TABLE 3 Estimates of species durations

Taxonomic group

Half-life, Ma

Mean duration, Ma

Acritarch cohort M1

1390a

1960a

Acritarch cohort N2

75

102

Acritarch cohort C3

5.4

7.7

Planktonic foraminifera

5

7

Dinoflagellates

9

13

Diatoms

5.5

7.9

All invertebrates

7.7

11.1

Half-life (median species duration) and mean duration were calculated according to Raup (1978, 1985). Data for Phanerozoic protists and invertebrates are from Van Valen (1973) and Raup (1985).

a Of course, species that originated <1700 Ma ago cannot have true durations that exceed this age. Durations presented were calculated from the mean extinction rate (the slope of the cohort survivorship curve), which is very low.

However, it is unlikely that any combination of sampling, taxonomic, or geochronometric errors exerts a major control on the principal pattern revealed by this analysis—that of doubling in diversity and ten-fold increase in rates of origination and extinction near the Mesoproterozoic–Neoproterozoic boundary and again in the Early Cambrian. Indeed, this exercise quantifies what we have long known as biostratigraphers—that acritarch-based correlation is difficult among Mesoproterozoic successions, somewhat easier in the Neoproterozoic, and precise in the Lower Cambrian.

Comparisons with Previous Diversity Estimates. The diversity trends noted here are broadly similar to those outlined a decade ago by Vidal and Knoll (1983), indicating an overall stability of pattern despite substantial increases in the numbers of species and assemblages described. There is one difference between the two estimates, however, and it is a major one; Vidal and Knoll's compilation lacks any inkling of the short-lived diversity peak in N7. That peak first appears in the literature in 1988 in a figure by Zang (1988), who discovered highly diverse N7 acritarchs in the Pertatataka Formation, Australia. Since then, even more diverse assemblages have been recognized in rocks of this age (Figure 1). The N7 spike cautions us that despite the overall stability of Proterozoic and early Cambrian acritarch diversity trends, short-lived diversification and extinction episodes may be missed at current levels of temporal sampling density.

A different view of Proterozoic protistan diversity was presented by Schopf (1992), who showed a broad diversity peak 1000–850 Ma ago, followed by a strong and continuing decline until the end of the eon.



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