Assemblage Of Organic-walled Microfossils Research Articles

Organic-walled microfossils from the Ediacaran Sete Lagoas Formation, Bambuí Group, Southeast Brazil: taxonomic and biostratigraphic analyses

Abstract This work presents a detailed taxonomic study on organic-walled microfossils from the Ediacaran Sete Lagoas Formation (Bambuí Group) at the Barreiro section in the Januária area of the São Francisco basin, Brazil. Seven species are described, including Siphonophycus robustum (Schopf, 1968), Ghoshia januarensis new species, Leiosphaeridia crassa (Naumova, 1949), Leiosphaeridia jacutica (Timofeev, 1966), Leiosphaeridia minutissima (Naumova, 1949), Leiosphaeridia tenuissima Eisenack, 1958, and Germinosphaera bispinosa Mikhailova, 1986. These taxa are recovered for the first time in the Sete Lagoas Formation. They occur abundantly in the lower portion of the studied section, but only Ghoshia januarensis is present in the upper part of the studied section, probably due to environmental or taphonomic changes. Leiosphaeridia species, particularly Leiosphaeridia minutissima, dominate the organic-walled microfossil assemblage. Although most taxa described here have long stratigraphic ranges, they are consistent with a terminal Ediacaran age as inferred from detrital zircon data and tubular fossils (e.g., Cloudina and Corumbella) from the Sete Lagoas Formation. UUID: http://zoobank.org/7f92b900-0176-4da6-93a3-fd51edb22cbf

The Proterozoic Qinggouzi microfossil assemblage and its biostratigraphic constraints on the Great Unconformity in northeastern North China Craton

The Great Unconformity at the Precambrian-Cambrian boundary has been recognized on several continents and may provide critical insights into the environmental context of the Cambrian explosion. However, the geodynamic drivers, geochronological duration, and paleogeographic extent of the Great Unconformity remain unconstrained in many cases. Recently, a major depositional gap of varying magnitudes has been reported around the Precambrian-Cambrian boundary in several regions of North China, leading to the interpretation that the Great Unconformity may have been widespread but diachronous across the North China Craton. However, the magnitude of the Great Unconformity remains unknown in northeastern North China, largely because of the poor age constraints on the thick Proterozoic–Cambrian successions in this region. Here we use biostratigraphic data to constrain the depositional age of Proterozoic strata below the Great Unconformity in northeastern North China. We report a diverse organic-walled microfossil assemblage from the Qinggouzi Formation in southern Jilin Province, northeastern margin of the North China Craton. Typical late Mesoproterozoic to early Neoproterozoic microfossils, including Trachyhystrichosphaera aimika and Proterocladus antiquus, are present in Qinggouzi assemblage, providing a solid biostratigraphic constraint on the Qinggouzi Formation. Given that the overlying Shuidong Formation contains typical early Cambrian small shelly fossils, our new data indicate that the Great Unconformity in northeastern North China represents a major depositional gap of >200 Myr. Thus, the new data, along with other available biostratigraphic and geochronological data from over the world, provide robust constraints on the paleogeographic extent, chronostratigraphic duration, and diachronous nature of the Great Unconformity.

New record of the green algal fossil Proterocladus and coexisting microfossils from the Meso-Neoproterozoic Diaoyutai Formation in southern Liaoning, North China

