The Palaeozoic corals, II: structure, variation and palaeoecology

Abstract

Palaeozoic coral faunas were dominated by two orders of Zoantharian corals, the Rugosa and the Tabulata. Almost all rugose and tabulate corals developed an epitheca or holotheca around the corallum and possessed small to minute attachment scars. A limited number had an encrusting life style and many others could facultatively encrust to some extent when settling by chance on a hard substrate. However, most were effectively free living on a soft substrate as mature coralla. A high percentage of rugose corals were solitary and of the colonial genera, few exhibited physical integration of modules within the colony. In contrast, most of the exclusively modular tabulate corals possessed some degree of integration. Colony formation in tabulate corals was, with doubtful exceptions, solely by non-parricidal increase, whereas both non-parricidal and parricidal increase, the latter in a minor role, occur in the Rugosa. The sources of variation in solitary and colonial Palaeozoic corals are reviewed and data on growth rates, measured on the basis of cyclomorphic variation expressed as density band couplets, are assessed. Growth-form variation in solitary corals and its relationship to stability on the substrate are explored. Growth strategies in colonial corals are described and their adaptive success under different environmental conditions is discussed. Colonial growth-forms were a function of the interaction between growth strategy and prevailing conditions during astogeny, although some species had genetically constrained, relatively invariate growth-forms. Factors controlling the distribution of Palaeozoic corals in the environment–substrate and turbidity, water energy, depth, light, temperature and variations in salinity and oxygenation - are reviewed. Palaeozoic coral diversity was much lower than that of Recent hermatypic scleractinian corals in comparable environments. Most Palaeozoic corals were adapted to soft substrates in warm, shelf seas, and made a limited contribution to reef frameworks. It is concluded that none of them developed a symbiosis with algae in contrast to zooxanthellate scleractinian corals.