FOSSIL INVERTEBRATES/Corals and Other Cnidaria 331 Figure 11 Reef forming scleractinian corals (A) Reef edge dominated by Acropora palmata; open ocean to the right; East Rio Bueno, Jamaica (B) Reef flat with phaceloid, partly polycentric colony of Lobophyllia hemprichii in the centre, flanked by ramose, plocoid colonies of Acropora spp; Heron Island, Great Barrier Reef, Queensland, Australia specialized forms, rugose and tabulate corals showing secure encrusting attachment or overgrowth as mature coralla are rare This reflects the presence of an external wall (epitheca or holotheca) around the corallum and the lack of an extensive edge zone of soft tissue, the reverse of the situation in most scleractinians Instead, the Palaeozoic corals were predominantly adapted to life on soft substrates They were subsidiary contributors to reef cores, although common in peri-reefal communities, and reached their greatest importance in bioherms and biostromes, with little evidence of frame building and were often associated with mudgrade carbonate environments Many show evidence of instability and smothering as soft substrates were disturbed by storm activity and sediment movement They reached their maximum diversity in warm shallow waters, with diversity decreasing with depth and towards the contemporary poles In addition, there was a distinctive low-diversity community of small, thick-walled, tabulate and solitary rugose corals characteristic of dysaerobic, dysphotic and other marginal environments that persisted from the Silurian to the Figure 12 Generalized ecological and palaeoecological ranges of (A) the Scleractinia, (B) the Tabulata, and (C) the Rugosa Depth not to scale, and divisions diagrammatic For further discussion see text Modified from Scrutton (1998, Figure 30) The Palaeozoic corals, II: structure, variation and palaeo ecology Proceedings of the Yorkshire Geological Society, 52: 57 with the permission of the Yorkshire Geological Society