The Mesoproterozoic-Neoproterozoic transition represents a crucial interval of evolutionary innovation and phylogenetic diversification. A spotty fossil record indicates that several clades of multicellular eukaryotes, including red and green algae, as well as fungi, may have diverged and diversified at this transition. However, the Meso-Neoproterozoic fossil record remains poorly documented, particularly at the assemblage level, greatly limiting our understanding of the marine ecosystem in this critical time interval. Here we report a well-preserved assemblage of organic-walled microfossils from the Meso-Neoproterozoic Diaoyutai Formation (ca. 1050–950 Ma) in Southern Liaoning Province of the North China Block. Raman geothermometric analysis of these organic-walled microfossils suggests that the Diaoyutai Formation experienced relatively low-grade metamorphism, with apparent peak metamorphic temperatures of ∼ 215 ℃. The Diaoyutai assemblage consists of 14 species belonging to 9 genera, as well as several unnamed or unidentified forms. In addition to simple forms such as aggregated coccoidal and unbranched filamentous fossils that are often interpreted as cyanobacteria, a moderate diversity of eukaryotes is also observed. Probable eukaryotic fossils in this assemblage include the branched multicellular alga Proterocladus antiquus that has been previously interpreted as a green alga, several morphologically complex spheroidal taxa (e.g., Germinosphaera bispinosa, Pterospermopsimorpha insolita, P. pileiformis and Squamosphaera colonialica), and relatively large leiospheres (50–700 μm in diameter). We note that the occurrence of Proterocladus antiquus in the Diaoyutai assemblage predates its previously reported occurrence from the overlying Nanfen Formation. Overall, the Diaoyutai assemblage presents a glimpse of the coastal marine ecosystem at the Meso-Neoproterozoic transition, which consisted of benthic microbial mats dominated by filamentous cyanobacteria, millimeter-sized benthic green algae with thallus elevating above the sediment–water interface, and micrometer-sized planktonic prokaryotic and eukaryotic organisms. From the perspective of primary producers, the rise of photosynthetic eukaryotes in the Meso-Neoproterozoic represents a transformative innovation to modernize the coastal marine ecosystem in terms of ecological functions.

Early Cambrian microfossils from the cherts in carbonates of the Kuruktag area, the Tarim block

The basal Cambrian is characteristic for the appearance of small shelly fossils and acanthomorphic acritarchs with relatively small vesicles. In the Tarim Block (NW China) and South China, this microfossil assemblage is dominated by small acanthomorphs and was referred to as the ‘Micrhystridium-Paracymatiosphaera-Megathrix’ (MPM) association, based on microfossils from the lower Cambrian Yurtus and Xishanblaq formations of the Tarim Block. This study provides the new early Cambrian microfossil assemblage preserved in cherts of carbonates in Tikebuladahuang section of the Kruktag area, Tarim Block. These early Cambrian microfossil assemblages come from upper part of the lower Cambrian Xishanblaq Formation in Tikebuladahuang section correlated with the Cambrian Stage 2, and the microfossil association dominated by Heliosphaeridium dissimilare, sponge spicules, Leiosphaeridia spp., and Siphonophycus sp. also occurs. The organic-walled microfossil assemblage found in this study can be compared with the microfossils in the early Cambrian Chert of the Yangtze plate. The Asteridium–Heliosphaeridium–Comasphaeridium (AHC) association in South China and the Tarim block may mix two fossil associations of different ages and fossil compositions.

Late Ediacaran occurrences of the organic-walled microfossilsGranomarginataand flask-shapedLagoenaforma collarisgen. et sp. nov.

AbstractNew occurrences of flask-shaped and envelope-bearing microfossils, including the predominantly Cambrian taxonGranomarginata, are reported from new localities, as well as from earlier in time (Ediacaran) than previously known. The stratigraphic range ofGranomarginataextends into the Cambrian System, where it had a cosmopolitan distribution. This newly reported Ediacaran record includes areas from Norway (Baltica), Newfoundland (Avalonia) and Namibia (adjacent to the Kalahari Craton), and puts the oldest global occurrence ofGranomarginatain the Indreelva Member (< 563 Ma) of the Stáhpogieddi Formation on the Digermulen Peninsula, Arctic Norway. AlthoughGranomarginatais rare within the assemblage, these new occurrences together with previously reported occurrences from India and Poland, suggest a potentially widespread palaeogeographic distribution ofGranomarginatathrough the middle–late Ediacaran interval. A new flask-shaped microfossilLagoenaforma collarisgen. et sp. nov. is also reported in horizons containingGranomarginatafrom the Stáhpogieddi Formation in Norway and the Dabis Formation in Namibia, and flask-shaped fossils are also found in the Gibbett Hill Formation in Newfoundland. TheGranomarginata–Lagoenaformaassociation, in addition to a low-diversity organic-walled microfossil assemblage, occurs in the strata postdating the Shuram carbon isotope excursion, and may eventually be of use in terminal Ediacaran biostratigraphy. These older occurrences ofGranomarginataadd to a growing record of body fossil taxa spanning the Ediacaran–Cambrian boundary.

