AbstractYunnanozoans are a key group of Cambrian fossils, and many specimens of these blade-shaped, soft-bodied animals show exceptional preservation. Debate continues over whether they were stem vertebrates, non-vertebrate chordates, stem deuterostomes or even protostome invertebrates. Previous studies of the yunnanozoan circulatory system were limited to optical observations and remain controversial. Here, we re-study the relevant soft-tissue structures using a suite of high-resolution three-dimensional imaging and chemical analytical techniques. These preserved structures exhibit tubular shapes, three-dimensional nature and iron-rich and proteinaceous chemical signatures, collectively consistent with the dorsal and ventral aortae of vertebrates. Similar, smaller vessels are revealed within four pairs of ‘circular structures’ behind the pharynx. Their vessel-rich nature, anatomical position and uniform spherical morphology favour an interpretation of these circular structures as kidney glomeruli rather than previously suggested gonadal or glandular tissues, although the absence of evidence for a heart in yunnanozoans complicates the physiological inference. This work provides the first micrometre-scale characterization of putative blood vessels in Cambrian animals and offers new insights into the early evolution of vertebrate-type circulatory and urogenital systems.

Cambrian Burgess Shale-type (BST) fossil biotas document nearly complete snapshots of the oldest Phanerozoic marine ecosystems1-4. However, the rarity of deposits bearing high-diversity BST biotas5 has restricted our understanding of the evolutionary and ecological dynamics of the Cambrian explosion. Here we report the Huayuan biota-a lower Cambrian (Stage 4, approximately 512 million years ago) BST Lagerstätte from an outer shelf, deep-water setting of the Yangtze Block in Hunan, South China. The Huayuan biota yields remarkable taxonomic richness, comprising 153 animal species of 16 phylum-level clades dominated by arthropods, poriferans and cnidarians, among which 59% of species are new. The biota is comprised overwhelmingly of soft-bodied forms that include preserved cellular tissues. The complex ecosystem contained diverse radiodonts and pelagic tunicates, filling a gap of high-diversity BST biotas from the Cambrian Stage 4. Critically, multivariate ordination based on a global dataset of Cambrian BST biotas places the Huayuan biota within a main transition of marine animal ecosystems between Cambrian Age 3 and Age 4. Network analysis reveals close faunal connections between the Huayuan and Burgess Shale biotas, indicating transoceanic dispersal. Dated shortly after the Sinsk event6-8, the Huayuan biota illuminates differences in the impacts of this extinction in shallow- versus deep-water settings during the first Phanerozoic mass extinction and offers critical insights into the transformation of global ecosystems in the early Cambrian.

The Shuram/DOUNCE event, the largest negative carbonate carbon isotope excursion in Earth’s history, has been linked to global ocean oxygenation and early animal evolution. While this anomaly is often attributed to dissolved organic carbon oxidation, the feasibility of this hypothesis remains debated due to uncertain oxidant supply. Here, we construct a refined global seawater δ13C curve for the event using an extensive carbonate δ13C database and published geochronological constraints. Applying an inverse approach within an Earth Evolution Model, we reassess the dissolved organic carbon oxidation timeline and oxidant demand. Our results show that oxidizing ~9×1019 mol of dissolved organic carbon could account for the negative δ13C excursion, but cessation of oxidation alone cannot explain the rapid recovery, which requires enhanced productivity. These findings support continental sulfate as the major, sufficient oxidant source, while suggesting that a later period of enhanced productivity drove the rapid δ13C rebound. The continental sulfate supply is sufficient to support the dissolved organic carbon oxidation hypothesis for the Shuram event, according to an inverse Earth evolution model based on a refined carbon isotope curve.

