Rudists are a group of bizarrely shaped marine heterodont bivalves that arose during the Jurassic, and became so diverse during the Cretaceous that they were major reef-building organisms in the Tethys Ocean. The Jurassic forms were elongate, with both valves being similarly shaped, often pipe or stake-shaped, while the reef-building forms of the Cretaceous had one valve become a flat lid, with the other valve becoming an inverted spike-like cone. They were among the many animal groups that perished during the Cretaceous–Tertiary extinction event .
The rudists are, according to different systematic schemes, placed in the orders Hippuritoida or Rudistes (sometimes Rudista). Their "classic" morphology consisted of a lower, roughly conical valve that was attached to the seafloor or to neighboring rudists, and a smaller upper valve that served as a kind of lid for the organism.
Their classification as true reef-builders is controversial, yet they were one of the most important constituents of reefs during the Cretaceous period. At one point, rudist reefs fringed the North American coast from the Gulf of Mexico to the present-day Maritime Provinces. Because of their high porosity, rudist reefs are highly-favored oil traps.