Corals are carnivorous marine organisms with stinging cells and tentacles. They are related to jellyfish and sea anemones. The term coral is also applied to their skeletons, which can be organic (soft) or composed of minerals (hard).

In species with a stony exterior, the outer body wall secretes calcium carbonate (lime). The skeleton forms in intricate patterns as the coral animal grows. The skeleton persists after its organism dies.

Corals live in colonies or are solitary, depending on the species. (Some species live either way). Skeletons of colonial corals, in particular, can accumulate and form massive structures, such as reefs.

More than 2300 species of coral live today. Corals appeared about 500 million years ago or more, at the same time that many other kinds of marine organisms were evolving. Thirty million years later, reef-like structures dominated by coralline skeletons were widespread.

One fossil reef has been particularly well studied on Manitoulin Island, Ontario. There, several kilometres from an ancient shoreline, many kinds of corals grew in warm water about 1 m (3 ft.) below the level of low tide. Some of the coral colonies were overturned and broken, which indicates the repeated passage of storms.

At that time (about 425 million years ago, during what is known as the Silurian Period) the area was in the tropics, south of the equator. The continent was oriented so that the equator passed through what is now Canada, from Baffin Island through southern British Columbia. Most of North America was covered by a warm, shallow sea.

The kinds of coral that formed the Manitoulin Island reef are now extinct. The corals that form reefs in tropical waters today first appeared in the Middle Triassic Period, about 240 million years ago.

Paleozoic coral reefs are known from North America, northern South America (Venezuela, Colombia), Europe (Britain, northern France, Belgium), Asia (Mongolia, China, Kazakhstan, Turkey), north Africa (Algeria, Morocco, Sahara), and Australia.