Cnidaria: Fossil Record
Many of the very best cnidarian fossils date back to the time when animals first appear in the fossil record, the Vendian. Since then, the fossil record of cnidarians without mineralized skeletons is quite sparse, and restricted to unusual sites with excellent fossil preservation.
On the other hand, cnidarians which possessed hard skeletons, in particular the corals, have left a significant legacy of their existence. While a few mineralized coral-like fossils have turned up in the Cambrian Period, identifiable corals began an evolutionary radiation in the Early Ordovician. These Paleozoic corals included taxa known as tabulate corals, rugose corals, and heliolitid corals. All these forms were wiped out at the end of the Permian Period, in a mass extinction event that claimed something like 95% of all marine invertebrate species.
Scleractinian corals first appear in the Middle Triassic, about 15 milion years after the Permian extinction. They rapidly expanded into ecological niches once dominated by tabulate and rugose corals, and became the dominant hermatypic (reef-building) organisms in shallow tropical marine habitats. Because corals are sensitive to changes in light, temperature, water quality, and salinity, their fossils provide information that can be used to interpret climate and geography of past environments.