Shale-hosted biota from the Dismal Lakes Group in Arctic Canada supports an early Mesoproterozoic diversification of eukaryotes

AbstractThe Mesoproterozoic is an important era for the development of eukaryotic organisms in oceans. The earliest unambiguous eukaryotic microfossils are reported in late Paleoproterozoic shales from China and Australia. During the Mesoproterozoic, eukaryotes diversified in taxonomy, metabolism, and ecology, with the advent of eukaryotic photosynthesis, osmotrophy, multicellularity, and predation. Despite these biological innovations, their fossil record is scarce before the late Mesoproterozoic. Here, we document an assemblage of organic-walled microfossils from the 1590–1270 Ma Dismal Lakes Group in Canada. The assemblage comprises 25 taxa, including 11 morphospecies identified as eukaryotes, a relatively high diversity for this period. We also report one new species,Dictyosphaera smauginew species, and one unnamed taxon. The diversity of eukaryotic forms in this succession is comparable to slightly older assemblages from China and is higher than worldwide contemporaneous assemblages and supports the hypothesis of an earlier diversification of eukaryotes in the Mesoproterozoic.

A diverse organic-walled microfossil assemblage from the Mesoproterozoic Xiamaling Formation, North China

The mid-Proterozoic (ca. 1.85–0.85 Ga) might have been environmentally “boring”, but was a crucial interval of time for the early establishment of eukaryotic life on Earth. The Mesoproterozoic shale-dominated Xiamaling Formation (ca. 1.4–1.35 Ga) in North China preserved abundant organic-walled microfossils, some of which were of eukaryotic affinity but their taxonomic diversity and evolutionary implications remain unclear. In this paper, we present a palynological study of the lower Xiamaling Formation in the northern Tianjin Municipality. Similar to other Mesoproterozoic microbiotas, this assemblage consists of well-preserved and diverse spheroidal to ellipsoidal vesicles, filamentous microfossils and cellular aggregates. In total, 36 species belonging to 28 genera are identified, including 1 new species Quadrimurus clavatus gen. et sp. nov., and 5 unnamed forms. Majority of these taxa are recognised for the first time from this formation, indicating a richer assemblage than previously known. Although most taxa are of unknown biological affinities, this microbiota is thought to be composed of prokaryotic and eukaryotic microorganisms, in which 12 taxa are inferred to be eukaryotic. These microfossils indicate a moderate diversity of eukaryotic life documented in the Xiamaling Formation, which is consistent with other microbiotas in the mid-Proterozoic. The presence of Jixiania lineata Yan, 1986 (=Lineaforma elongata Vorob’eva and Sergeev, 2015) in North China extends its geographic distribution and further enhances its potential as index fossil for the early Mesoproterozoic strata. The Xiamaling assemblage provides one of the broader pictures of the Mesoproterozoic biosphere, in which eukaryotes were predominantly protistan-grade microorganisms.

