The fossil studied was first described in 1959 as Aeolisaccus kotori Radoicic, a new species of a problematic fossil worm, Aeolisaccus Elliott. In 1975 De Castro recognized the true identity of this microbial fossil: a cyanobacterium related closely to the modern genus Scytonema. The fossil is common in the sediments of the Mesozoic carbonate platforms of southern Europe. This contribution confirmed De Castro's interpretation and determined, using the high resolution of the SEM, the extent to which these fossils preserved or diagenetically modified their original architecture. In addition, it investigated their presumed modern counterparts among the abundant mat-forming species of Scytonema on the intertidal flats of Andros Island, a part of the Bahama carbonate platform. The systematic affinities of the fossil and the environments it inhabited were reconstructed by comparing its morphology to that of the modern counterparts, along with their respective sedimentary contexts. Based on these comparisons, we conclude that the organism lived in a peritidal environment and was buried and fossilized in the shallow waters of an ancient carbonate platform. A formal transfer of the fossil to a new genus of fossil cyanobacteria designated as Decastronema gen. nov., and thereby honoring the contribution of Prof. Piero De Castro, to paleontology, is proposed.