1. The shells of marine molluscs are made of calcium carbonate (CaCO3).
2. Corals build reefs.
3. Their hard outer skeletons are made of calcium carbonate.
4. When marine molluscs and corals die, debris from their shells and skeletons accumulate on the sea floor as sediment.
5. Over geological eras, this sediment forms limestone.
6. The shells of marine molluscs and coral skeletons may become fossilised in limestone.