Dmitriy V. Grazhdankin

Carbonate-hosted Avalon-type fossils in arctic Siberia

Avalon-type fossils are crucial to understanding the origin of Phanerozoic ecosystems, but their usual occurrence in volcaniclastic and siliciclastic facies greatly limits their paleobiological resolution. The unique carbonate-hosted assemblage of the Khatyspyt Formation, on the Olenek uplift of north-central Siberia, promises a much enhanced anatomical and paleo-ecological view of these enigmatic organisms. Avalon-type fossils are preserved by authigenic carbonate cementation in intervals of finely laminated nodular limestones (Khatyspyt-type taphonomic window). Interbedded silicified calcareous mudstones yield diverse carbonaceous compressions, occasionally with taphonomic phantoms of Avalon-type taxa (Miaohe-type taphonomic window). Styles of moldic preservation do not appear to be taxon selective, and therefore cannot alone be responsible for the morphological distinctiveness of Ediacaran macrofossils and the profound disparity in the taxonomic composition between fossil assemblages. On the other hand, the exclusion of Avalon-type fossils from carbonaceous compressions (Miaohe-type preservational window) is a real taphonomic signal that provides an important constraint on the properties of certain Ediacaran tissues.

The oldest evidence of bioturbation on Earth

Detailed study of the Khatyspyt Formation in arctic Siberia has shown that, contrary to common expectations, the earliest ichnofabric is of late Ediacaran age, records a food-seeking behavior, and is found in a distal carbonate ramp setting where it co-occurs with fossils of Avalon-type Ediacaran soft-bodied organisms. Preserved depth of bioturbation reaches 5 cm. The ichnofabric solely comprises Nenoxites , the oldest known meniscate trace fossil, which is interpreted as burrows with terminal backfill structure formed as a result of active displacement of sediment to form a tunnel within the substrate, followed by emplacement of the material behind the animal as it progressed through the sediment. In addition to being the most reliable paleontological evidence for the existence of bilaterians at ca. 555 Ma, the late Ediacaran bioturbation is regarded as a key step in the escalatory “engineering” of Phanerozoic ecospace leading to sudden diversification of macroscopic organisms and macrocommunities.

Arumberia-type structures in the Upper Vendian of the Urals

Problematic arumberia-type structures are widespread in Upper Proterozoic sedimentary rocks deposited in depositional environments subject to periodic desiccation and fluctuating salinity. Here we report two new occurrences of arumberia-type structures in the Krutikha Member of the Upper Vendian Chernyi Kamen Formation on the western slope of the Central Urals (Us’va River) and the Zigan Formation of the Asha Group in the South Urals (new road Sterlitamak-Beloretsk). The results of the study of these structures significantly expand paleontological characteristics of the Upper Vendian non-marine rocks and allow us to re-examine the paleontological importance of the hastily forgotten group of arumberia-type organisms. We distinguished six varieties of arumberia-type structures that for the sake of convenience were given formal Latin names in binominal nomenclature although they are not biological species. Two species—Arumberia banski and A. vindhyanensis—were erected by previous researchers, whereas the A. beckeri, A. ollii, A. usvaensis, and A. multykensis varieties are recognized for the first time. Despite similarity to erosional structures on the bottom of turbidite beds, erosion does not explain the genesis of the entire spectrum of arumberia-type structures including those preserved on ripple marks, with evidence of soft-body deformation, or with ribbon and filamentous fabric replaced by clay minerals. The diversity of arumberia could be only a manifestation of living systems. The species of Arumberia are distinguished by their different biostratinomy, the distance between the main structural units, the preservation of the main structural units, and the presence of additional structures between the main structural units. Most likely, the diversity of arumberia reflects not only taphonomic, but also ontogenetic and phenotypic variation. The level of morphological complexity and biostratinomic features of arumberia apparently exceed the organization of microbial mats and represent a non-actualistic type of microbial communities. The occurrence in extremely shallow-water depositional setting suggests that arumberia had evolved an adaption to periodic desiccation and that arumberia-type organisms, along with microbial mats, had been an important factor of sediment biostabilization in the Late Vendian non-marine environments.

