Bivalves were fairly rare before the P–Tr extinction but became numerous and diverse in the Triassic, and one group, the rudist clams, became the Mesozoic's main reef-builders.
Os bivalvos eran bastante raros antes da extinción P-Tr pero fixéronse numerosos e diversos no Triásico, e un grupo, o das ameixas rudistas, converteuse no principal construtor de arrecifes do Mesozoico.