ary, the morphological diversity and turnover rates of acritarch-producing protists increased significantly, apparently as part of a larger increase in eukaryotic diversity that included heterotrophs as well as algae. Most notably, the Proterozoic and Early Cambrian record of acritarchs suggests that radiating animals had a profound effect on both diversity and turnover within clades already present in marine communities, implying an important role for ecology in fueling the Cambrian explosion and, perhaps, earlier protistan diversification.

In rocks of late Paleoproterozoic and Mesoproterozoic age (ca. 1700–1000 million years ago), probable eukaryotic microfossils are widespread and well preserved, but assemblage and global diversities are low and turnover is slow. Near the Mesoproterozoic–Neoproterozoic boundary (1000 million years ago), red, green, and chromophytic algae diversified; molecular phylogenies suggest that this was part of a broader radiation of ''higher" eukaryotic phyla. Observed diversity levels for protistan microfossils increased significantly at this time, as did turnover rates. Coincident with the Cambrian radiation of marine invertebrates, protistan microfossils again doubled in diversity and rates of turnover increased by an order of magnitude. Evidently, the Cambrian diversification of animals strongly influenced evolutionary rates within clades already present in marine communities, implying an important role for ecology in fueling a Cambrian explosion that extends across kingdoms.

I thank Zhang Yun, Yin Leiming, Kathleen Grey, Zang Wenlong, Malcolm Walter, Tadas Jankauskas, Nina Volkova, Tamara German, Alexei Veis, Vladimir Sergeev, Nicholas Butterfield, and—especially—Gonzalo Vidal and Malgorzata Moczydlowska for access to and discussions about Proterozoic and Cambrian microfossils. George Miklos, Kenneth Campbell, and J. William Schopf provided helpful criticisms of an earlier draft. This work was sponsored in part by National Aeronautics and Space Administration Grant NAGW-893 and National Science Foundation Grant BSR 90-17747.

Allison, C. W. & Awramik, S. M. (1989) Organic-walled microfossils from earliest Cambrian or latest Proterozoic Tindir Group rocks, northwest Canada. Precambrian Res. 43, 253–294.

Aseeva, E. A. (1983) Vendian microfossils of the Ukraine, in Vend Ukrainy [The Vendian of the Ukraine], eds. Velikanov, V. A., Aseeva, E. A. & Fedonkin, M. A. (Naukova Dumka, Kiev, pp. 102–127).