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On the basis of data on the geochemical characteristics ((La/Yb)N, Eu/Eu*, and content of Th) of clay rocks of Podolian Transnistria, Southeastern Poland, Belarus and Volyn, Lithuania, the vicinity of St. Petersburg, Southeastern White Sea region, and Arctic Norway, categories of rivers that transported fine-grained aluminosilicate clastic material to sinks that existed during the Vendian and Early Cambrian were reconstructed in the west of the East European Platform. The distribution of data points of clay rocks of the Volhynian time on the (La/Yb)N–Eu/Eu* diagram with the fields of the composition of the pelitic fraction of bottom sediments in the mouth of modern rivers of various categories allows us to assume that, for Lithuania and Podolian Transnistria, the feeding provinces were, among other things, paleowatersheds composed of volcanic formations. For Eastern Belarus and the Southeastern White Sea region, the influence of erosion products supplied by rivers flowing through igneous/metamorphic terranes (crystalline basement) is noticeable. A significant part of the clastic material was carried by rivers that drained sedimentary formations, as well as large rivers, i.e., rivers whose length exceeded 1000 km and whose drainage area was more than 100 000 km2. The existence of the latter is confirmed by the presence of detrital zircon in the rocks of the Zhukov Formation of Southeastern Poland, borrowed possibly from the rocks of Fennoscandia. In the Redkinian time, along with large rivers (rivers of category 1) and rivers fed by the products of erosion of sedimentary formations (rivers of category 2), transport of fine-grained clastic rocks was also carried out by rivers that drained rocks of the crystalline basement (rivers of category 3; this is typical of Belarus and Volyn, the Southeastern White Sea, and Arctic Norway) and by rivers flowing through areas of distribution of volcanic associations (rivers of category 4). The distribution of data points of the clay rocks of the Kotlin stage on the (La/Yb)N–Eu/Eu* diagram suggests that the main agents of transport of fine-grained aluminosilicate clastic material to sinks at that time were large rivers and rivers fed by fine-grained particulate matter owing to the erosion of predominantly sedimentary rocks. The Early Cambrian paleowatersheds were apparently composed not only of crystalline rocks but also of sedimentary formations. All of the above is in fairly good agreement with the previously established fact of a gradual increase in the contribution of erosion products of sedimentary rocks to the formation of Vendian–Early Cambrian sedimentary sequences from Podolian Transnistria to Arctic Norway. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hydrological circulations of Early Cambrian intrashelf basins are restricted. • Hydrodynamic processes affected the Early Cambrian marine redox environment. • High primary productivity and sluggish oceanic circulation enriched organic matter. • Enrichment mechanisms of redox-sensitive trace elements vary temporally. There was a strong relationship between paleo-oceanic conditions and organic matter accumulation during the critical period of the Early Cambrian. The Yangtze Platform in South China provides an opportunity to study this relationship, but the basin architecture and paleo-hydrographic dynamics remain poorly understood, which hinders one's understanding of the evolution in oceanic redox state and its relationship to organic matter accumulation. To fill the knowledge gap, a newly obtained data set from the Lower Cambrian Niutitang Formation shales (Late Cambrian Stage 2-Stage 3) of the intrashelf basin is used. Results show that the molybdenum–uranium covariation, along with the abnormally low Mo contents and Mo/TOC ratios in the anoxic (ferruginous and euxinic) Niutitang shales of the intrashelf basin, indicate a hydrographic circulation that is moderately to strongly restricted. The hydrographic evolution at different stages, influencing redox conditions to a certain extent, in conjunction with variations in primary productivity, controls the enrichment of organic matter. In particular, the relatively low sea level during Interval I (Late Stage 2) resulted in weak connectivity between the intrashelf basin and open ocean; this, coupled with high primary productivity and enhanced chemical weathering, formed stable stratified water (euxinic bottom-water) conditions that were favorable for organic matter enrichment. In contrast, Interval II (Early–Middle Stage 3) and Interval III (Late Stage 3) involved less organic matter enrichment due to weakened restricted hydrographic circulation (Interval II) and oxic bottom-water conditions (Interval III). This study highlights how hydrodynamic processes affected the marine environment during the Early Cambrian and reveals that sluggish oceanic circulation combined with high primary productivity led to anomalous enrichment of organic matter. This may be a common feature of basins with restricted hydrographic circulation in deep time. [ABSTRACT FROM AUTHOR]

An increase in atmospheric p O2 has been proposed as a trigger for the Cambrian Explosion at ∼539–514 Ma but the mechanistic linkage remains unclear. To gain insights into marine habitability for the Cambrian Explosion, we analysed excess Ba contents (Baexcess) and isotope compositions (δ138Baexcess) of ∼521-Myr-old metalliferous black shales in South China. The δ138Baexcess values vary within a large range and show a negative logarithmic correlation with Baexcess, suggesting a major (>99%) drawdown of oceanic Ba inventory via barite precipitation. Spatial variations in Baexcess and δ138Baexcess indicate that Ba removal was driven by sulfate availability that was ultimately derived from the upwelling of deep seawaters. Global oceanic oxygenation across the Ediacaran–Cambrian transition may have increased the sulfate reservoir via oxidation of sulfide and concurrently decreased the Ba reservoir by barite precipitation. The removal of both H2S and Ba that are deleterious to animals could have improved marine habitability for early animals. [ABSTRACT FROM AUTHOR]

The Early Cambrian represents a critical time period characterized by extraordinary biological innovations and dynamic redox conditions in seawaters. Nitrogen isotopic signatures of ancient sediments have the potential to elucidate the evolutionary path of marine redox states and the biogeochemical nitrogen cycle within the water column of the Early Cambrian ocean. While existing research on this topic has predominantly focused on South China, the exploration of other continental margins has been limited, leaving contradictory hypotheses untested. In this study, paired δ15N and δ13Corg analyses were performed on the Lower Cambrian successions from the Shiairike section (inner ramp) and Well Tadong 2 (deep shelf/basin) in the northwestern and eastern Tarim Basin, respectively. Our data from the Shiairike section reveal a discernible shift in the operation of different nitrogen cycles for the black chert-shale unit, also referred to as the black rock series in Chinese literature, of the Yurtus Formation (Fortunian stage to lower Stage 3). Oscillating δ15N values for its lower part are suggestive of alternating anaerobic assimilation of NH4+ and denitrification/anammox. This is likely attributed to a shallow, unstable chemocline consistent with the upwelling and incursion of deep, anoxic waters during a major transgression. In contrast, aerobic nitrogen cycling, indicated by positive δ15N values of >2‰, dominated the upper part alongside a reduction in upwelling intensity. On the other hand, the δ15N signatures of Xishanbulake and Xidashan Formations of Well Tadong 2, which encompass a time interval from the Cambrian Fortunian Age to Age 4, are indicative of N2 fixation by diazotrophs as the major nitrogen source. The two studied intervals, although not time-equivalent, exhibit separated states of nitrogen cycling at least during the deposition of the Yurtus black rock series. The spatially different nitrogen cycling of the studied sections is compatible with a redox-stratified ocean during the deposition of the Yurtus black rock series. The build-up of a NO3− reservoir and aerobic nitrogen cycling in seawater was largely restricted to near-shore settings whereas anaerobic nitrogen cycling dominated by N2 fixation served as the main nitrogen uptake pathway in off-shore settings. [ABSTRACT FROM AUTHOR]

Selenium (Se) is an essential trace element for humans and animals, and an excess of or deficiency in Se is harmful to health. Research on the selenium enrichment zone began in the late 1970s in Shuang'an, Ziyang, southern Shaanxi Province. Naore village is only one selenosis area in Shuang'an, Ziyang, China. Different scholars have conducted systematic studies on the occurrence of selenium, its organic geochemistry and biomarkers, and its content and enrichment patterns in this area. This study applied the TIMA (TESCAN integrated mineral analyzer) for the first time to conduct detailed mineralogical work. The minerals included quartz, carbonate minerals (calcite and dolomite), feldspar (plagioclase, albite, and orthoclase), biotite and muscovite, clay minerals (chlorite and kaolinite), hematite, pyrite, and accessory minerals (almandine, olivine, zircon, and apatite) in Naore village, Ziyang, Shaanxi Province. The ATi index (100 × apatite/(apatite + tourmaline)) is used to determine the source of heavy minerals and the degree of heavy minerals' weathering. The content POS (100 × (pyroxene + olivine + spinel)/transparent heavy mineral) of olivine, pyroxene, and spinel in heavy minerals can reflect the contribution of basic and ultrabasic rocks in the source area. The ATi and POS indexes for the heavy minerals in the research area were 91.83~99.96 and 0.01~18.75, respectively, reflecting the abundance of volcanic rock material in their source. In addition, the migration, transformation, bioavailability, and toxicity of selenium in the environment are closely related to its species. The species of selenium in various selenium-rich areas (Naore, Wamiao, and Guanquan) mainly include unusable residues and organic forms, followed by humic-acid-bound selenium. The proportions of water-soluble, exchangeable, and carbonate-bound selenium are relatively small, and the proportion of Fe-Mn oxide-bound selenium is the lowest. [ABSTRACT FROM AUTHOR]

