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Approximately 185 occurrences of fossil blue-green algae (excluding stromatolites and similar organo-sedimentary structures) have been reported since 1855. More than half of these occurrences have been described during the past 5 years and more than 75% since 1950. This recent, major expansion of the known cyanophytic fossil record is a direct result of the recent upsurge of interest in Precambrian paleobiology; nearly two thirds of all occurrences and 95% of those reported during the past 5 years are of Precambrian age. The majority of reported fossil cyanophytes are cellularly preserved in microcrystalline cherts. Evidence of ecologic setting, growth habit, general morphology, detailed cellular anatomy, and mode of reproduction is rather commonly present. Comparison of ossil and living taxa indicates that in all of these features, and presumably in ultrastructure and biochemistry as well, many of these primitive prokaryotes have evolved little or not at all since the Precambrian. The marked evolutionary conservatism of the Cyanophyceae is attributable to the wide ecologic tolerance, versatile physiology, and unusually stable genetic system characteristic of the class; a suitable ecologic niche, relatively free from competitors, has been accessible to these highly adaptive microorganisms since early in earth history. Evidence now available suggests that the earliest blue-green algae were unicellular coccoids, first appearing during the early Precambrian; that mat-building, filamentous cyanophytes had become established as early as 2.8 b. y. ago; that th class reached its zenith in evolutionary diversification and ecologic importance during the late Precambrian; and that the subsequent appearance of heterotrophic, mobile eukaryotes (protozoans and metazoans) resulted in adjustment of ecologic relations and a marked reduction in distribution and abundance of cyanophytic communities early in the Paleozoic.
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