Late Vendian Miaohe-type ecological assemblage of the east European platform

Upper Vendian rocks of the East European Platform (EEP) are characterized by the presence of the White Sea fossil biota, which colonized the region from the southeastern White Sea area to the Central Urals [1]. The White Sea biota includes ecological assemblages of the Avalon (Newfoundland), Ediacara (South Australia), and Nama (Namibia) types, each related to certain environmental conditions [2]. In 2006, we found a previously unknown and morphologically diverse assemblage of carbonaceous macroscopic fossils in the fine-grained aluminosiliciclastic rocks of the Perevalok Formation (Sylvitsa Group) in the Central Urals. Together with the organic-walled macrofossils from the rocks of the Lyamtsa Formation (Valdai Group) in the southeastern White Sea area, the carbonaceous fossils from the Perevalok Formation represent a new (fourth) ecological assemblage of the White Sea fossil biota. The ecological assemblage is older than 557‐558 Ma [3, 4] and includes macroscopic microbial colonies, multicellular and coenocytic eukaryotic macroalgae. In the Late Vendian history of the EEP, this assemblage predated the appearance of the world’s most diverse soft-bodied assemblage, which was found in the overlying rocks of the White Sea area (Verkhovka and Erga formations) and Central Urals (Chernokamen Formation) [1, 2, 5]. The carbonaceous fossils are confined to a thick (200‐400 m) transgressive sequence at the base of the Upper Vendian succession in the southeastern White Sea area and Central Urals (Fig. 1). The lower part of the sequence (laminated mudstones with layers of volcanic tuffs) is gradually replaced upsection by thinly interbedded siltstones and mudstones with rare layers of wave-bedded sandstones. The sequence was formed by the advance and periodic retreat of storm-dominated coastal depositional setting into subaqueous muddy planes with relatively quiet sedimentation in the course of oscillating wane in transgression. Fossiliferous intervals contain thin laminae of phosphorites and organic matter and mark the peak of shallow-water transgression over the platform. In the southeastern White Sea area, this sequence correlates with the Lyamtsa Formation and lower part of the Verkhovka Formation; in the Central Urals, with the upper part of the Staropechny and Perevalok formations [6, 7] (Fig. 1).

Trace Fossils and the Upper Vendian Boundary in the Southeastern White Sea Region

Among numerous localities of Late Proterozoic (Vendian) fossilized soft-bodied organisms, the southeastern White Sea region is the most informative one owing primarily to high taxonomic diversity of the biota and perfect preservation of its remains. The absence of paleontologically characterized Vendian‐ Cambrian boundary sections, a drawback of this region, hampers the correlation and restricts possibilities of paleontological studies. Based on lithological correlation between sections of the southeastern White Sea region and northwestern East European Platform, the existence of Cambrian sediments in the northeastern White Sea region was repeatedly suggested in the 1950s and 1960s [1‐4]. However, this assumption could not be confirmed because of the poor geological knowledge of sediments attributed now to the Padun Formation that crowns the Valdai Group section in the southeastern White Sea region. Recently, we were able to solve this problem. In the Padun Formation, we detected trace fossils Diplocraterion , which indicate the Cambrian age of host sediments and allowed us to carry out a more reliable correlation of Vendian reference sections of the White Sea region with coeval sequences not only in the neighboring regions, but also beyond the East European Platform. In 2004, we studied small isolated outcrops of redbrown and light gray fine-grained quartz sandstones partly bleached by surface weathering in the Bol’shaya Yura River, a right tributary of the Severnaya Dvina River (Fig. 1). The sections include numerous sandstone layers with thin horizontal lamination (Fig. 2) that form lenticular interbeds from 4‐8 cm to 0.5 m thick. The thin lamination pattern is provided by alternation of millimeter-scale sandstone laminae that differ in grain-size and the shape of detrital material. Some interbeds are characterized by convolute lamination, probably related to water-saturation and deformation of finely laminated sediments. The sections enclose abun

A geochemical study of the Ediacaran discoidal fossil Aspidella preserved in limestones: Implications for its taphonomy and paleoecology