The first trilobite mass extinction, known as the Redlichiid-Olenellid extinction, occurred at the Cambrian Series 2-3 boundary and is associated with the Redlichiid-Olenellid Extinction negative Carbon isotope Excursion (ROECE). The causes of the ROECE event remain debated, with some researchers attributing it to the Kalkarindji volcanism and others linking it to a coeval large-scale transgression event. Continuous seawater sulfur isotope records across this boundary are crucial for resolving this debate. In this study, we present sulfur isotope data of carbonate-associated sulfate (CAS) across the Cambrian Series 2-3 boundary from the Xiaoerbrak section, Tarim Basin and reconstruct a seawater S34S variation curve for this critical period. Our findings show that seawater S34S values were likely positively coupled with S13C values during Cambrian Series 2 but showed an abrupt increase across the Series 2-3 boundary, marking a shift from coupled to decoupled carbon-sulfur isotope behaviour. According to the prevailing ROECE hypothesis, this decoupling of carbon and sulfur isotopes may be caused by isotopically light carbon inputs to surface water from Kalkarindji volcanism or from anoxic bottom water. These two hypotheses are tested quantitively using a biogeochemical box model. The modelling results show that light carbon emission from volcanism is insufficient to explain the recorded seawater isotope variations. Instead, the best fit comes from a dissolved organic carbon (DOC) oxidation hypothesis, an updated version of the anoxic bottom water shoaling/upwelling hypothesis. This hypothesis posits that DOC-enriched anoxic bottom water was upwelled and oxidised by oxygen during a large-scale transgression, leading to a negative seawater S13C excursion. Along with oxygen consumption and expanded anoxia, increased pyrite burial resulted in a sharp rise in seawater S34S at the Cambrian Series 2-3 boundary. The proposed DOC oxidation hypothesis effectively explains the decoupled carbon and sulfur isotope behaviour at this boundary and highlights the significant role of reduced oxygen levels in the Redlichiid-Olenellid extinction.

The Ediacaran Weng'an Biota of South China yields embryo-like microfossils preserved with sub-cellular fidelity, previously interpreted as the oldest evidence of animals. Megasphaera dominates the assemblage and preserves the earliest stages of binary reductive division. It is assumed to develop into Megaclonophycus, which is composed of hundreds to thousands of cells; however, this developmental link has not been tested. We used synchrotron and computed tomography to characterize hundreds of specimens of Megaclonophycus and determine cell division patterns. Specimen cell counts range from 14 to 10 201, with counts clustering around 2048 and 4096, consistent with binary reductive cell division. However, the specimens have asynchronous binary cell division with cell sizes within a specimen varying by two- to threefold. The volume of Megaclonophycus is variable during development, showing no evidence of gastrulation, characteristic of metazoan development. Megaclonophycus and Megasphaera exhibit a similar sequence of development, size and taphonomy indicating a developmental sequence of early to later stages of the same organism. These findings are incompatible with the crown group metazoan affinity suggested for this taxon, and so molecular clock estimates for the origin of animals must rely on fossil calibration from sources other than Megaclonophycus.

The Precambrian stratigraphic archive of the North Qilian Accretionary Belt (NQAB) is crucial for understanding the tectonic evolution of the Proto-Tethys Ocean. Although various tectonic evolution models of the NQAB have been proposed, no consensus has yet been reached due to the ambiguous Precambrian chronostratigraphic framework and tectonic nature of this belt. In this study, we present LA-ICP-MS U-Pb dates of detrital zircons from the Precambrian iron-formation-bearing Jingtieshan and Zhulongguan groups in the northwestern NQAB. The new results, together with the published dates, indicate that the depositional ages of the two groups are 1400-1230 Ma and 740-580 Ma, respectively. Meanwhile, the Precambrian stratigraphic sequences and detrital zircon age distributions show strong similarities to their age-equivalent units in the western Central Qilian Block (CQB), demonstrating a close tectonic affinity between them in Precambrian. The provenance shift in early Paleozoic, together with the Ediacaran ophiolite suites and associated rift-related basaltic rocks in the NQAB, indicates that the previously identified North Qilian Block, where the Jingtieshan and Zhulongguan groups deposited, was separated from the CQB by continental rifting and subsequent ocean spreading during the late Ediacaran. These findings shed new lights on the tectonic evolution of the Proto-Tethys Ocean in the Neoproterozoic.

