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Clade Perseverance From Mesozoic to Present: a Multidisciplinary Approach to Interpretation of Pattern and Process

British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge, CB3 OET, United Kingdom

Two clades of marine bryozoans, cyclostomes and cheilostomes, exemplify the benefits of applying a multidisciplinary approach to the interpretation of long-term evolutionary patterns. The cyclostome bryozoans were dominant in the Mesozoic; since that era, they have decreased in absolute terms and the cheilostomes have come to exceed them in both abundance and diversity. Many studies of living assemblages of the encrusting members of these two clades indicate that cheilostomes are superior space competitors, but paleontological studies suggest that competition between the two taxa has not been escalating over geological time. Both clades occur throughout the world’s oceans and seas, and recent work in the geographical extremes has shown that the relative success of the clades varies markedly from place to place. In this study, the importance of differential patterns of recruitment and cumulative space occupation in the two clades was evaluated over four years and in two environments, one temperate and one polar. In both of these environments, peaks of recruitment and space occupation by the two clades were out of phase. The different strategies and outcomes of spatial competition are examined, largely using data from the literature. Only recently has it been realized that tied outcomes of competition are stable alternative results and not simply transitory phases. Many competitive encounters involving cyclostomes result in ties, implying that their strategy is based on persistence rather than dominance. When different indices and models are used to analyze competition data from the two clades, the interpretation varies markedly with methodology. The differences in patterns of recruitment, space occupation, and spatial competition have influenced both our understanding of how the two clades have persisted alongside each other and our perception of cheilostome superiority. Analysis of fluid dynamics has shown that small differences in the mechanical structure of typical members of each clade lead to fundamental differences in water movement. For animals that rely on water motion for transport of nutritional and excretory elements (suspension feeders), small changes in current velocity and direction can have a major impact. Preliminary chemical analysis of the excurrent stream leaving cheilostome colonies has shown it to be laden with excretory products, which can interfere and mix with a neighbor’s feeding currents. Clearly, spatial competition involves more than a simple mechanical "showdown."