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Rudists are a group of bizarrely shaped 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, yetthey were one of the most important constituents of reefs during the Cretaceous period.[1] 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.

Comparison of Volviceramus grandis and Durania maxima