What event marks the end of both the Paleozoic Era and Mesozoic Era?

The Permian-Triassic (P-Tr) Extinction Event is widely recognised as the greatest extinction event on Earth. It is also known as the "Great Dieing".

It occurred 251 million years ago marking the end of the Permian Period and the start of the Triassic Period.It also marks the end of the Paleozoic Era and the start of the Mesozoic Era. Estimates have suggested that 96% of marine species and 70% percent of land vertebrate species became extinct. It is also the only known mass extinction of insect with 56% of insect families and 83% of insect genera becoming extinct.

Little is known of the cause of the mass extinction but theories range from meteorite impacts, to volcanism, to sea level fluctuations.