Cnidaria is a diverse phylum which includes corals, jellyfish, and other aquatic critters with stinging cells called nematocysts. Many cnidarians live in the ocean but some can be found in freshwater settings such as lakes and rivers (UCMP). The cnidarian fossil record extends back to the Precambrian (~580 million years ago) and can still be found in bodies of water today.
Anthozoa is a cnidarian class that includes corals, sea anemones, sea fans, and sea pens. Most recognizable are the corals, which have been major reef building animals throughout Earth’s history (UCMP). Anthozoa dates back to the Precambrian (~580 million years ago) and are still found today. However, corals living today (Scleractinia) first appear around the time dinosaurs evolved in the Triassic (~243 million years ago). The other two major coral groups, Rugosa and Tabulata, went extinct at the end Permian Mass Extinction event (~251 million years ago). There is a gap in our understanding of coral evolution because of this interval called ‘the coral gap’ where there are no documented anthozoans in the fossil record.
Rugosa, previously referred to as Tetracorallia, is an extinct group of solitary and colonial corals that lived from the Ordovician to the Permian (~480-251 million years ago). Solitary (single) rugose corals are often called ‘horn corals’ because of their distinctive shape that looks familiar to a bull’s horn. Colonial rugose corals do not take this same diagnostic shape (Digital Atlas of Ancient Life).