DISPARATE RATES, DIFFERING FATES - TEMPO AND MODE OF EVOLUTION CHANGED FROM THE PRECAMBRIAN TO THE PHANEROZOIC

Over the past quarter century, detailed genus- and species-level simil arities in cellular morphology between described taxa of Precambrian m icrofossils and extant cyanobacteria have been noted and regarded as b iologically and taxonomically significant by numerous workers worldwid e. Such similarities are particularly well documented for members of t he Oscillatoriaceae and Chroococcaceae, the two most abundant and wide spread Precambrian cyanobacterial families. For species of two additio nal families, the Entophysalidaceae and Pleurocapsaceae, species-level morphologic similarities are supported by in-depth fossil-modern comp arisons of environment, taphonomy, development, and behavior. Morpholo gically and probably physiologically as well, such cyanobacterial ''li ving 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 G. G. Simpson's [Simpson, G. G. (1944) Tempo and Mode in Ev olution (Columbia Univ. Press, New York)] ''rule of the survival of th e relatively unspecialized.'' In both tempo and mode of evolution, muc h of the Precambrian history of life-that dominated by microscopic cya nobacteria and related prokaryotes-appears to have differed markedly f rom the more recent Phanerozoic evolution of megascopic, horotelic, ad aptationally specialized eukaryotes.