The 'Great Dying'

The Paleozoic era culminated 251.9 million years ago in the most severe mass extinction recorded in the geologic record. Known as the 'great dying,' this event saw the loss of up to 96% of all marine species and around 70% of terrestrial species, including plants and insects.

source https://www.sciencedaily.com/releases/2021/05/210519163702.htm