First record of organic-walled microfossils from the Tonian Shiwangzhuang Formation of the Tumen Group in western Shandong, North China

Organic-walled microfossils offer important information on the biospheric evolution in pre-Cryogenian and provide biostratigraphic implications for many Proterozoic fossiliferous sequences that are poorly age constrained for the lack of reliable radiometric date. Recently, macroscopic carbonaceous compression fossils have been reported for the first time from the Tonian Shiwangzhuang Formation of the Tumen Group in western Shandong, North China. However, organic-walled microfossils have never been discovered from this formation up till now. To improve our knowledge about Proterozoic biodiversity in North China, we conducted a micropaleontological survey on the argillaceous limestone samples of the Shiwangzhuang Formation, which also contain macroscopic carbonaceous compression fossils, from the Baishicun section in Anqiu, western Shandong, North China. Our investigation shows that the Shiwangzhuang microfossil assemblage is dominated by smooth-walled sphaeromorphic acritarchs and cyanobacterium-like filaments and relatively low abundance of other acritarchs, including 16 taxa, such as Polysphaeroides filliformis, Ostiana microcystis, Simia annulare, ?Jacutianema sp., Arctacellularia tetragonala, Pellicularia tenera, Polythrichoides lineatus, and Navifusa actinomorpha. The Shiwangzhuang organic-walled microfossil assemblage, although consisting of long-ranging and not age diagnostic taxa, is consistent with a Tonian age suggested by macroscopic carbonaceous compression fossils, including the Chuaria-Tawuia and Sinosabellidites-Protoarenicola-Pararenicola assemblages, revealed from the same fossiliferous horizon of the Shiwangzhuang Formation and by organic-walled microfossil assemblage, including the late Mesoproterozoic to Tonian index fossil Trachyhystrichosphaera aimika, from the underlying Tongjiazhuang Formation. However, it is also worth noting that a Cryogenian or Ediacaran age cannot be completely excluded based just on the Shiwangzhuang microfossils because of their limited biostratigraphic utility.

Organic walled microfossils from the Neoproterozoic Owk Shale, Kurnool Group, South India

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.

Unusual preservation of an Ordovician (Floian) arthropod from Peary Land, North Greenland (Laurentia)

Preservation of a fragment of an arthropod from starved trough sediments of the Bøggild Fjord Formation (Ordovician, Floian) of Johannes V. Jensen Land in north Peary Land, North Greenland, recalls that of the lower Cambrian Sirius Passet Lagerstätte of extreme north-west Peary Land and may suggest a second locality for exceptional preservation in North Greenland. A prominent petaloid pattern on the tergopleurae reflects impression onto the internal mould of terrace lines from the cuticle exterior. The arthropod is associated with poorly preserved sponges and a depauperate assemblage of organic-walled microfossils. It is tentatively compared to Mollisonia, originally described from the Burgess Shale Lagerstätte (middle Cambrian, Miaolingian Series) of Canada.

A new record of late Ediacaran acritarchs from La providencia group (Tandilia System, Argentina) and its biostratigraphical significance

A well preserved assemblage of organic-walled microfossils is described from the Alicia and Cerro Negro formations of the La Providencia Group. The microbiota is dominated by Leiosphaeridia species (L. minutissima, L. tenuissima, L. crassa and L. jacutica) and comprises colonial sphaeroids (Synsphaeridium sp.) and filamentous microfossils of the genus Siphonophycus. The assemblage is assigned to the Late Ediacaran Leiosphere Palynoflora (LELP), which is consistent with the occurrence of the discoidal fossil Aspidella (<565 Ma) in the overlying unit. The distribution of microfossils within the Alicia Formation is mainly controlled by the shallowing upward trend and thus, environmental factors. This is indicated by the abundance and size of filaments, which tend to occur near to the paleoshore in shallow water settings. Furthermore, large leiospheres also occur in shallower facies together with filaments, in accordance with existing models for acritarch ecology and distribution in the Proterozoic marine successions. The absence of acanthomorphic acritarchs is thus interpreted as a real evolutionary trend, and not as a result of preservational or environmental biases. Regarding the preservation of microfossils, a higher Thermal Alteration Index for the Alicia Formation, compared with older and younger units, is interpreted as a result of hydrothermal overprint and/or contact metamorphism by basic dikes.

