Ediacara Biota Research Articles

Biostratinomy of the enigmatic tubular organism Aulozoon soliorum, the Rawnsley Quartzite, South Australia

The Ediacaran fauna (574–539 Ma) are the first macroscopic community forming organisms on Earth, thus providing unique insight into the origins and evolution of complex life. However, the assignment of most Ediacaran organisms to specific phyla and kingdoms remains difficult due to their commonly non-analogous body plans and soft-bodied nature, which often results in the loss of diagnostic features through the processes of death and fossilization. Notably, this taphonomic overprint is not imparted equally to all Ediacaran organisms and is particularly exaggerated for the morphotypic grouping of tubular organisms. This morphogroup is comprised of macroscopic organisms with elongate, hollow, and simple body plans and is the most abundant Ediacaran morphogroup. However, the simple and hollow morphology of tubular organisms results in a broadly low preservation potential. In turn, this commonly limits the reconstruction of in vivo morphology, muddles taxonomic assignment, obfuscates phylogenetic affinities, and inhibits understanding of their broader significance. Fortunately, previous studies of abundant tubular taxa have demonstrated that taxon-level biostratinomic investigations can, in part, account for this low preservation potential. Such studies inform a more complete understanding of the morphogroup, providing novel insight into the paleobiology, paleoecology, and biomaterials of tubular taxa. To contribute to this biostratinomically-geared assessment of Ediacaran tubular organisms, this study analyzes the preservation of Aulozoon soliorum, an abundant tubular organism from the Rawnsley Quartzite of South Australia. This investigation demonstrates that Aulozoon is preserved via five preservational modes, which inform a novel in vivo reconstruction of Aulozoon as a sessile, fluid-filled, cylindrical organism with a holdfast and a thin, organic, and featureless body wall. The preservational modes identified here additionally inform a taphonomic framework under which Aulozoon fossils can be interpreted, thus furthering holistic understanding of the morphogroup-specific preservational variability of Ediacaran tubular organisms.

A Great late Ediacaran ice age.

The emergence of the Ediacara biota soon after the Gaskiers glaciation ca. 580 million years ago (Ma) implies a possible glacial fuse for the evolution of animals. However, the timing of Ediacaran glaciation remains controversial because of poor age constraints on the ∼30 Ediacaran glacial deposits known worldwide. In addition, paleomagnetic constraints and a lack of convincing Snowball-like cap carbonates indicate that Ediacaran glaciations likely did not occur at low latitudes. Thus, reconciling the global occurrences without global glaciation remains a paradox. Here, we report that the large amplitude, globally synchronous ca. 571-562 Ma Shuram carbon isotope excursion occurs below the Ediacaran Hankalchough glacial deposit in Tarim, confirming a post-Shuram glaciation. Leveraging paleomagnetic evidence for a ∼90° reorientation of all continents due to true polar wander, and a non-Snowball condition that rules out low-latitude glaciations, we use paleogeographic reconstructions to further constrain glacial ages. Our results depict a 'Great Ediacaran Glaciation' occurring diachronously but continuously from ca. 580-560 Ma as different continents migrated through polar-temperate latitudes. The succession of radiation, turnover and extinction of the Ediacara biota strongly reflects glacial-deglacial dynamics.

Spatial distributions of Tribrachidium, Rugoconites, and Obamus from the Ediacara Member (Rawnsley Quartzite), South Australia

