Figure 2 Total taxonomic richness (histogram), first appearances (solid dots), and last appearances (open dots) of protistan microfossil species for the 17 intervals recognized in this study (Tables 1 and 2). V marks the Varanger ice age.

Abbreviations are as in Figure 1.

fossils known as acritarchs (Figure 4). Structural features leave little doubt that all or nearly all were eukaryotic. Most were the vegetative and reproductive walls of unicellular protists, although the reproductive cysts of multicellular algae and even egg cases of early animals may be included.

The total number of clades that contributed to the observed record is unknown, but probably small. Some of the Early Cambrian microfossils included here are clearly the phycomata of green algal flagellates (Tappan, 1980). (The phycoma is a nonmotile vegetative stage of the flagellates' life cycle characterized by a wall that contains the degrada-



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