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Permian/Trias crisis

The Permian ended with the greatest mass extinction of all time: 95% of sea species and 70% of terrestrial species disappeared there following cataclysmic volcanic eruptions that poisoned the ocean and the atmosphere.

It occurred about 252 million years ago. It delimits the geological periods of the Permian and the Triassic, therefore the limit between the Paleozoic (the primary era) and the Mesozoic (the secondary era).

Some species escaped the Permian-Triassic extinction: archosaurs, which later include dinosaurs and pterosaurs, as well as some synapsids, such as Lystrosaurus, ancestors of modern mammals.
Later, 250 million years ago, a completely new group of animals appeared: the dinosaurs.