GUILD-STRUCTURE AND EVOLUTION OF MESOZOIC BENTHIC SHELF COMMUNITIES

On the basis of the relative abundance of species in 334 literature-derived paleocommunities the evolutionary history of Mesozoic benthic marine macroinvertebrate associations is evaluated quantitatively. The data base is analyzed within a framework of five broad environmental subdivisions (salinity-controlled environments, coarse-grained and fine-grained shallow shelf, mid to outer shelf, and oxygen-controlled environments), six time units, and 13 guilds. The guild-spectrum tends to be broadest in habitats controlled by biological factors. Under more rigorous and variable physical conditions ecospace is utilized less intensely than in stable and predictable environments. The quantitatively documented long-term patterns confirm the results of previous studies which applied a more qualitative approach. Both persisting and evolving aspects are evident: The permanence of various guild-assemblages and the evolutionary stability of morphotypes in all studied environments are outstanding features. Superimposed on these conservative aspects is a general trend towards increasingly shallow to moderately deep infaunal suspension-feeding life habits. This infaunalization can be traced back to the success of siphonate heterodont bivalves. Their evolutionary history illustrates the causal relation between major innovations, polyphyletic radiations, and the rise in relative abundance and biomass of an ecological group. The net increase of infaunal organisms was largely controlled by a rise in Mesozoic predation pressure and the adoption of new strategies.