The Ediacara biota features the rise of macroscopic complex life immediately before the Cambrian explosion. One of the most abundant and widely distributed elements of the Ediacara biota is the discoidal fossil Aspidella, which is interpreted as a subsurface holdfast possibly anchoring a frondose epibenthic organism. It is a morphologically simple fossil preserved mainly in siliciclastic rocks, which are unsuitable for comprehensive stable isotope geochemical analyses to decipher its taphonomy and paleoecology. In this regard, three-dimensionally preserved Aspidella fossils from upper Ediacaran limestones of the Khatyspyt Formation in the Olenek Uplift of northern Siberia offer a rare opportunity to leverage geochemistry for insights into their taphonomy and paleoecology. To take advantage of this opportunity, we analyzed δ13 Ccarb , δ18 Ocarb , δ13 Corg , δ34 Spyr , and iron speciation of the Khatyspyt Aspidella fossils and surrounding sediment matrix in order to investigate whether they hosted microbial symbionts, how they were fossilized, and the redox conditions of their ecological environments. Aspidella holdfasts and surrounding sediment matrix show indistinguishable δ13 Corg values, suggesting they did not host and derive significant amount of nutrients from microbial symbionts such as methanogens, methylotrophs, or sulfide-oxidizing bacteria. δ13 Ccarb , δ18 Ocarb , and δ34 Spyr data, along with petrographic observations, suggest that microbial sulfate reduction facilitated the preservation of Aspidella by promoting early authigenic calcite cementation in the holdfasts before matrix cementation and sediment compaction. Iron speciation data are equivocal, largely because of the low total iron concentrations. However, consideration of published sulfur isotope and biomarker data suggests that Aspidella likely lived in non-euxinic waters. It is possible that Aspidella was an opportunistic organism, colonizing the seafloor in large numbers when paleoenvironments were favorable. This study demonstrates that geochemical data of Ediacaran fossils preserved in limestones can offer important insights into the taphonomy and paleoecology of these enigmatic organisms living on the eve of the Cambrian explosion.

Ediacaria in the Siberian hypostratotype of the Riphean

The study of stratigraphy and paleontology of the Riphean/Vendian boundary strata is fundamental to decoding the transitional (Neoproterozoic) stage in the evolution of the biosphere. During this stage, the Proterozoic-style biota with limited morphological diversity, small size of individual organisms, lack of biogeographic zonation, and low rates of evolutionary turnover was replaced by the diverse Phanerozoic-style biota, with bioprovinciality and high dynamics of macroevolutionary processes [1]. The study of the transitional period invokes the analysis of the most representative Upper Proterozoic sections along the periphery of the Siberian Craton. Here, the uppermost Riphean was separated into the Baikalian Complex [2], which was proposed as a Regional Stage [3] or, later, as an Erathem of the General Stratigraphic Chart for the Precambrian of Northern Eurasia and equivalent to the Cryogenian of the International Stratigraphic Chart for the Precambrian [4]. Ediacaran fossils recently discovered in the Ui Group suggest that the biosphere shift commenced during the Baikalian. In 2005, large isolated outcrops of light- and yellowish gray quartzitic and arkosic sandstones, light gray siltstones and greenish gray mudstones were studied in the middle reaches of the Maya River on a segment between the mouths of the Malyi Kandyk and UlakhanKrestyakh Creeks (eastern slope of the Aldan Shield) (Fig. 1). The sections comprise the stratotype of the Kandyk Formation of the Ui Group, Upper Riphean [5, 6]. The total thickness of the succession exceeds 300 m (Fig. 1). The upper part of the sequence can be traced in large-blocky Felsenmeere, screes, and isolated outcrops at the mouth of the Yudoma River, where it is overlain by gray flaggy dolostones and oncolitic limestones of the Yudoma Group of Vendian age [2, 6, 7]. The studied sedimentary succession of the Kandyk Formation consists of two large depositional systems. The first depositional system, up to 100 m in thickness (Fig. 1, Member 1), has a distinctive complexly rhythmic stratification pattern composed of alternating sheets of wavy-bedded sandstones (0.1‐0.4 m), intervals of siltstone‐mudstone couplets, and sheets of wavy-laminated limestones (up to 0.7 m). The second depositional system, over 200 m thick (Fig. 1, members 2‐4), has a relatively simple structure: thick (up to 70 m) packages of thin- and wavy-laminated and thick-bedded and cross-bedded sandstones are interstratified with thick (up to 65 m) intervals of siltstone‐mudstone couplets.

