One of the two principal orders of Palaeozoic corals, the other being the Tabulata. Rugose corals first appeared in the Middle Ordovician. The small number of known Cambrian corals include at least one species with superficial similarity, but it is not considered a direct ancestor of the Rugosa. The Rugosans diversified steadily during the Early and Middle Paleozoic to reach their greatest diversity in the Middle Devonian. A series of extinction events in the Late Devonian almost resulted in the disappearance of the group, but a handful of survivors rediversified to almost match peak diversity by the Late Mississippian. Thereafter, the group declined, slowly at first but more rapidly in the Permian, and became extinct at the end of the Permian.