Cambrian Fortunian Research Articles

Widespread coastal upwelling along the marginal Yangtze Platform (South China) during the early Cambrian: Implications for hyper-enrichment of organic matter

Early Cambrian Earth history witnessed significant changes in marine environments and biological evolution contemporaneous with extensive accumulation of organic-rich shale. However, major factors controlling hyper-enrichment of organic matter (OM) of lower Cambrian shale deposits remain controversial. Black shale of the lower Cambrian Niutitang Formation (NTT) of the marginal Yangtze Platform (South China) deposited during the Cambrian middle Age 2 are especially organic-rich, with total organic carbon (TOC) concentrations of as much as 13.0 wt%. Geochemical evidence suggests that cool, dry paleo-climatic conditions that prevailed on the Yangtze Platform during the Cambrian Fortunian-late Age 2, induced vigorous coastal upwelling. Paleo-productivity level assessments indicate that the magnitude of primary productivity contemporaneous with deposition of the NTT shale deposits exceeded that of modern upwelling systems (e.g., Peruvian Margin). The widespread occurrence of phosphate nodules within these deposits and decreased Co-EF × Mn-EF values of associated lower Cambrian black shale successions deposited along the margin of the Yangtze carbonate platform suggest extensive coastal upwelling. Widespread and strong coastal upwelling in tandem with elevated surface water primary productivity and anoxic (even euxinic) bottom water conditions are manifested by deposition of OM hyper-enriched black shale during Cambrian Fortunian to middle Age 2 time. However, weakened seasonal upwelling that appears to have prevailed during the late Cambrian Age 2 was accompanied by accumulation of NTT shale deposits of diminished TOC content. In summary, this study provides robust evidence of extensive coastal upwelling along the marginal Yangtze Platform during early Cambrian time that favored accumulation of OM hyper-enriched shale. These results help to elucidate the distribution of high-quality lower Cambrian natural gas source rocks in South China.

Growth patterns and affinities of Huizenodus oligaspinosus (?Cnidaria) from the lower Cambrian of South China

Helmet-like small shelly fossils from the Cambrian Fortunian Stage of South China are a class of morphological taxa with uncertain affinities. Here, we redefine the genera Amoebinella and Huizenodus and consider Huizenodus oligaspinosus to be a senior synonym of Amoebinella echinata. In addition, we report two compound fossils of H. oligaspinosus for the first time from the Fortunian Zhangjiagou Lagerstätte in southern Shaanxi. Their multi-branched, dendritic, rigid structure is similar to the skeletons in several groups of colonial anthozoans, and thus, H. oligaspinosus may represent the skeletal structure of an ancestral anthozoan. H. oligaspinosus could have further constructed its skeleton by producing secondary individuals from the main branches or the sub-branches of the primary individuals to accommodate more polyps.

Three Indeterminate Forms of Scalidophoran Worms from the Cambrian Fortunian of South China

AbstractA wide variety of scalidophoran worms have been found in the Cambrian Fortunian in recent years, suggesting that scalidophorans diversified during this age. Here, we report new scalidophoran material obtained from the 2nd member of the Kuanchuanpu Formation (Fortunian Stage) in Zhangjiagou Lagerstätte, Xixiang County, Shaanxi Province, South China. Three indeterminate forms of scalidophoran worms are described in detail. The morphology of these worms differs from that of any coeval worms and may represent new taxa. However, because these fossils are fragmented and lack detailed information on the arrangement pattern of introvert scalids, they are named here in open nomenclature. This study enriches the diversity of the earliest scalidophorans to 7 genera and species and 10 indeterminate forms. Based on the new fossils, Fortunian scalidophoran worms can be divided into three categories: (1) worms with microannuli; (2) worms with macroannuli; and (3) worms without annuli.

AGE CONSTRAINTS AND SOURCE AREAS FOR THE PRECAMBRIAN TO CAMBRIAN STRATA OF THE SOUTHERN YENISEI RIDGE (REDKOLESNAYA AND OSTROVNOY FORMATIONS)

The Precambrian to Cambrian strata at the southwestern margin of the Siberian Platform are ubiquitously characterized by a gradual transition from red-colored alluvial and deltaic deposits to shallow-marine carbonates, reflecting a transgressive infill of the accommodation space of the foreland basin. However, a supposed synchronicity of the Early Precambrian marine transgressive events calls for verification by geochronological and bio- and chemostratigraphic dating. The herein-presented data on small skeletal fossils, trace fossils and variations in the carbon isotope composition of carbonates from the mixed carbonate-terrigenous succession (Redkolesnaya and Ostrovnoy formations) in the southern Yenisei ridge show that the base of the Cambrian Fortunian stage is placed within the Redkolesnaya formation. Age distribution of detrital zircons in the studied succession suggests the Siberian craton to be a major source of the clastic material in the foreland basin during the development of the Redkolesnaya formation. An increase in the amount of zircons derived from "non-Siberian" sources, located at the southwestern periphery of the paleocontinent at that time, in the coeval strata of the northern part of the southern Yenisei ridge, indicates a complex morphology of the paleobasin and considerable differentiation among the sources of clastic material therein.