The expansion of marine anoxia is widely invoked as a primary driver for the late Cambrian Steptoean Positive Carbon Isotope Excursion (SPICE) and associated trilobite extinctions; however, the redox state of the shallow water remains contentious. Here, we re-evaluate these dynamics using high-resolution carbonate-associated sulfate sulfur isotopes (delta S-34(CAS)) and iodine content (I/(Ca + Mg)) datasets from the Xishan and Jiulongshan sections of the North China Platform (NCP). Both sections document a positive delta S-34(CAS) excursion (from similar to 31 parts per thousand to similar to 49 parts per thousand) superimposed onto a long-term decreasing trend, marking the first identification of this excursion alongside the SPICE delta C-13 record in the NCP. Simultaneously, a pronounced nadir in I/(Ca + Mg) (<0.5 mu mol/mol) during the rising limb of the SPICE in the Jiulongshan section provides robust evidence for transient shallow-water deoxygenation, coinciding with the observed turnover of shallow-water trilobites. The long-term decreasing trend in delta S-34(CAS) was likely driven by intensified weathering inputs of isotopically light sulfur, whereas the positive delta C-13 and delta S-34(CAS) excursions reflect the seafloor anoxia expansion and enhanced organic carbon and pyrite burial. While this increased burial potentially served as a substantial atmospheric oxygen source, our iodine data highlight that local eutrophication, fueled by intensified continental weathering and nutrient fluxes, likely counteracted this global oxygenation trend to induce shallow-water deoxygenation and trilobite turnovers. Given the occurrence of similar low iodine concentrations in other regions, this shallow-water deoxygenation likely exerted a widespread influence, particularly within the marginal settings such as North China and South China. The subsequent recovery of iodine levels during the falling limb reflects an increase in dissolved oxygen levels, representing the cumulative result of enhanced organic carbon and pyrite burial throughout the SPICE event. These findings highlight the complex interplay between global biogeochemical cycles and localized environmental stressors in shaping Cambrian marine ecosystems.

Chemical weathering is a critical Earth system process that regulates climate, ocean chemistry and the long-term carbon cycle. However, the intensity and variability of chemical weathering remain insufficiently constrained for the mid-Proterozoic (similar to 1.8-0.8 Ga), greatly limiting our understanding of the environmental context to early eukaryotic evolution. Here, we report the first coupled positive seawater Sr-87/Sr-86 (similar to 0.0007) and delta Li-7 (similar to 5 parts per thousand) isotope excursions of the Mesoproterozoic Era (1.6-1.0 Ga), which we argue signifies a substantial weathering event at similar to 1.57 Ga, characterised by increased silicate weathering rates and decreased weathering congruency. Drawing on independent geological evidence, we posit that enhanced volcanic CO2 degassing, possibly alongside accretional orogenesis, increased denudation rates and invigorated the hydrological cycle, amplifying silicate weathering and secondary clay formation. This weathering pulse broadly coincided with coeval ocean oxygenation and carbon cycle disruption, implicating it in the appearance of the earliest known decimetre-scale, multicellular eukaryotic fossils.

The Cambrian–Ordovician strata are well preserved in the Quruqtagh area of the Tarim Basin. While extensive work on the biostratigraphy and sedimentology of this region has been done, comprehensive studies integrating biostratigraphic and carbon isotope stratigraphic data remain relatively scarce. In this study, a detailed, integrated latest Cambrian stratigraphic sequence was established for the Wuligezitage area, and the first systematic carbon isotope was analyzed herein.The new findings clearly record the TOCE (Top of Cambrian Excursion) event in the northern part of the Quruqtagh area, facilitating extensive global carbon isotope stratigraphic correlations. The TOCE may have been triggered by a marine oxygenation pulse, which released light carbon via the oxidation of marine dissolved organic carbon and/or reduced organic carbon burial resulting from the contraction of oxygen-deficient intermediate water masses.This study establishes a direct and potentially causal relationship between TOCE, marine oxygenation, and sea-level fall, providing critical insights into the co-evolution of carbon cycle perturbations and environmental changes during late Cambrian.