The early Cambrian period is a critical time in Earth's history, marked by the second oxygenation of the atmosphere (known as the Neoproterozoic Oxidation Event) and the rapid evolution of animals, as well as the worldwide large-scale deposits. Polymetallic Ni-Mo deposits have been previously reported in the black rocks at the base of the Cambrian strata on the Yangtze Block, South China, while their genesis is still controversial, and their global occurrences are uncertain. Therefore, we conduct a comprehensive analysis of trace elements and rare earth elements in the black rocks at the base of the Yurtus Formation in the Sugaitblak section (SGT) in Aksu, Tarim Block, and then make a comprehensive study by combining our and other data from the Tarim and Yangtze Block. The distribution patterns of the trace and rare earth element data from our study and another two sections in the Aksu area have shown that the black rocks in the Tarim were strongly enriched in V, Sr, Mo, Ba, and U, moderately enriched in Zn, Cu, Cr, and Pb, and slightly depleted in Ni, and Co during the early Cambrian. These enriched elements could mainly originate from the waning hydrothermal fluids, and then precipitate and preserve in sediments under anoxic environment. The comparison of trace elements in the black rocks between the Tarim and the Yangtze Block suggests that the types of enriched metal elements in these two regions were different, and metal concentrations in the Yangtze Block were much higher than those in the Tarim by 1-3 orders of magnitude. This result indicates that there are no polymetallic Ni-Mo enrichment layers in the black rocks in the Tarim during the early Cambrian, and the widespread polymetallic Ni-Mo enrichments in South China have been more likely influenced by the local hydrothermal activity. It could be further speculated that the early Cambrian period may have been an active period for seafloor hydrothermal activity, with locally different chemical components in hydrothermal fluids. Abundant metal nutrients brought by these hydrothermal activities may have been a potential factor for the rapid evolution of life during this period. [ABSTRACT FROM AUTHOR]

At the end of Early Cambrian time, the Sichuan basin (South China) was located in a wide carbonate platform, with hundreds of meters of carbonate deposited. The Longwangmiao Formation carbonate in Sichuan basin is partially to completely dolomitized, displaying a mottled texture in the northern area of the exposure. The mottled dolomitic limestone developed parallel to bedding, with shape irregular boundaries with limestone that has not been dolomitized. The mottled dolomite is composed of powder crystalline and finely crystalline dolomite, while the matrix limestone is composed of micritic calcite. the isotopic composition of mottled dolomite (δ13C = +0.29‰PDB, δ18O = −1.15‰PDB) is similar to that of micrite calcite (δ13C = −0.49‰PDB, δ18O = −1.45‰PDB). Both isotopic values and trace element data indicate that the dolomitized fluid is originated from sea water. Some beds contain gypsum pseudomorphs and mud cracks, indicating a shallow and evaporative environment with local high salinity during deposition. Dolomitization likely took place early, in part as a result of sea water salinity concentration. Trace fossils thalassinoides horizontalis, thalassinoides callianassa and planolites developed in the Longwangmiao Formation, and the sharp edges of mottled dolomite are similar to these trace fossils. The beds are intensely bioturbated. In the burrow network, the sediments and burrow fill were coarse and loose with little clay, and it is interpreted here as being easier to be dolomitized than the surrounding sediments. Partial dolomitization is thus interpreted to have occurred in the burrow system, and the degree of dolomitization was related to the degree of bioturbation, which is controlled by the trace-making creatures.

A polymetallic layer is usually developed at the bottom of the early Cambrian black shale in Guizhou Province. The mineral that makes up the polymetallic layer is related to the sedimentary facies. To analyze the differentiation mechanism between polymetallic deposits (Ni‐Mo and V), the Zhijin Gezhongwu profile located in the outer shelf and the Sansui Haishan V deposit located in the lower slope are selected to study the in situ sulfur isotopes and trace elements of pyrite. The results show that δ34S values of pyrite vary widely from −7.8 ‰ to 28 ‰ in the Gezhongwu profile, while the δ34S values are relatively uniform (from 27.8 ‰ to 38.4 ‰) in the Haishan profile. The isotopic S composition is consistent with the transition that occurs in the sedimentary phase from the shelf to the deep sea on the transgressive Yangtze platform; this indicates that the δ34SO42– values in seawater must be differently distributed in depositional environments. The sulfur in the Ni‐Mo layer is produced after the mixing of seawater and hydrothermal fluid, while the V layer mainly originates from seawater. Overall, the Ni‐Mo and V deposits have been differentiated primarily on the basis of the combined effect of continental weathering and hydrothermal fluid. [ABSTRACT FROM AUTHOR]

Molting (ecdysis) is a ubiquitous behaviour and critical process in the growth and development of extant ecdysozoans. The origin of ecdysis can be traced back to the early Cambrian period, primarily in arthropods and priapulid-like worms. However, understanding the molting process in palaeoscolecidomorphs, the most diversified group of Cambrian priapulid-like worms, is challenging owing to the limited number of fossils preserved with molting features. Here, we report abnormal palaeoscolecidomorph Cricocosmia jinningensis (from the early Cambrian Chengjiang Lagerstätte, South China) that is distinguished by conjoined and 3–4 longitudinal rows of sclerites along the trunk, showing pre-molting features. The molting process of C. jinningensis could be reconstructed as follows: the new cuticle initially forms and lies beneath the old one; the new cuticle separates from the old one by dislocating and staggering the sclerites; the new worm frees itself by shedding the old cuticle from the broken anterior body, similar to that observed in living priapulid worms. To our knowledge, this is the first in the act instance of molting behaviour in C. jinningensis. The ecological purpose of molting in adult C. jinningensis may be removal of symbiotic organisms attached to its trunk and replacement of the worn-out sclerites. [ABSTRACT FROM AUTHOR]

Field data from recent geological mapping over a major part of the north-central Scandinavian Caledonides combined with published information give a detailed view of early Cambrian basin successions, comprising the Gärdsjön formation (Gf; Jämtland supergroup) in the Lower Allochthon and autochthonous equivalents (Dividal Group). The Gf comprises ten units of sandstone and siltstone or mudstone (Gf I—X, > 300 m thick). Green siltstones with red layers (Gf VI, c. 521 to 519 Ma) and green–grey siltstones at the top (Gf X, c. 516.5 to 513.5 Ma) are regional key horizons. Gf V, VI, VII, IX, and X deposition may be related to eustatic events. Restoration of Caledonian shortening reveals a major "Hornavan-Vattudal basin" (HVB; > 330 km NW–SE, > 400 km NE-SW) between the Grong–Olden culmination in the S and the Akkajaure–Tysfjord culmination in the N. Published zircon ages imply the latter separated the HVB from those shed from the Timan orogen in the N. The eastern basin margin straddles the present Caledonian erosional margin. Basement highs identified here within the Nasafjället, Bångonåive, and Børgefjellet "basement" windows define the western margin. They separate the HVB from the outer shelf towards the Iapetus Ocean in the W. The onset of sedimentation is time-related with E–W extension at c. 544–534 Ma. NNE–SSW-directed extension occurs after c. 518 Ma, perhaps related with Timan late-orogenic extension. The HVB is distinctly younger (c. 535–513.5 Ma) than Rodinia break-up and Iapetus ocean formation (> 550 Ma), comparable with post-rift basins in inner parts of modern passive margins. [ABSTRACT FROM AUTHOR]

The early Cambrian period is a critical time in Earth’s history, marked by the second oxygenation of the atmosphere (known as the Neoproterozoic Oxidation Event) and the rapid evolution of animals, as well as the worldwide large-scale deposits. Polymetallic Ni-Mo deposits have been previously reported in the black rocks at the base of the Cambrian strata on the Yangtze Block, South China, while their genesis is still controversial, and their global occurrences are uncertain. Therefore, we conduct a comprehensive analysis of trace elements and rare earth elements in the black rocks at the base of the Yurtus Formation in the Sugaitblak section (SGT) in Aksu, Tarim Block, and then make a comprehensive study by combining our and other data from the Tarim and Yangtze Block. The distribution patterns of the trace and rare earth element data from our study and another two sections in the Aksu area have shown that the black rocks in the Tarim were strongly enriched in V, Sr, Mo, Ba, and U, moderately enriched in Zn, Cu, Cr, and Pb, and slightly depleted in Ni, and Co during the early Cambrian. These enriched elements could mainly originate from the waning hydrothermal fluids, and then precipitate and preserve in sediments under anoxic environment. The comparison of trace elements in the black rocks between the Tarim and the Yangtze Block suggests that the types of enriched metal elements in these two regions were different, and metal concentrations in the Yangtze Block were much higher than those in the Tarim by 1–3 orders of magnitude. This result indicates that there are no polymetallic Ni-Mo enrichment layers in the black rocks in the Tarim during the early Cambrian, and the widespread polymetallic Ni-Mo enrichments in South China have been more likely influenced by the local hydrothermal activity. It could be further speculated that the early Cambrian period may have been an active period for seafloor hydrothermal activity, with locally different chemical components in hydrothermal fluids. Abundant metal nutrients brought by these hydrothermal activities may have been a potential factor for the rapid evolution of life during this period.

