The extinction line
In a former quarry near Meishan, China, a brass marker is driven into a cliff face of layered rock. This is a Global Boundary Stratotype Section and Point (GSSP), a location that formally defines a boundary in the geologic time scale. This specific GSSP, established in 2001, marks the official end of the Permian Period and the Paleozoic Era, and the beginning of the Triassic Period and the Mesozoic Era, around 251.9 million years ago.
The boundary is not arbitrary. It is placed at the very base of a thin layer of light-colored claystone known as Bed 27c. Its position was determined by the first appearance of a specific fossil, the tooth-like remains of a conodont animal named Hindeodus parvus. This unassuming fossil signals the dawn of a new world, one that emerged after the most severe extinction event in Earth's history. Below this line, the limestone layers are filled with fossils from a diverse and bright marine ecosystem, containing over 330 species. Immediately above it, the rock layers are comparatively barren, a silent evidence to a global catastrophe. The entire extinction event is recorded in a section of rock that is less than a meter thick, with the main pulse of death occurring over a span of about 60,000 years.
A world in turmoil
The cause of the Great Dying was a series of massive volcanic eruptions in a region now known as the Siberian Traps. For roughly two million years, fissures in the Earth's crust poured out an estimated four million cubic kilometers of lava, covering an area of Siberia larger than western Europe.
These eruptions were flows of lava; they released colossal amounts of carbon dioxide, sulfur dioxide, and other volatiles into the atmosphere. The magma also ignited and burned through vast underground coal deposits, adding even more greenhouse gases. Atmospheric carbon dioxide levels may have risen from around 400 parts per million to 2,500 ppm. Global temperatures soared, with equatorial ocean temperatures possibly exceeding 40°C (104°F).
The chemical consequences were devastating. The carbon dioxide dissolved into the oceans, causing rapid acidification that made it impossible for organisms with calcium carbonate shells and skeletons to survive. The warming also led to widespread anoxia, or the loss of oxygen in the seawater, creating vast dead zones. Geochemical analysis of the Meishan rocks shows evidence of these anoxic and sulfidic water conditions. The extinction was so thorough that it took life on Earth as long as 10 million years to fully recover its previous levels of biodiversity.