Vindhyan akinites: an indicator of mesoproterozoic biospheric evolution.

A wide size range of rod-shaped, ellipsoidal akinites assignable to Archaeoellipsoides are reported from the Newari locality of the Mesoproterozoic Kheinjua Formation of the Semri Group, Vindhyan Supergroup. These akinites of heterocystous cyanobacteria (Archaeoellipsoides) represent the smallest of the forms reported from any other assemblage to date and are well comparable to the akinites of modern bloom forming Anabaena species. Like any other Mesoproterozoic microfossil assemblage, The Newari microfossil assemblage is also dominated by cyanobacterial population, but the presence of Archaeoellipsoides (akinites) or heterocyst forming Nostocales and Stigonematales are rather rarely reported. These fossils set a minimum date for the evolution of derived cyanobacteria, capable of marked cell differentiation, and they corroborate geochemical evidence indicating that atmospheric oxygen level was above 1% of present day level during Mesoproterozoic time. In the presence of oxygen a protected environment for nitrogenase (an oxygen sensitive nitrogen fixing enzyme) is produced by these akinites, which were abundant in coastal communities of Mesoproterozoic shallow marine carbonates. It is therefore interpreted that the presence of Vindhyan's akinites indicate Mesoproterozoic biospheric evolution.