The Biostratigraphic Paradox of Precambrian Cyanobacteria: Distinguishing the Succession of Microfossil Assemblages and Evolutionary Changes Observed among Proterozoic Prokaryotic Microorganisms

Resumo

Precambrian cyanobacteria demonstrate unprecedented evolutionary conservatism and have remained practically unchanged for the last 2 Ga, considering that ancient forms have counterparts among genera and even species of modern microorganisms. However, Proterozoic cyanobacteria and other prokaryotes form unique assemblages of restricted geochronological range and broad spatial distribution. The most remarkable of these assemblages are the Gunflint-type Paleoproterozoic microbiotas and the Early Riphean Archaeoellipsoides-dominant Kotuikan-type assemblages. Their observed taxonomic uniqueness and short geochronological ranges reflect irreversible changes in the Earth’s global environments, rather than evolutionary innovations. Nonetheless, the fossil blue-green algae demonstrate some evolutionary changes throughout the Proterozoic: the stalked cyanobacteria Polybessurus appeared in the Middle Riphean and the spiral cylindrical cyanobacteria Obruchevella emerged in the Late Riphean.