Abstract

The degradation-resistant organic-walled cell envelopes of acritarchs are the most abundant microfossils in Proterozoic and Cambrian rocks. These microfossils reveal diversity fluctuations that illuminate the nature of the record of primary producers near the Proterozoic/Phanerozoic boundary. Neoproterozoic radiations, some 1000-542 m.y. ago, reached levels comparable to those observed in the Cambrian Period. The microbiotas from rock successions from 13 Cambrian biochrons display significant fluctuations in the total number of microfossil taxa belonging to discrete microfossil assemblages. The assemblages reveal that Cambrian protist assemblages evolved over relatively short time spans, apparently out of low-diversity remnant populations after gradual declines in diversity. The characteristic microbiotas of the terminal Neoproterozoic and the Early, Middle, and Late Cambrian blossomed over relatively narrow time ranges, subsequently collapsing to nearly the initial levels. By virtue of the decreasing time spans involved in the late Vendian, Early, Middle, and Late Cambrian respectively, the tempo of specific turnover appears to have varied considerably. Speciation levels gradually decreased during Early and Middle Cambrian times and during Early Cambrian times were accompanied by rising levels of extinction. This latter feature seems to have reversed during Middle Cambrian times, lasting well into Late Cambrian times. Acritarchs were at the base of the marine trophic chain together with bacteria and other protists that are largely unrepresented in the fossil record. For this reason, the rise of diverse Cambrian protistan plankton must have been essential for early marine metazoan differentiation. Indeed, patterns of total diversity, speciation, and extinction of Cambrian acritarchs clearly mirror those of contemporaneous marine invertebrate faunas at the generic level.

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