The basal unit of the Early Cambrian black shale sequence of South China hosts sulfide-rich polymetallic units, non-sulfidic vanadium-rich black shales, sapropelic alginite (combustible shale), barite, and phosphorite. This rock spectrum occurs in a paleoceanographic similar, and stratigraphically correlated, transgressive upwelling setting on the passive continental margin of the Neoproterozoic Yangtze Platform. Several centimeter-thick polymetallic sulfidic units (3–13 wt% Mo+Ni, 100–600 ppm U) have relatively light Mo (δ98/95Mo = 1.1 ± 0.2‰) and relatively heavy U isotope composition (δ238/235U = 0.2 ± 0.1‰). Several meter-thick V-rich shales with multiple ore-grade layers (0.1–0.8 wt% V, < 100 ppm U, Mo and Ni ~ 100 ppm) have isotopically lighter Mo (δ98/95Mo = 0.3 ± 0.4‰) and heavier U composition (δ238/235U = 0.4 ± 0.2‰ and up to 0.7‰). The inverse Mo versus U isotope correlation suggests that both metals were enriched by removal from anoxic to strongly euxinic bottom water in restricted basins along the rifted continental margin. Metal replenishment probably occurred via the cycling of Fe–Mn-oxyhydroxide particles across the redox boundary, with sorption/desorption of Mo (and likely Ni) in a stratified water column. In contrast, V enrichment with much lower Mo, Ni, and U contents, but more fractionated Mo and U isotope composition, reflects non-sulfidic anoxic depositional conditions in a partly open system with higher bottom water renewal rates. While Mo isotope fractionation likely occurred in the water column, U isotope fractionation may dominantly have occurred at the water-sediment interface, perhaps in a benthic organic flocculent layer. These findings indicate that local hydrodynamic control and stratified water column redox conditions may explain the observed variation of metal enrichment (Mo–Ni versus V) in the black shales. Furthermore, the high δ98/95Mo values up to 2.6‰ of the black shales studied and the correlated U and Mo isotope data suggest that Early Cambrian seawater was at least episodically broadly similar to modern seawater. [ABSTRACT FROM AUTHOR]

The complex of felsic to intermediate volcanic rocks intruded by plagiogranitic bodies is first revealed in the western part of the Tekturmas ophiolite zone of Central Kazakhstan. The U–Pb (SIMS) geochronological study of one of the plagiogranitic bodies dated back to the Early Cambrian (537 ± 5 Ma) is conducted for the first time. The peculiarities of the volcanic rocks and the composition of plagiogranites indicate their formation in a suprasubduction setting within the Early Cambrian primitive volcanic island arc. The formation of this arc was not related to the evolution of the Ordovician oceanic structures, the complexes of which participate in the structure of the Tekturmas zone. [ABSTRACT FROM AUTHOR]

Cambrian vetulicolians have mosaic characteristics of both deuterostomes and protostomes, which has important implications for the origin and early evolution of the Deuterostomia. They are intriguing in their bizarre body plan with a series of pharyngeal gill slits. The anterior section is characterized by five pairs of gill pouches, while the paddle‐like posterior part is composed of seven or more segments. Although the overall external morphology and some internal anatomies of vetulicolians, such as pharyngeal cavity, gill system and alimentary canal, have largely been clarified, mysteries remain regarding some other internal structures, and their functional interpretations are highly controversial. In this study we identify an important but hitherto unrecognized feature: a sub‐rounded, wrinkled structure located ventrally at the posterior section (or 'tail') of weakly sclerotized members of vetulicolians, including Xidazoon stephanus and Didazoon haoae, from the early Cambrian Chengjiang biota. The enigmatic structure consistently appears at or between the third and fourth segments of the posterior section. We show that the new structure is an internal organ in the body cavity of vetulicolians and infer that it might have functioned for reproduction, excretion or digestion. The finding of this enigmatic structure from X. stephanus and D. haoae enriches our understanding of vetulicolians and might facilitate further exploration of the anatomy and physiology of early deuterostomes. [ABSTRACT FROM AUTHOR]

Molecular clock estimates suggest the origin of Annelida dates back to the Ediacaran period, which is in discordance with the first appearance of this taxon in the early Cambrian, as evidenced by the fossil records of stem-group and basally branching crown-group annelids. Using new material from the early Cambrian Guanshan biota (Cambrian Series 2, Stage 4), we re-interpret Gaoloufangchaeta bifurcus Zhao, Li & Selden, 2023, as the earliest known errantian annelid. Gaoloufangchaeta has a prominent anterior end bearing three pairs of putatively sensory appendages and a pair of anterior eyes; a muscular eversible pharynx with papillae is identified. The presence of enlarged parapodia with acicula-like structures and long capillary chaetae suggests a pelagic lifestyle for this taxon. Our phylogenetic analyses recover Gaoloufangchaeta within the Phyllodocida (Pleistoannelida, Errantia), extending the origin of Errantia back to the early Cambrian. Our data are in line with the hypothesis that Annelida diverged before the Cambrian and indicate both morphological and ecological diversification of annelids in the early Cambrian.

The Igoudine and Amouslek formations (TerreneuvianâCambrian Epoch 2 boundary) in the western Anti-Atlas of Morocco record the replacement of stromatolite-dominated microbial consortia by thrombolite-metazoan consortia. Carbonate and calcareous shales of both formations have been analyzed for major, trace, and rare earth elements to study their geochemical characteristics and evaluate the provenance of the terrigenous fraction and paleoredox conditions. Discrimination diagrams for the source rocks based on major elements and selected trace elements indicate that the terrigenous fractions of the sediments were likely derived from predominantly felsic rocks, and the source rocks have been identified to be the PaleoproterozoicâNeoproterozoic granites and metasediments of the Kerdous inlier. Paleoredox proxies such as U/Al, V/Al and Mo/Al suggest that the Igoudine and Amouslek formations were deposited in the oxic environment. Our data show that the local water column was prevailingly oxidized before, during and after the transition from the microbial consortium (stromatolite-dominated biota) to the thrombolite-archaeocyathan consortium and shelly metazoans within the studied interval. This implies that the seawater redox status was not driving this change in these biological communities.

Selenium (Se) is an essential trace element for humans and animals, and an excess of or deficiency in Se is harmful to health. Research on the selenium enrichment zone began in the late 1970s in Shuang’an, Ziyang, southern Shaanxi Province. Naore village is only one selenosis area in Shuang’an, Ziyang, China. Different scholars have conducted systematic studies on the occurrence of selenium, its organic geochemistry and biomarkers, and its content and enrichment patterns in this area. This study applied the TIMA (TESCAN integrated mineral analyzer) for the first time to conduct detailed mineralogical work. The minerals included quartz, carbonate minerals (calcite and dolomite), feldspar (plagioclase, albite, and orthoclase), biotite and muscovite, clay minerals (chlorite and kaolinite), hematite, pyrite, and accessory minerals (almandine, olivine, zircon, and apatite) in Naore village, Ziyang, Shaanxi Province. The ATi index (100 × apatite/(apatite + tourmaline)) is used to determine the source of heavy minerals and the degree of heavy minerals’ weathering. The content POS (100 × (pyroxene + olivine + spinel)/transparent heavy mineral) of olivine, pyroxene, and spinel in heavy minerals can reflect the contribution of basic and ultrabasic rocks in the source area. The ATi and POS indexes for the heavy minerals in the research area were 91.83~99.96 and 0.01~18.75, respectively, reflecting the abundance of volcanic rock material in their source. In addition, the migration, transformation, bioavailability, and toxicity of selenium in the environment are closely related to its species. The species of selenium in various selenium-rich areas (Naore, Wamiao, and Guanquan) mainly include unusable residues and organic forms, followed by humic-acid-bound selenium. The proportions of water-soluble, exchangeable, and carbonate-bound selenium are relatively small, and the proportion of Fe-Mn oxide-bound selenium is the lowest.

