Palaeobiology of Mesoproterozoic Salkhan Limestone, Semri Group, Rohtas, Bihar, India: Systematics and significance

Mesoproterozoic (similar to 1600 Ma old), Salkhan Limestone (Semri Group) of the Vindhyan Supergroup, exposed in Rohtas district of Bihar, India, preserves an abundant and varied ancient microbial assemblage. These microfossils are recorded in three distinctly occurring cherts viz., bedded chert, stromatolitic chert and cherty stromatolites. 27 morphoforms belonging to 14 genera and 21 species have been recognized. Six unnamed forms are also described. The microbial assemblage, almost exclusively composed of the remnants of cyanobacteria, is dominated by entophysalidacean members and short trichomes and can be termed as 'typical Mesoproterozoic microbiotas'. The assemblage includes characteristic mat-forming scytonematacean and entophysalidacean cyanobacteria. Eoentophysalis is the dominant organism in the assemblage. Ellipsoidal akinetes of nostocalean cyanobacteria (Archaeollipsoides) and spherical unicells also occur; both are distinct from mat forming assemblage, allochthonous and possibly planktic. Co-occurrence of the microbiotas and precipitates is related to the depositional environment of the Mesoproterozoic tidal flats with high carbonate saturation.