BIODIVERSITY, SPECIATION, AND EXTINCTION TRENDS OF PROTEROZOIC AND CAMBRIAN PHYTOPLANKTON

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/Phanero zoic boundary, Neoproterozoic radiations, some 1000-542 m.y, ago, reac hed levels comparable to those observed in the Cambrian Period. The mi crobiotas from rock successions from 13 Cambrian biochrons display sig nificant fluctuations in the total number of microfossil taxa belongin g to discrete microfossil assemblages. The assemblages reveal that Cam brian protist assemblages evolved over relatively short time spans, ap parently out of low-diversity remnant populations after gradual declin es in diversity. The characteristic microbiotas of the terminal Neopro terozoic and the Early, Middle, and Laee Cambrian blossomed over relat ively narrow time ranges, subsequently collapsing to nearly the initia l levels. By virtue of the decreasing time spans involved in the late Vendian, Early, Middle, and Late Cambrian respectively, the tempo of s pecific turnover appears to have varied considerably. Speciation level s gradually decreased during Early and Middle Cambrian times and durin g Early Cambrian times were accompanied by rising levels of extinction . This latter feature seems to have reversed during Middle Cambrian ti mes, lasting well inter Late Cambrian times. Acritarchs were at the ba se oi the marine trophic chain together with bacteria and other protis ts that are largely unrepresented in the fossil record. For this reaso n, the rise of diverse Cambrian protistan plankton must have been esse ntial for early marine metazoan differentiation, Indeed, patterns of t otal diversity, speciation, and extinction of Cambrian acritarchs clea rly mirror those of contemporaneous marine invertebrate faunas at the generic level.