The Permian-Triassic (P-Tr) extinction event, informally known as the Great Dying, occurred 252.28 million years ago and–as the name suggests–marks the transition from the Permian period to the Triassic, as well as that between the Paleozoic and Mesozoic eras. It is the P-Tr extinction, rather than the K-T extinction, that is the most devastating extinction event in Earth’s known history: 96% of all marine animals and 70% of all terrestrial vertebrate species became extinction during this time, and it is also the only known mass extinction of insects. Approximately 57% of all taxonomic families and 83% of all genera at the time died out; this enormous loss of biodiversity caused life on Earth to take significantly longer to recover than after any other extinction event, possibly as many as 10 million years.
There are several theories as to exactly how the P-Tr extinction happened, though most pertain to environmental change as opposed to overhunting by one species or an outside effect such as an asteroid strike; the earlier phases of the extinction were likely gradual, while the later phases were sudden and catastrophic. Suggestions as to the actual mechanisms of the extinction, primarily in the later phases, include increased volcanic eruptions, coal or gas fires and explosions from the Siberian Traps, the sudden release of methane clathrate from the sea floor, a shift in ocean circulation due to climate change, and gradual changes in the seal level which increased aridity. One of the most common suggestions is that the formation of Pangaea caused the continental environment to become extremely arid, causing a drought which lasted millions of years.