Which group of organisms survived mass extinctions that marked the end of both the Paleozoic era and Mesozoic era?

The answer is Gastropods. The earliest representatives of this group appeared in the Late Cambrian. There are diverse gastropods alive today, commonly known as snails and slugs. There are many thousands of species of sea snails and sea slugs, as well as freshwater snails, freshwater limpets, land snails and land slugs.