AbstractThe spatial distribution of in situ sessile organisms, including those from the fossil record, provides information about life histories, such as possible dispersal and/or settlement mechanisms, and how taxa interact with one another and their local environments. At Nilpena Ediacara National Park (NENP), South Australia, the exquisite preservation and excavation of 33 fossiliferous bedding planes from the Ediacara Member of the Rawnsley Quartzite reveals in situ communities of the Ediacara Biota. Here, the spatial distributions of three relatively common taxa, Tribrachidium, Rugoconites, and Obamus, occurring on excavated surfaces were analyzed using spatial point pattern analysis. Tribrachidium have a variable spatial distribution, implying that settlement or post-settlement conditions/preferences had an effect on populations. Rugoconites display aggregation, possibly related to their reproductive methods in combination with settlement location availability at the time of dispersal and/or settlement. Additionally, post-settlement environmental controls could have affected Rugoconites on other surfaces, resulting in lower populations and densities. Both Tribrachidium and Rugoconites also commonly occur as individuals or in low numbers on a number of beds, thus constraining possible reproductive strategies and environmental/substrate preferences. The distribution of Obamus is consistent with selective settlement, aggregating near conspecifics and on substrates of mature microbial mat. This dispersal process is the first example of substrate-selective dispersal among the Ediacara Biota, thus making Obamus similar to numerous modern sessile invertebrates with similar dispersal and settlement strategies.

The role of iron in the formation of Ediacaran 'death masks'.

The Ediacara biota are an enigmatic group of Neoproterozoic soft-bodied fossils that mark the first major radiation of complex eukaryotic and macroscopic life. These fossils are thought to have been preserved via pyritic "death masks" mediated by seafloor microbial mats, though little about the chemical constraints of this preservational pathway is known, in particular surrounding the role of bioavailable iron in death mask formation and preservational fidelity. In this study, we perform decay experiments on both diploblastic and triploblastic animals under a range of simulated sedimentary iron concentrations, in order to characterize the role of iron in the preservation of Ediacaran organisms. After 28 days of decay, we demonstrate the first convincing "death masks" produced under experimental laboratory conditions composed of iron sulfide and probable oxide veneers. Moreover, our results demonstrate that the abundance of iron in experiments is not the sole control on death mask formation, but also tissue histology and the availability of nucleation sites. This illustrates that Ediacaran preservation via microbial death masks need not be a "perfect storm" of paleoenvironmental porewater and sediment chemistry, but instead can occur under a range of conditions.

Phosphatized microfossils from the Miaohe Member of South China and their implications for the terminal Ediacaran biodiversity decline

The upper Ediacaran Miaohe Member (∼550 Ma) in South China is well known for exceptionally preserved macroscopic carbonaceous compression fossils (i.e., the Miaohe biota) that are pivotal in understanding the marine ecology and environment during the terminal Ediacaran Period. However, micro-organisms, which are also important biotic components of marine ecosystems, are poorly documented and largely limited to simple leiospheres and filaments in the Miaohe Member and equivalent strata in South China and around the world. Here we report a moderately diverse assemblage of cellularly preserved microfossils from phosphorites and siliceous phosphorite nodules and bands of the Miaohe Member at the Maxi section, Hubei Province, South China. Ten named species of eight genera, four open nomenclatures, and two unnamed taxa have been identified, including several multicellular algal fossils and various coccoidal and filamentous cyanobacteria. Abundant spheroidal fossils are morphologically analogous to Megasphaera, which has previously been interpreted as a putative metazoan embryo. Together with previously described microfossils from equivalent strata of Miaohe Member, these microfossils expand our understanding of the evolution of marine ecosystems prior to the Cambrian explosion. The late Ediacaran Maxi assemblage and the early Ediacaran Weng’an biota, both preserved through phosphatization, share several taxa. Notably, Doushantuo-Pertatataka-type acanthomorphs (DPAs) are absent from the Maxi assemblage but abundant in the Weng’an biota, consistent with a late Ediacaran decline of DPAs, which may have ushered in the terminal Ediacaran decline of soft-bodied macro-organisms of the Ediacara biota during the Kotlin crisis or in the last 10 million years of the Ediacaran Period.

