on Land
On land, the post-extinction world was bleak. Terrestrial ecosystems were dominated for millions of years by a handful of extremely common “disaster taxa” that exploited the ecological vacancies left behind. The most famous is Lystrosaurus, a stocky, pig-sized herbivorous relative of mammals. In the Karoo Basin of South Africa, Lystrosaurus comprised about 54 percent of all vertebrate specimens even before the extinction peaked and rose to roughly 70 percent in the extinction’s aftermath.24PubMed Central. Evidence from South Africa for a protracted end-Permian extinction on land That early rise in abundance suggests Lystrosaurus was not simply filling empty niches after other species vanished but was actively benefiting from the changing environmental conditions.
Whether Lystrosaurus truly qualifies as a “disaster taxon” has become a point of debate. Some researchers have argued that the genus does not meet formal definitions of a disaster taxon based on its biostratigraphy and evolutionary relationships, and that individual species of Lystrosaurus are better described as opportunistic.25Frontiers in Earth Science. The Disaster Taxon Lystrosaurus: A Paleontological Myth Either way, the picture is the same: Early Triassic terrestrial ecosystems had dramatically low diversity and were dominated by just a few widespread species, a pattern that persisted for millions of years.26PubMed Central. Delayed recovery of non-marine tetrapods after the end-Permian mass extinction tracks global carbon cycle
How the Great Dying Permanently Reshaped Life
Most mass extinctions thin the ranks of existing groups, but the survivors eventually rebuild ecosystems that look broadly similar to what came before. The end-Permian event was different. It caused a permanent shift in which types of organisms dominate the world’s oceans, a transition paleontologists call the shift from the Paleozoic Fauna to the Modern Fauna.
Before the Great Dying, seafloors were ruled by brachiopods, crinoids (sea lilies), and tabulate and rugose corals. After it, bivalves and gastropods, the clams and snails familiar to anyone who has walked a beach, rose to ecological dominance. This changeover had been slowly building in the late Paleozoic, with molluscs already moving into offshore environments and mixing with brachiopod communities.27Geology. Ecological effects of the Paleozoic-Modern faunal transition: Comparing predation on Paleozoic brachiopods and molluscs But the end-Permian extinction made it permanent by preferentially eliminating the organisms least equipped to handle hot, low-oxygen, acidic oceans. Because bivalves and gastropods carried more effective respiratory proteins and generally had higher metabolic flexibility, they weathered the crisis and inherited a world their Paleozoic predecessors could no longer occupy.
The recovery took a remarkably long time. Environmental stresses like extreme warming, ongoing ocean anoxia, and acidification persisted well into the Early Triassic, suppressing the radiation of new species for millions of years after the eruptions themselves had ended.28Nature. The timing and nature of marine ecosystem recovery following the Permian-Triassic mass extinction Full marine ecosystem recovery did not occur until the Middle Triassic, some five to ten million years after the boundary. On land, low-diversity ecosystems dominated by a few abundant forms persisted for a comparably long stretch. The Great Dying did not just kill; it kept the world impoverished for an era afterward, creating the conditions from which the Age of Dinosaurs would eventually emerge.
Mercury in the Rocks as a Volcanic Fingerprint
One of the more useful forensic tools for studying the Great Dying is mercury. Volcanic eruptions are among the largest natural sources of mercury in the atmosphere, and mercury spikes in sedimentary rocks have become a standard way to trace volcanic activity through the geologic record. At and around the Permian-Triassic boundary, both marine and terrestrial sedimentary sequences around the world show pronounced mercury enrichment, providing a chemical fingerprint of the Siberian Traps eruptions that can be correlated across continents.29Geology. Ecological disturbance in tropical peatlands prior to marine Permian-Triassic mass extinction These mercury spikes often coincide with negative carbon isotope shifts and the ultraviolet-absorbing compound excursions linked to ozone collapse, reinforcing the picture of volcanic eruptions as the common root of all the environmental crises. The ability to trace a single chemical signal across marine limestone in Italy, peat deposits in the tropics, and terrestrial mudstones in the southern hemisphere is part of what makes the Siberian Traps hypothesis so robust: the evidence shows up everywhere researchers look.