The Ecology of the Cambrian Radiation

Calcified microbes expanded rapidly in abundance and diversity from NemakitDaldynian to Tommotian. This rapid diversification near the base of the Cambrian reflects a burst of cyanobacterial evolution, and commencement of an environmentally facilitated Cyanobacterial Calcification Episode that continued into the Ordovician. No new genera appeared during the Middle-Late Cambrian, and apparent diversity declined. Correlation between generic diversity and number of studies suggests that this decline might be a monographic artifact. Calcified microbes remained important components of shallow marine carbonates throughout the Cambrian. Most groups represent cyanobacteria (Angusticellularia, Botomaella, Girvanella, and Obruchevella groups), or probable cyanobacteria (Epiphyton, Proaulopora, and Renalcis groups). Chabakovia, Nuia, and Wetheredella are Microproblematica. Calcified microbes created rigid, compact reef frameworks. During the Early Cambrian they were commonly associated with archaeocyaths, but they continued their successful reef-building role into the Middle-Late Cambrian in the absence of a significant metazoan contribution. Distribution patterns suggest that filamentous and dendritic forms (Angusticellularia, Epiphyton, and Girvanella groups) preferred high-energy conditions and formed reefs in grainy locations; whereas botryoidal forms (Renalcis Group) formed mudstone-associated reefs in shelf and midramp environments. There is no evidence that calcified microbes were affected by metazoan grazing, disturbance, or competition during the Cambrian. Conversely, these microbes may have inhibited metazoan larval settlement and growth. Cambrian calcified algae are very scarce and are much less diverse than cyanobacteria. Amgaella, Mejerella, and Seletonella may be dasycladaleans. They are known only from the Middle (Amgaella) and Late (Mejerella and Seletonella) Cambrian of Russia and adjacent regions.

THE LONG-TERM HISTORY of microbes and metazoans has been seen as a displacement of prokaryotes by eukaryotes (Garrett 1970). In the Cambrian, it is tempting to

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