Organic-walled microfossils from the late Mesoproterozoic to early Neoproterozoic lower Shaler Supergroup (Arctic Canada): Diversity and biostratigraphic significance

The diversification of acritarchs (organic-walled vesicular microfossils of unknown affinity), filamentous, and multicellular microorganisms, happened during a time of profound environmental, biological, and ecological change. The Mesoproterozoic to Neoproterozoic transition is a key interval, notably for eukaryotic organisms. New assemblages of organic-walled microfossils from the ca. 1230 Ma to 900 Ma lower Shaler Supergroup of Arctic Canada record an impressive diversity, including macroscopic compressions of Chuaria circularis and numerous unambiguous eukaryotic taxa. The index taxon Trachyhystrichosphaera aimika co-occurs with eukaryotic taxa hitherto reported only from earlier (Dictyosphaera, Gigantosphaeridium, Satka favosa – sensu Javaux and Knoll, 2016) or later (Microlepidopalla) time intervals. Five new taxa, comprising the spheroidal acritarchs with inner wall sculpture Nunatsiaquus cryptotorus n. gen., n. sp. and Daedalosphaera digitisigna n. gen., n. sp.; the acanthomorphs (spiny acritarchs) with regularly distributed processes Herisphaera arbovela n. gen., n. sp. and Herisphaera triangula n. sp.; and the process-bearing multicellular Ourasphaira giraldae n. gen., n. sp.; are reported, along with three unnamed forms. Collectively, these remarkable microfossils (63 taxa, including 25 eukaryotic forms) demonstrate the greatest diversity of eukaryotes ever recorded for this time interval.

Implications of selective predation on the macroevolution of eukaryotes: evidence from Arctic Canada.

Existing paleontological data indicate marked eukaryote diversification in the Neoproterozoic, ca. 800 Ma, driven by predation pressure and various other biotic and abiotic factors. Although the eukaryotic record remains less diverse before that time, molecular clock estimates and earliest crown-group affiliated microfossils suggest that the diversification may have originated during the Mesoproterozoic. Within new assemblages of organic-walled microfossils from the ca. 1150 to 900 Ma lower Shaler Supergroup of Arctic Canada, numerous specimens from various taxa display circular and ovoid perforations on their walls, interpreted as probable traces of selective protist predation, 150-400 million years before their first reported incidence in the Neoproterozoic. Selective predation is a more complex behavior than phagotrophy, because it requires sensing and selection of prey followed by controlled lysis of the prey wall. The ca. 800 Ma eukaryotic diversification may have been more gradual than previously thought, beginning in the late Mesoproterozoic, as indicated by recently described microfossil assemblages, in parallel with the evolution of selective eukaryovory and the spreading of eukaryotic photosynthesis in marine environments.

Microfossils from the late Mesoproterozoic – early Neoproterozoic Atar/El Mreïti Group, Taoudeni Basin, Mauritania, northwestern Africa

