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The following HTML text is provided to enhance online readability. Many aspects of typography translate only awkwardly to HTML. Please use the page image as the authoritative form to ensure accuracy. TABLE 2 Species richness, rates of origination, and rates of extinction for Proterozoic and early Cambrian acritarchs
appearances also increase throughout these intervals and exceed first appearances at the end of the Early Cambrian. Table 2 shows calculated rates of cladogenetic evolution for each of the intervals under consideration. For intervals M1 though N1, both total and per taxon rates of first and last appearances are low, indicating not only that diversity was low but also that constituent species were long lasting. (The calculated rates of first appearance for M1 may be misleading, in that all species are recorded as first appearances. The presence of simple acritarchs in rocks that may be older than 1700 Ma indicates that at least some of these forms may have originated earlier.) By 900–800 Ma ago (N2), total rates of origination had increased by an order of magnitude to a level at which they remained for the duration of the pre-Varanger Neoproterozoic. Interestingly, after an increase during interval N2, per taxon rates of origination returned to levels comparable to earlier intervals; both total and per taxon extinction rates increased toward the Varanger ice age. Another order of magnitude increase in origination and extinction
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