Biological Interactions and Precambrian Eukaryotes

Evidence of Precambrian biotic interactions can be sought in the paleo- cological distribution of ancient microbial fossils. Within Proterozoic stromatolitic microbiotas, for example, discrete recurrent associations of taxa are found, various mat-dwelling microbes often occurring in specific association with a single type of mat builder (Knoll, 1981, 1982). It can be inferred from this that some mat-dweller populations took particular advantage of microenvironments created by the activity of mat-building cyanobacteria, a type of biological interaction analogous to the relationship of reef-dwelling invertebrates to framework builders in the same community. The distribution of microbial mat associations themselves, however, appears to have been in large part physically controlled, with mat communities distributed along environmental gradients in intertidal to shallow subtidal coastal areas. Planktonic microfossils in Late Precambrian sedimentary sequences also exhibit diversity differences between inshore and offshore environments, and again the simplest explanation for their paleoecological distribution involves responses to physical environmental factors, including salinity (Vidal, 1976; Knoll, 1983; Vidal and Knoll, 1983).