Schopf's compilations of mean assemblage diversity for plankton and eukaryotes emphasize the inferred early Neoproterozoic diversity peak even more strongly. This discrepancy arises for at least three reasons: (i) Schopf's estimates of species richness for early Neoproterozoic assemblages from Russia significantly exceed those accepted here, (ii) most of the fossils that determine the diversity levels of intervals N4 to N7 in the present paper do not appear in Schopf's data set, largely because of recent discovery, and (iii) Schopf's calculation of mean assemblage diversity is swamped by low diversity assemblages of limited paleobiological value. For these reasons, I believe that the diversity trends shown in Figures 13 of the present paper better reflect the known record of early protists.

Intimations of Mode? As noted above, the increase in acritarch diversity and tempo near the Mesoproterozoic–Neoproterozoic boundary coincides with the appearance of identifiable red, green, and probable chromophyte algae in the record. Branching patterns in molecular phylogenies of the eukaryotes suggest that these algal taxa, along with stramenopiles (ciliates, dinoflagellates, and plasmodia), fungi, and the ancestors of animals, diverged rapidly relatively late in the history of the domain (Sogin et al., 1989). The paleontological data suggest that the radiation implied by molecular phylogenies occurred near the Mesoproterozoic–Neoproterozoic boundary; phylogenetic data, in turn, suggest possible explanations for the acceleration of evolutionary tempo documented by the fossils.

Nuclear introns, multicellular development that includes coordinated growth and cellular differentiation, and life cycles in which classical meiosis plays a prominent role are all characters displayed by higher eukaryotes but not earlier branching clades (Cleveland, 1947; Margulis et al., 1989; Tibeyrence et al., 1991; Palmer and Logsdon, 1991). The evolutionary relationships among these features are poorly understood, but possibly not coincidental. Either sexual life cycles or the exon shuffling made possible by introns could increase genetic variation and, thereby, accelerate evolutionary tempo (Schopf et al., 1973; Knoll, 1992a). This would be true of nuclear introns whether they first evolved at the time of higher protistan differentiation (Palmer and Logsdon, 1991) or were simply retained more readily in lineages characterized by sexual life cycles (Hickey, 1982).

Given the population genetic possibilities of such changes, it is surprising that the greater increase in acritarch diversity and tempo is concentrated at the beginning of the Cambrian Period. At this time, there is no evidence of genetic reorganization. New faster evolving clades may enter the acritarch record, but groups such as the prasino-

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