Mukund Sharma

Late Palaeoproterozoic (∼1800–1600 Ma) stromatolites, Cuddapah Basin, southern India: cyanobacterial or other bacterial microfabrics?

Abstract Late Palaeoproterozoic stromatolites from the Vempalle Formation of the Cuddapah Supergroup, south-eastern India, contain well-preserved microfabrics. These exhibit a range of complex clotted to bush-like micritic fabrics set in microsparite and sparite cements. No calcified microfossils confidently referable to calcified cyanobacteria have been recognized, although micritic masses crudely resembling the Phanerozoic calcified cyanobacterium Angusticellularia are present. It is suggested that these Vempalle stromatolitic microfabrics compare more closely with modern calcified non-cyanobacterial biofilms. This interpretation questions the widespread assumption that cyanobacteria were important components of stromatolite-building microbiotas during the Proterozoic and instead emphasizes the possibility that examples similar to those studied were dominated by non-cyanobacterial bacteria. It is generally thought likely that cyanobacteria were important components of stromatolite-building microbiotas during the Proterozoic, and that early lithification by carbonate precipitation was widespread and intense. If these assumptions are correct then the apparent absence of diverse calcified cyanobacteria suggests that these organisms, which are conspicuous in the Early Palaeozoic, underwent evolutionary radiation close to the Neoproterozoic–Cambrian boundary.

Fossil evidence of iron-oxidizing chemolithotrophy linked to phosphogenesis in the wake of the Great Oxidation Event

The oxygenation of Earth’s atmosphere allowed for the diversification of metabolisms to include those that rely on oxygen and its derivatives. For example, chemolithotrophic oxidation of sulfide and iron both require oxygen or nitrate as terminal electron acceptors. A growing number of oxygen-utilizing chemolithotrophs are known to accumulate intracellular polyphosphate as an energy reserve that allows them to adapt to the fluctuating redox conditions in their distinctive-gradient habitats. Polyphosphate is also thought to play an important role in the formation of phosphatic mineral deposits. Here we present fossil evidence of iron-oxidizing bacteria preserved as filamentous iron oxides within phosphatic Paleoproterozoic stromatolites. The filaments include twisted stalks similar to those produced by modern iron-oxidizing bacteria that are known to metabolize polyphosphate and inhabit steep redox gradients. Fossil iron-oxidizing bacteria preserved within some of the oldest known phosphorites serve as indicators of O 2 -Fe(II) gradients that may have supported microbially mediated phosphogenesis via polyphosphate metabolism and/or an active iron redox pump.

Archaean microbiota from the Donimalai formation, Dharwar supergroup, India☆

Abstract Syngenetic microbial trichomes are recorded from a black chert band interlayered with the banded iron formation belonging to the > 2.6-Ga-old Sandur Schist Belt of the Dharwar sequence, Karnataka, India. The iron formation has interbeds of mafic pillow lava and tuffs. Although the fossil microbial community consists of highly biodegraded mats, some well-preserved structurally mineralized trichomes are also present. Electron microprobe analysis shows high percentage of SiO 2 and P 2 O 5 in the filaments and the presence of carbon along the margins of some filaments. Rare earth element and isotopic data indicate a higher oxygen concentration than hitherto considered for Archaean seawater. In the present context there is presumptive evidence to suggest that this oxygen was produced by the photosynthetic activity of microbial biocoenoses.

Megascopic carbonaceous compression fossils from the Neoproterozoic Bhima Basin, Karnataka, South India

Abstract An abundant well-preserved assemblage of annulated carbonaceous compressions and impressions has been recorded from the intra-cratonic argillaceous sedimentary sequence of the Bhima basin in south India. Impressions and carbonaceous compressions recorded in the Hulkal Formation belong to annulated forms similar to the previously reported Sinosabellidites huainanensis, Protoarenicola baiguashanensis and Pararenicola huaiyuanensis from China. This paper discusses the diversity, systematics, affinity, biostratigraphical potential and global significance of these remains. The previously proposed worm-like body fossil affinity for these organisms, based on similarity with the Chinese assemblage, has been reinterpreted as with pre-Ediacaran epibenthic organisms. Fresh investigations of the Indian assemblage of such specimens reveal their close proximity to the algal affinity. The occurrence of global marker events, such as phosphatization, the presence of complicated annulated carbonaceous remains in the Hulkal Formation and the absence of stromatolites in the carbonate sequences of the Bhima basin, collectively indicate a Neoproterozoic, possibly pre-Sturtian age for the Bhima Group.

