Problematic fossils from the Palaeo-Neoproterozoic Vindhyan Supergroup, India

Abstract

Fossils of the Vindhyan Supergroup exhibit extensive diversity and variable biologic affinities represented by: bacteria, cyanobacteria, algae, fungi, acritarchs, metaphytes and metazoans (including members of the Ediacaran Fauna) and ranging from less than a micron to almost a metre in size. Besides identified fossils, a number of bizarre morphologies (due to deviation of morphology from conventional structures), present in various stratigraphic horizons, have been observed. It is very difficult to identify and decide their biologic affinities. In thin sections of Lower Vindhyan cherts, microfossils resembling lichen-like or fungal forms in which a sac encompassing a coiled filament may possibly indicate a symbiotic relationship, a Volvox colony-like structure and a vase-shaped body without an opening are unique. Among the carbonaceous fossils, very unusual and interesting fossil is a transparent disc comprising numerous appendages of an unidentified mesoscopic insect-like organism. Megascopic branching and associated Grypania-like structure is another form preserved as impression on micritic limestone. Petrographic thin sections of chert belonging to the Sirbu Shale Formation, exhibit presence of microscopic bizarre forms. The assemblage includes acritarchs and acanthomorphs of variable morphology and a dividing cell-like structure interpreted to be of rhodophycean affinity or a cleaving embryo of an animal affinity. Other peculiar morphologies among the carbonaceous fossils are: branched filaments that have attached sporangia-like vesicles, Chuaria-like body comprising cluster of very small-sized spheroids resemble scale-like structure, a chrysophycean alga or a multicellular tissue of a metaphyte. Another carbonaceous fossil represents a possible metazoan exhibiting an elongate body and a mid-gut-like structure or a Vaucheriacean alga. Although the biologic affinities of these bizarre forms can be a matter of debate, their biogenic nature is almost undoubted. The presence of such forms in the Vindhyans indicates advancement in morphology and a gradual evolution of life during the Palaeoproterozoic–Neoproterozoic period. In addition, presence of Ediacaran fossils in Bhander Group and large-sized acritarchs especially Trachyhystrichosphaera sp. in petrographic thin section of chert from the Sirbu Shale Formation, Bhander Group, Upper Vindhyans, suggests Ediacaran age as an upper age limit of the Vindhyan Supergroup.