Conclusions

On the whole, Vendian (pre-Nemakit-Daldynian) benthic communities were dominated by microbial ecosystems. Only in temperate conditions, usually coupled with active hydrodynamics, were more-complex communities developed. Although the Ediacara fauna is known from shallow-water onshore and offshore sandstones, deep-water turbidites, and shallow-water carbonates (Fedonkin 1987), it may be attributed to a single community of sessile filter and/or suspension feeders and to a few deposit feeders independently of the systematic interpretation of its members (Lipps et al. 1992).

Cambrian communities evolved by further partitioning ecologic niches and by replacing older forms by new ones within established trophic guilds. This is especially noticeable in ecospace utilization, as recognized by developed tiers and diverse feeding strategies (tables 10.1 and 10.2; see also figures 10.1 and 10.2). In this respect, the Cambrian biota was very different from the Neoproterozoic one, but also from younger Paleozoic and Meso-Cenozoic biotas (Bambach 1977; Ausich and Bot-tjer 1982; Bottjer and Ausich 1986). The relatively diverse Early Cambrian benthos was replaced during the Middle Cambrian by a simpler, trilobite-dominated Cambrian sensu stricto Evolutionary Fauna. Taxonomic impoverishment of Cambrian

Figure 10.4 Distribution of major marine groups composing the Cambrian biota, relative to salinity. Source: Modified after Debrenne and Zhuravlev 1997.

communities is observed in siliciclastic as well as in carbonate level-bottom communities. A similar trend is obvious in reefal communities (Zhuravlev 1996). In terms of taxonomic composition, proximal communities were the most conservative; distal, deep-water communities grew permanently by addition of new elements (Conway Morris 1989); and intermediate shallow shelf subtidal communities were the most changeable.

Displacement of communities was common. The earliest deep-water communities were derived from former shallow-water water elements (Bottjer et al. 1988; Crimes, this volume). On the other hand, a trilobite-lingulate community, which during the Middle Cambrian occupied normal marine conditions, at first appeared in dysaero-bic Early Cambrian basins (see figure 10.2.3).

Although Cambrian organisms occupied various depositional regimes, they were largely restricted to normal marine and to shallow subtidal conditions (figures 10.3 and 10.4). Possibly, the further spreading into these environments was one of the sources of higher Ordovician and Mesozoic-Cenozoic radiations.

Acknowledgments. We thank Mary Droser and Peter Crimes for helpful comments and Fran├žoise Pilard and Henri Lavina for the preparation of figures. This paper is a contribution to IGCP Project 366.

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