The DOUNCE (DOUshantuo Negative Carbon isotope Excursion) was marked by a significant shift in delta C-13(carb) from similar to + 5 parts per thousand down to similar to - 12 parts per thousand in the upper part of the Ediacaran Doushantuo Formation of South China. As an equivalent event of the Shuram/Wonoka anomaly, the DOUNCE event is the largest negative delta C-13(carb) excursion in geological history and denotes a global ocean oxygenation event. Consequently, it has been widely used as a chemostratigraphic tool for correlating the Ediacaran strata globally. Nonetheless, the DOUNCE exhibits variable stratigraphic expressions across sections and depositional environments, raising questions about its representation as a primary indicator of the Ediacaran seawater delta C-13 value. Such variability casts doubt on the reliability of the DOUNCE for global correlation, and its implications for the carbon cycle, oceanic oxygenation, and biological evolution during the Ediacaran period. To elucidate the DOUNCE event as a synchronous global occurrence and a chemostratigraphic tool, we have compiled the "DOUNCEraq", a global-scale database comprising 9375 valid delta C-13(carb) analyses from 156 sections/boreholes documenting the DOUNCE/Shuram/Wonoka event. Our meta-analysis of DOUNCEraq highlights the global scope of the DOUNCE event and reveals the presence of an instant rise stage post the abrupt delta C-13(carb) decline as an inherent feature of the DOUNCE pattern. Moreover, it also emphasizes the impacts of palaeolatitude, palaeocontinent, water depth, and lithology on the DOUNCE's pattern and variability: (1) lower pre-DOUNCE delta C-13(carb) values and smaller shift magnitudes within 30-0 degrees N compared to the southern hemisphere; (2) compared to the shallower sections, deep-water sections exhibit lower pre-DOUNCE and DOUNCE nadir delta C-13(carb) values with smaller shift magnitudes relative to shallower sections; (3) dolostones demonstrate lower pre-DOUNCE values, higher values at the DOUNCE nadirs, and smaller shift magnitudes compared to limestones. Such local impacts on the DOUNCE pattern provide important constraints on the causes of the DOUNCE event and could be explained within the DOC-oxidation hypothesis via regulating oxidants supply. Overall, the present meta-analysis enhances our understanding of the DOUNCE's global stratigraphic expressions and provides important constraints on the DOUNCE causes.

The mandibulate euarthropods are the most speciose animal group, but the evolutionary gaps in origin of mandibulate body plan remain unresolved. Marrellomorphs, a common Paleozoic euarthropod group, had a long evolutionary history from Cambrian to Devonian. With computed microtomography, here we report the fine-scale soft-bodied morphoanatomy of the oldest marrellomorph Primicaris larvaformis, a millimeters-sized euarthropod from the ~ 518-million-year-old Chengjiang biota, China. Primicaris possesses a body plan featuring morphologically similar post-antennular biramous appendages, but also mandibulate diagnostic features including multi-segmented exopodites, a well-developed and differentiated hypostome-labrum complex, and a pancrustacean-like topological configuration of frontalmost three pairs of appendages. Phylogenetic analysis resolves Acercostraca and Marrellida as stem-Mandibulata. The undifferentiated post-antennular appendages in Primicaris suggest a possibility that the head appendages acquired a crown-mandibulate configuration before their morphological specialization in mandibulate origin. The emergence of novel appendage morphotypes in Acercostraca and Marrellida reveals that the complexity of limb tagmatization evolved independently in different Euarthropoda clades.

Chemical weathering is a critical Earth system process that regulates climate, ocean chemistry and the long-term carbon cycle. During the mid-Proterozoic (~1.8‒0.8 Ga), chemical weathering is generally considered to have been relatively muted, but this perception remains largely untested, limiting our understanding of the drivers of purported oxygenation events and coeval biological evolution. Here, we report the first coupled positive seawater 87Sr/86Sr (~0.0007) and δ7Li (~5‰) isotope excursion of the Mesoproterozoic Era (1.6‒1.0 Ga). Geochemical box modelling suggests the concurrent Sr - Li isotope excursions signify a substantial weathering event at ~1.57 Ga, characterised by increased silicate weathering rates and decreased weathering congruency. Drawing on independent geological evidence, we posit that enhanced volcanic CO2 degassing, possibly alongside accretional orogenesis, increased denudation rates and invigorated the hydrological cycle, amplifying silicate weathering and secondary clay formation. This weathering pulse coincided with the ocean oxygenation and carbon cycle disruption, implicating it in the coeval preservation of the earliest known decimetre-scale, multicellular eukaryotic fossils.