Frond orientations with independent current indicators demonstrate the reclining rheotropic mode of life of several Ediacaran rangeomorph taxa

AbstractFossils from the deep-sea Ediacaran biotas of Newfoundland are among the oldest architecturally complex soft-bodied macroorganisms on Earth. Most organisms in the Mistaken Point–type biotas of Avalonia—particularly the fractal-branching frondose Rangeomorpha— have been traditionally interpreted as living erect within the water column during life. However, due to the scarcity of documented physical sedimentological proxies associated with fossiliferous beds, Ediacaran paleocurrents have been inferred in some instances from the preferential orientation of fronds. This calls into question the relationship between frond orientation and paleocurrents. In this study, we present an integrated approach from a newly described fossiliferous surface (the “Melrose Surface” in the Fermeuse Formation at Melrose, on the southern portion of the Catalina Dome in the Discovery UNESCO Global Geopark) combining: (1) physical sedimentological evidence for paleocurrent direction in the form of climbing ripple cross-lamination and (2) a series of statistical analyses based on modified polythetic and monothetic clustering techniques reflecting the circular nature of the recorded orientation of Fractofusus misrai specimens. This study demonstrates the reclining rheotropic mode of life of the Ediacaran rangeomorph taxon Fractofusus misrai and presents preliminary inferences suggesting a similar mode of life for Bradgatia sp. and Pectinifrons abyssalis based on qualitative evidence. These results advocate for the consideration of an alternative conceptual hypothesis for position of life of Ediacaran organisms in which they are interpreted as having lived reclined on the seafloor, in the position that they are preserved.

The rangeomorph Pectinifrons abyssalis: Hydrodynamic function at the dawn of animal life

Rangeomorphs are among the oldest putative eumetazoans known from the fossil record. Establishing how they fed is thus key to understanding the structure and function of the earliest animal ecosystems. Here, we use computational fluid dynamics to test hypothesized feeding modes for the fence-like rangeomorph Pectinifrons abyssalis, comparing this to the morphologically similar extant carnivorous sponge Chondrocladia lyra. Our results reveal complex patterns of flow around P.abyssalis unlike those previously reconstructed for any other Ediacaran taxon. Comparisons with C. lyra reveal substantial differences between the two organisms, suggesting they converged on a similar fence-like morphology for different functions. We argue that the flow patterns recovered for P.abyssalis do not support either a suspension feeding or osmotrophic feeding habit. Instead, our results indicate that rangeomorph fronds may represent organs adapted for gas exchange. If correct, this interpretation could require a dramatic reinterpretation of the oldest macroscopic animals.

Species of Dickinsonia Sprigg from the Ediacaran of South Australia

AbstractAn iconic member of the Ediacara Biota, Dickinsonia Sprigg is one of few such taxa with multiple species. Here we use Gaussian finite mixture models to assess the validity of species distinctions for this genus. Our results indicate that the five described species of Dickinsonia from the Ediacara Member, South Australia are better classified as two based on multiple approaches. Two different methods for dimension reduction both provide strong support for two groups, with overlapping but distinct mixture models. The variable selection method produces the most biologically realistic clusters, indicating that the two species can be primarily differentiated based on the greater relative size of the anterior most unit of Dickinsonia costata Sprigg compared with Dickinsonia tenuis Glaessner & Wade. Despite differences in aspect ratio and number of modules, both species regulated growth to maintain overall shape. The greater likelihood of preservation of a midline and an irregular outer margin in D. tenuis highlights differential structural integrity and flexibility. Co‐occurrence in the Ediacara Member indicates that both species occupied the same environments and temporal distribution. Smaller maximum and average size for D. costata, combined with higher abundance, may suggest a comparatively shorter lifespan and increased rates of reproduction.