Simple Summary: Priapulida form a small relict group of marine invertebrates characterized by a vermiform shape, an annulated trunk, and an eversible anterior part. Worms with a comparable body plan were diverse and abundant throughout the Cambrian Era, although uncertainties remain concerning their relation to modern priapulids. The problem lies in the lack of morphological detail (ornament, symmetry) available from Cambrian worms and the fact that a comprehensive phylogeny of Cambrian worms with a robust homology framework is not available to define priapulids. The exceptionally preserved worm Ercaivermis sparios, described here from the early Cambrian of China, displays an unusual octagonal symmetry, suggesting that different symmetry types may have co-existed in the early history of Priapulida, before five-fold symmetry was naturally selected to become overwhelmingly dominant. The vast majority of early Paleozoic ecdysozoan worms are often resolved as stem-group Priapulida based on resemblances with the rare modern representatives of the group, such as the structure of the introvert and the number and distribution of scalids (a spiny cuticular outgrowth) and pharyngeal teeth. In Priapulida, both scalids and teeth create symmetry patterns, and three major diagnostic features are generally used to define the group: 25 longitudinal rows of scalids (five-fold symmetry), 8 scalids around the first introvert circle and the pentagonal arrangement of pharyngeal teeth. Here we describe Ercaivermis sparios gen. et sp. nov., a new priapulid from the early Cambrian Chengjiang Lagerstätte, characterized by an annulated trunk lacking a sclerotized ornament, four pairs of anal hooks and 16 longitudinal rows of scalids along its introvert and eight scalids around each introvert circle, giving the animal an unusual octoradial symmetry. Cladistic analyses resolve Ercaivermis as a stem-group priapulid. Ercaivermis also suggests that several biradial symmetry patterns (e.g., pentagonal, octagonal) expressed in the cuticular ornament, may have co-existed among early Cambrian priapulids and that the pentaradial mode may have become rapidly dominant during the course of evolution, possibly via the standardization of patterning, i.e., the natural selection of one symmetry type over others. [ABSTRACT FROM AUTHOR]

A large enigmatic fossil is described from the early Cambrian (Series 2, Stage 3) Heatherdale Shale in the Fleurieu Peninsula, South Australia. It has an almost circular outline about 150 mm across, with an outer rim and 27–30 evenly-spaced rays that extend about two-thirds of the distance to the centre of the structure; it shows radial symmetry. There is a featureless central area with a width about one-third that of the entire specimen. The fossil is preserved as pale-coloured material that stands out slightly above the surrounding mudstone. XRF data indicate that there is little compositional difference between the fossil and the enclosing rock. The fossil may appear to be a transverse section of a larger organism, but it does not continue down into the surrounding rock. This suggests it is a non-mineralised compression fossil that retains the remnants of a three-dimensional structure. Possible affinities with radiodont oral cones, ctenophores, eldonioids, scyphozoa and archaeocyaths are discussed, but no definite assignment can be made. [ABSTRACT FROM AUTHOR]

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The Cambrian is a critical period in the evolution of life and environment. Reconstructing the marine sedimentary environment, notably redox conditions during this period is key to explore the spatial and temporal evolutionary characteristics of seawater chemistry and the relationship between biological evolution and environmental changes. In this study, the major elements, trace elements and organic carbon content (TOC) of the Lower Cambrian Xishanbulake-Xidashan formations in well Tadong 2 in the Tarim Basin were analyzed, and the sedimentary environment of the formations was reconstructed. The results show that the water body was in weak restriction condition when the formation was deposited. The redox state of bottom waters was dominated by anoxia, but the degree of anoxia varied obviously. Specifically, the middle part of the Xishanbulake Formation showed elevated anoxia (euxinia) in bottom waters compared to the lower and the upper parts, while the Xidashan Formation showed a decrease in anoxia from the middle and the lower parts to the upper part and an expansion of bottom water oxidability. The results are consistent with the observation in previous studies that the Early Cambrian seawaters was characterized by dynamic changes in the redox state. The analysis of organic matter enrichment mechanism of Xishanbulake-Xidashan formations in well Tadong 2 shows that organic matter enrichments in the formations is not controlled by a single factor. The anoxic/euxinic environment in the middle and the lower parts of the Xishanbulake Formation is more conducive to the preservation of organic matter, while the higher level of primary productivity played a key role in the sedimentation of the upper part of the Xishanbulake Formation and the Xidashan Formation.

The North Qaidam belt is an important polymetallic metallogenic belt in the northwestern region of China. However, the tectonic setting and age of related VMS deposits remain debated. Here we performed an integrated analysis of field relationship, geochemistry, and geochronology for hosting rocks of the Lüliangshan VMS-type Cu deposit and surrounding mafic-ultramafic rocks. These rocks, including serpentinite, pyroxenite, chromitite, mafic dykes with associated meta-plagiogranite, lava, chert, and limestone, constitute a relatively complete ophiolite complex, indicating that the Lüliangshan Cu deposit can also be introduced as an ophiolite-hosted VMS deposit. Geochemical data show that the meta-dolerite and ore-hosting lava exhibit geochemical features similar to tholeiitic forearc basalt and are probably generated by partial melting of a depleted mantle source metasomatized by hydrous fluids. Some lavas have boninitic compositions and are formed by partial melting of residual mantle after extraction of forearc basalt. Some ore-hosting lavas also have geochemical affinities to island arc tholeiites as a result of more SSZ components involved in their magma source. The chert samples have remarkably high Fe2O3T contents and are classified as iron-rich one of hydrothermal origin, which is deposited in a ridge-proximal environment. These rocks, together with chromitites with subducted-related geochemical features, collectively indicate that the ophiolite-hosted VMS-type Cu deposit was formed in the forearc setting. Meta-gabbros intruding the ore-hosting lavas yield zircon U-Pb ages mainly ranging from 527 Ma to 518 Ma. The new ages of forearc ophiolite and the oldest age of island-arc rocks (514 Ma) suggest that the Lüliangshan Cu deposit formed in the early Cambrian during early-stage subduction of Proto-Tethys Ocean.

Simple Summary: Small skeletal fossils are reported for the first time from the early Cambrian Guojiaba Formation, southern Shaanxi, China. All specimens were recovered from bioclastic limestone interbeds and encompass a wide variety of skeletal clades, including brachiopods, sphenothallids, archaeocyaths, bradoriids, sponge spicules, echinoderm plates, and trilobite spines. The archaeocyaths described herein are considerably older than those described from the Xiannvdong Formation, which was previously assumed to contain the lowest archaeocyath-bearing horizons in South China. The brachiopod Lingulellotreta yuanshanensis is recorded for the first time from the Fucheng area, with previous records confined mainly to the Chengjiang Fauna-bearing Yu'anshan Formation in the lower Cambrian, the eastern Yunnan Province. Micro-morphological and elemental analyses of the small skeletal fossil assemblages were carried out using SEM, BSEM, and Micro X–ray fluorescence. The skeletal fauna in the Guojiaba Formation resembles the assemblages recovered from the upper Yu'anshan Formation (Chengjiang Fauna) in eastern Yunnan Province. Based on the recovered assemblage, the biostratigraphic age of the Guojiaba Formation correlates with the Chiungchussuan Stage (Stage 3 of Cambrian Series 2). A small skeletal fossil assemblage is described for the first time from the bioclastic limestone interbeds of the siltstone-dominated Guojiaba Formation, southern Shaanxi, China. The carbonate-hosted fossils include brachiopods (Eohadrotreta zhujiahensis, Eohadrotreta zhenbaensis, Spinobolus sp., Kuangshanotreta malungensis, Kyrshabaktella sp., Lingulellotreta yuanshanensis, Eoobolus incipiens, and Eoobolus sp.), sphenothallids (Sphenothallus sp.), archaeocyaths (Robustocyathus sp. and Yukonocyathus sp.), bradoriids (Kunmingella douvillei), chancelloriids sclerites (Onychia sp., Allonnia sp., Diminia sp., Archiasterella pentactina, and Chancelloria cf. eros), echinoderm plates, fragments of trilobites (Eoredlichia sp.), and hyolithelminths. The discovery of archaeocyaths in the Guojiaba Formation significantly extends their stratigraphic range in South China from the early Tsanglangpuian at least to the late Chiungchussuan. Thus, the Guojiaba Formation now represents the lowest known stratigraphic horizon where archaeocyath fossils have been found in the southern Shaanxi area. The overall assemblage is most comparable, in terms of composition, to Small skeletal fossil (SSF) assemblages from the early Cambrian Chengjiang fauna recovered from the Yu'anshan Formation in eastern Yunnan Province. The existing position that the Guojiaba Formation is correlated with Stage 3 in Cambrian Series 2 is strongly upheld based on the fossil assemblage recovered in this study. [ABSTRACT FROM AUTHOR]

[Display omitted] • We discovered the oldest ∼521 Ma ophiolitic gabbro in the Chinese Tianshan. • The gabbro formed in a fore-arc tectonic setting related to initiation of subduction. • We propose a large-scale subduction event at 530–520 Ma in the southern CAOB. The long-lived accretion of the southwestern Central Asian Orogenic Belt is directly related to the subduction and demise of multiple Paleozoic oceanic basins. However, the evolution of those different oceans, and their relationships, remain poorly constrained. In this paper, we report on an Early Cambrian ophiolite discovered in the North Tianshan suture, China, which represents the oldest oceanic crustal fragments documented in the North Tianshan belt. Two gabbroic blocks collected from the Gangou ophiolitic mélange yielded consistent zircon U–Pb ages of ca. 521 Ma. These gabbros have similar whole-rock major and trace element and Sr–Nd isotopic compositions to those of gabbroic oceanic crust from mid-ocean ridges. In addition, high-An (∼92) plagioclase and high-Mg# (∼91) clinopyroxene indicate that the gabbros crystallized under hydrous conditions, likely in a subduction-related setting. Our data and those for Early Cambrian subduction-related ophiolites in the Junggar, Western Tianshan, and Beishan orogenic belts record the development of the earliest, large-scale, subduction system within the southwestern Central Asian Orogenic Belt, which was probably associated with assembly of Gondwana. This system may have also connected the Early Cambrian subduction systems between the Western Tianshan and Beishan belts. [ABSTRACT FROM AUTHOR]