The well-preserved Meso-Neoproterozoic shallow marine succession of the Atar/El Mreïti Group, in the Taoudeni Basin, Mauritania, offers a unique opportunity to investigate the mid-Proterozoic eukaryotic record in Western Africa. Previous investigations focused on stromatolites, biomarkers, chemostratigraphy and palaeoredox conditions. However, only a very modest diversity of organic-walled microfossils (acritarchs) has been documented. Here, we present a new, exquisitely well-preserved and morphologically diverse assemblage of organic-walled microfossils from three cores drilled through the Atar/El Mreïti Group. A total of 48 distinct entities including 11 unambiguous eukaryotes (ornamented and process-bearing acritarchs), and 37 taxonomically unresolved taxa (including 9 possible eukaryotes, 6 probable prokaryotes, and 22 other prokaryotic or eukaryotic taxa) were observed. Black shales preserve locally abundant fragments of organic-rich laminae interpreted as benthic microbial mats. We also document one of the oldest records of Leiosphaeridia kulgunica, a species showing a circular opening interpreted as a sophisticated circular excystment structure (a pylome), and one of the oldest records of Trachyhystrichosphaera aimika and T. botula, two distinctive process-bearing acritarchs present in well-dated 1.1Ga formations at the base of the succession. The general assemblage composition and the presence of three possible index fossils (A. tetragonala, S. segmentata and T. aimika) support a late Mesoproterozoic to early Neoproterozoic (Tonian) age for the Atar/El Mreïti Group, consistent with published lithostratigraphy, chemostratigraphy and geochronology. This study provides the first evidence for a moderately diverse eukaryotic life, at least 1.1billion years ago in Western Africa. Comparison with coeval worldwide assemblages indicates that a broadly similar microbial biosphere inhabited (generally redox-stratified) oceans, placing better time constraints on early eukaryote palaeogeography and biostratigraphy.

Organic-walled microfossils of the mid-Neoproterozoic Alinya Formation, Officer Basin, Australia

AbstractEstimates of Precambrian eukaryotic diversity and disparity indicate broad trends of increase in the Mesoproterozoic Era, leading to a peak and then rapid decline by ca. 750 Ma. The organic-walled microfossil assemblage presented here is representative of that mid-Neoproterozoic height of eukaryotic species richness. Organic-rich shales and siltstones of the mid-Neoproterozoic upper Alinya Formation, eastern Officer Basin, Australia, preserve an abundant and diverse assemblage of organic-walled microfossils deposited in a low-latitude, shallow marine setting. Use of scanning electron microscopy (SEM) revealed an unexpected level of morphological detail not visible in transmitted light microscopy. This led to the recognition of new species as well as establishment of degradational sequences, which aid in fossil recognition. In total, 26 taxa are described here; these include 21 previously named forms, four newly described species (Caelatimurus foveolatus, Culcitulisphaera revelata,Karenagare alinyaensis, andMorgensternia officerensis), and one new combination (Vidalopalla verrucata).

Systematics of organic-walled microfossils from the ca. 780–740 Ma Chuar Group, Grand Canyon, Arizona

AbstractThe ca. 780–740 Ma Chuar Group, Grand Canyon, Arizona, provides an exceptional record of life during the diversification of crown-group eukaryotes, just prior to the first Cryogenian glaciation. We document in detail the assemblage of organic-walled microfossils preserved in fine-grained siliciclastics throughout the unit. In contrast with earlier studies, we primarily used SEM to document fossil morphologies, augmented by transmitted light microscopy, FIB-SEM, and TEM. This resulted in the discovery of new species and the recognition of broad-ranging, intraspecific biological and taphonomic variation in other species. Twenty-two species and five unnamed morphotypes are described, including three new species:Kaibabia gemmulella,Microlepidopalla mira, andVolleyballia dehlerae; two new combinations:Galerosphaera walcottiiandLanulatisphaera laufeldii; and 17 previously described forms. The possible colonial green algaPalaeastrum dyptocranumButterfield in Butterfield, Knoll, and Swett, 1994 and the index fossilCerebrosphaera globosa(Ogurtsova and Sergeev, 1989) Sergeev and Schopf, 2010 (=C. buickiiButterfield, 1994) are described for the first time from Chuar rocks.Lanulatisphaera laufeldii, a locally abundant and globally widespread species characterized by submicrometer filamentous processes that form a reticulate network, may be a useful marker for the time interval just before the appearance of vase-shaped microfossils (VSMs) ca. 740 Ma.Organic-walled microfossil assemblages decline in diversity upsection, coincident with the appearance of VSMs and intermittent euxinia within the basin. Whether this pattern is due to preservational bias related to greater water depth or the higher TOC of upper Chuar rocks or instead reflects biotic turnover related to the spread of euxinic water masses in the basin is unknown.