THE ROLE OF SURFICIAL BIOTURBATION IN THE LATEST EDIACARAN: A QUANTITATIVE ANALYSIS OF TRACE FOSSIL INTENSITY IN THE TERMINAL EDIACARAN–LOWER CAMBRIAN OF CALIFORNIA

ABSTRACT Bioturbating activities have played a vital role in shaping the marine ecosystem throughout metazoan history, influencing the abundance and preservation potential of body fossil-producing taxa and driving major environmental and geochemical changes. The earliest trace making behaviors arose during the late Ediacaran Period (∼ 560–541 Ma), disrupting the substrate previously occupied by dominantly sessile organisms. Simple dwelling and grazing behaviors exploited the organic-rich matgrounds, expanding into the underutilized microbial mat ecosystem. In the western United States, trace assemblages from Ediacaran–Cambrian boundary-spanning deposits document a thriving trace-maker ecosystem. One boundary-spanning deposit in this region, the lower member of the Wood Canyon Formation, crops out along the California-Nevada boundary and contains both trace and body fossil assemblages. The Chicago Pass section of the lower Wood Canyon Formation contains a suite of dominantly simple Ediacaran traces, which become commonplace in the upper part of the stratigraphic section, documenting the onset of prevalent trace-making behaviors in this region. While traces have been previously described from this locality, the addition of the complex trace Lamonte trevallis and quantification of trace fossil density of simple Ediacaran traces provides a more comprehensive ichnological view of the Chicago Pass section. Although Chicago Pass does not yield abundant tubicolous body fossils, as are found elsewhere in the region, the low diversity ichnoassemblages document both burgeoning surficial trace making groups and mat-targeted mining in the latest Ediacaran. The behaviors present at Chicago Pass are similar to those of the Dengying Formation in South China, and highlight the need for petrographic-based trace fossil studies. Additionally, studies of Nama Group trace fossils of the same age from Namibia report higher diversity and complexity in trace-making activities than what has been observed at Chicago Pass, but with similar, low Ediacara biota body fossil diversity. If Ediacara biota diversity is anticorrelated with trace-making behaviors, Chicago Pass represents a low-complexity end-member of the same phenomenon observed in Namibia. The effect of surface sediment disruption on the sessile Ediacaran communities may have been decoupled from complexity of the traces, more so influenced by the presence of general trace-making behaviors in aggregate, including simple traces.

Ediacaran biota from Ougnat Massif (Eastern Anti-Atlas, Morocco): Paleoenvironmental and stratigraphic constraints

The Ediacaran period records the appearance of the first multicellular and complex organisms in Earth's history. Within the West African Craton, just a few simple discoidal structures have been previously reported within the supposed Ediacaran successions. Here, we describe for the first time a slightly diversified Ediacaran assemblage of micro- and macro-fossils from Ediacaran volcano-sedimentary rocks of Northwest Africa. Fossils occur in shallow water carbonate-bearing siliciclastic sediments of the Izelf Formation (567–550 Ma), in the Moroccan Anti-Atlas. Macrofossils are represented by Aspidella, ivesheadiomorphs and other problematic structures with putative biotic origin. The macrofossil assemblage is dominated by taphomorphs that indicate different degrees of preservation due to progressive decaying processes, possibly extending the effacement preservation mode outside of Avalonia assemblages. Microbially induced sedimentary structures (MISS), stromatolites and spheroidal microfossils are also reported. In particular, spheroidal microfossils may occur as isolated individuals or as concatenated spheres with potentially cell-division processes and were preserved through carbonate and silica permineralization. Morphologically, spheroidal microfossils are compared to sphaeromorph acritarchs and sulfur-oxidizing bacteria such as genus Thiomargarita. The depositional environment and age interval provide new information concerning paleogeographic and paleoenvironmental conditions of Ediacaran biota living on the West African Craton; thus, fills a gap in the Precambrian fossil record from the WAC.

