An assemblage of microfossils of moderate diversity, with the remarkable occurrence of ECAP acritarchs, is reported from the Ediacaran Krol ‘A’ (= the Mahi Formation) succession of Lesser Himalaya, India. Microfossils occur in the chert nodules exposed in Solan district, Himachal Pradesh. Two microfossils, Barogophycus symmetricus n. gen. n. sp. and Botominella lineata are new to the well-established Krol assemblage. The paper concentrates predominantly on fossil eukaryotic filamentous and coccoidal micro-organism of simple morphology. The assemblage is dominated by remains of prokaryotic cyanobacteria and demonstrates the diversity achieved by microorganisms at the beginning of the Ediacaran Period in the aftermath of the Marinoan glaciation. Filamentous and coccoidal microorganisms differ in taxonomic composition from the Mesoproterozoic microbiotas. The Krol microfossils assemblage has biostratigraphic potential and usefulness in broadly demarcating the different levels of the Ediacaran strata. Because of the presence of ECAP acritarchs, along with the appearance of other eukaryotic microorganisms of filamentous and coccoidal morphology, the general level of the Krol ‘A’ assemblage is considered as Ediacaran.

Subtidal cherts from phosphorite-bearing basal strata of the Early Cambrian Shabakta (Dzhylandy) Formation, Tamda Group of the Maly Karatau Range of South Kazakhstan, contain a diverse assemblage of acanthomorph acritarchs and other well-preserved organic-walled microfossils. Acanthomorphs of the Shabakta lowermost layers are taxonomically essentially identical to those of many similarly aged Early Cambrian organic-walled compression-preserved and permineralized microbiotas preserved in rocks deposited in relatively open marine settings and are biostratigraphically correlative with acanthomorph-containing microfloral assemblages of the Vergale and Rausve Horizons (Regional Stages) of the East European Platform as well as with other assemblages known worldwide from the upper Atdabanian (Series 2 of the International Union of Geological Sciences [IUGS]). Although lower Atdabanian deposits of the Talsy (Lukati) horizon are missing from the Maly Karatau succession studied here, Nemakit-Daldynian through Tommotian (the IUGS Terranovian) microbiotas both of the Shabakta-underlying Berkuta Member of the Kyrshabakta Formation and phosphoritic Chulaktau Formation are comparable to those of the pre-trilobite Rovno and Lontova Horizons (Regional Stages) of the East European Platform. Thus, the phosphatized and chert-permineralized Shabakta assemblages date from of the Early Cambrian part of the Precambrian-Cambrian phosphogenic event, an interpretation supported also by associated small shelly fossils. The studies reported here of the Shabakta microfossils - based on the use of optical microscopy, confocal laser scanning microscopy (CLSM) and Raman spectroscopy - provide information in three dimensions at high spatial resolution about their organismal morphology, cellular anatomy, kerogenous composition, taphonomy, and mode and fidelity of preservation. The combined use of CLSM and Raman, techniques introduced to Precambrian paleobiology only rather recently, is shown to reveal morphological characters of taxonomic, biostratigraphic and palaeoenvironmental significance.

An assemblage of organic walled microfossils (OWM) of 17 taxa belonging to 10 genera is reported from the Neoproterozoic Owk Shale of the Kurnool Group, South India. The assemblage comprises sphaeromorphs, colonial aggregates, filamentous forms, spiral cylindrical filaments belonging to cyanobacteria, problematic acanthomorphic acritarchs, Netromorphic, Sphaeromorphic and Acantomorphic groups. The assemblage includes cyanobacteria: Siphonophycus kestron, S. robustum, S. typicum, S. solidum, Polytrichoides lineatus; Netromorphic acritarch: Arctacellularia tetragonala, Navifusa majensis, Jacutianema solubila; Sphaeromorphic acritarch: Ostiana microcystis, Synsphaeridium spp., Leiosphaeridia minutissima, L. tenuissima, L. crassa, L. jacutica, L. ternate; and Acanthomorphic acritarch: Cavaspina aff. C. acuminata and Variomargosphaeridium aff. V. litoschum. The age conundrum of Kurnool Group (Mesoproterozoic versus Neoproterozoic) is discussed. On the basis of the reported OWM assemblage the age of the Kurnool Group is established as Neoproterozoic.