New Qinscyphus material from the Fortunian of South China

Microscopic medusozoans from the Cambrian Fortunian stage of South China are well known for their exceptionally preserved embryos and elongated tubes. However, additional details of their morphology remain unclear. This paper describes new medusozoan fossils showing the whole apical complement and covering the morphological integrity of Qinscyphus. The apical part of Qinscyphus is considered to be soft during early ontogeny, and the inverted pentagonal pyramid may gradually form with growth and development. This discovery of the apical complement of Qinscyphus is novel and essential to complement the external morphology of early medusozoan fossils. More importantly, the new specimens have different annuli with triangular thickenings, providing a more comprehensive view on the developmental sequence of Qinscyphus. Therefore, this work allows a better understanding of early medusozoans ecology and evolution.

A new species of Decimoconularia (Cnidaria, Medusozoa) from the Lower Cambrian of South China

Hexangulaconulariids, a family of biradially symmetrical medusozoan cnidarians, have been widely reported from the Lower Cambrian of South China. The four currently recognized genera of hexangulaconulariids differ from each other mainly in the number of faces in the abapical region of the periderm. However, previously published illustrations of the monospecific type genus, Hexangulaconularia, clearly show two distinct morphotypes, one with six faces and the other with 10. Specimens with 10 faces are herein reassigned to the genus Decimoconularia. In addition, the new species D. anisfacialis is described from the Kuanchuanpu Formation (Cambrian Fortunian Stage) in the Kuanchuanpu and Shizhonggou sections in Ningqiang County, southern Shaanxi Province, China. Also described are additional specimens of H. formosa from the same formation in the Zhangjiagou section in Xixiang County, southern Shaanxi Province, and from Member 2 of the Yanjiahe Formation (Cambrian Fortunian Stage) in western Hubei Province. The discovery of D. anisfacialis extends the known stratigraphical range of Decimoconularia, now composed of two species, downward from Cambrian Stage 2 into the Fortunian Stage. Additionally, certain specimens previously assigned to H. formosa are reassigned to D. anisfacialis. The diagnoses of Hexangulaconularia, Decimoconularia, and Hexangulaconulariidae are emended accordingly. In accordance with the rule of time priority, the previously designated type genus and species, Hexaconularia He and Yang, 1986 and Hexaconularia sichuanensis He and Yang, 1986, are replaced herein by Hexangulaconularia He, in Xing et al., 1983.

The morphology and affinity of anabaritids revealed by their internal molds from the Cambrian Fortunian, southern Shaanxi, China

Anabaritids are a group of tubular skeletal fossils known from the lower Cambrian worldwide. Many previously collected specimens of anabaritids lack both the apical and apertural regions, and thus their complete morphology and growth patterns are not fully understood. Here we describe a number of internal molds of Anabarites from the lower Cambrian (Fortunian Stage) Zhangjiagou Lagerstätte, southern Shaanxi Province. Our new specimens preserved the apex, the aperture with three lobes, and three longitudinal grooves initiating apically rather than adapically as previously reported. This work enriches the morphology of anabaritids, casting doubt on the previous proposition that anabaritids originated from a cylindrical tubular ancestor. It is proposed here that a triradial symmetry may have been established in very early ontogenetic stages of some anabaritids; furthermore, the aperture with three lobes resembles those of olivooids and thus supports a possible close relationship between anabaritids and olivooid scyphozoans.

