Figure 1 Species richness of selected protistan microfossil assemblages in 17 stratigraphic intervals running from the latest Paleoproterozoic era (P), through the Mesoproterozoic (Meso), Neoproterozoic, and Early Cambrian. Numbers 1–33 identifying assemblages refer to Table 1. V marks the Varanger ice age. Species richnesses are based on my taxonomic evaluation and do not necessarily reflect published tabulations. With a single exception (assemblage 31), all assemblages have been examined first-hand, resulting in uniform systematic treatment. Stratigraphic and systematic data for all figures and tables are available from the author.

The Nature and Limitations of the Record

Stratigraphic and Geochronometric Framework. The time interval considered here is 1700–520 Ma; that is, the latest Paleoproterozoic Eon to the end of the Early Cambrian Period (Figure 1 and Table 1). U-Pb dates on accessory minerals in volcanic rocks of known relationship to fossiliferous strata are limited for this interval—but then, such data are also limited for younger Paleozoic fossils on which much greater paleobiological demands are placed. Quantitative analysis of the Paleozoic fossil record is possible because a well-defined chronostratigraphic, or relative, time-scale has been calibrated by radiometric data in a few key sections.

The Proterozoic-Cambrian time scale is developing along the same



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