The Ediacaran to Cambrian transition (ECT) represents a critical period in Earth's history, marked by the rapid emergence and diversification of metazoan life. Understanding the precise timing and dynamics of this bio-event as well as their links to paleoenvironmental changes requires a well-defined stratigraphic framework. The Anti-Atlas platform in Morocco offers a key region for unravelling the tectono-sedimentary setting, chemostratigraphic and geochemical features of ECT in northern Gondwana margin. Stratigraphic record of this transitional interval lying in the Lower Adoudou Formation (Tabia-Tifnount Members transition) is well-constrained by delta 13Ccarb chemostratigraphy and geochronology. However, the basin evolution analysis based on sequence stratigraphy has received little attention, as well the exact location of the Ediacaran-Cambrian boundary in this region remains uncertain, partly due to the absence of typical small shelly fossils and trace fossil assemblages. In this study, we apply an integrated approach combining sedimentary, stratigraphic, paleontological, and geochemical analyses to investigate the Tabia section, one of the most continuous ECT sequences in northern Gondwana. The stratigraphic record reveals four key stages in the tectono-sedimentary evolution. The synrift stage comprises three tectonic pulses, each corresponding to distinct depositional sequences within the Tabia Member. The progressive extension starts with rift initiation forming a narrow rift branch and culminates during rift propagation with wide rift branch during the following two tectonic pulses. The postrift stage coincides with a shift to the stable carbonate platform of Tifnout Member. This geodynamic and paleogeographic transition is well documented by geochemical evidence. In contrast to the postrift strata, synrift sequences preserve increased syn-sedimentary fault-driven hydrothermal activity and related dolomitization process (e.g., higher 87Sr/86Sr ratios, Eu/Eu* anomalies, and elevated concentrations of metals). In addition, our results offer new fossils and delta 13Ccarb chemostratigraphy constraints on the precise placement of the Ediacaran-Cambrian boundary across the platform, suggesting that the synrift Tabia Member and postrift lower part of Tifnout member including Tamjout dolomite are late Ediacaran in age. The first negative excursion in the Lower Tifnout Member is interpreted as the basal Cambrian carbon isotope excursion (BACE), coinciding with the transition from synrift to post-rift settings in the western Anti-Atlas platform.

Vendotaenia, a simple ribbon-like fossil, is classified within the informal vendotaenid group alongside morphologically similar fossils. These fossils, which are frequently encountered near the Precambrian-Cambrian boundary, have been hypothesized to possess potential for stratigraphic correlation. However, their taxonomy is often ambiguous and the occurrences in older strata challenge their biostratigraphic significance. In this study, we first report vendotaenid fossils from the Late Ediacaran Tabia Member of the Adoudou Formation in Morocco. Based on meticulous morphological examination of the abundant specimens, these fossils are identified into three distinct morphological taxa, including Vendotaenia, characterized by a constant width and smooth curvature; Tyrasotaenia, also of constant width but commonly folded and twisted; and the lanceolate form Lanceoforma. Furthermore, we have reassessed the related fossil reports within a comprehensive database to evaluate their spatiotemporal distribution. The result shows that all three fossil genera exhibit long temporal ranges with the first appearances predating the Ediacaran. Given their simplistic and often indistinguishable morphology, Vendotaenia and related vendotaenids are deemed unsuitable for stratigraphic correlation of the uppermost Ediacaran.

Body size greatly affects how organisms interact with their environments. However, the macroevolutionary patterns of body size across many major metazoan clades and their constraining mechanisms remain elusive. A new high-resolution body size dataset covering 2435 species from 1091 genera of Cambrian and Ordovician trilobites reveals that body size evolution changes episodically, with three marked reductions in size. Such a pattern rules out a persistent Cope's rule dynamic. Rather, we find a strong temporal link between body size changes and major fluctuations in marine redox, supporting the hypothesis that marine oxygen levels exerted a primary control on the tempo and mode of trilobite body size evolution. These further imply a dominant role for marine oxygen in early animal evolution.

During the Cambrian Explosion, episodic radiations of major animal phyla occurred in concert with repeated coupled carbon‐sulfur isotope excursions. These isotope patterns are thought to reflect oscillations in atmospheric and shallow‐marine O 2 , which promoted animal diversification events. However, the driver for oxygenation pulses is unclear. Here we show that these synchronous carbon‐sulfur isotope cycles and marine oxygenation pulses can be driven by long‐period orbital forcing through effects on continental weathering and nutrient delivery. The impact of orbital forcing is explored using a combined climate‐biogeochemical model. When forced with latitudinally‐resolved insolation signals, the model produces long‐term variations in nutrient weathering and carbon burial, which reproduces the co‐variation of carbon‐sulfur isotopes. We conclude that the oxygen‐driven evolutionary changes in the early Cambrian can be explained by recurrent nutrient inputs to the ocean, resulting from climate change caused by long‐period orbital cycles.