Nitrogen isotope evidence for stratified ocean redox structure during late Ediacaran to Cambrian Age 3 in the Yangtze Block of South China

The Ediacaran–Cambrian (E–C) transition witnessed profound biological and oceanic changes, among which the appearance of explosive radiation of skeletonized animals is considered as the most remarkable one. Although the prominent rise of oxygen concentration in atmosphere and ocean is considered to be the major trigger of the “Cambrian Explosion”, the links between the oxygenated process of deep ocean and the diversification of Cambrian metazoans are still under debate. In this study, δ15N and δ13Corg values, TOC and TN contents, as well as trace elements concentrations of fresh shale core samples were analyzed to explore the redox condition and nitrogen cycle of deep oceans and their relationships with biological evolution. The Mo concentrations, Mo–U covariations and Th/U ratios of our samples indicate suboxic/anoxic waters in the mid-depth ocean during the late Ediacaran to Cambrian Fortunian age, widespread anoxic/euxinic waters with temporary sulfidic waters during Cambrian Age 2 to early Age 3, and suboxic waters again during the middle to late Age 3. The positive δ15N values from late Ediacaran to Cambrian Age 2 indicate a large NO3− reservoir with a deep chemocline. In contrast, the low δ15N values (<0‰) in the deepwater slope regions suggest a dominant biological N2-fixation, and/or partial NH4+-assimilation in the photic zone during Cambrian Age 3. Integrated δ15N values of this study and early publications suggest a continuous redox stratified paleo-ocean with anoxic/suboxic deep waters from late Ediacaran to Cambrian Age 3. It is also worthy to note that although the deep ocean may not experience substantial oxygenation during the main phase of Cambrian diversification, the expanding oxic surface waters and supply of bioavailable fixed nitrogen may satisfy complex high O2-demanding animals, and thus at least have promoted the Cambrian biodiversity in shallow waters.

The evolutionary relationships of the earliest known cycloneuralians and a new record from the Cambrian Fortunian of South China

The Cambrian Fortunian Kuanchuanpu Formation and coeval strata in southern Shaanxi and northern Sichuan Provinces contain phosphatic carbonate that has yielded unambiguous body fossils of cycloneuralians. To date, six genera and species as well as nine indeterminate forms have been reported, and they provided information on the earliest known cycloneuralians and ecdysozoans. Among them, an indeterminate form is critical for understanding the evolution of the Fortunian cycloneuralians, and here I formally name it as Xinliscolex intermedius n. gen. n. sp. It has a series of at least seven longitudinal spinose sclerites on the dorsal side, and sensory structures such as pores and tubules distributed randomly among the annuli. The trunk end has two pairs of bilaterally arranged caudal spines. Micro-CT analysis reveals the preservation of a pharynx with inverted introvert with internally hollow scalids, an esophagus, midgut and hindgut, and a terminal anus. The digestive tract of Xinliscolex intermedius resembles that of modern priapulids, implying that the earliest known cycloneuralians have evolved a relatively derived digestive system, and this lays a foundation for their further diversification and evolution.

Reconstruction of oceanic redox structures during the Ediacaran-Cambrian transition in the Yangtze Block of South China: Implications from Mo isotopes and trace elements

To explore paleo-ocean redox variations and Mo biogeochemistry across the Ediacaran–Cambrian transition (E-C), continuous fresh core shale samples of Ediacaran to Cambrian were collected from a well drilled in the upper slope region of the Yangtze Block in South China and analyzed for δ98/95Mo and δ13Corg values, TOC, as well as the concentrations of trace and rare earth elements. The Mo–U covariations, Th/U and Re/Mo ratios as well as published iron speciation data indicate a redox stratified ocean with oxic surface waters and anoxic/euxinic deep waters prevailing across the E-C transition. During the time of Cambrian Fortunian and Stage 2, the bottom water in the upper slope became oxic due to the deepening oxycline. Afterwards, widespread anoxia and/or euxinia occurred during latest Age 2 and early Age 3. The Mo concentrations and Mo/TOC ratios suggest that the Yangtze Block was well connected with the open ocean during the late Ediacaran, and then became strongly restricted at the beginning of Cambrian. Following a large-scale transgression during Cambrian Age 2, its connection with the open ocean was enhanced again. The negative δ98/95Mo values may reflect preferential adsorption of isotopically light Mo by Mn oxides particles above the chemocline and their subsequent shuttling to euxinic sediments in the redox stratified late Ediacaran ocean; whereas the relatively low δ98/95Mo values during the Cambrian Fortunian and Age 2 indicate the decrease of δ98/95MoSeawater values in the restricted basin. The operation of local Mn particulate shuttles could also be stimulated by inflow of oxic waters into restricted basin during marine transgression, resulting in negative δ98/95Mo values. In conjunction with previous published Mo isotope data, the variations of sedimentary δ98/95Mo values from shallow shelf to deepwater slope regions draw the outline of ‘sandwich’-like model with euxinic waters (e.g., [H2S]aq > 11 μM) sandwiched by oxic surface waters and anoxic deep waters during the E-C transition. The shuttling of isotopically light Mo to euxinic sediments by Mn oxides in the redox stratified Ediacaran-Cambrian ocean may play a role in driving heterogeneous heavy seawater Mo isotopes and variable sedimentary δ98/95Mo values.