Unveiling the body architectures of Cambrian problematic fossils would provide novel insights into the radiation of metazoan body plans during the 'Cambrian Explosion' and the ancestral traits of major living animal clades. Xianguangia sinica, from the celebrated Chengjiang biota (∼518 Ma), is a typical Cambrian problematicum with disputable body architecture, particularly about its 'column' part. The contradictory interpretations of the 'column' of X. sinica have led to at least three hypotheses regarding its affinity in the diploblastic clade. Here we depict the detailed anatomy of the 'column' based on new, exquisitely preserved material. The 'column' of X. sinica is formed by 18 longsword-shaped tentacle-sheath complexes that can either close or be in a flowering state. There is no partitioned cavity internally when the 'column' is closed, invalidating the homology with the true column of living sea anemones. Each tentacle tapers distally and includes a distal flexible portion at about one-fourth the length of the tentacle. The proximal portion is stiff, bearing a set of paired dark stains, and is enveloped by a single outer sheath. Pinnules carrying a row of large cilia are fringed on both sides along the whole length of the tentacles. The body plan of X. sinica is accordingly revised as consisting of a calyx and 18 unique tentacle-sheath complexes that radially surround the mouth. Our result corroborates previous observations that suggest a close relationship between Xianguangia, Daihua and Dinomischus, all of which are here formally assigned to the family Dinomischidae, a monophyletic clade recovered in our phylogenetic analyses. Xianguangia sinica likely employs cilia-bearing pinnate tentacles for sieving particle matter down to 21 µm, indicating that its nutrition source is suspended micro-planktonic organisms or other organic matter in the bottom water. [ABSTRACT FROM AUTHOR]

The Late Neoproterozoic to Early Cambrian is a remarkable time-interval that witnessed significant biolo-gical and environmental evolutions, during which, the deep ocean was extensively anoxic, whereas the surface ocean was characterized by considerable fluctuations in redox conditions. The Lower Cambrian Yanjiahe and Shuijingtuo formations in the Luojiacun section from the Three Gorges area situated in Yichang city of central China were studied in this paper. According to biostratigraphic and carbonate carbon isotope stratigraphic correlation, the Yanjiahe Formation and Shuijingtuo Formation were assigned to the Cambrian Terreneuvian Stage 1-2 and Series 2 Stage 3, respectively. In view of this time-frame, the response of inorganic and organic carbon isotopes decoupling to water redox environment transition was systematically studied. The rise of anoxic bottom water during transgression provided sufficient nutrients and essential trace elements for primary productivity in the surface ocean. The increase of net productivity led to oxygen consumption increase and the changes of redox environment in the water column. Photosynthetic oxygen temporarily oxygenated the surface seawater, and dissolved oxygen was gradually consumed by the oxidation of DOC reservoirs, resulting in gradual expansion of oceanic anoxia, which was conducive for the preservation of organic matters. The oxidation of DOC in the surface water accounted for the formation of authigenic carbonates that were 13C-depleted, while the large oceanic DOC reservoir in the Early Cambrian buffered the change in δ13Corg, which eventually manifested as decoupled carbon isotope during the transition time of Cambrian Stage 2-3.

Abstract The Lower Cambrian Barut Formation is one of the most important lithostratigraphic units in reconstructing the paleogeography of northern Iran during the early Cambrian time. Herein, we present detailed facies analysis data of this formation in the Barut Aghaji Section in southwest Zanjan to analyze the palaeoenvironment. The Barut Formation in this section consists of about 500 m thick mixed siliciclastics and carbonate rocks and its upper and lower contacts with both Soltanieh and Zaigun formations are gradational. Seven recognized lithofacies/microfacies include: red, fine- to medium-grained arkosic sandstone containing physical and biogenic tidal indicators (F1), red to green, finely laminated mudrock containing tidal biogenic structures (F2), dolomitic stromatolite boundstone (F3), dolomitic mudstone with evidence of former evaporites (F4), dolomitic thrombolite boundstone (F5), carbonate breccia/conglomerate (F6), and dolomitic oncoid wackestone (F7). These facies are arranged in shallowing-up cycles consisting of mixed flat sandstones overlain by mud flat mudrocks and supratidal carbonates. Evidence of evaporite minerals in the Barut Formation along with other contemporaneous early Cambrian evaporite basins in the Middle East, India and Pakistan, indicates a dry and warm palaeo climate during the deposition of the sediments. Keywords: Early Cambrian, Barut Formation, Barut Aghaji Section, Tidal flat, Evaporite basins. Introduction The Late Neoproterozoic to Early Cambrian is one of the most important time intervals in the history of Earth. Extreme climate changes, major tectonic events such as the formation of the East African Orogens, oxygenation events, change of seawater composition and rising nutrient input, as well as the emergence and diversification of multicellular life, are among the most important events of this time (Zhang et al. 2014). A large part of the reconstruction of these important events has been obtained from the study of sedimentary rocks. In the palaeogeographic reconstructions of Late Neoproterozoic–Early Cambrian, Iran is placed along the northeastern margin of Gondwana, between the Arabia and India plates. The oldest sedimentary rocks in Iran are siliciclastics with subordinate carbonate rocks known as Kahar, Bayndor, Soltanieh, Barut and Zaigun formations, which range in age from Ediacaran to lower Cambrian (Stocklin et al. 1964; Horton et al. 2008). Researchers believed that this sedimentary succession was deposited mainly in a shallow marine environment. The Barut Formation in the northwest of the Soltanieh Mountains, 18 Km southwest of Zanjan and northwest of Barut Aghaji village, consists of about 500 m of purple mudrocks and carbonates. The lower and upper boundary of this formation, with the Soltanieh and Zaigun formations, respectively, is conformable and gradational. The depositional ages assigned to the Barut Formation range from Late Neoproterozoic (e.g., Horton et al. 2008) to early Cambrian (Salehi et al. 1998; Devaere et al. 2018). Despite the importance of the Barut Formation in the reconstruction of the palaeogeography of Iran during the Ediacaran–Early Cambrian, this formation has received little sedimentological study. For the first time, this study examines the lithology and palaeodepositional conditions of the Barut Formation in the Barut Aghaji section (Type section). Our findings provide new data on the palaeogeographical conditions of this area in northern Iran during the Early Cambrian time. Material & Methods In this study, detailed field investigations were carried out to prepare a stratigraphic section, sampling and recognizing lithofacies. The main lithofacies were recognized using lithology, geometry, the nature of bed boundaries, and sedimentary structures during field studies according to Stow (2005). A total of 100 siliciclastic and carbonate samples were collected at regularly spaced intervals. After preparing thin sections, petrographic studies were performed on thin sections according to the classification of Folk (1974), Tucker (2009), and Dunham (1962). Lithofacies were interpreted according to Stow (2005), Flügel (2010), Einsele (2000), Nichols (2009), and Davis (2012), and then were classified into facies associations that reflect the palaeoenvironments. Discussion of Results & Conclusions Seven lithofacies have been identified in the studied section, which can be grouped into two main facies associations: (1) Siliciclastic-dominated facies association and (2) Carbonate-dominated facies association. Siliciclastic-dominated facies association includes fine- to medium-grained red arkosic sandstone containing tidal bedding with flaser and lenticular laminations, herringbone cross stratifications, mud drapes on cross stratifications and ripples, and Microbially Induced Sedimentary Structures (MISS) (F1) and red to green, finely laminated mudrock containing MISS (F2). Carbonate-dominated facies association includes dolomitic stromatolite boundstone (F3), dolomitic mudstone with evidence of former evaporites (F4), dolomitic thrombolite boundstone (F5), carbonate breccia/conglomerate (F6), and dolomitic oncoid wackestone (F7). These facies contain several physical and biological tidal criteria and are arranged as shallowing upward cycles. Each cycle starts with mixed flat sandstones overlain by mud flat mudrocks and supratidal carbonates that are subjected to subaerial exposure. The results of this study indicate that tidal flat sedimentation conditions prevailed in the study area throughout the early Cambrian, from the Upper Dolomite Member of the Soltanieh Formation to the Zaigun Formation. In addition, evidence of evaporite minerals in the Barut Formation along with other contemporaneous Early Cambrian evaporite basins in the Middle East, India and Pakistan, indicates dry and warm palaeoclimate during the deposition of sediments. More information on palaeotectonic would help us to establish a greater degree of accuracy on this matter.

