Figure 5 Cohort survivorship curves for species that originated during intervals M1 (1700–1400 Ma), N2 (900–800 Ma), and C3 (531–528 Ma). The abscissa denotes time since cohort origin.

rates attended the brief post-Varanger diversification event, after which terminal Proterozoic turnover returned to average Neoproterozoic levels. During the peak of the Cambrian acritarch radiation, origin and extinction rates both increased to levels an additional order of magnitude above the Neoproterozoic mean.

Cohort survivorship provides another means of evaluating evolutionary tempo (Van Valen, 1973; Raup, 1978, 1985). A comparison of the three cohort survivorship curves in Figure 5 shows that species originating in early Cambrian interval C3 turned over much more rapidly than those of Neoproterozoic cohort N2, which in turn decayed more quickly than Paleo- and Mesoproterocoic cohort M1. Very rough estimates of mean species duration and half-life (median species duration) confirm the order of magnitude increases in tempo between the Mesoand Neoproterozoic and again between the Neoproterozoic and Early Cambrian (Table 3). Thus, relative to earlier intervals, Cambrian acritarch assemblages contain more and more species that survive for shorter and shorter periods of time. Mean species duration and half-life for Cambrian acritarchs is similar to values computed for both younger protistan groups with good fossil records and Phanerozoic animal taxa (Table 3).

Discussion

How seriously should we take these figures? The general problems of sampling, data set size, and age estimation have already been noted. Imprecisions could easily alter estimates of tempo by a factor of two.



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