Cochleatina: an enigmatic Ediacaran–Cambrian survivor among small carbonaceous fossils (SCFs)

AbstractConspicuously few body‐fossil taxa are known to span the Ediacaran–Cambrian boundary, a pattern usually taken to signal either a terminal Proterozoic mass extinction, or taphonomic failure. We draw attention to the emerging record of small carbonaceous fossils (SCFs), which exhibit continuous preservation spanning this critical interval. Here we focus on the enigmatic SCF Cochleatina, a morphologically complex coil‐shaped problematicum that ranges across the Ediacaran–Cambrian divide, and is potentially among the oldest fossil occurrences of metazoans. We report new material of Cochleatina canilovica from the Ediacaran of Estonia and Ukraine, which offers new characters for assessing its palaeobiology. Significantly, new specimens include sets of three‐alike triplets of Cochleatina adhering to organic sheets, suggesting a clustering habit, or grouping of elements within an individual during life; an important step in constraining the morphology and ecology of this Ediacaran–Cambrian problematicum. We present revised systematic descriptions for Cochleatina and C. canilovica, and critically evaluate previous biological interpretations, drawing comparisons with metazoan, algal and protistan analogues. We reject hypotheses supporting Cochleatina as a metazoan mouthpart, and suggest new grounds for viewing Cochleatina as a potential multicomponent predator that trapped protists among microbial mats. Most occurrences are from Baltica, but we synthesize sporadic reports of Cochleatina from other palaeocontinents, pointing to its global distribution during the latest ˜10 myr of the Ediacaran and majority of the earliest Cambrian Fortunian Stage. As a rare example of an ‘Ediacaran survivor’, Cochleatina highlights the broader significance of SCFs as a novel means of tracking evolutionary patterns through the Proterozoic–Phanerozoic transition.

A new scalidophoran animal from the Cambrian Fortunian Stage of South China and its implications for the origin and early evolution of Kinorhyncha

Kinorhyncha is a cycloneuralian phylum with about 240 extant species, and it was not until 2015 that the first and only fossil kinorhynch species Eokinorhynchus rarus was reported. Here, we describe a new scalidophoran animal, Zhongpingscolex qinensis gen. et sp. nov., from the Cambrian Fortunian Kuanchuanpu Formation at Zhangjiagou section, southern Shaanxi Province, South China. The holotype and only specimen possesses morphology comparable to that of E. rarus. They both possess heterogeneous trunk annuli, and compared with E. rarus, Z. qinensis gen. et sp. nov. lacks a neck. Phylogenetic analysis resolves Z. qinensis gen. et sp. nov. as the sister group of E. rarus, both constituting stem-lineage derivatives of Kinorhyncha. The evolutionary scenario from the last common ancestor of crown-group Scalidophora to the last common ancestor of total-group Kinorhyncha includes the acquisition of a much shortened body with a relatively low body length to width ratio, heterogeneous trunk annuli, and macroannuli with simple armatures. The spines, large spinose sclerites, and annular rings of rectangular plates are autapomorphic to E. rarus, and segments (zonites) with independent cuticular plates as in crown-group kinorhynchs were evolved later.

Intraspecific variation of radial symmetry number of a 535 million-year-old jellyfish

Although modern jellyfishes are stably tetramerous, intraspecific variation of radial symmetry number is not uncommon and spans from unimery to octamery. However, intraspecific variation of radial symmetry number among fossil cnidarians is rarely reported. We report a single new specimen of Quadrapyrgites quadratacris from the Cambrian Fortunian Kuanchuanpu Formation at the Zhangjiagou Lagerstätte in southern Shaanxi Province, South China. Quadrapyrgites quadratacris has previously been interpreted to be a coronate scyphozoan with stable tetramery. The new specimen has morphology well comparable with the previously reported specimens of Q. quadratacris, with the exception that it is pentamerous. This is the first report of intraspecific variation of radial symmetry number among the Fortunian cnidarians. It demonstrates that tetramery is stable for Quadrapyrgites, with pentamery as less common intraspecific variation, and that the Fortunian scyphozoans have acquired the ability of regulating and changing their radial symmetry number during their alternation of generations.

