Rudists are a clade of marine bivalves that dominated the Tethyan benthic realm in the Late Jurassic and most of the Cretaceous. In traditional systematics they are divided into subclass Heterodonta and order Hippuiritoida. The rudists are characterized by thick shells (compared to extant bivalves) and a cone-like valve capped a smaller "free valve".These valves vary in size and especially shape, exhibiting cylindrical, coiled, twisted and hornlike forms.
Named "Rudists" by Lamarck in 1819, these enigmatic organisms were previously described as corals, cephalopods and the horns of animals. In 1850, Alcide D'Orbigny classified three rudist genera and placed them within the Brachiopoda. Today there are about one thousand described rudist morphospecies and an unresolved debate among paleontologists regarding the role of these bivalves as reef-builders.
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