An extinct group of epifaunal bivalves that constructed reefs during the late Mesozoic. The valves of rudists are coiled to cylindrical in external form, and the shapes and sizes of rudist shells are extreme relative to other members of the class Bivalvia. Rudists lived as solitary individuals or in large clusters on predominantly carbonate substrates in low to mid-paleolatitudes until their demise near or at the end of the Cretaceous.