RUDISTS AS GREGARIOUS SEDIMENT-DWELLERS, NOT REEF-BUILDERS, ON CRETACEOUS CARBONATE PLATFORMS

It has sometimes been claimed that rudist bivalves competitively displ aced corals from reef frameworks during the Cretaceous. This hypothesi s combines two assertions: (1) that the autecology of rudists was conv ergent with that of reef-building corals; and (2) that rudist formatio ns commonly developed as reefs. We dispute both assertions, and thus r eject the hypothesis of competitive displacement. We argue instead tha t mobile sediments, rather than frameworks, dominated the margins and tops of most of the extensive carbonate platforms of the period, and t hat it was on these, non-reefal, biotopes that the rudists flourished. Definitions of reefs tend to combine two major elements: (1) a robust biogenic framework (with accompanying sedimentary and diagenetic comp onents); and (2) topographical relief. Such definitions are clearly ro oted in Recent coral reefs, in which endosymbiotic zooxanthellae permi t the extensive growth of colonial coral frameworks in shallow but rel atively nutrient-poor waters, coralline algae and cementation may cont ribute significantly to the growth of rigid structures, and topography is largely the legacy of Pleistocene changes in sea-level. In rudist formations, in contrast, individual rudist congregations are volumetri cally limited, relative to sediment. They are often loosely constructe d, and they evidently showed little, if any, original relief. Tabular and small lenticular units predominate. These differences in structure and palaeoenvironmental situation between rudist and modern coralgal associations reflect the different autecologies of the constituent org anisms. The clonal growth of corals predisposes them to the developmen t of frameworks projecting above the sediment surface (herein termed s uperstratal growth). By contrast, the aclonal development of rudists w as better suited to the opportunistic occupation of a variety of tempo rarily available substrata, by large numbers of individuals. In partic ular, elevator rudists (in which the entire commissure exhibited upwar d growth) evidently grew with their shells largely embedded in, and su pported by, the ambient sediment (herein termed constratal growth), Mo reover, the tolerances and growth responses of rudists to such factors as water turbidity, nutrients and current regime were quite different from those of the majority of reef-building corals. Despite repeated assertions in the literature that rudists possessed zooxanthellae, onl y a few species show any evidence for such a symbiosis and other evide nce suggests that most lacked them. In view of these differences in th eir preferred biotopes, competition between rudists and corals is doub tful, even though members of both groups co-occur in many areas. The r elative decline of coral frameworks in the Cretaceous was thus probabl y independently caused.