Reefs are found in the fossil record and are thought to be about two billion years old. These reefs were built by calcareous algae and not corals. The first true corals did not appear until about 300–500 million years ago. They apparently flourished until a devastating extinction occurred killing many groups of corals. Most of the corals that compose extant (still living) reefs were found around 65 million years ago. They are still a vital part of the living environment. In addition to their ecological role as a foundation for a wide variety of life forms, the coral reefs have become a haven for tourists and scuba divers.

Corals are animals that belong to the monophyletic group called the Cnidaria (formerly called a Phylum). They are named for specialized stinging structures that emit long, venomous barbs when stimulated by the presence of prey or...