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Rudist Reefs

About 100 million years ago, during the heyday of the dinosaurs, reefs were built by mollusks called rudist clams. They looked very different from today's coral reefs. Discover more about the evolution of marine life in our Ocean Over Time interactive.

Tags: Scientific illustrations Mussels, Oysters & Relatives Paleobiology Geologic periods
Color illustration of an ancient bivalve.
Preview Notice the pink tentacles, which “recliners” used to filter feed.
Preview A rendering of an underwater marine scene depicting life ~145-65 million years ago, when rudist clams were the major reef builders.
Color illustration of an ancient bivalve.

Elevator Rudists

Credit: Mary Parrish/Smithsonian Institution

These "elevator" rudists, an ancient bivalve, used one long heavy valve to anchor themselves in the sediment. They used their tentacles (shown here in pink) to filter food from the sea water. And many often grew together to form early reefs. Learn more about these rudist reefs.

Notice the pink tentacles, which “recliners” used to filter feed.

Recliner Rudists

Credit: Mary Parrish/Smithsonian Institution

Where ocean currents were strong, ancient rudist “recliners” lay unattached on the seabed. Notice the pink tentacles, which were used to filter feed. Learn more about ocean life throughout deep time in our Ocean Over Time interactive or an image gallery. 

A rendering of an underwater marine scene depicting life ~145-65 million years ago, when rudist clams were the major reef builders.

Carbonate Reef (Cretaceous Period: 145 - 65 Million Years Ago)

Credit: Smithsonian Institution

This illustration shows the edge of a warm inland sea during the Cretaceous Period, heyday of the dinosaurs. Constantly shifting sediment supported new groups of organisms, including rudist clams—major molluscan reef builders. Various organisms have taken a turn as the dominant tropical reef builders. Visit a tropical reef today and you’ll see coral. But if you visited during the Cretaceous Period, you might have seen reefs built by mollusks: rudist clams. Rudists enjoyed a brief tenure as the ocean’s reef chiefs. They died out about 65 million years ago, the same time dinosaurs became extinct. Explore the ancient ocean in an image gallery in our Ocean Over Time interactive.