Carbonate microbial stromatolites occur today (Fig. 1a, b, d) that in size, shape, and laminar structure are much like those known from the Precambrian (Fig. 1c, e,

f). Such modern stromatolites are usually restricted to refugia, environments such as hypersaline lagoons (Fig. 1a, b, d) in which the slow-growing microbial mats are not disrupted by grazing and burrowing Pevonedistat metazoans. For this reason, stromatolites are not particularly abundant in sediments of the Phanerozoic, deposits laid down in environments dominated by diverse types of metazoans. But in the absence of grazing and burrowing animals, as was the situation until the very end of the Precambrian, stromatolites were abundant and morphologically varied in shallow-water carbonate deposits worldwide. Known earliest from rocks ~3,500 Ma in age, their distribution over time selleck chemicals parallels that of the surviving Precambrian rock record—that is, stromatolite-bearing rock units become less and less abundant as the record of increasingly older rocks gradually

peters out. Such structures establish the presence of flourishing photosynthesis-based microbial communities, but only rarely do they preserve the cellular fossils that might indicate whether the stromatolite-building photoautotrophs were oxygenic, like cyanobacteria, or anoxygenic, like photosynthetic bacteria. Fig. 1 Modern and fossil stromatolites. a Modern stromatolites at Shark Bay (Hamelin Pool), Western Australia. b Modern Shark Bay

columnar and domical stromatolites for comparison with (c) fossil stromatolites from the ~2,300-Ma-old Transvaal Dolomite, Cape see more Province, South Africa. d–f Modern and fossil vertically sliced columnar to domical stromatolites showing upwardly accreted microbial laminae from Shark Bay (d), the ~1,300-Ma-old Belt Supergroup of Montana (e), and the ~3,350-Ma-old Fig Tree Group of the eastern Transvaal, South Africa (f). Scale for a and c shown by the geological hammers Isotretinoin enclosed by red circles Archean stromatolites As is shown in Fig. 2, an impressive number of Archean-age geological units—of particular interest because of their potential bearing on the time of origin of oxygenic photosynthesis—are known to contain microbially produced stromatolites. Shown in Fig. 3 are representative examples: carbonate sediments of the ~2,723-Ma-old Fortescue Group of Western Australia contain domical, pseudocolumnar and branching stromatolites (Fig. 3a and b); those of the ~2,985-Ma-old Insuzi Group of South Africa include stratiform and conical forms (Fig. 3c and d); and those of the ~3,388-Ma-old Strelley Pool Chert of Western Australia contain close-packed conical stromatolites patchily distributed over many tens of square kilometers (Fig. 3e through g). The presence of conical stromatolites in such deposits, like those shown in Fig. 3c through g and reported from 17 of the 48 units listed in Fig. 2 (Hofmann et al.