In the present paper, well-preserved specimens of taxonomically distinctive Proterozoic eukaryotic fossil Tappania Yin are recorded for the first time from the rocks of the Saraipali Formation of the Singhora Group, Chhattisgarh Supergroup, India. In the global context, among the various species of this genus, Tappania plana is widely distributed in the latest Palaeoproterozoic (Statherian) to the early Mesoproterozoic (Calymmian) organic-walled microfossil assemblages. Tappania plana of the Saraipali is subjected to transmitted light microscopy, confocal laser scanning microscopy and laser Raman spectroscopy and these results are presented. Collectively, the occurrence of remarkable microfossil Tappania and other associated microfossils in the Saraipalli Formation of rocks demonstrate the Calymmian age for the lower sediments of the Chhattisgarh Supergroup.

The study of stromatolites of the Vendian Chencha Formation (Ediacaran) of the Baikal-Patom Highland (Central Siberia) was carried out using an integrated approach: biogenic microformations were studied using a scanning electron microscope and the traditional method of optical microscopy. This approach made it possible to identify remains of communities of stromatolite-building cyanobacteria and morphologically complex eukaryotic microorganisms. Such a study eliminates the incorrect determination of biota and expands our understanding of the microbial structure and participation of microorganisms in the formation of stromatolites.

The association of organic-walled microfossils of the Kaltasy Formation, which includes morphologically complex large forms, is considered on the basis of previous and new materials. In general, the Kaltasy microbiota is similar to many associations of the Neoproterozoic (Late Riphean and Vendian) microfossils and, originally, was erroneously interpreted as Late Riphean, in spite of missing Late Riphean index taxa and geological evidence of its Early Riphean age. The Early Riphean age of the formation was supported by results of Re–Os, U–Pb, and other modern methods of isotope geochronology, whereas the low 87Sr/86Sr ratios and almost invariant δ13C values of its rocks are comparable with isotopic characteristics of the Mesoproterozoic sequences of >1300 Ma. The application of geochemical methods using high-reagent Fe (FeHR) and C and S isotopes has showed that relatively deep rocks of the Arlan Subformation of the Kaltasy Formation accumulated under aerobic oxidation conditions (ratio FeHR/FeT < 0.38). This contradicts a common concept of the presence of an anoxic ocean in the Mezoproterozoic, but explains the presence of morphologically complex large eukaryotic forms of microbiota, which were abundant in Neoproterozoic oxygenic basins.

The Rhynie chert provides sublime fossils of cyanobacteria from an Early Devonian hot spring ecosystem. However, only a small fraction of the diversity has been documented. Rhyniotaxillus devonicus nov. gen. et sp., a minute coccoid cyanobacterium from the Rhynie chert, is characterized by cuboid to somewhat irregular colonies of up to 64 cells (cell diameter: 2.5-3.5 mu m) arranged into sarcinoid packages, and held together by prominent gelatinous envelopes. Colony morphology parallels that seen in certain present-day species of Eucapsis (Synechococcales) and Entophysalis (Chroococcales), as well as Eoentophysalis belcherensis. a Precambrian fossil equivalent of Entophysalis. The systematic affinities of R. devonicus remain unresolved. Nevertheless, this discovery expands our knowledge of cyanobacterial diversity in early non-marine ecosystems. (C) 2019 Elsevier B.V. All rights reserved.

