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.