New constraints for the age of Vendian glacial deposits (Central Urals)

ISSN 1028334X, Doklady Earth Sciences, 2013, Vol. 449, Pa rt 1, pp. 303–308.

Opening up a window into ecosystems with Ediacara-type organisms: preservation of molecular fossils in the Khatyspyt Lagerstätte (Arctic Siberia)

The Khatyspyt Formation in Arctic Siberia is one of only two carbonate settings with Ediacara-type fossils. As a potential hydrocarbon source rock, it contains abundant molecular fossils that may help to expand our understanding of these ecosystems. Unfortunately, however, the molecular fossil record in geological materials is commonly biased by secondary processes such as thermal maturation, migration of bitumen compounds or surface contamination. In this study, we evaluate the preservation of molecular fossils in a sample from the Khatyspyt Formation and elucidate their paleobiological meaning. Our results reveal that the organic matter is remarkably immature (oil window maturity) and shows little effect of biodegradation. Petrographic observations, exterior/interior experiments, and the similarity between free bitumen, mineral-occluded bitumen, and kerogen pyrolysate point to the syngeneity of the molecular fossils. Abundant hopanes, cyclohexylalkanes, and methyl-branched alkanes indicate a bacterial source of the organic matter, likely including cyanobacteria and anaerobic bacteria. At the same time, a carbonaceous compression fossil on top of the sample and abundant steranes indicate the presence of eukaryotes. The steranes show typical distributions for the Ediacaran (i.e., dominance of stigmastane). Given the exceptional preservation of the body fossils, trace fossils, and molecular fossils, the Khatyspyt Formation can be considered a fossil lagerstätte sensu Seilacher (1970: Begriff und Bedeutung der Fossil-Lagerstätten. Neues Jahrbuch für Geologie und Paläontologie, Monatshefte: 34–39). The combined analysis of sedimentary facies, paleontology (body, trace, and molecular fossils), and biogeochemistry will provide a more complete understanding of ecosystems with Ediacara-type fossils.KurzfassungDie Khatyspyt-Formation im arktischen Sibirien ist eines von lediglich zwei karbonatischen Ablagerungsräumen mit Ediacara-Fossilien. Gleichzeitig ist sie auch ein potentielles Kohlenwasserstoff-Muttergestein. Nicht zuletzt aufgrund der daher reichlich enthaltenen molekularen Fossilien birgt sie das Potential, unser Verständnis dieser Ökosysteme zu verbessern. Das Inventar an in geologischen Materialien erhaltenen molekularen Fossilien ist jedoch häufig durch diverse sekundäre Prozesse, wie z. B. thermische Maturierung, die sekundäre Migration von Kohlenwasserstoffen oder Oberflächen-Kontamination, verfälscht. In dieser Studie evaluieren wir die Erhaltung molekularer Fossilien in einer Probe der Khatyspyt-Formation und diskutieren ihre paläobiologische Bedeutung. Unsere Ergebnisse zeigen, dass das organische Material eine bemerkenswert niedrige Reife aufweist (Ölfenster) und kaum durch Biodegradation beeinflusst wurde. Petrographische Beobachtungen, Exterieur/Interieur-Experimente, und die Gleichheit zwischen freiem Bitumen, mineral-gebundenem Bitumen und Kerogen-Pyrolysat unterstreichen die Syngenität der enthaltenen molekularen Fossilien. Hopane, Cyclohexane, und and methylverzweigte Alkane deuten auf die Anwesenheit von Bakterien, wahrscheinlich inklusive Cyanobakterien und anaeroben Bakterien. Gleichzeitig belegen ein organisch erhaltenes Makrofossil auf der Probenoberseite und häufige Sterane, welche ein für das Ediacarium typisches Verteilungsmuster aufweisen (Dominanz von Stigmastan), die Existenz von Eukaryoten. Aufgrund der außergewöhnlichen Erhaltung von Körper-, Spuren- und molekularen Fossilien kann die Khatyspyt-Formation als Fossillagerstätte sensu Seilacher (1970: Begriff und Bedeutung der Fossil-Lagerstätten. Neues Jahrbuch für Geologie und Paläontologie, Monatshefte: 34–39) interpretiert werden. Die zukünftig gemeinschaftliche Analyse von sedimentärer Fazies, Paläontologie (Körper-, Spuren- und molekulare Fossilien) und Biogeochemie wird ein kompletteres Bild von Ökosystemen mit Ediacara-Fossilien ermöglichen.

