Tubular forms

The tubular components of the Tirohan stromatolitic biota mostly consist of aggregating masses of entangled filaments, interpreted to represent filamentous cyanobacteria [52,56]. Furthermore, scattered among the stromatolitic layers and the intercolumnar debris are isolated tubular structures. Most of these are less than 10 μm in width and are probably sheaths of filamentous bacteria or chains of bacterial cells. Some tubes, however, are considerably larger and represent potential eukaryotic organisms. Being rare, these have not been observed in petrographic sections but have been recovered by manual selection from acid residues.

The larger tubes vary in width from 58 to 275 μm. Similar forms from the Tirohan phosphorites were initially reported and interpreted as Cambrian taxa, Cambrotubulus decurvatus and Hyolithellus vladimirovae [55]. The ensuing controversy about the age of the Lower Vindhyan and the provenance of the fossils was resolved by the demonstration that the Tirohan fossil tubes were morphologically analogous to modern filamentous algae rather than to the named Cambrian taxa and that Pb–Pb isochron analysis confirmed a late Palaeoproterozoic rather than Cambrian age of the phosphorites [52]. However, the biological nature of these enigmatic fossils has not until now been seriously investigated.

Our microtomography data clarify the internal structure of the tubular fossils, suggesting that they represent two significantly different organisms, here named Rafatazmia chitrakootensis n. gen., n. sp. and Denaricion mendax n. gen., n. sp. Rafatazmia ranges in width from 58 to 175 μm, Denaricion from 130 to 275 μm. Rafatazmia consists of concatenated cell-like compartments, often collapsed or distorted, resulting in buckling and folding of the exterior, whereas Denaricion has a solid internal structure similar to a stack of coins, upholding the integrity of the outer shape.