Timing the origin of cyanobacteria: sedimentology and stromatolites of the 2.72Ga Tumbiana formation

Geochemical evidence indicates that biogenic oxygen production had altered the redox state of Earth’s surface environments by 2.45Ga. Oxygenic photosynthesis in cyanobacteria is thought to have evolved somewhat earlier, but the evidence for this is poor. The early fossil record consists solely of microfossils and microbialites which offer few clues to the identities of the organisms that built them, however tufted microbial mats prese t a rare opportunity to constrain the behaviour and biological affinities of ancient microbialite building microorganisms. Here, the oldest convincing fossil tufted microbial mats are described alongside the other stromatolites and sedimentology of the 2.72Ga Tumbiana Formation, Fortescue Group, Western Australia. Extant tufted microbial mats growing in the hypersaline embayments of Shark Bay, Western Australia are described and compared to other ancient and modern examples. Significantly, all modern tufted microbial mats are built by filamentous and motile cyanobacteria