phytes that appear to have been important on both sides of the Proterozoic–Cambrian boundary also document the acceleration of cladogenetic tempo. Of course, the sharp increase in acritarch diversity and turnover coincides with a comparable evolutionary burst in animals. The nearly simultaneous radiation in two such phylogenetically, developmentally, and trophically disparate groups suggests the importance of ecology in determining the tempo of Cambrian (and later) evolution. Evolving animals would have contributed in several ways to the complexity of environments perceived by acritarch-producing protists: for example, through predation, the disturbance of pre-existing physical environments, the creation of new physical environments, and the alteration of nutrient fluxes in marine platform and shelf waters. Diversifying protists would have had reciprocal effects on animals. Diversity levels reached by Early Cambrian animals and protists were later eclipsed by continuing diversification, but the increased rates of turnover established at this time have persisted for the past 500 Ma (Table 2; Van Valen, 1973; Raup, 1985).

This is interesting in light of evidence that turnover in Phanerozoic marine communities may be coordinated among species and concentrated at times of environmental disturbance represented sedimentologically by sequence boundaries (Brett et al., 1990; Morris et al., 1992; Miller, 1993). This suggests that the basal Cambrian increase in the biological complexity of environments may have lowered the response thresholds of populations to physical fluctuations, perhaps by decreasing population sizes and effective niche breadth.

The short-lived acritarch radiation in N7 stands out as anomalous. Is this when faster evolutionary tempo was established in protists, only to be cut off by mass extinction? Might it correspond to an epoch of cryptic animal diversification that presaged the Ediacaran faunas of the next interval? Is the acritarch diversification causally related to oceanographic changes that accompanied the end of the Varanger glaciation, and if so, why aren't comparable changes observed in the wake of earlier Neoproterozoic ice ages?

Conclusions

We still glimpse early biological history through a glass darkly, but broad patterns are beginning to come into focus. These patterns suggest that on the time scale of eukaryotic evolution as a whole, evolutionary tempo has increased episodically. Morphological diversity and turnover rates were low for the earliest recorded period of early protistan evolution, an interval that lasted longer than the entire Phanerozoic Eon. Near the Mesoproterozoic–Neoproterozoic bound-



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