While some fossil corals from the Precambrian (more than 540 million years ago) have been found, they are not abundant until later. By about 450 million years ago, several groups of corals were common on Earth. Early corals included Rugosa and Tabulata corals, both with calcite skeletons. Some Rugosa (the horn corals) were solitary, while others lived in large colonies. Tabulata formed colonies with hexagonal patterns like honeycombs. Sponges and bryozoans were still the main reef builders, but these colonial corals gradually surpassed them. The Permian corals went extinct at the end-of-Permian mass extinction event, but soft-bodied, anemone like animals (“naked corals”) survived. Naked corals gave rise to modern calcified corals (scleractinians), formed when they adapted t geochemical changes in the ocean by secreting stony skeletons. During the Triassic (about 250 million years ago), stony corals assumed their role that continues today as the main reef builders on Earth.

Fossils of soft-bodied Cnidarians are rare, although they may be some of the earliest fossils of complex animals. Paleontologists have debated whether fossils from the Ediacaran (more than 540 million years ago) that look like jellyfish and sea pens are Cnidarians. The first definite Cnidarian fossils are from the Cambrian (about 500 million years ago). Some Cnidarians begin to make mineralized skeletons then, which were more likely to fossilize than the soft body parts underneath. During the Paleozoic, Cnidarians underwent a diversification into many forms. Jellyfish, hydrozoans, and corals emerged as distinct lineages. While various extinction events impacted Cnidarians along the way, these lineages survived to the present day. Colonial corals, whose skeletons accumulate into large reefs, ended up leaving a substantial fossil record. Because corals are sensitive to environmental variables, such as temperature and water quality, they have been used as indicators of past climate and other conditions.