See text Rudists are a group of extinct box-, tube- or ring-shaped marine heterodont bivalves belonging to the order Hippuritida that arose during the Late Jurassic and became so diverse during the Cretaceous that they were major reef-building organisms in the Tethys Ocean, until their complete extinction at the close of the Cretaceous.

Order: †Hippuritida Bieler, Carter & Coan in 2010 also named the non-Hippuritid families Megalodontoidea and Chamoidea, of Megalodontida and Venerida respectively, as "Rudists", but this classification was not monophyletic.

[5] During the Cretaceous, rudist reefs were so successful that they may have driven scleractinian corals out of many tropical environments, including shelves that are today the Caribbean and the Mediterranean.

During this period tropical waters were between 6°C and 14°C warmer than today and also more highly saline, and while this may have been a suitable environment for the rudists, it was not nearly so hospitable to corals and other contemporary reef builders.

[1] These rudist reefs were sometimes hundreds of meters tall and often ran for hundreds of kilometers on continental shelves; in fact at one point they fringed the North American coast from the Gulf of Mexico to the present-day Maritime Provinces.