Corals are marine animals with simple body structures. The mouth of a coral’s sac-like body is surrounded by a ring of tentacles. The living coral animal, the polyp, secretes a cup-like skeleton called the corallite. Many corallites cemented together make up the entire skeleton, or corallum. Inside the corallite, a radial divider, called a septum (plural, septa) grows vertically from the attachment base and helps support the soft tissues. Many coral polyps contain algal cells, which use photosynthesis to produce food for themselves and the coral.
Corals can live together in large colonies, or reefs, which can be hundreds of miles across. Coral reefs are among the most complex ecosystems on Earth because many thousands of species other than corals make the reef their home. Corals themselves require specific living conditions, so fossil coral reefs tell us a great deal about the environmental conditions at the time of reef formation. Living coral reefs are confined to subtropical regions in shallow waters that are warm and clear. Thus, Wisconsin’s Silurian reefs provide evidence that the state was once covered by a warm, subtropical sea. Although ancient corals formed the reef itself, many other organisms flourished in the small habitats the reef provided. Brachiopods covered the reef structure, gastropods fed on the abundant algae and detritus, cephalopods hunted for prey, and crinoids swayed in the agitated waters.