A diverse Ediacara assemblage survived under low-oxygen conditions

The Ediacaran biota were soft-bodied organisms, many with enigmatic phylogenetic placement and ecology, living in marine environments between 574 and 539 million years ago. Some studies hypothesize a metazoan affinity and aerobic metabolism for these taxa, whereas others propose a fundamentally separate taxonomic grouping and a reliance on chemoautotrophy. To distinguish between these hypotheses and test the redox-sensitivity of Ediacaran organisms, here we present a high-resolution local and global redox dataset from carbonates that contain in situ Ediacaran fossils from Siberia. Cerium anomalies are consistently >1, indicating that local environments, where a diverse Ediacaran assemblage is preserved in situ as nodules and carbonaceous compressions, were pervasively anoxic. Additionally, δ238U values match other terminal Ediacaran sections, indicating widespread marine euxinia. These data suggest that some Ediacaran biotas were tolerant of at least intermittent anoxia, and thus had the capacity for a facultatively anaerobic lifestyle. Alternatively, these soft-bodied Ediacara organisms may have colonized the seafloor during brief oxygenation events not recorded by redox proxy data. Broad temporal correlations between carbon, sulfur, and uranium isotopes further highlight the dynamic redox landscape of Ediacaran-Cambrian evolutionary events.

Guts, gut contents, and feeding strategies of Ediacaran animals

The oldest animals appear in the fossil record among Ediacara biota communities. They prelude animal-dominated ecosystems of the Phanerozoic and may hold clues to the appearance of modern animal phyla in the Cambrian explosion. However, little is known about the phylogeny of the Ediacaran organisms and even less about their diet and feeding behavior.1,2,3 An exception is mollusc-like Kimberella, for which a fossilized gut, feeding traces, and even potential coprolites have been found.4,5 By contrast, Ediacaran organic-walled tubes, such as Sabellidites and Calyptrina, are thought to belong to tube worms comparable with modern Siboglinidae that have no gut but gain their nutrition from symbiotic bacteria.6,7 Here, we examine the gut contents of Ediacaran animals using biomarker molecules. We show that 558-million-year (Ma)-old tube worm-like Calyptrina and mollusc-like Kimberella possessed a gut and shared a diet of green algae and bacteria. Despite their ancient age, sterol metabolism within the gut of both organisms was already comparable to extant invertebrates.8Dickinsonia, one of the key Ediacaran animals, show no traces of dietary molecules, indicating a different feeding mode and possible external digestion analogous to modern Placozoa. Lipid biomarkers uncover a range of feeding strategies in Ediacaran communities, highlighting true eumetazoan physiology of some Ediacaran animals.

Environmental drivers of the first major animal extinction across the Ediacaran White Sea-Nama transition

The Ediacara Biota-the oldest communities of complex, macroscopic fossils-consists ofthree temporally distinct assemblages: the Avalon (ca. 575-560 Ma), White Sea (ca.560-550 Ma), and Nama (ca. 550-539 Ma). Generic diversity varies among assemblages, with a notable decline at the transition from White Sea to Nama. Preservation and sampling biases, biotic replacement, and environmental perturbation have been proposed as potential mechanisms for this drop in diversity. Here, we compile a global database of the Ediacara Biota, specifically targeting taphonomic and paleoecological characters, to test these hypotheses. Major ecological shifts in feeding mode, life habit, and tiering level accompany an increase in genericrichness between the Avalon and White Sea assemblages. We find that ∼80% of White Sea taxa are absent from the Nama interval, comparable to loss during Phanerozoic mass extinctions. The paleolatitudes, depositional environments, and preservational modes that characterize the White Sea assemblage are well represented in the Nama, indicating that this decline is not the result of sampling bias. Counter to expectations of the biotic replacement model, there are minimalecological differences between these two assemblages. However, taxa that disappear exhibit a variety of morphological and behavioral characters consistent with an environmentally driven extinction event. The preferential survival of taxa with high surface area relative to volume may suggest that this was related to reduced global oceanic oxygen availability. Thus, our data support a link between Ediacaran biotic turnover and environmental change, similar to other major mass extinctions in the geologic record.

