In Morocco, the limestones are 600 million years old, and contain many invertebrate fossils. They fall broadly into three categories: shells, crinoids and segmented shells. The evolution of shelled creatures is demonstrated with the flatworm, which eventually changed its body shape when burrowing became a necessity for either food or safety. It then evolved shielded tentacles and the casings eventually enveloped the entire body: these creatures are the brachiopods. The most successful shelled animals are the molluscs, of which there are some 80,000 different species. Some are single-shelled such as the cowrie, while others are bivalves that include the scallop and the giant clam. One species that has remained unchanged for millions of years is the nautilus: it features flotation chambers within its shell, which in turn formed the basis for the ammonites. Crinoids are illustrated by sea lilies, starfish and sea urchins on the Great Barrier Reef. Segmented worms developed to enable sustained burrowing, and well preserved fossils are found in the Rocky Mountains of British Columbia. These developed into trilobites and crustaceans, and the horseshoe crab is shown nesting in vast numbers on Delaware Bay. While the robber crab breeds in the sea, it is in all other respects a land animal and Attenborough uses it to exemplify the next evolutionary step.
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