Long Before Coral, Mollusks Built the Ocean's Reefs

Rudist clams were major reef builders during the heyday of dinosaurs.

Credit: Smithsonian Institution

About 100 million years ago, during the heyday of the dinosaurs, reefs were built by mollusks called rudist clams. Like modern clams, rudists were bivalves, with two shells (or valves) joined at a hinge. But they sure didn’t look like modern clams!

One major group of rudists grew upright, like big ice cream cones standing on end. The bottom valve was anchored in the ocean floor. Only the upper few inches poked above the sediments. The second major group of rudists had horn-shaped shells that lay flat on the ocean floor, preventing strong currents from washing them away.

Like the dinosaurs, rudist clams became extinct 65 million years ago, when an asteroid impact changed the global climate. Millions of years later, corals took over as the dominant tropical reef builders.

Mollusks, Paleobiology, Geologic time, Adaptations

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Submitted by Deborah (not verified) on

Its an interesting History based on the sea waters :) Very Interesting

Submitted by Tom Kapitany (not verified) on

Long before rudist clams , Stromstolites and microbial mats were the real reef builders . In the Archeon and Proterozoic, stomatolite biohermes dominated the oceans building massive reefs . Stomatolite reefs reefs exist today , though they only occur in isolated pockets . They are the only reef builders today in the Antarctic.

Submitted by Anonymous (not verified) on