Taxonomy and palaeoenvironmental distribution of palaeopascichnids

AbstractPalaeopascichnida is a problematic group of extinct organisms that is globally distributed in Ediacaran sequences of Avalonia, Baltica, Siberia, South China and Australia. The fossils related to Palaeopascichnida consist of serially or cluster-like arranged, millimetre- to centimetre-scale globular or allantoid chambers, which are characterized by substantial differences in preservation, leading to no consistent diagnosis for these organisms. Here we integrate morphometric variation, stratigraphic distribution and habitat settings of more than 1200 specimens from all known fossil localities. The results of the morphological analysis demonstrate variation in chamber shape and size, and allow us to recognize six valid species within the group. Statistical analysis of the specimen distribution with respect to sedimentary environments indicates a significant difference in palaeoecological settings between species, making a significant contribution to the evolution and systematic palaeontology of these problematic organisms and perspective on their use in Neoproterozoic biostratigraphy. Our revision and systematic study sheds new light on one of the least studied groups of the late Ediacaran biota.

THE ASSOCIATION OF BENTHIC MACROBIOTA OF THE LATE EDIACARAN OF PODILLIA IN THE DEPOSITS OF THE MOHYLIV-PODILSKYI GROUP IN THE QUARRY NEAR THE DNIESTER HPP

The maximum level of Ediacaran-type biota diversity in the Volyn-Podilskyi sedimentation basin existed in the time interval that corresponds to the Lomoziv and Yampil Members of the Mohyliv Formation. Studies of the isotopic age showed that this period ended 557 million years ago. Environmental conditions that prevented the existence of classical Ediacaran biota prevailed during the accumulation of the Lyadova Member. A less numerous biotic association of the transitional type with individual representatives of the Ediacara-type association is observed in the deposits of the Bernashivka Member. Paleoecological conditions in the sedimentation basin were reconstructed on the basis of the study of the section in the quarry near the Dniester HPP. More than a thousand specimens of fossils of soft-bodied organisms and tracefossils were collected there. More than three dozen species were discovered in Podilla for the first time. The study of a significant number of new species continues. It was established that the biotic association of the Mohyliv Formation was dominated by problematic frondomorphic and spherical organisms, which sometimes reached significant sizes.The existence of probable ancestors of several groups of biota of the Phanerozoic type:Cnidaria, Porifera, Lophotrochozoa, Mollusca and Metaphita were found in the Late Ediacaran shallow water ecosystem. Thefossil remains of the earliest probable members of the phylum Chordata of the subphylum Tunicata are collected here. These arethe most likely ancestors of vertebrates according to data from several research groups. It was established that some Ediacaran organisms used life strategies typical for representatives of later biota: combinations of different types of reproduction, activeresponse to external environmental factors, etc. New methods of fossil research were proposed and important results were obtained in the process of their use. It was established that quite sharp changes in the taxonomic composition of biotic associations in the section of sediments are caused by facies conditions and evolutionary processes. This makes it possible to use the fossilized remains of the Ediacaran biota for the needs of biostratigraphy and correlation with the same age deposits in other regions.

The Ediacaran record of glaciogenic dropstones, diamictites and cap carbonates associated with non-metamorphosed banded iron formations (BIFs) in Iran