To deeply analyze the thalassochemical conditions and organic matter enrichment mechanism during the Early Cambrian (541-509 Ma)and actively evaluate the potential of shale gas resources of the Lower Cambrian Qiongzhusi Formation(Fm) in the southwestern Sichuan Basin, based on the analysis of the petrology, organic geochemistry, element characteristics, pore structure and adsorption capacity of Qiongzhusi Formation, Well W-207, Weiyuan area, this study has discussed the Early Cambrian paleo-ocean environment, organic matter enrichment control factors and gas-bearing properties of shale gas in the upslope area of the Upper Yangtze Platform. Sedimentary cycle shows that multiple interactive conversioncycles of deep-water continental shelf and shallow-water continental shelf are developed during the fine-grained deposition period of Qiongzhusi Formation under the control of eustasy. In particular, the slope turbidite (fan) and gravity flow sediments indicate that shallow-water continental shelf facies are dominant, and the wells in the upslope of the Weiyuan area are not in deep-water for a long time, with the sedimentary thickness of organic-rich black shale limited. Organic geochemistry evidence indicates that the organic matter of Qiongzhusi Formation in Well W-207 is mainly Type-Ⅰ kerogen, with a high degree of thermal evolution, fewer residual hydrocarbons and a low hydrocarbon generation capacity. The redox parameters indicate that the marine environment on the upslope has a medium restrictive degree, and there is a certain degree of upwelling. The seawater has experienced the transformation process of "anoxic-oxidation-anoxic-secondary oxidation- oxidation". Therefore, the paleo-ocean productivity level in the upslope area is generally low, with an obvious downward trend from bottom to top. The pore structure and nitrogen adsorption curve show that the reservoirs of the Qiongzhusi Formation are mainly complex and irregular slit pores. The methane adsorption capacity is positively correlated with TOC but negatively correlated with temperature, indicating that the high-pressure and high-temperature conditions generally faced by the Qiongzhusi Fm are not suitable for methane adsorption. As a result, the geological conditions of shale gas for Qiongzhusi Fm in the upslope area are complex. With high exploration risk, this study suggests that the resource evaluation direction should change to the intracratonic sag (downslope area), which is characterized by deep-water continental shelf facies.

The Ediacaran to Cambrian transition witnessed great biological and environmental turnovers from the Precambrian to Phanerozoic. These changes are also reflected by the fossil records during that time. However, the fossil distribution was highly heterogenous. While diverse fossils were constantly found from the shallow shelf including restricted basins, reports from deep-water areas, which are characterized by chert-dominated formations straddling the Ediacaran-Cambrian boundary, were scarce. Among them was the Liuchapo Formation that has been considered barren. In this study, a fossiliferous assemblage was found from the Liuchapo Formation in east Guizhou Province, South China. This assemblage comprises Palaeopascichnus jiumenensis, Horodyskia minor, multicellular algae that possibly related with red algae, cyanobacteria, sponge spicules, small shelly fossils, membrane structures of uncertain affinity, and fossils with structures resemble the Ediacaran Megasphaera. Palaeopascichnus and Horodyskia have been widely reported and abundant in the Liuchapo Formation and equivalents, while the others have seldomly, or never, been found from this formation or, more widely, the deep-water chert of transitional interval. The discovery not only contributes to the rare paleontological records preserved in deep-water sediments, but also expanded the geographic distribution of the fossils, providing new materials of biological diversity during this critical interval. [ABSTRACT FROM AUTHOR]

Organic matter pores are considered to be the most important type of pore for preserving hydrocarbon gases in shale gas reservoirs. The organic matter in each over-mature marine shale sample was separated into two organic fractions with densities of greater than and less than 1.25 g/cm3, and then their molecular compositions and pore characteristics were quantitatively evaluated using solid state 13C-nuclear magnetic resonance (NMR) and gas (N2 and CO2) adsorption analyses, respectively. The results revealed that aromatic carbon is the dominant molecular composition of the over-mature organic matter in the Lower Cambrian Niutitang shale. During the overmature stage, the organic fractions with densities of greater than and less than 1.25 g/cm3 have no significant differences in molecular composition. The organic fractions with densities of greater than and less than 1.25 g/cm3 do have significant differences in their organic pore characteristics. In contrast to the high density organic fraction, the low density fraction contained abundant micropores and lacked mesopores and macropores. The organic pore structures of the different occurrence states of organic matter were significantly different. The C/O of organic matter in different occurrence states are obviously different, which proves that the organic pore structure is closely related to both the occurrence state and density of the organic matter. However, these relationships are still unclear and require further study. [ABSTRACT FROM AUTHOR]

The early Cambrian witnessed profound environmental changes and biological evolution in Earth' history. During this period, organic-rich shales were widely distributed over almost the entire Yangtze Block. However, the dominant factor that drove the significant accumulation of organic matter (OM) remains controversial and is still debated. Here, we analyzed TOC, organic carbon isotopes, iron speciation, major and trace elements for the lower Cambrian Niutitang Formation in the upper slope Meiziwan section, to investigate the dominant factor controlling OM accumulation. High contents of TOC and Baxs reveal an OM-enriched feature of the Niutitang Formation, and the coupled relationship between them suggest a strong production control on OM accumulation at Meiziwan. Meanwhile, negative relationships between TOC and chemical index of alteration (CIA) values as well as Al contents suggest that influence of chemical weathering and terrestrial input on OM accumulation were limited. Fairly low CoEF × MnEF values provide strong evidence that the deposition of organic-rich shales was under the control of oceanic upwelling event. The upwelling event would bring nutrient-rich deep waters into surface water, stimulating phytoplankton bloom and primary productivity in surface water and facilitating OM enrichment. Meanwhile, enhanced accumulation of OM would have promoted subsequent bacterial sulfate reduction, leading to the occurrence of occasional euxinia (evidenced by iron speciation and redox-sensitive trace element data) and promoting preservation of OM. Taken together, our results shed light on the critical role of oceanic upwelling on the marine primary productivity on the earliest Cambrian Yangtze Platform. [ABSTRACT FROM AUTHOR]

The Kuanchuanpu biota (ca. 535 Ma) from the basal Cambrian in South China yields various phosphatized, three-dimensionally preserved microscopic fossils, such as animal embryos, cyanobacteria, and algae. This provides an exceptional window for investigating the rapid divergent evolution of animal phyla in the early Cambrian ocean. However, the fossil eukaryotic algae are poorly documented and are not properly incorporated into the conceptual food web of the Cambrian ocean ecosystem. In this study, we report a new taxon of calathiform alga, Calathophycus irregulatus Tang gen. et sp. nov., and an indeterminate alga, from the Kuanchuanpu biota. Calathophycus irregulatus can be distinguished from other reported phosphatized multicellular algal fossils from the Kuanchuanpu biota and the Ediacaran Weng’an biota (Guizhou Province, China), based on the external morphology and features of cell clump aggregation. The observation of C. irregulatus and the indeterminate alga indicates a certain diversity of algae at the beginning of the Cambrian. Concerning the regularly or irregularly shaped multicellular algal fossils from the Kuanchuanpu Formation, co-occurring spherical fossils, previously interpreted as “cleavage stage” of animal embryos, should be reconsidered in affinity.

Simple Summary: The collection of early Cambrian microfossils leads to the amassing of a pile of thousands of tiny tubes, grains and various fragments. Rare type of microfossils with high academic value are mingled with numerous ordinary fossils and the traditional way of manual selection is very inefficient. Many artificial intelligence (AI) technologies have already been applied in fossil image recognition, but current methods largely depend on a great number of fossil images to "train" the AI model. However, usually only a handful of samples are available for specific rare fossil taxa and these cannot provide enough photos for AI. In this study, we fine-tuned a new convolutional neural network, combining pre-trained models from a nature image database to solve the problem of the lack of training materials. Through many tests, this new model was proved valid. It presented relatively high accuracies in recognizing specific micro fossil taxa, while the required number of corresponding fossil images is very low. Various microfossils from the early Cambrian provide crucial clues for understanding the Cambrian explosion and the origin of animal phyla. However, specimens with important anatomical structures are extremely rare and the efficiency of retrieving such fossils by traditional manual selection under a microscope is quite low. Such a contradiction has hindered breakthroughs in micropaleontology for a long time. Here, we propose a solution for identifying specific taxa of Cambrian microfossils using only a few available specimens by transferring a model pre-trained on natural image datasets to the field of paleontological artificial intelligence. The method employs a 34-layer deep residual neural network as the underlying framework, migrates the ImageNet pre-trained model, freezes the low-layer network parameters and retrains the high-layer parameters to build a microfossil image recognition model. We built training sets with randomly selected images of varied number for each taxon. Our experiments show that the average recognition accuracy for specific taxa of Cambrian microfossils (50 images for each taxon) is higher than 0.97 and it can reach 0.85 with only three training samples per taxon. Comparative analyses indicate that our results are much better than those of various prevalent methods, such as the transpose convolutional neural network (TCNN). This demonstrates the feasibility of using natural images (ImageNet) for the training of microfossil recognition models and provides a promising tool for the discovery of rare fossils. [ABSTRACT FROM AUTHOR]

Cnidarians form a disparate phylum of animals and their diploblastic body plan represents a key step in animal evolution. Cnidarians are split into two main classes; anthozoans (sea anemones, corals) are benthic polyps, while medusozoans (hydroids, jellyfishes) generally have alternating life cycle stages of polyps and medusae. A sessile polyp is present in both groups and is widely regarded as the ancestral form of their last common ancestor. However, the nature and anatomy of the ancestral polyp, particularly of medusozoans, is controversial, owing to the divergent body plans of the extant lineages and the scarcity of medusozoan soft tissues in the fossil record. Here, we redescribe Conicula striata Luo & Hu from the early Cambrian Chengjiang biota, south China, which has previously been interpreted as a polyp, lophophorate or deuterostome. Through re-examination of the holotype and 51 exceptionally preserved specimens, we show that C. striata possessed features of both anthozoans and medusozoan polyps. A conical, annulated organic skeleton (periderm) fully encasing a polyp is found in fossil and living medusozoans, while a tubular pharynx extending from the mouth into a gut partitioned by c. 28 mesenteries, resembling the actinopharynx of anthozoans. Our phylogenetic analyses recover C. striata as a stem-group medusozoan, implying that the wealth of medusozoan diversity derived, ultimately, from an anemone-like ancestor. [ABSTRACT FROM AUTHOR]