A diverse and exquisitely preserved organic-walled microfossil assemblage from the Meso–Neoproterozoic Mbuji-Mayi Supergroup (Democratic Republic of Congo) and implications for Proterozoic biostratigraphy

A well preserved and diversified microfossil assemblage is reported from the Meso–Neoproterozoic Mbuji-Mayi Supergroup in the Kasai oriental Province, central part of Democratic Republic of Congo. A total of 49 taxa belonging to 27 genera were identified, including 11 species of unambiguous eukaryotes, 10 species of possible eukaryotes or prokaryotes and 28 species of probable bacteria. This assemblage is more diverse than previously reported but includes taxa reported in coeval worldwide assemblages. It is characterized by abundant sphaeromorphs, filamentous colonial aggregates and filamentous forms, as well as a relatively low diversity of acanthomorphs including the Late Mesoproterozoic and Early Neoproterozoic index fossil – Trachyhystrichosphaera aimika – reported for the first time in Central Africa. This species co-occurs with other taxa also reported for the first time in Africa: Trachyhystrichosphaera botula, Jacutianema solubila, cf. Tappania sp., Valeria elongata and numerous other taxa. Correlation with other geochronologically constrained successions that contain Trachyhystrichosphaera confirms T. aimika as promising index fossil to define the Late Mesoproterozoic–Early Neoproterozoic interval. The available biostratigraphic data enable to suggest a minimum Tonian age for the Mbuji-Mayi Supergroup. This age is consistent with the published and new geochronological data. Comparison with worldwide Proterozoic assemblages permits to define microfossil assemblages useful for biostratigraphy. This study significantly improves our understanding of the diversity of the Late Mesoproterozoic–Early Neoproterozoic biosphere, and in particular the diversification of early eukaryotes, preserved in the Democratic Republic of Congo rock record and more broadly in Africa where micropaleontological investigations are sparse.

Organic-walled microfossils from the Tonian Gouhou Formation, Huaibei region, North China Craton, and their biostratigraphic implications

The Meso-(?) and Neoproterozoic Huaibei Group in the Huaibei region, North China Craton (NCC), is emerging as a target for geobiological and tectonic studies due to its thick unmetamorphosed sediments with well-preserved organic-walled fossils. However, the lack of accurate age constraints for this sedimentary sequence dramatically hampers our ability to take full advantage of the rich geological and paleontological history recorded in the Huaibei Group. Particularly, the depositional age of the uppermost unit of the Huaibei Group, the Gouhou Formation, is a controversial issue. Although it is widely accepted that there is an unconformity between the Gouhou Formation and the overlying early Cambrian Houjiashan Formation, the magnitude of this unconformity and hence the scale of the underlying tectonic event are unconstrained due to the uncertainty of the sedimentary age for the Gouhou Formation which has been variously interpreted as Cambrian, Cryogenian–Ediacaran (∼720Ma to ∼541Ma), and Tonian (1000Ma to ∼720Ma). To improve our knowledge about this sedimentary gap, we carried out a biostratigraphic study of the Gouhou Formation. Our investigation using a low manipulation maceration technique revealed a diverse organic-walled microfossil assemblage. A total of 22 taxa have been revealed, including a new species—Dictyosphaera tacita n. sp. Importantly, the co-occurrence of Trachyhystrichosphaera, Valeria, and Dictyosphaera in the Gouhou Formation suggests a likely Tonian age. The biostratigraphic data indicate an infra-Cambrian depositional gap of >200myr between the Tonian Gouhou Formation and Cambrian Houjiashan Formation, perhaps driven by a major tectonostratigraphic event.