The Ediacaran period is marked by global icehouse episode, greenhouse period, increasing levels of oxygen-carbon dioxide, syn-glaciogenic structures-BIFs, typical Ediacaran biota and a marked negative carbon isotope excursion. On this global point, field glaciogenic structures, petrographic textures, Ediacaran biota, U-Pb zircon ages, C-O isotopes and geochemical data record an Ediacaran glaciogenic BIFs event in Iran. The Ediacaran (578–547 or 578–541.8 Ma) glaciogenic sequences in Bafq and Hormuz Island mining districts include submarine syn-glaciovolcanic rhyolitic rocks, rhyolitic tuffs, shaly tuffaceous carbonates, dropstone-bearing jaspilitic BIFs, salt minerals, ferruginous diamictites, and cap carbonates. The presence of polygonal, bullet-shape and flatiron forms of gneiss, greenschist, quartz schist, quartzite, marble and magnetite-hematite clasts in a matrix of jaspilitic magnetite-hematite, muddy sandstones and muddy dolomitic rocks records an Ediacaran glaciogenic event. The dropstones occur as lone-stones, dump-stones and deflect their underlying layers. The diamictites contain similar clasts to those of the dropstones, are gradationally covered by buff to dark-grey cap carbonates ± dropstones and resemble the Ghaub, Chuos and Kingston Peak Formation diamictites. C-O isotopes, REE+Y diagrams, high Y/Ho ratios, positive Eu-Y anomalies and subtle Ce anomaly possibly reflect a possible hydrothermal origin for the cap carbonates. Delta 13C values (−0.42 to −6.33‰) in cap carbonates show similar negative signatures to the Ediacaran Shuram glaciogenic event. Average delta 18O (−8.34‰) in the least-altered cap carbonates gives a precipitation temperature of 252.7 °C in the waning stage of the glaciogenic BIFs-hydrothermal system.

Expanded deepwater euxinia recorded in the Ediacaran–Cambrian boundary interval in South China

• The Ediacaran-Cambrian boundary interval at Pingyin in South China records a deepwater euxinia. • Expanded deepwater euxinia occurred in the Nanhua Basin during the Ediacaran-Cambrian transition. • The expanded deepwater euxinia probably resulted in the extinction of Ediacaran fauna. Global anoxia of shallow seawater during the Ediacaran–Cambrian (EC) transition has been interpreted as being caused by the upwelling of H 2 S-bearing deep seawater. This hypothesis implies the possible widespread occurrence of H 2 S-rich deep seawater during the EC transition. However, due to a lack of detailed investigation of environmental information recorded across the EC boundary (ECB) at deep-water facies sections, the occurrence of widespread H 2 S-rich deep seawater in paleo-basins during the EC transition is uncertain. The Pingyin section in South China is a deep-water-facies section for which the position of the ECB has been constrained by high-precision zircon chemical abrasion–isotope dilution–thermal ionization mass spectrometry U–Pb ages. Therefore, this section is a favourable site for obtaining information about deep seawater during the EC transition. In this paper, we present redox sensitive element (RSE) concentrations and Fe speciation of cherts from this section to study the redox condition of deep seawater recorded in the ECB at Pingyin. The results show dramatic differences of redox proxies in the cherts in the ECB from those underlying ones. The enrichment factors of U, V and Mo of the underlying samples are 2.08–6.89, 0.82–1.52 and 3.36–7.32, much lower than those of cherts in the ECB which are 16.52, 3.65 and 48.60, respectively. The underlying samples have Th/U ratios of 0.69–2.29, much higher than the cherts in the ECB with Th/U ratios of <0.30. The V/(V + Ni) ratios of most of the samples below the ECB are <0.84, whereas those of the samples at the ECB are >0.84. The Ce/Ce* values of the chert samples show an upward-decreasing trend below the ECB, but change to an increasing tendency immediately above the boundary. Almost all of the chert samples from the topmost Liuchapo Formation have Fe HR /Fe T ratios of >0.38. The Fe PY /Fe HR ratios of the samples underlying the ECB are lower than 0.80, whereas those of the samples around the ECB are higher than 0.80. These various indicators show that the ECB at Pingyin records an ancient deep-water redox turnover from anoxic ferruginous to sulphidic conditions. Combining the new findings with previous data, we conclude that the ECB in South China records the extensification of euxinia in deep water of the investigated paleo-basin. This event, which may have been global in extent, could have had a profound influence on early marine life and was likely a major cause of Ediacaran faunal extinction.

