Why is the Permian extinction also called the Permian-Triassic extinction?

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Line graph showing the number of marine animal families from the Precambrian era to the present, with illustrations of key groups such as trilobites, ammonoids, marine reptiles, and diatoms. The graph highlights major extinction events and a rise to 1,900 living families today. — Permian extinction The diversity of marine animal families since late Precambrian time. The data for the curve comprise only those families that are reliably preserved in the fossil record; the 1,900 value for living families also includes those families rarely preserved as fossils. The several pronounced dips in the curve correspond to major mass-extinction events. The most catastrophic extinction took place at the end of the Permian Period.

The Permian extinction is called the Permian-Triassic extinction because it occurred at the boundary between the Permian Period and the Triassic Period.12

This event marks the transition from the end of the Paleozoic Era to the beginning of the Mesozoic Era. Geologically, the extinction event is identified by significant changes in the carbon cycle and environmental conditions that crossed the boundary from the final age of the Permian into the Induan Age, which is the first age of the Triassic. Because this crisis devastated life on Earth—resulting in the loss of 85 to 95 percent of marine invertebrate species and 70 percent of terrestrial vertebrate genera—it serves as a critical marker in the geologic time scale that separates these two distinct periods.3412