Kotuikan Formation assemblage: A diverse organic-walled microbiota in the Mesoproterozoic Anabar succession, northern Siberia

Abundant and diverse microfossils from shales of the Lower Member of the ∼1500-Ma-old Kotuikan Formation in northern Siberia document early Mesoproterozoic life along the Siberian Platform. Similar to many Meso- and Neoproterozoic microbiota, the Kotuikan assemblage is dominated by prokaryotic cyanobacteria, both filamentous (oscillatorialeans and nostocaleans, which are represented primarily by cellular trichomes and by empty sheaths) and coccoidal (chroococcaleans, including solitary and colonial specimens). However, unlike Mesoproterozoic microbiota of shallow-water settings, the Kotuikan compressed organic-walled microfossil assemblage in the open shelf facies includes diverse microscopic eukaryotes: large (up to ∼1-mm diameter) megasphaeromorph acritarchs, branched filaments that are most likely of eukaryotic algae and coccoidal monostromatic colonies of chlorococcaleans, as well as other morphologically complex microorganisms.The Kotuikan assemblage contains numerous taxa previously recorded from Neoproterozoic successions (e.g., Elatera, Eosolena, Palaeastrum, Pterospermopsimorpha, Rugosoopsis and Navifusa), 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 mixed assemblage of prokaryotic and eukaryotic microorganism remains including new taxa unknown in younger deposits. The Kotuikan assemblage documents the status of the marine biosphere at an early Mesoproterozoic time preceding the primary radiation of eukaryotes and representing an evolutionary stage transitional between the predominantly prokaryote-dominated Paleoproterozoic and the eukaryote-dominated Neoproterozoic microbiota.As reported in this study, 35 taxa were identified in the Kotuikan assemblage (of which five forms are described informally) that are assigned to 18 genera of microscopic prokaryotes and eukaryotes. Two new genera and five new species are proposed: Hirudiforma lancetica gen. and sp. nov., H. simmetrica gen. and sp. nov., Lineaforma elongata gen. and sp. nov., Elatera minor sp. nov., Eosolena minuta sp. nov.

Organic-walled microfossils from the early Neoproterozoic Liulaobei Formation in the Huainan region of North China and their biostratigraphic significance

Biostratigraphic subdivision and correlation of early Neoproterozoic strata is hampered by the limited number of widely distributed and age diagnostic fossils. The acanthomorphic acritarch Trachyhystrichosphaera aimika, among a few other microfossil taxa, has emerged as a potential early Neoproterozoic index fossil, but its temporal and spatial ranges have not been thoroughly documented. To improve our knowledge about early Neoproterozoic biodiversity, we carried out a micropaleontological investigation of the early Neoproterozoic Liulaobei Formation in northern Anhui of North China. Our investigation using a low-manipulation maceration technique revealed a diverse assemblage of organic-walled microfossils dominated by sphaeromorphs and filaments, with relatively few acanthomorph taxa. A total of 23 taxa were recovered, including three new species (Eotylotopalla? grandis n. sp., Siphonophycus gigas n. sp., and Trachyhystrichosphaera botula n. sp.). Also present in the Liulaobei Formation are Trachyhystrichosphaera aimika (a species widely present in pre-Cryogenian Neoproterozoic and latest Mesoproterozoic rocks) and Pololeptus rugosus (a species characterized by an ellipsoidal vesicle with transverse annulations). The new data add to the growing diversity of early Neoproterozoic fossils and strengthen the basis for improved biostratigraphic correlation of early Neoproterozoic strata. Correlation with other geochronologically dated successions that contain Trachyhystrichosphaera confirms Trachyhystrichosphaera and T. aimika as promising index fossils to define and subdivide the pre-Cryogenian Neoproterozoic Era in terms of Global Boundary Stratotype Section and Point (GSSP), as opposed to current system using Global Standard Stratigraphic Age (GSSA). Available biostratigraphic data, including the occurrence of Trachyhystrichosphaera, Chuaria, Tawuia, and Sinosabellidites, suggest that the Liulaobei Formation is of pre-Cryogenian Neoproterozoic age, not Cryogenian–Ediacaran age as some have suggested in the past.