Decoupled oxygenation of the Ediacaran ocean and atmosphere during the rise of early animals

The Ediacaran Period (∼635 to 541 Ma) witnessed the early diversification and radiation of metazoans, in the form of the Ediacaran Biota. This biological revolution, beginning at ∼575 Ma, has been widely attributed to a temporally restricted episode of deeper ocean oxygenation, potentially caused by a contemporaneous rise in atmospheric oxygen levels. However, quantitative geochemical-record-driven estimates of Ediacaran atmospheric and oceanic redox evolution are lacking, and hence possible links between oceanic and atmospheric oxygenation remain speculative. Here, after screening for possible post-depositional alteration, we utilize paleogeographically-diverse carbon and sulfur isotope records from South China, Oman and USA-Mexico, to develop a biogeochemical isotope mass balance model to quantify Ediacaran atmospheric oxygen and oceanic sulfate evolution. Model results from all three continents indicate that Ediacaran atmospheric oxygen levels rose monotonically between ∼630 Ma and ∼590 Ma, and subsequently remained relatively stable at around 0.6 present atmospheric level for the remainder of the Ediacaran. By contrast, the marine sulfate reservoir appears to have remained relatively stable before ∼575 Ma, with a subsequent large pulse where sulfate concentrations rose to ∼8 mM. These quantitative results indicate that Ediacaran oceanic and atmospheric oxygenation were decoupled, which is consistent with published geochemical records. We propose that the early Ediacaran rise of atmospheric oxygen levels, driven by increased net burial of organic carbon and pyrite, may not have established widespread deep-ocean oxygenation. Instead, later pulsed input of oxidizing power (mainly sulfate) from the continents drove transient episodes of seafloor oxygenation that accompanied radiations of the Ediacaran Biota.

Biofilms as agents of Ediacara-style fossilization

Earth’s earliest fossils of complex macroscopic life are recorded in Ediacaran-aged siliciclastic deposits as exceptionally well-preserved three-dimensional casts and molds, known as “Ediacara-style” preservation. Ediacara-style fossil assemblages commonly include both macrofossils of the enigmatic Ediacara Biota and associated textural impressions attributed to microbial matgrounds that were integral to the ecology of Ediacara communities. Here, we use an experimental approach to interrogate to what extent the presence of mat-forming microorganisms was likewise critical to the Ediacara-style fossilization of these soft-bodied organisms. We find evidence that biofilms can play an instrumental role in fostering fossilization. Rapid silica precipitation associated with macroorganism tissues is enhanced in the presence of mat- and biofilm-forming microorganisms. These results indicate that the occurrence of microbial mats and biofilms may have strongly shaped the preservational window for Ediacara-style fossils associated with early diagenetic silica cements, and therefore influenced the distribution and palaeoecological interpretation of the Ediacara Biota fossil record.

Global Hg cycle over Ediacaran–Cambrian transition and its implications for environmental and biological evolution

The Ediacaran–Cambrian (E–C) transition witnessed remarkable environmental changes, the extinction of the Ediacaran biota, and subsequent rapid diversification of Cambrian animals. However, the linkages between environmental and biological evolution are still under debate at this critical time. Here, we present new Hg abundance and Hg isotopes in sediments from South China, which are then combined with those published from the Indian craton to explore the co-evolution of environment and complex life during the E–C transition. In both areas, high Hg/TOC ratios and near-zero Δ199Hg of the Late Ediacaran sediments suggest enhanced volcanism, whereas relatively high Hg/TOC ratios and positive Δ199Hg shift upsection indicate volcanic-sourced atmospheric Hg(II) deposition in the earliest Cambrian. The dramatically decreasing Hg/TOC ratios and positive Δ199Hg of early Cambrian Age 2 to 3 sediments indicate scavenging of dissolved seawater Hg(II) by organic matter particulates. Our Hg results suggest volcanism may have likely played a significant role in the extinction of Ediacaran biota and global negative carbon excursions near the E–C boundary. Furthermore, our Hg data provides new evidence of extensive OM burial in the early Cambrian Age 2 to 3 oceans, leading to a rapid increase of Earth-surface O2 levels that coincided with appearance of more complex large-body animals. Our study provides new insights of the global Hg cycle into the co-evolution of the environment and complex life at this critical time.