• Two evolutionary stages are identified in the Lower Cambrian Xiaoerbrak Formation carbonates. • Depositional architecture of the carbonate platform margin changes from base to top in the Xiaoerbrak Formation. • Tectonic and paleomorphologic evolution and sea-level change control the evolution of carbonate platforms. The objective of this study is to systematically document the depositional architecture and evolution of the carbonate platform margin in the Lower Cambrian Xiaoerblak Formation of the northwestern Tarim Basin. This study uses the following approaches: (1) seismic reflector identification; (2) lithofacies and paleoenvironmental interpretation based on Paleozoic outcrops; and (3) thin section examination. Identification of seismic reflectors and determination of lithofacies associations in Paleozoic outcrops reveal two phases of architectural evolution of the Xiaoerblak Formation platform margin. Phase 1 corresponds to the Lower Xiaoerblak Formation, characterized by discontinuous to semicontinuous moderate- to high-amplitude reflectors, revealing a uniform, gentle ramp platform margin thinning toward the basin. The dolomudstone and laminate lithofacies associations in the outcrops show a middle-ramp low-energy depositional environment. Phase 2 corresponds to the Upper Xiaoerblak Formation. The seismic stratigraphic units display upwardly convex irregular reflectors, indicating the development of a rimmed carbonate platform margin system. The lithofacies associations reveal reef-shoal interbedding, suggesting a high-energy marginal marine environment. The tectonic and paleomorphic evolution of the Tarim Basin, along with Paleozoic outcrop features, suggest that paleomorphic inheritance from the Neoproterozoic created a homogeneous, broad, low-angle shelf. This, combined with the continuous sea-level fall in the early Cambrian and the tropical environment, provided an ideal depositional environment for carbonate platform development in the Xiaoerblak Formation. [ABSTRACT FROM AUTHOR]

Organic-rich shale related uranium (U) mineralization has attracted widespread attention as a strategic and clean alternative to fossil energy and a paleoceanic redox proxy. A comprehensive study of mineralogy, organic geochemistry and elemental geochemistry of the lower Cambrian Niutitang Formation organic-rich shales in northeastern Guizhou, South China was conducted to address this issue. SEM-EDS results showed that the U occurrence state in the Niutitang Formation shales is dominated by nanoscale U minerals, mainly including coffinite, brannerite, U-bearing apatite and U-bearing xenotime. Various states of U are closely associated with organic matter, apatite, pyrite and clay minerals. Based on geochemical indexes, paleomarine condition and event deposition jointly drove the U enrichment in the Niutitang shales, in which hydrothermal activities and/or volcanic eruptions increased U fluxes in seawater and euxinic environment with high primary productivity provided a favorable condition for U reduction. The adsorption, complexation and reduction of UO 2 2+ by organic matter and the formation of uranyl phosphate complexes by the combination of phosphate and UO 2 2+ accelerated U immobilization and enrichment from bottom water to sediments. The microbial activities (e.g., BSR) in this process created favorable euxinic conditions and released inorganic-phase P to provide available ligands for UO 2 2+. The U enrichment models of the lower Cambrian Niutitang shales in northeastern Guizhou were established. This paper provides a new insight into the U enrichment mechanism in organic-rich shales, and may also have implications for the applicability of U as redox proxy and the exploration of organic-rich shale related U deposits. • The U occurrence states in Niutitang shales were studied through SEM-EDS. • Paleomarine condition and event deposition jointly drove the U enrichment. • Organic matter, phosphate and microbial participation accelerated the U enrichment. [ABSTRACT FROM AUTHOR]

The early Cambrian was generally considered the earliest time interval while the archaeocyaths could act as the constructors in building reef framework although commonly coexisting with the microbial constructors (Epiphyton and Renalcis) in the course of the "Cambrian explosion"; however, this kind of metazoan-microbial buildups has not yet been reported in Tarim Basin. To delineate facies (associations), spatiotemporal variations, reef-building processes of metazoan-microbial constructed ramps/shelves and evolution, detailed anatomy was carried out upon the lower Cambrian Xiaoerbulak Formation (or Stage 3) exposed continuously along a marginal transect (∼2 km wide) in Aksu area, northwestern Tarim Basin. Three platform types with distinctive facies associations are distinguished: (1) sponge-microbial biostrome/mound-colonized homoclinal and distally-steepened ramps recorded respectively in the Lower Member and basal Middle Member, (2) Archaeocyath-Epiphyton reef- and microbial reef-rimmed platforms (or shelves) in the mid-upper Middle Member, and (3) microbial biostrome-colonized ramp in the Upper Member, thus illustrating the evolution from the ramp to reef-rimmed platform and final reversion to the ramp system. Four and a half third-order depositional sequences are further identified: one in the Lower Member, two in the Middle Member and one and half (TST) in the Upper Member, although, at least, one sequence was absent in the platform/ramp interior. All these can be well correlated with those in equivalent successions identified elsewhere around the world, implying a basic control of eustatic changes on platform development and evolution. Moreover, syndepositional faulting/sliding that occurred off the ramp margin, as demonstrated by slope failure/collapse, may have enhanced the relief difference between the inner and mid-outer ramps, promoting metazoan-microbes to colonize and build up wave-resistant architecture on the uplifting margin, forming the reef-rimmed platform (or shelf) system as typified in the Middle Member. Meanwhile, the development of wave-resistant, metazoan-microbial reef complexes also benefited from their concurrently-gained calcifying ability, which likely resulted from the instinctive biotic responses to the changes in climate and oceanic chemistry (i.e., p CO 2 , seawater Mg/Ca ratio) in the early Cambrian. The new datasets of this study fill previous information gap on the early Cambrian metazoan-involved buildups that had controlled the platform configuration and evolution in Tarim Basin, which thus are critically important for better understanding of climatic/environmental controls on their occurrence and evolution of the time-specific reef complexes in the course of "Cambrian Explosion". Additionally, these datasets would provide a useful analogue for perceiving the facies-constrained porosity and heterogeneity in the deeply buried counterparts in the basin, helpful for making exploration strategy for the deeply-buried hydrocarbon resources. • Sponge-microbial buildups were firstly reported in the Cambrian Stage 3, Tarim Basin. • Archaeocyath - Epiphyton reefs appeared in the course of the "Cambrian Explosion". • Platforms evolved from the ramp to reef-rimmed platform and final reversion to ramp system. • Syndepositional faulting and eustatic fluctuations played key roles in shaping platform configuration, spatiotemporal facies variations. • Climatic-oceanic coupling promoted the calcifying ability of both metazoans and microbes to build rigid reef framework. [ABSTRACT FROM AUTHOR]

The role of environmental change as a significant force driving biological evolution during the Ediacaran–Cambrian transition remains a subject of extensive debate. In this study, we present high-resolution C-isotope data of organic matter (δ13C org) and multiple S-isotope compositions of pyrite (δ34S py and Δ33S py) obtained from a continuous drill core (ZK1505) in South China, spanning the interval from the Ediacaran to Cambrian Stage 2. Our δ13C org and δ34S py data, combined with published data from four sections deposited across a shelf-to-basin transect, reveal substantial gradient from shallow to deep water in δ13C org and sustained positive δ34S py values in the early Cambrian. The vertical δ13C org gradient suggests chemical stratification of the early Cambrian ocean, characterized by oxic shallow water and anoxic deep water. The sustained positive δ34S py values in the same period suggest low seawater sulfate concentrations. Furthermore, we observed intermittent occurrences of negative Δ33S py values coupled with positive δ34S py , suggesting a dominant deep-water anoxia during early Cambrian in the Yangtze Block. We propose that protracted deep-water anoxia may have contributed to the extinction of the Ediacaran biota and played a significant role in the evolution of Cambrian animals. • Vertical δ13C org gradient reveals chemical stratification of the early Cambrian ocean. • Positive δ34S py may have resulted from low seawater sulfate concentrations in the early Cambrian ocean. • Negative Δ33S py coupled with positive δ34S py suggests a dominant deep-water anoxia. • Deep-water anoxia may have hindered the flourishing of early Cambrian animals on the seafloor. [ABSTRACT FROM AUTHOR]

The vast majority of early Paleozoic ecdysozoan worms are often resolved as stem-group Priapulida based on resemblances with the rare modern representatives of the group, such as the structure of the introvert and the number and distribution of scalids (a spiny cuticular outgrowth) and pharyngeal teeth. In Priapulida, both scalids and teeth create symmetry patterns, and three major diagnostic features are generally used to define the group: 25 longitudinal rows of scalids (five-fold symmetry), 8 scalids around the first introvert circle and the pentagonal arrangement of pharyngeal teeth. Here we describe Ercaivermis sparios gen. et sp. nov., a new priapulid from the early Cambrian Chengjiang Lagerstätte, characterized by an annulated trunk lacking a sclerotized ornament, four pairs of anal hooks and 16 longitudinal rows of scalids along its introvert and eight scalids around each introvert circle, giving the animal an unusual octoradial symmetry. Cladistic analyses resolve Ercaivermis as a stem-group priapulid. Ercaivermis also suggests that several biradial symmetry patterns (e.g., pentagonal, octagonal) expressed in the cuticular ornament, may have co-existed among early Cambrian priapulids and that the pentaradial mode may have become rapidly dominant during the course of evolution, possibly via the standardization of patterning, i.e., the natural selection of one symmetry type over others.

