The Permian–Triassic (P–T, P–Tr) extinction event, also known as the End-Permian extinction event and colloquially as the Great Dying, forms the boundary between the Permian and Triassic geologic periods, and with them the Paleozoic and Mesozoic eras respectively, approximately 251.9 million years ago. It is the Earth's most severe known extinction event, with the extinction of 57% of biological families, 83% of genera, 81% of marine species and 70% of terrestrial vertebrate species. It is the largest known mass extinction of insects. There is evidence for one to three distinct pulses, or phases, of extinction. The scientific consensus is that the main cause of extinction was the large amount of carbon dioxide emitted by the volcanic eruptions that created the Siberian Traps, which elevated global temperatures, and in the oceans led to widespread anoxia and acidification. Proposed contributing factors include: the emission of much additional carbon dioxide from the thermal decomposition of hydrocarbon deposits, including oil and coal, triggered by the eruptions; and emissions of methane by novel methanogenic microorganisms, perhaps nourished by minerals dispersed in the eruptions. The speed of recovery from the extinction is disputed. Some scientists estimate that it took 10 million years (until the Middle Triassic), due both to the severity of the extinction and because grim conditions returned periodically over the course of the Early Triassic, causing further extinction events, such as the Smithian-Spathian boundary extinction. However, studies in Bear Lake County, near Paris, Idaho, and nearby sites in Idaho and Nevada showed a relatively quick rebound in a localized Early Triassic marine ecosystem, taking around 3 million years to recover, while an unusually diverse and complex ichnobiota is known from Italy less than a million years after the end-Permian extinction. Regional differences in the pace of biotic recovery suggest that the impact of the extinction may have been felt less severely in some areas than others, with differential environmental stress and instability. Dating Previously, it was thought that rock sequences spanning the Permian–Triassic boundary were too few and contained too many gaps for scientists to reliably determine its details. However, it is now possible to date the extinction with millennial precision. U–Pb zircon dates from five volcanic ash beds from the Global Stratotype Section and Point for the Permian–Triassic boundary at Meishan, China, establish a high-resolution age model for the extinction – allowing exploration of the links between global environmental perturbation, carbon cycle disruption,