Although micropaleontological study of the Proterozoic was already underway nearly a century ago (Walcott 1899), significant progress has been of surprisingly recent vintage. Indeed, of the 2800 occurrences of authentic microfossils now known from sediments of Proterozoic age (Table 22.3), more than 85% have been reported during the past two decades (Section 5.2). Reported chiefly from shales (62% of occurrences) and stromatolitic cherts (38%), planktonic (1580 occurrences in 248 formations) and benthic taxa (1219 occurrences in 208 formations) are now well known from units worldwide (Sections 5.4, 5.5; Chapter 22).

Unfortunately, however, meaningful interpretation of these newly available data is difficult, largely because of the lithology-related differing systems of classification and taxonomy of such microfossils that are currently in use. In particular, the majority of microfossils preserved as essentially two-dimensional compressions in Proterozoic shales, extracted from their surrounding matrix by palynological techniques and studied in acid-resistant residues, have been classified as “acritarchs”– organic-walled microfossils, commonly spheroidal, of uncertain systematic position–and have therefore been grouped into morphological categories by use of an “artificial” (non-biologic based) system of classification. In contrast, because of their three-dimensional preservation and morphological comparability to extant bacteria, cyanobacteria, and eukaryotic microalgae, permineralized microfossils detected in Proterozoic cherts and studied in petrographic thin sections have generally been grouped into the same categories as those used for classification of living microorganisms.