Purnima Srivastava

Microfossils from the Kheinjua Formation, Mesoproterozoic Semri Group, Newari area, central India

Abstract A well preserved and diversified microfossil assemblage is reported from a new locality, Newari, Son Valley area, Uttar Pradesh from the black bedded chert belonging to the Fawn Limestone Member of the Kheinjua Formation (∼ 1200 Ma), Semri Group (Lower Vindhyans). In all, 28 species belonging to 18 genera representing both coccoid and filamentous forms are described. These are associated with microbial laminites. Excluding Archaeoellipsoides and Bactrophycus, and forms of acritarchean affinity, the diameter range among coccoids is between 2 and 24 μm, averaging 10.4 μm, and the width of the filamentous forms is from 1.5 to 28 μm, averaging 6.8 μm. Eoentophysalis and Siphonophycus are the two most dominating forms. In comparison to filamentous forms the coccoid forms show more morphological diversity. Well preserved cellular filaments are few in number. In both coccoid and filamentous forms there is a possibility that many forms represent degradational variants of a common biological entity. When the Newari assemblage is compared with other Proterozoic microbial assemblages, it shows maximum resemblance with the assemblage described from the Meso- to Neoproterozoic Deoban Limestone, Garhwal Lesser Himalaya, India, with 12 genera being present in both assemblages. It has 9 genera in common with the Tindir Creek Group, 8 genera with the Balbirini Dolomite, 8 genera with the Draken Conglomerates assemblage, 8 genera with the Bitter Springs Formation and 6 genera with the Belcher Islands assemblage. The following assemblage is reported: (1) the coccoid forms Eoentophysalis belcherensis, E. magna, Myxococcoides minor, M. grandis, Sphaerophycus parvum, S. medium, Huroniospora sp., Eosynechococcus moorei and E. isolatus, cf. Palaeopleurocapsa sp., Tetraphycus major, T. congregatus, Glenobotrydion aenigmatis, Coniunctiophycus gaoyuzhuangense, Archaeoellipsoides sp., Bactrophycus oblongum, cf. Kheinjuasphaera vulgaris, cf. Trachysphaeridium sp., Leiosphaeridia jacutica; (2) the filamentous forms Palaeolyngbya sp., Oscillatoriopsis breviconvexa, O. sp., O. grandis n. sp., O. constrictum n. sp., Siphonophycus kestron, S. robustum, S. septatum and cf. Eomicrocoleus crassus.

Middle to Late Proterozoic microbiota from the Deoban Limestone, Garhwal Himalaya, India

A well-preserved Middle to Late Proterozoic microbial assemblage from the Deoban Limestone (Formation), Chakrata region, Garhwal Himalaya, is described from petrographic thin sections of black bedded chert. The present assemblage contains both filamentous and coccoid forms which are mostly of cyanobacterial affinity. The assemblage includes 28 species belonging to sixteen genera and two forms of uncertain affinity described as unnamed forms A and B. Eoentophysalis, Glenobotrydion and Myxococcoides are the three most abundant coccoid forms constituting about 40% of the total coccoid population. The diameter of coccoids ranges between 1.5 and 65 μm. Gunflintia, Biocatenoides and Eomycetopsis are the three dominating filementous genera constituting about 90% of the filamentous population. The width of the filaments ranges between 0.5 and 18 μm. The diversity in coccoids is greater in comparison with filamentous forms. The total assemblage comprises: Coccoid forms—Myxococcoides minor, M. inornata, Eosynechococcus isolatus, Glenobotrydion aenigmatis, G. majorinum, Melasmatosphaera media, M. parva, Tetraphycus major, T. conjunctum, Globophycus rugosum, Huroniospora psilata, H. microreticulata, Caryosphaeroides pristina, Eoentophysalis magna, E. belcherensis, E. cumulus, Gloeodiniopsis gregaria, G. lamellosa, G. sp., Sphaerophycus parvum, unnamed form A and unnamed form B. Filamentous forms—Gunflintia grandis, G. minuta, Biocatenoides sp., Eomycetopsis robusta, E. filiformis, E. siberiensis, Siphonophycus kestron and Oscillatoriopsis sp. The assemblage shows an abundance of small coccoids and thin filaments as compared to larger coccoids and thicker filaments. The abundance of forms decreases with the increase in diameter in coccoid forms and width in filamentous forms.

Vindhyan akinites: an indicator of mesoproterozoic biospheric evolution.

A wide size range of rod-shaped, ellipsoidal akinites assignable to Archaeoellipsoides are reported from the Newari locality of the Mesoproterozoic Kheinjua Formation of the Semri Group, Vindhyan Supergroup. These akinites of heterocystous cyanobacteria (Archaeoellipsoides) represent the smallest of the forms reported from any other assemblage to date and are well comparable to the akinites of modern bloom forming Anabaena species. Like any other Mesoproterozoic microfossil assemblage, The Newari microfossil assemblage is also dominated by cyanobacterial population, but the presence of Archaeoellipsoides (akinites) or heterocyst forming Nostocales and Stigonematales are rather rarely reported. These fossils set a minimum date for the evolution of derived cyanobacteria, capable of marked cell differentiation, and they corroborate geochemical evidence indicating that atmospheric oxygen level was above 1% of present day level during Mesoproterozoic time. In the presence of oxygen a protected environment for nitrogenase (an oxygen sensitive nitrogen fixing enzyme) is produced by these akinites, which were abundant in coastal communities of Mesoproterozoic shallow marine carbonates. It is therefore interpreted that the presence of Vindhyan’s akinites indicate Mesoproterozoic biospheric evolution.

Problematic Worms and Priapulid-like Fossils from the Nagaur Group, the Marwar Supergroup, Western Rajasthan, India

Worm-like problematic megafossils, comparable with priapulids, are reported from the Lower Cambrian Nagaur Formation of the Marwar Supergroup, Western Rajasthan, India. These fossils occur as positive epireliefs on the top surface of siliciclastic sandstone beds in the Dulmera area of Bikaner district, western Rajasthan. Occurrence of these structures is significant, as the fossil bearing strata also exhibit the preservation of trilobite traces and Treptichnus pedum (three-dimensional subhorizontal burrows). The latter is a Precambrian-Cambrian boundary marker fossil, which has a worldwide distribution. Recently, it has been experimentally demonstrated that priapulids (ecdysozoa) or similar organisms are responsible for the construction of treptichnid burrows. Co-occurrence of these body fossils and treptichnid burrows in the same stratigraphic horizon indicate their close relationship and supports the inferences that these organisms are responsible for the formation of T. pedum. It also projects strong evidence demarcating the Precambrian/Cambrian boundary in the Nagaur Group. These faunal remains are considered to be a significant tool in comparative studies of the evolution from invertebrate to vertebrate animals. Detailed study may certainly shed light on different aspects of macroevolution and radiation among animal clades, and the palaeoecology prevailing at the time of the Nagaur Sandstone deposition.

Problematic fossils from the Palaeo-Neoproterozoic Vindhyan Supergroup, India

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.

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

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).