In the present study well-preserved specimens of Obruchevella Reitlinger- a helically coiled cyanoprokaryote microfossil are recorded for the first time from the carbonaceous shale of the Mesoproterozoic Saradih Limestone of the Raipur Group, Chhattisgarh Supergroup. Three species of Obruchevella viz., O. delicata, O. exilis and O. parva are recorded as organic-walled microfossils. hi the global context, recovered species of Obruchevella are widely distributed in Mid-Neoproterozoic (Cryogenian) to Devonian silicified and organic-walled microfossils assemblages. Therefore, the occurrence of Obruchevella in the Raipur Group sediments has biostratigraphic implications for the claim of the recently assigned Mesoproterozoic age for the Chhattisgarh Supergroup.

The sedimentary succession of the Anabar Uplift (North Siberia) contains the Early Riphean abundant and diverse microfossil assemblages. The Ust-Il’ya and lower Kotuikan organic-walled assemblages of prokaryotic and eukaryotic microorganisms from open shelf facies reflect the level of the marine biosphere evolution in the early Mesoproterozoic. The silicified microbiotas of the upper subformations of Kotuikan and Yusmastakh formations are represented by prokaryotic cyanobacteria only.

Based on previously obtained and new materials, the association of organ-walled microfossils represented by morphologically complex and large forms from the Kaltasa Formation is considered. In general, the Kaltasa microbiota resembles many associations of Neoproterozoic microfossils (late Riphean and Vendian). It was mistakenly interpreted as Late Riphean in its initial description, although the late Riphean index taxons were absent, and geological data indicated its early Riphean age. The early Riphean age of the Kaltasinskaya Formation has been confirmed in a series of studies in recent years using Re–Os, U–Pb and other modern methods of isotopic geochronology. Additionally, the low ratios of 87 Sr/ 86 Sr and almost invariant ō13C values in the surrounding sediments are comparable with the isotopic characteristics of Mesoproterozoic strata that are older than 1300 million years. The use of geochemical methods using high-reagent iron (FeHR), as well as carbon and sulfur isotopes showed the presence of aerobic conditions during the deposition of relatively deep-water sediments of the Arlan Member of the Kaltasinskaya Formation (FeHR/FeT ratio <0.38). This contradicts the general concept of the existence of an anoxic ocean in the Mesoproterozoic, but explains the presence in the microbiota composition of morphologically complex and large eukaryotic forms that are widely distributed in oxygenic environments of the Neoproterozoic.

The lower Vendian microbiota of the Ura Formation, Baikal-Patom Highland, eastern Siberia, contains remains of acanthomorph acritarchs almost 1 millimeter in diameter that is extremely large for Precambrian microfossils. The microremains have been described as the new genus and species Stellarossica ampla gen. et sp. nov., and altogether with the morphologically and dimensionally similar Pertatataka-type (ECAP) acanthomorph acritarchs Keltmia, Weissiella and Galeasphaeridium from the Vendian deposits of the East European Platform, are placed in the formal infragroup Keltmiides. Lower Vendian acanthomorph acritarchs are either interpreted as remains of giant algae or eggs (diapause egg cysts) of the earliest invertebrates, and the new finds tend to support the latter hypothesis.

Precambrian stromatolites were studied with a complex approach including two complementary methods. The biogenic origin of ultramicrostructures examined with SEM was supported by the traditional optical microscopy. The paper addresses columnar-stratiform stromatolites of the Vendian Chencha Formation in the Ura Uplift (Baikal–Patom Highland, Central Siberia) and fossilized remains of coccoidal and filamentous microorganisms therein: cyanobacteria Eoentophysalis , hormogonian cyanobacteria Siphonophycus and, probably, Eomicrocoleus . The unraveled community of stromatolite builders includes the major organisms commonly observed in the Precambrian microbiota. Stromatolites of the Chencha Formation contained not only cyanobacteria, but also remains of eukaryotic microorganisms, including likely testate amoebae and acanthomorphic acritarchs. It is shown that the complex approach rules out incorrect determination of biota and significantly enhances concepts of the origin and formation of stromatolites, as well as the participation of microorganisms in their formation.