New macrobenthic cycloneuralians from the Fortunian (lowermost Cambrian) of South China

Although morphology-based phylogenetic analyses and molecular data suggested that the cycloneuralians might have emerged in the Ediacaran Period, the oldest known fossil remains of cycloneuralians were first reported from the Cambrian Fortunian Stage of South China, and to date four genera and species as well as nine indeterminate forms have been described. Here, we report the latest discoveries of cycloneuralians from the Fortunian Kuanchuanpu Formation at Zhangjiagou Lagerstätte, Dahe Village, Xixiang County, southern Shaanxi Province, South China. A new genus and species Dahescolex kuanchuanpuensis n. gen. n. sp. is described, and a previously reported genus and species Eopriapulites sphinx is re-described. Dahescolex kuanchuanpuensis n. gen. n. sp. is suggested to be a stem-lineage derivative of Scalidophora, whereas E. sphinx may represent a stem-lineage derivative of Cycloneuralia. From the fragments discovered, the new specimens are suggested to have reached millimeters in dimension, and the original individuals are estimated to have dimensions in centimeter, hence falls within the size range of macrobenthos. Therefore, at least some of the Fortunian cycloneuralians can be considered macrobenthic, and it is suggested here that the Cycloneuralia may have originated in the Fortunian macrobenthos.

Diverse radial symmetry among the Cambrian Fortunian fossil embryos from northern Sichuan and southern Shaanxi provinces, South China

The Cambrian Fortunian fossil embryos exhibit embryonic development of ancient animals and hence have important bearings on evolutionary developmental biology. They have radial symmetry, and may be early representatives of cnidarians. Here we report new material of three-dimensionally phosphatized fossil embryos from the Fortunian Kuanchuanpu Formation and coeval strata in northern Sichuan and southern Shaanxi provinces, South China. The new material includes previously reported fossil embryos assignable to Pseudooides prima with biradial symmetry or pseudo-hexaradial symmetry, Quadrapyrgites quadratacris with tetraradial symmetry, and Olivooides multisulcatus with pentaradial symmetry. Additionally, we recovered two new types of fossil embryos, i.e., Embryo I with hexaradial symmetry and Embryo II with octaradial symmetry, and they are tentatively suggested to represent new cnidarians. In contrast to the diverse radial symmetry of the Fortunian cnidarians, modern cnidarians exhibit stable tetraradial symmetry in medusozoans, biradial symmetry in anthozoans, and bilateral symmetry in siphonophores (Hydrozoa). The current study supports the view that the tetraradial symmetry of modern medusozoans is a surviving remnant of their Fortunian relatives.

New armoured scalidophorans (Ecdysozoa, Cycloneuralia) from the Cambrian Fortunian Zhangjiagou Lagerstätte, South China

AbstractAlthough molecular studies estimated that cycloneuralians might have originated in the Ediacaran Period and diversified in the early Cambrian, it was not until 2014 that they were first reported from the Cambrian Fortunian Stage. To date only two species and five indeterminate forms have been described. Here, we report new three‐dimensionally phosphatized microfossils of cycloneuralians from the Fortunian Zhangjiagou Lagerstätte, South China. Two new genera and species, Qinscolex spinosus gen. et sp. nov. and Shanscolex decorus gen. et sp. nov., and Indeterminate Forms 1–7 are described and tentatively assigned to total‐group Scalidophora, greatly increasing the diversity of the Fortunian cycloneuralians. These Fortunian cycloneuralians have diverse cuticular spines, plates, and spinose sclerites of various sizes on the trunk, thus assignment of co‐occurring fragmented trunk parts or disassociated spines or sclerites to a specific species is currently impossible. Some of the Fortunian cycloneuralians might have reached a centimetre in length; they may have been temporary meiobenthos in their juvenile or younger adult stages, only joining the macrobenthos when they reached macroscopic dimensions. It is proposed that an introvert with hexaradially symmetric and internally hollow scalids might be the plesiomorphic state for cycloneuralians.

