(Knoll, 1992b). Observations such as these suggest that differential preservation is not a principal determinant of observed diversity and turnover patterns.
In general, Proterozoic and Cambrian acritarch species have wide (and known) paleoenvironmental distributions and show little evidence of provincialism. This minimizes the likelihood that observed patterns are influenced strongly by differential sampling of facies among time intervals.
Perhaps the best indicators of sample quality are the degree of similarity among assemblages within an interval and the incremental taxonomic richness accompanying each new assemblage reported. The total number of assemblages known for the interval from 1700 to 1000 Ma is low, but the taxonomic similarity among samples is high. Insofar as knowledge of the age and environmental setting of an assemblage permits the prediction of taxonomic composition, the Proterozoic and Early Cambrian acritarch record appears to be sufficiently well sampled to permit the broad estimation of evolutionary tempo.
Despite my confidence that the existing record is governed more by evolution than by sampling, this paper should be read as a dispatch from the trenches and not as a definitive history. The events most likely to have escaped notice to date are short bursts of diversification and extinction of the type recorded in interval N7. The time intervals most likely to yield new assemblages that will modify the conclusions drawn here are those between 750 and 600 Ma ago, just prior to and including the Varanger ice age.
Acritarchs occur in rocks as old as 1900–1700 Ma (Zhang, 1986). The fossils are morphologically simple, but sedimentological distributions, size frequency distributions, and inferred excystment structures ally these remains to unequivocally eukaryotic microfossils that extend upward from this interval to the present. Independent evidence for the occurrence of late Paleoproterozoic to early Mesoproterozoic eukaryotes comes from significant sterane concentrations in bitumens (Summons and Walter, 1992) and problematic macrofossils (Walter et al., 1990; Han and Runnegar, 1992). There is no reason to assume that these remains document the evolutionary first appearance of the Eucarya nor even any necessary reason to insist that they mark the emergence of clades capable of synthesizing preservable walls or cysts. What can be concluded is that eukaryotic organisms were significant parts of marine ecosystems in the late Paleoproterozoic Era and that the fossil record of earlier periods is poor.