Estimates of Evolutionary Tempo

Figures 1 and 2 depict assemblage and total diversity through the interval from 1700 to 520 Ma ago. (Figure 3 plots total diversity data on a linear time scale to show more clearly the length of the initial interval of low diversity and the rapidity of subsequent changes.) The similarity of the figures is not surprising, given the wide facies and geographic distributions of many taxa. Species richness began low and rose only slightly during the first 700–800 Ma of the acritarch record—an interval longer than the entire Phanerozoic Eon. A burst of first appearances 900–800 Ma nearly doubled both assemblage and total diversity, bringing them to a new level that would persist with limited change until the Varanger ice age. The figures show diversity peaking 750–700 Ma ago and then declining to a minimum during the Varanger interval. However, intervals N5 and N6 are the most poorly sampled of the entire period under consideration. Taxa whose currently known last appearance is in N4 or N5 may well be discovered in closer proximity to Varanger strata, while some of the many acritarch taxa whose first known appearance is in N7 may be found in earlier intervals. For example, the large acritarchs Papillomembrana compta and Ericiasphaera spjeldnaessi, both conspicuous components of N7 assemblages, occur in clasts of the Biskopås Comglomerate, Norway, that underlie Varanger tillites (Vidal, 1990). On the other hand, few pre-Varanger taxa occur in the beautifully preserved assemblages that characterize N7, and in places like northwestern Canada, assemblages deposited just before the ice age contain only characteristically pre-Varanger taxa (Allison and Awramik, 1989). Thus, the marked change in assemblage composition across the Varanger interval is probably a stable feature of the record, and the extinctions inferred from the figures may have been concentrated in a brief interval before or during the Varanger ice age.

The high diversity of immediately post-Varanger acritarch assemblages is apparent from the figures. A burst of first appearances lifted both assemblage and total species richness to their Proterozoic maxima, and an ensuing maximum in last appearances subsequently reduced diversity to levels resembling those of the Mesoproterozoic and earliest Neoproterozoic. Seventy-five percent of recorded species disappeared, including most if not all large morphologically distinctive forms.

The first four intervals of the Early Cambrian exhibit sharp increases in numbers of first appearances; species richness within assemblages eclipsed its Proterozoic maximum in C3 (ca. 531–528 Ma), and total diversity peaked one interval later (C4, ca. 528–524 Ma ago). Last

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