Corals are a diverse group of marine invertebrates that first appeared in the Cambrian Period, some 550 million years ago (MYA). Fossils of these primitive tabulate corals are found in sedimentary rocks throughout the Paleozoic Era; a second group of ancient corals, the rugose corals, arose in the Silurian (400 MYA) and both groups apparently died out during the Permian Extinction, 225 MYA.
Early Triassic rocks are devoid of coral fossils but they reappear during the mid-late Triassic, some 210 MYA. These scleractinian corals are the ancestors of all modern corals, having undergone cycles of expansion and near extinction as well as major periods of diversification, especially during the Jurassic (150 MYA) and the Miocene (25 MYA) Periods. It was during the latter Period that the Great Barrier Reef began to form off the northeast coast of Australia, now composed of almost 3000 reefs that harbor at least 500 species of coral.
Marine biologists have cataloged about 70,000 species of coral across the globe. While the great majority of corals are found in shallow, clear, warm waters of tropical and subtropical seas, there are coral species that occupy deep water and cold water habitats as well. Unfortunately, these diverse communities are significantly threatened by human activity, primarily due to pollution and the effects of global warming. At least 10% of modern coral reefs are dead and an increasing concentration of carbon dioxide in our oceans, which acidifies the seawater, impairs the formation of calcium carbonate shells, thus threatening corals and other shell-forming marine life.