Precambrian cyanobacteria demonstrate unprecedented evolutionary conservatism and have remained practically unchanged for the last 2 Ga, considering that ancient forms have counterparts among genera and even species of modern microorganisms. However, Proterozoic cyanobacteria and other prokaryotes form unique assemblages of restricted geochronological range and broad spatial distribution. The most remarkable of these assemblages are the Gunflint-type Paleoproterozoic microbiotas and the Early Riphean Archaeoellipsoides-dominant Kotuikan-type assemblages. Their observed taxonomic uniqueness and short geochronological ranges reflect irreversible changes in the Earth’s global environments, rather than evolutionary innovations. Nonetheless, the fossil blue-green algae demonstrate some evolutionary changes throughout the Proterozoic: the stalked cyanobacteria Polybessurus appeared in the Middle Riphean and the spiral cylindrical cyanobacteria Obruchevella emerged in the Late Riphean.

Abundant and diverse microfossils from shales of the 1500-Ma-old Ust'-Il'ya Formation in northern Siberia document early Mesoproterozoic life along the Siberian Platform. Similar to many Meso-and Neoproterozoic microbiota, the Ust'-Il'ya assemblage is dominated by cyanobacteria, both filamentous (oscillatorialeans and nostocaleans, represented primarily by cellular trichomes and empty sheaths) and coccoidal (chroococcaleans, including solitary and colonial specimens). Microbiotas from shallow-water settings that contain morphologically simple and small microfossils were originally taken to be representative of Mesoproterozoic life. However, the Ust'-Il'ya and other compressed organic-walled microfossil assemblages in open shelf facies include diverse microscopic eukaryotes: large (up to 1-mm diameter) megasphaeromorph acritarchs, branched filaments that are most likely of eukaryotic algae and other morphologically complex microorganisms. The Ust'-Il'ya assemblage contains numerous taxa previously reported from Tonian and Cryogenian successions (e.g., Elatera, Pterospermopsimorpha, Rugosoopsis and Chuaria), but these are long-ranging genera and no Neoproterozoic index fossils have been recorded. The microbiota is considered to be Mesoproterozoic in age because it contains a few taxa reported from the Calymmian microfossil assemblages (e.g., Lineaforma aff. L. elongate, Satka favosa and Elatera aff. E. minor). The Ust'-Il'ya mixed assemblage of prokaryotic and eukaryotic microorganisms records an evolutionary stage transitional between the prokaryote -dominated Paleoproterozoic and the eukaryote-dominated Neoproterozoic world. As reported in this study, 38 taxa were identified in the Ust'-Il'ya assemblage (of which six forms are described informly) that are assigned to 17 genera of microscopic eukaryotes (presumably 17-20 entities) and prokaryotes. A new species is proposed: Heliconema ustilense.

It is demonstrated on the basis of the first monographic study of multiple and taxonomically variable organic-walled microfossils from the Ust’-Il’ya Formation of the Anabar Uplift that both prokaryotic and eukaryotic forms are present in the composition of this microbiota. They are divided into four formal groups on the basis of the specifics of the morphological indicators of the identified taxa. The review of the data on the isotopic age of hosting deposits showed that the Ust’-Il’ya Formation is of the Early Riphean in age which are currently evaluated as 1750 ± 10–1400 Ma. Relatively large and morphologically complex eukaryotic forms present in the Ust’-Il’ya Formation served in due time as the basis for an erroneous conclusion on the Late Riphean age of the specified formation and the overlying Lower Kotuikan Subformation of the Anabar Uplift. The paper provides a global comparative analysis of the Early Riphean microbiotas, demonstrates the position of the Ust’-Il’ya and Kotuikan microbiotas amidst the microbiotas of the same age, and shows that the relatively large acanthomorphic acritarchs Tappania , Valeria , Dictiosphaera , Satka , and Shuiyousphaeridium appeared in the geological history already during the Early Riphean Erathem. Moreover, the paper discusses the recently published data on the distribution of aerobic and anaerobic conditions in the Early Riphean paleobasins and provides the conclusion on the impact of the lateral change of these conditions on the taxonomic composition of the microbiota.