Over the past quarter-century, detailed genus - and species-level similarities in cellular morphology between described taxa of Precambrian microfossils have been noted and regarded as biologically and taxonomically significant by numerous workers worldwide. Such similarities are particularly well-documented for members of the Oscillatoriaceae and Chroococcaceae, the two most abundant and widespread Precambrian cyanobacterial families.For species of two additional families, the Entophysalidaceae and Pleurocapsaceae, species-level morphologic similarities are supported by in-depth fossil-modern comparisons of environment, taphonomy, development, and behavior. Morphologically and probably physiologically as well, such cyanobacterial living fossils have exhibited an extraordinarily slow (hypobradytelic) rate of evolutionary change, evidently a result of the broad ecologic tolerance characteristic of many members of the group (and a striking example of Simpson's "rule of the survival of the relatively unspecialized"). In both tempo and mode of evolution, much of the Precambrian history of life - that dominated by microscopic cyanobacteria and related prokaryotes - appears to have differed markedly from the more recent Phanerozoic evolution of megascopic, horotelic, adaptationally specialized eukaryotes.