New material of three-dimensionally phosphatized and microscopic cycloneuralians from the Cambrian Paibian Stage of South China

AbstractSome rare microscopic cycloneuralians are present in the Cambrian of South China, represented by Eopriapulites and Eokinorhynchus (both early Cambrian), fossil embryos of Markuelia (middle to late Cambrian), and palaeoscolecids (early to late Cambrian). Among them, palaeoscolecids are relatively diverse and abundant. Here, we describe new material of three-dimensionally phosphatized and microscopic cycloneuralians from the Paibian Stage of Wangcun Lagerstätte, western Hunan, South China. New material includes fossil embryos assignable to Markuelia sp., two other types of fossil embryos, and three species of palaeoscolecids, including Dispinoscolex decorus Duan, Dong, and Donoghue, 2012, Schistoscolex hunanensis Duan, Dong, and Donoghue, 2012, and Austroscolex sinensis new species. The palaeoscolecid fragments differ mainly in size and armor of the trunk annuli. Since Eokinorhynchus and Eopriapulites occurred the earliest among the Cambrian cycloneuralians, it is proposed here that: (1) cycloneuralians originated in the Cambrian Fortunian small shelly faunas rather than in the early Cambrian macrobenthos, (2) ancestral cycloneuralians may have simple trunk armor, and (3) Eopriapulites represents an ancestral cycloneuralian.

New material of scalidophoran worms in Orsten-type preservation from the Cambrian Fortunian Stage of South China

AbstractScalidophoran worms diversified in the Cambrian Fortunian, as indicated by recent reports from this stage, with two described species and two more unnamed forms exclusively from Orsten-type Lagerstätten yielding three-dimensionally phosphatized fossils. Here, we report new material of scalidophoran worms in Orsten-type preservation from the Cambrian Fortunian Xinli section in northern Sichuan Province, South China. At least five forms of scalidophoran worms were recovered from this location, includingEokinorhynchus rarusZhang et al., 2015 and four unnamed taxa—Forms A, B, C, and D. Co-occurring disassociated spinose small shelly fossils might also be isolated cuticular elements of these early scalidophoran worms. The ontogeny ofEokinorhynchus rarusis revised. Forms A, C, and D are assigned to total-group Scalidophora to indicate their uncertain positions within Scalidophora, while Form B might be a close relative ofEokinorhynchus rarus. The current work highlights the significance of Orsten-type Lagerstätten in uncovering the morphology, ontogeny, and taxonomy of early Scalidophora and Cycloneuralia, made particularly available by the new finds in China.

A new scyphozoan from the Cambrian Fortunian Stage of South China

AbstractAnimals with radial symmetry are abundant in the Cambrian Fortunian Stage of South China, but with relatively low diversity: representatives include Olivooides, Quadrapyrgites, carinachitiids, hexangulaconulariids and Pseudooides. Here, we report a new radial animal, Qinscyphus necopinus gen. et sp. nov., from the Fortunian small shelly fauna of southern Shaanxi Province, South China. Qinscyphus necopinus has a cup‐shaped profile, with slightly raised annuli and five groups of triangular thickenings in pentaradial symmetry. This organism has a comparable morphology to, and thus a close affinity with, Olivooides and Quadrapyrgites, and is interpreted as a coronate scyphozoan. This discovery adds a new crown‐group cnidarian to the Cambrian Explosion.

Marine redox evolution in the early Cambrian Yangtze shelf margin area: evidence from trace elements, nitrogen and sulphur isotopes

AbstractNitrogen is an essential element for biological activity, and nitrogen isotopic compositions of geological samples record information about both marine biological processes and environmental evolution. However, only a few studies of N isotopes in the early Cambrian have been published. In this study, we analysed nitrogen isotopic compositions, as well as trace elements and sulphur isotopic compositions of cherts, black shales, carbonaceous shales and argillaceous carbonates from the Daotuo drill core in Songtao County, NE Guizhou Province, China, to reconstruct the marine redox environment of both deep and surface seawater in the study area of the Yangtze shelf margin in the early Cambrian. The Mo–U covariation pattern of the studied samples indicates that the Yangtze shelf margin area was weakly restricted and connected to the open ocean through shallow water flows. Mo and U concentrations, δ15Nbulk and δ34Spy values of the studied samples from the Yangtze shelf margin area suggest ferruginous but not sulphidic seawater and low marine sulphate concentration (relatively deep chemocline) in the Cambrian Fortunian and early Stage 2; sulphidic conditions (shallow chemocline and anoxic photic zone) in the upper Cambrian Stage 2 and lower Stage 3; and the depression of sulphidic seawater in the middle and upper Cambrian Stage 3. Furthermore, the decreasing δ15N values indicate shrinking of the marine nitrate reservoir during the middle and upper Stage 3, which reflects a falling oxygenation level in this period. The environmental evolution was probably controlled by the changing biological activity through its feedback on the local marine environment.