Mesozoic-Cenozoic scleractinian corals were not derived by descent from late Paleozoic rugose corals. Rather, they probably originated from a group of Paleozoic sea anemones that survived into the Mesozoic after the extinction of the Rugosa at the end of the Paleozoic. This conclusion is based on three arguments: 1. all scleractinians have cyclic septal insertion in contrast to the serial insertion of rugosans; there are no intermediates; 2. scleractinians have aragonite skeletons; those of rugosans are calcite; 3. there are no Lower Triassic corals. It is unlikely that two important characters (points 1 and 2) would change so drastically during the only stage in the history of the corals in which no corals are known (point 3).