Approximately 252 million years ago, Earth experienced its most severe extinction event, marking the boundary between the Permian and Triassic geological periods. This event, known as the Permian-Triassic extinction and often called “The Great Dying,” represents the closest life has ever come to being completely erased from the planet. It was a period of environmental upheaval that altered the course of evolution. The sheer scale of this catastrophe distinguishes it from all other mass extinctions in Earth’s history.
The Scale of Extinction
The Great Dying was a global phenomenon that devastated both marine and terrestrial ecosystems. In the oceans, the losses were staggering, with estimates suggesting the extinction of up to 81% of all marine species. The extinction particularly affected organisms with calcium carbonate skeletons, which struggled with changing ocean chemistry. Among the most notable marine casualties were the trilobites, a highly successful group of arthropods that vanished completely.
On land, the devastation was also immense, with about 70% of terrestrial vertebrate species becoming extinct. This included the disappearance of most large amphibians and sauropsid reptiles, the dominant land animals of the Permian. Predators like the large, saber-toothed gorgonopsians were wiped out, leaving a void at the top of the food chain. The event is also recognized as the largest known mass extinction of insects.
Primary Causes of the Catastrophe
The scientific consensus points to a massive and prolonged series of volcanic eruptions in modern-day Siberia as the primary trigger. This event, the Siberian Traps, was one of the largest known volcanic events in the last 500 million years, spewing vast quantities of lava and gas for roughly two million years. The eruptions released immense volumes of greenhouse gases into the atmosphere. The magma also ignited vast underground coal deposits, further intensifying the release of carbon.
This massive injection of greenhouse gases initiated a cascade of environmental disasters. Atmospheric carbon dioxide levels are thought to have surged from around 400 parts per million (ppm) to 2,500 ppm, leading to extreme global warming. Global temperatures rose by as much as 8°C (14°F), with equatorial ocean temperatures potentially exceeding a lethal 40°C (104°F). This warming had profound effects on the oceans.
The warmer water held less dissolved oxygen, creating vast “dead zones” known as anoxic zones. Concurrently, the absorption of massive amounts of atmospheric CO2 led to ocean acidification. This acidification made it difficult for marine organisms that form shells and skeletons from calcium carbonate to survive. The combination of extreme heat, oxygen-depleted oceans, and acidification created a deadly environment for marine life. While other theories have been considered, the evidence strongly supports the Siberian Traps volcanism as the central cause.
Life in the Aftermath
The world that emerged after the Great Dying was a desolate and unbalanced landscape. For millions of years, ecosystems were impoverished, dominated by a few hardy survivors known as “disaster taxa.” The most prominent of these was Lystrosaurus, a pig-sized, beaked, plant-eating therapsid. Fossils of Lystrosaurus are so abundant in early Triassic rocks that they sometimes constitute 90% of the terrestrial vertebrate fossils found.
This mass extinction cleared the evolutionary slate, leaving countless ecological niches vacant. The disappearance of dominant reptiles and their predators created an opportunity for new groups to rise in a world with little competition, spurring a new wave of evolutionary diversification.
It was in this empty world that a group of reptiles called archosaurs began to flourish. Initially small, these animals rapidly diversified to fill the voids left by the Permian fauna. This group would eventually give rise to a range of creatures, including crocodiles, pterosaurs, and the dinosaurs. The Triassic period, therefore, became the dawn of a new era, setting the stage for the Age of Dinosaurs, which would dominate the planet for the next 150 million years.