Media Gallery: Photos Oldest

Two brown and purple nudibranchs (Risbecia tryoni), crawling in tandem on a surface encrusted with pink-colored coralline algae. These "sea slugs" live in the tropical Western Pacific Ocean. You can learn more about coral reef ecosystems by exploring our Coral Reefs featured story.

A shark swims in the waters near Fiji.

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.

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. Discover more about the ancient ocean at our feature Ocean Over Time.