The Palaeozoic corals, I: origins and relationships

The status, origins and relationships of the various groups of Palaeozoic corals are reviewed. Five rders are currently recognized: Rugosa, Tabulata, Heterocorallia, Cothoniida and Kilbuchophyllida, to which I add the Tabulaconida and Numidiaphyllida. The Rugosa and Tabulata are considered to be broadly onophyletic clades, and the Tabulata are confirmed as zoantharian corals. Morphological features, particularly aspects of septal insertion in both groups, are discussed as clues to their likely origins and relationships. They are not considered to have had a skeletonized common ancestor, but they may have arisen as separate skeletonization events from the same broad group of anemones, represented by the living Zoanthiniaria. The Rugosa are not considered to be ancestral to the Scleractinia. The latter, together with the Permian Numidiaphyllida, are considered to have evolved through skeletonization events among a group of anemones derived from the Actiniaria/Corallimorpharia, a member of which also gave rise to the Kilbuchophyllida in the Ordovician. The pattern of septal insertion in the Heterocorallia is controversial and the relationship of these corals to contemporary coral groups remains uncertain. The increasingly important record of Cambrian coralomorphs is assessed, and considered to include everal genera of zoantharian corals. However, although similarities are apparent, none is regarded as directly ancestral to the post-Cambrian coral clades. The history of diversification and extinction of corals through the Palaeozoic is briefly reviewed.