Acritarchs are widespread and abundant in late Paleoproterozoic and Mesoproterozoic rocks, and in all known assemblages morphological diversity is limited to a few taxa of smooth-walled spheromorphs (leiosphaerids; Figure 4A) differentiated on the basis of size, spheroids bearing a single filament-like emergence, and/or somewhat lumpy or irregular vesicles (Keller and Jankauskas, 1982; Jankauskas, 1989).
Morphological diversification began in the late Mesoproterozoic Eon with the appearance of striated spheromorphic vesicles (Valeria lophostriata ) and the first acanthomorphic acritarchs (Jankauskas, 1989; Knoll, in press). Chinese rocks poorly dated at ca. 1100 Ma contain the oldest known large (>100 µm) acanthomorphs (Yan and Zhu, 1992; Figure 4D)—a type of microfossil characteristically found in Neoproterozoic assemblages (Semikhatov, 1991; Knoll, in press). The 900- to 1000-Maold Lakhanda biota of Siberia (German, 1990) contains a moderate diversity of both acanthomorphs and lightly ornamented spheromorphs (Figure 4C). Latest Mesoproterozoic to early Neoproterozoic acritarch diversification is complemented by the first appearance of multicellular eukaryotes that can be placed in extant phyla. Red algae that display clear cellular differentiation are locally abundant in silicified peritidal carbonates of the Hunting Formation, arctic Canada (Butterfield et al., 1990); probable chromophytic algae are beautifully preserved in Lakhanda mudstones (German, 1990); and several green algal taxa occur in the younger Svanbergfjellet Formation of Svalbard (Butterfield et al., in press).
Acritarchs increase in both total and assemblage diversity in 900-to 800-Ma-old rocks (Figures 1–3), due largely to the differentiation of ornamented forms. Vase-shaped protistan tests also proliferate at this time. Most taxa that originated during the early-to-middle Neoproterozoic Era disappeared by the time of the great Varanger ice age (?650–590 Ma ago), but post-Varanger successions on three continents contain diverse assemblages of new and highly ornamented acritarchs (Yin, 1987; Knoll, 1992a; Zang and Walter, 1992; Moczydlowska et al., 1993; Figure 4F and G). This postglacial diversification is all the more remarkable because it is so short-lived. Even exceptionally preserved latest Proterozoic acritarch assemblages are limited to a handful of leiosphaerids and small acanthomorphs. The extinction correlates stratigraphically with the appearance of diverse Ediacaran-type metazoans; where diverse acanthomorphs and Ediacaran remains occur in the same succession, the former lie stratigraphically beneath the latter. However, correlation to the independent chronostratigraphic record of C and Sr isotopic variation suggests that the two types of assemblage coexisted for a limited interval (Kaufman and Knoll, in press). Acritarchs again diversified rapidly during the Early Cambrian (Volkova et al., 1983; Moczydlowska, 1991; Figure 4H).