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.