Occurrence of helically coiled microfossil Obruchevella in the Owk Shale of the Kurnool Group and its significance

The present study reports occurrence of helically coiled microfossil Obruchevella Reitlinger from the Owk Shale of the Kurnool Group from the peninsular India. The age of the Kurnool Group is poorly constrained due to the absence of direct radiometric dating and meager palaeobiological data. Occurrence of Obruchevella is considered as a typical Vendian marker genus recorded mostly from close to the Precambrian–Cambrian boundary successions. Hence, the present assemblage is important to ascertain the age of the basin. In the Owk Shale, four species of Obruchevella, viz., O. delicata, O. parva, O. minor and O. valdaica are recorded as organic walled microfossils. Among them O. valdaica is the largest in terms of size parameters. On the global scale, the recovered species occur in the Late Neoproterozoic to Early Cambrian sediments. Therefore, the occurrence of microfossil Obruchevella in the Owk Shale and known burrow structures in the Narji Limestone suggest Ediacaran age close to the Cambrian for the Kurnool Basin and challenges the recently assigned Mesoproterozoic age of the basin.

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

Mesoproterozoic (∼ 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.

The evolution and distribution of life in the Precambrian eon-Global perspective and the Indian record

The discovery of Precambrian microfossils in 1954 opened a new vista of investigations in the field of evolution of life. Although the Precambrian encompasses 87% of the earth’s history, the pace of organismal evolution was quite slow. The life forms as categorised today in the three principal domains viz. the Bacteria, the Archaea and the Eucarya evolved during this period. In this paper, we review the advancements made in the Precambrian palaeontology and its contribution in understanding the evolution of life forms on earth. These studies have enriched the data base on the Precambrian life. Most of the direct evidence includes fossil prokaryotes, protists, advanced algal fossils, acritarchs, and the indirect evidence is represented by the stromatolites, trace fossils and geochemical fossils signatures. The Precambrian fossils are preserved in the form of compressions, impressions, and permineralized and biomineralized remains.

Megascopic Carbonaceous Remains from Proterozoic Basins of India

Megascopic carbonaceous remains, occurring on the bedding planes of the areno-argillaceous successions of the Precambrian, are the important evidence of the early life forms. A wide variety of such forms are described from the Proterozoic successions of India viz., the Vindhyan, Chhattisgarh, Kurnool and Bhima basins in peninsular India and some of the carbonate belts of the Lesser Himalaya. The Chuaria–Tawuia assemblage invariably constitutes the most important element of megascopic carbonaceous remains in these basins. Besides, simple films to morphologically complex forms are also recorded. In the present paper, knowledge and status of megascopic carbonaceous remains entombed in these successions of India with their significance in understanding the earth’s earliest biosphere are reviewed. Two principal questions are addressed: Are these carbonaceous mega-remains divulge any evolutionary steps and event in the Precambrian? Is there any discernable biostratigraphic potential in these carbonaceous mega-remains in space and time? The review reveals that coccoidal/spherical propkaryotic forms attained gigantism for the first time around Palaeoproterozoic/Mesoproterozoic boundary and this phenomenon was repeated in the geiological history around cryogenian. Eukaryotic carbonaceous remains show morphological changes from simple non processed to processed forms. A scheme of biozonation on the distribution pattern of the carbonaceous mega-remains in the Proterozoic successions, with form and function of the some of these remains, is presented.