Morphological Changes in Microscopic–Megascopic Life and Stromatolites Recorded During Late Palaeoproterozoic–Neoproterozoic Transition: The Vindhyan Supergroup, India

Abstract

The Palaeoproterozoic to Neoproterozoic era (1,700–542 Ma) is a significant time of transition, reflecting major biotic events in the evolution of life at global level. This time has been marked by the domination of cyanobacterial prokaryotic community, emergence of eukaryotes and their subsequent radiation, transition from microscopic to megascopic life, emergence of metazoan and metaphytes (evolution from plant to animal clades) and evolution from unicellular to multicellular organization. Recent palaeobiological studies in the Vindhyan Supergroup, India, provide substantial data to establish evolutionary history of the Proterozoic life. The available fossil record throws light on biological/morphological changes at microscopic to megascopic level in different time frames in the Vindhyan Supergroup. Palaeoproterozoic to Mesoproterozoic fossil record of the Lower Vindhyans is marked by the diversity and domination of benthic cyanobacterial communities such as Eoentophysalis and Siphonophycus (this is also valid to some extent for rarely well-preserved Neoarchean microfossil assemblages; Kazmierczak and Altermann, Science 298:2351, 2002). In Semri Group, Lower Vindhyans, Glenobotrydion, Glaeodiniopsis, Eosynechococcus and Sphaerophycus are the other commonly occurring benthic coccoid forms. On the contrary, the Neoproterozoic life in the Bhander Group, Upper Vindhyans, is dominated by the planktic communities, like Myxococcoides and acritarchs of variable morphologies and dimensions. Presence of akinites/Archaeoellipsoides is very common in Palaeoproterozoic–Mesoproterozoic Vindhyan microfossil assemblages, which are totally absent in Neoproterozoic assemblages. Diverse cellular filaments of cyanobacterial affinity and domination of planktic coccoidal form genera are the common features of Lower Vindhyans. Rapid precipitation instantly entombed fragile trichomes and preserving them as organic-walled fossils, which were silicified prior to the neomorphic alteration of host carbonates. Intermediate carbonaceous fossil forms, exhibiting super-imposed size range of microscopic and megascopic fossils and inferred as the missing link between the evolution from microscopic to megascopic life, have already been recorded (Srivastava, Curr. Sci. 86:644–646, 2004) from the Rewa Group, Upper Vindhyans (Mesoproterozoic age). Neoproterozoic (Upper Vindhyan) microfossil assemblages are marked by the presence of highly diversified eukaryotes, presence of very small-sized, helically coiled filamentous cyanobacteria Obruchevella, a possible Volvox colony, emergence of large-sized acanthomorphs and exceptionally large-sized dividing cell-like unit and many other unidentified complex morphologies. The most important is the recently reported, process-bearing age-marker acanthomorph (of Cryogenian 850–630 Ma), Trachyhystrichosphaera from the Sirbu Shale Formation, Bhander Group, of the Vindhyan Supergroup, Rajasthan (Srivastava, J. Earth Syst. Sci. 118:575–582, 2009).