Rudist

Rudists are extinct group of various heterodont bivalves. They formed a bizarre shells, one valve is usually cone-shaped (attached valve) and the second one is strongly reduced and it forms a flat operculum-like structure in some representatives. Both valves are connected with teeth and holes. The marked growth lines and prominent ornamentation is apparent at valve surface. The oldest rudists are known from the Late Jurassic (155 Ma) and they probably became extinct in the latest Cretaceous (68 Ma). The largest valves exceeded almost to 2 meters in length. They lived in warm tropical seas and built prominent reefs in the Cretaceous.

## Keywords
Mesozoic, Jurassic-Cretaceous, Bivalvia, Heterodonta