Why did bivalves become more similar to modern forms during the Mesozoic Era?

During the Mesozoic Era, bivalves underwent significant evolutionary changes that established the "stamp of modernity," with virtually all currently recognized families present by the end of the period.1

This transition was largely driven by the "Mesozoic Marine Revolution," an evolutionary arms race sparked by the rise of more effective predators, such as crustaceans, snails, and echinoderms. To survive increasing predation pressure, bivalves evolved various antipredation devices, including shell ridges, spines, and teeth, or adopted the habit of burrowing to greater depths within the sediment.23

This period of intense adaptive radiation allowed bivalves to repeatedly modify their forms to diversify into different habitats. Because similarity in habitat often matches similarity in structure, these adaptations resulted in the modern forms observed today. The inherent plasticity of the bivalve body plan, combined with these environmental pressures, ensured their continued success as a group.21