Figure 3 Histogram showing total species richness of protistan microfossils for the 17 intervals recognized in this study (Tables 1 and 2). The time scale along the abscissa is linear, underscoring the long initial interval of low diversity and the rapidity of later radiations. V marks the Varanger ice age. PAL, Paleoproterozoic; C; Cambrian.

tion-resistant polymer, sporopollenin.) Others, including most Neoproterozoic taxa, may also represent green algae (Tappan, 1980; Vidal and Knoll, 1983; Knoll et al., 1991), but phylogenetic relationships have not been established unequivocally.

The pre-Ediacaran record of seaweeds is too patchy for meaningful evaluation of evolutionary tempo, but these fossils do provide a paleobiological context for the interpretation of microfossil assemblages. In particular, fossils of multicellular algae relate the latest Mesoproterozoic and early Neoproterozoic diversification of acritarchs to the biological differentiation of ''higher" protists inferred from molecular phylogenies (Sogin et al., 1989; Knoll, 1992a).

If the view of eukaryotic evolution provided by molecular phylogenies is reliable, many of the protistan phyla that differentiated during the Proterozoic are not represented in the fossil record. Therefore, care must be exercised in ascribing generality to the preserved record. The prob-