The Great Dying, formally the Permian-Triassic extinction event, refers to the largest mass extinction of life on Earth in all history. It happened 252 million years ago (mya) at the end of the Paleozoic era between the Permian and Triassic periods, long before dinosaurs roamed.
The world looked much different during the Permian period. The continents had been pushed together by the forces of plate tectonics into a single supercontinent known as Pangea that stretched from the North Pole to the South Pole. The surrounding super-ocean of Panthalassa was full of sponges, corals, starfish, clams, sea scorpions and bony fish. Amphibians crawled in the wetlands and insects explored ferns and primitive trees, while therapsids or strange mammal-like reptiles that resembled forerunners to dinosaurs roamed. But in a time span of just 80,000 years, 95% of all life would become extinct.
Different theories exist to explain the Great Dying, but it may have been a combination of events that led to the mass extinction. The formation of Pangea itself choked off cool oceans that had formerly surrounded smaller continents that now lay with much of their land mass lodged in the hot, dry interiors. The average temperature increased steadily over millions of years as the supercontinent formed. Suitable habitats may have been difficult to find and competition for food may have thinned species, if not caused some outright extinctions. Pangea also changed ocean currents, salinity and weather patterns, upsetting the balance of how life had been evolving. However these changes occurred so slowly it is unlikely to have been more than a contributing factor.
A major event that did occur at the same time of the Great Dying was the creation of the Siberian Trapps, formed by volcanic eruptions that continued for one million years. The greatest volcanic event in known history, the gases released would have created acid rains, a greenhouse effect, and global warming. The oceanic repercussions from a rise in temperature along with changes in salinity could have resulted in the disruption of thermohaline circulation or global currents. Stagnation would have resulted in oxygen and nutrient depletions, leading to a global loss of marine life.
If the greenhouse effect created by prolonged volcanism raised oceanic temperatures enough, it would have triggered yet another repercussion for which there is scientific evidence: methane hydrate gasification.
Paul Wignall found that a carbon isotope signature in strata from Greenland dating to the time of the Great Dying indicated a marked increase of carbon-12, unaccountable by standard explanations. Geologist Gerry Dickens suggested that a rise in deep-sea temperatures would have released frozen methane hydrate from the seabeds, freeing catastrophic amounts of methane gas. This would have welled up through the oceans and been released into the atmosphere, accounting for the signature in the strata. Methane gas is yet another powerful greenhouse gas. The volume released would have raised the average temperature again another 9 degrees Fahrenheit (5 degrees Celsius). This would be sufficient to kill most life.
Theories with lesser evidence suggest an impact from a comet or meteor, but there is little to no proof of this as a cause of the Great Dying, while there is ample proof of such an impact being responsible for bringing the reign of the dinosaurs to an end some 187 million years later. Another theory suggests that a supernova event within ten parsecs (32.6 light-years) of Earth could have destroyed the protective upper ozone layer for several years. This layer of ozone filters out ultra-violet (UV) radiation from the sun. Without it, UV rays would kill nearly all life on land and in the seas. There is some geological evidence that brief periods of ozone destruction occurred, but the record is inconclusive on this theory.
The Great Dying or Permian-Triassic extinction was the largest known catastrophe in history. However it did clear the way for the next great event that would come 25 million years later: the birth of the dinosaur and the Age of the Reptiles.