Enigmatic large-sized tubular fossils from the Terreneuvian of Arctic Siberia

The atypically large tubular fossils occur in the upper Mattaia Formation of the Kessyusa Group cropping out along the Khorbusuonka River (N71°17′31′′, E123°46′40′′), the right tributary of the Olenek River, Arctic Siberia. The fossils occur ~ 4 m stratigraphically below a volcanic tuff with a U–Pb zircon date of 529.7 ± 0.3 Ma (Kaufman et al. 2012). They also slightly predate the first appearance of fossil molluscs Aldanella attleborensis and Watsonella crosbyi, candidates for the index-species to define the base of the Cambrian Stage 2, and are coeval with a diverse ichnofossil assemblage (Nagovitsin et al. 2015). The first appearance of the ichnospecies Treptichnus pedum within the uppermost Syhargalakh Formation of the Kessyusa Group, ca. 65 m further down the section, defines the base of the Terreneuvian. The middle part of the Syhargalakh Formation is coeval with the ~ 544 My-old tuff breccia of the Tas-Yuryakh volcanic complex (Bowring et al. 1993; Rogov et al. 2015) (Fig. 1). Eight specimens are available for study, all preserved in a form of truncated conical steinkerns (Fig. 1a–d, f), subcircular to ovoidal in cross-section (Fig. 1e, h) and consisting of bio-intraclastic sparstone, petrographically indistinguishable from the host rock (Fig. 1i). All the specimens are incomplete at both ends, aligned subparallel to bedding and constitute a densely packed allochthonous fossil assemblage. The longest of the specimens measured is 92 mm (Fig. 1a–c). The degree of convergence of the edges of the steinkerns varies, although it remains constant within each specimen (6°–10°). In the widest specimen (CSGM 2028189) the maximum measured diameter is 33 mm narrowing down to 30 mm over a distance of 35 mm. Yet in the narrowest specimen (CSGM 2028-190) the measured diameter increases from 14.5 to 24.5 mm over the distance of 90 mm. The surface of the fossils has a pattern of transverse fine annulations (seven per 10 mm) preserved on a thin (50 μm) yellowishto brownish-grey ferruginous crust (Fig. 1a–d, f). The annulations are separated by narrow ridges or furrows, discontinuous or barely discernible where the ferruginous crust is thin and poorly preserved (Fig. 1a–b). The fossils are interpreted as internal moulds of annulated tubes that were open at either end, had a circular cross-section and tapered posteriorly. The uniform nature of the infill implies that the tubes must have been unoccupied by the time of transport and burial and relatively rigid to withstand sedimentation processes. Transverse thin-sections reveal no relics of hard mineralized skeleton—the thin ‘wall’ (Fig. 1e) is composed by dark-brown coat of relic pyrite, Fe-oxides and -hydroxides (Fig. 1i). Co-occurring numerous small skeletal fossils with well-preserved calcite shells (Fig. 1g, i) suggest a non-calcitic composition of the tubes. Supposing the tubes consisted of aragonite that had been completely dissolved (Morse et al. 1980; Mucci 1983), and taking into account their morphological characters (straight cones with sub-circular cross-sections and poorly developed transverse ornamentation), the fossils from the Mattaia Formation resemble circothecidae, a family of problematic Cambrian fossils, sometimes treated as hyoliths of the order Orthothecida Marek (1966). The circothecid conchs exhibit a simple surface morphology and are devoid of dorsoventral differentiation (Sysoev 1972; Missarzhevsky 1989; Parkhaev and Demidenko 2010). Hyoliths are demonstrably lophophorates (Moysiuk et al. 2017), but the affinities and life habit of circothecids remain elusive (Dzik 1981; Webers and Yochelson 1989; Landing 1993; Kouchinsky 2001; Malinky 2009). Neither apices, nor unambiguous apertures, nor associated operculum are preserved in the tubular fossils from the Mattaia Formation precluding their further taxonomic identification. The closest analogue can be found Handling Editor: Jan-Peter Duda.