Palaeontological Institute, Russian Academy of Science, Profsoyuznaya 123, Moscow 117868, Russia

Abstract

The unique diversity of modern reef is maintained to a significant extent by frame-building organisms which due to their modular habit provide three-dimensional space suitable for tiny niche partitioning. Many principal groups of reef-builders in the Phanerozoic history of the Earth demonstrate a regular pattern of the appearance and progressive development of modularity from low-modular non-communicating forms to high modular communicating forms that are true colonies. Already the earliest skeletal modular organisms (archaeocyaths and other sponges, radiocyaths, coralomorphs, renalcids) show such a pattern. A comparison of Cambrian reef-building organisms with their relatives indicates the superiority of modular forms over solitary ones among them. Among advantages of reefal modular organisms over their solitary relatives there are: (1) larger individual size, (2) increased gregarity, (3) higher growth rate, (4) increased survivoship, and (5) strengthened anchoring to the substrate. The appearance of true colonies as utterly developed modular forms in different evolutionary lines may be evident for independence of development of coloniality and asexual reproduction.