Which group of organisms survived mass extinctions that marked the ends of both the paleozoic era and mesozoic era?

The group of organisms that survived mass extinctions that marked the ends of both the Paleozoic Era and the Mesozoic Era is the gastropods. Gastropods are one of the most diverse groups of animals, both in form, habitat, and habit. They are by far the largest group of molluscs, they include snails and slugs of all kinds and all sizes from microscopic to Achatina achatina. 

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