Modern medusae are primarily macroscopic, free‐swimming marine organisms with a tetraradial symmetry. But the early Cambrian medusozoans, such as biradial Hexaconularia sichuanensis and tetraradial Quadrapyrgites quadratacris from the basal Cambrian Kuanchuanpu Formation in South China, were millimeter‐scale and sedentary forms. Although well‐documented as common components in a shelf environment, little is known about their in situ life conditions since they are easily transported by tidal flow before burial. Specifically, the interactions between their exoskeleton shapes and the hydrodynamic conditions are poorly known. Here we use fluid–structure interaction computational fluid dynamics to analyze the functional performance of the two taxa using 3D mesh models constructed from scanned fossils. The simulation experiments reveal that: (a) due to their millimeter‐scale size, most sedentary small shelly fossils (SSFs), including those of medusozoans lived within the viscous sublayer range of tidal flow; (b) the biradial symmetry of Hexaconularia was more structurally stable than the tetraradial symmetry of Quadrapyrgites; (c) different orientations of Hexaconularia, relative to the water flow, resulted in different force distributions on the surface. This research provides quantitative data of the hydrodynamic conditions of SSFs in shelf environments and critical insights into the biotic replacement of the early Cambrian benthic medusozoan community. Plain Language Summary: Computational fluid dynamics (CFD) method in recent 5 years has been proved to be an effective tool to address functional morphology of macrofossils. However, it has never been applied to microfossils, particularly those from the early Cambrian, which have played increasingly important roles in understanding the origin of animal phyla and stepwise establish of complex food web during the Ediacaran‐ Cambrian transitional period. In this study we examined the fluid dynamics of millimeter‐scale sedentary medusozoan fossils (Quadrapyrgites quadratacris and Hexaconularia sichuanensis) from the Cambrian Kuanchuanpu Formation in South Chian using 3D mesh models and computational fluid dynamics. The results reveal that even microscopic morphological characteristics (shapes, symmetry patterns and orientations) can lead to significant differences in hydrodynamic effects, which may exert further influence on their lifestyles and evolution. This research first introduces CFD to the field of early Cambrian microfossils and paves the way for more sophisticated simulations in the future, leading us to a better understanding of the role hydrodynamics played in the natural selection of early Cambrian sedentary benthos. Key Points: Hydrodynamics related with the viscous flow layer profoundly affected the Cambrian millimeter‐scale benthic medusozoansThe biradial symmetry is more adaptable to shelf environments than the tetraradial formThis could help to explain the biotic replacement of the early Cambrian benthic medusozoan community [ABSTRACT FROM AUTHOR]

Recently, subsalt exploration in the Tarim Basin has been one of the most significant targets of the deep hydrocarbon exploration.The Lower Cambrian Xiaoerbulake Formation is the major reservoir in current exploration.However, the analysis of the detailed geochemical and paleoenvironmental characteristics are limited in Keping area in absence of studies in the interior of the basin.This paper focuses on the Well Shutan-1 in the Bachu Uplift of the northwestern Tarim Basin.Based on the analysis of petrology and the geochemical characteristics (major, trace, rare elements and carbon-oxygen isotope), depositional environment of the Lower Cambrian Xiaoerbulake Formation is explored.Our study indicates that the lower, middle and upper part of the Xiaoerbulake Formation in Well Shutan-1 comprise thick algal dolomite, mudstone interbedded with dolomitic limestone and fine to silty dolomite, respectively.The average value of Lan/Ybn is 1.09 in the Xiaoerbulake Formation, of which the lower and upper part show positive Eu anomaly.Silica cementation, silica fragment and dissolution can be seen in the thin section, indicative of hydrothermal influence.Redox-sensitive trace elements are enriched in the middle of the formation.V/Cr and Ni/Co range from 2 to 4.25 and 5 to 7.The grain size of the sediment is finer.These are indications of suboxic bottom-water condition.The RSTEs and their ratios are relatively low in the upper and lower Xiaoerbulake Formation.The grain size of sediment is coarser.These are indications of oxic bottom-water condition.The carbon isotope ranges from -1.3‰ to 2.7‰. It decreases firstly and then increase throughout the interval.Two positive and one negative isotope excursion are identified in the lower and middle part of the interval, respectively.Paleoenvironmental reconstructions indicate that paleo-salinity firstly increased and then decreased.Paleo-seawater temperature gradually increased.Dolomite reservoirs were well developed in the late stage due to moderate salinity, occurrence of algal dolomite along with later weathering and dissolution.The research results can provide a basis for oil and gas exploration in this area.

A small skeletal fossil assemblage is described for the first time from the bioclastic limestone interbeds of the siltstone-dominated Guojiaba Formation, southern Shaanxi, China. The carbonate-hosted fossils include brachiopods (Eohadrotreta zhujiahensis, Eohadrotreta zhenbaensis, Spinobolus sp., Kuangshanotreta malungensis, Kyrshabaktella sp., Lingulellotreta yuanshanensis, Eoobolus incipiens, and Eoobolus sp.), sphenothallids (Sphenothallus sp.), archaeocyaths (Robustocyathus sp. and Yukonocyathus sp.), bradoriids (Kunmingella douvillei), chancelloriids sclerites (Onychia sp., Allonnia sp., Diminia sp., Archiasterella pentactina, and Chancelloria cf. eros), echinoderm plates, fragments of trilobites (Eoredlichia sp.), and hyolithelminths. The discovery of archaeocyaths in the Guojiaba Formation significantly extends their stratigraphic range in South China from the early Tsanglangpuian at least to the late Chiungchussuan. Thus, the Guojiaba Formation now represents the lowest known stratigraphic horizon where archaeocyath fossils have been found in the southern Shaanxi area. The overall assemblage is most comparable, in terms of composition, to Small skeletal fossil (SSF) assemblages from the early Cambrian Chengjiang fauna recovered from the Yu’anshan Formation in eastern Yunnan Province. The existing position that the Guojiaba Formation is correlated with Stage 3 in Cambrian Series 2 is strongly upheld based on the fossil assemblage recovered in this study.

Natural gas generation is the result of organic matter degradation under the effects of biodegradation and thermal degradation. Early Cambrian black shales in the Upper Yangtze Region are rich in organic matter and have shown great shale gas potentiality in recent years. Nevertheless, the enrichment mechanism and distribution of organic matter in these black shales between different sedimentary settings, such as intra-platform basin, slope, and deep basin, are still poorly understood. In this paper, based mainly on elemental geochemistry, a comprehensive study of the marine redox conditions, primary productivity, sedimentation rate, terrigenous input, hydrothermal activity, and water mass restrictions was conducted on the Early Cambrian Niutitang black shale in the Upper Yangtze Region. Our data showed that an intra-platform basin received a higher terrigenous input and that it deposited under more restricted conditions than the slope and deep basin settings. The primary productivity in the slope and deep basin settings was higher than that in the intra-platform basin setting. In the intra-platform basin, the productivity increased from its inner part to its margin. For the slope and deep basin settings, the high paleoproductivity generated large amounts of organic matter and its preservation was synergistically affected by the redox conditions. In contrast to the slope and deep basin, the preservation of organic matter in the inner part of the intra-platform basin was mainly controlled by redox conditions because the paleoproductivity in it was much lower than in the slope and deep basin settings. The intra-platform basin margin was the most favorable area for accumulating organic matter. [ABSTRACT FROM AUTHOR]

The geochemistry of rare earth elements and yttrium (REY) in phosphorite has been widely studied. However, the effect of organic matter on REY enrichment has not been well determined. We utilized paired inorganic (δ13Ccarb) and organic (δ13Ccarb) carbon isotopes, total organic carbon (TOC), and REY content (∑REY) of the Zhijin Motianchong (MTC) phosphorite and compared them with those of Meishucun (MSC) phosphorite to reveal the effect of organic matter on REY. The δ13Ccarb of the MTC area (≈0‰) is heavier than that of the MSC area (−5.23‰ to −1.13‰), whereas δ13Corg is lighter (−33.85‰ to −26.34‰) in MTC than in MSC (−32.95‰ to −25.50‰). Decoupled δ13Ccarb and δ13Corg in MTC indicate the contribution of chemoautotrophic organisms or methanotrophic bacteria. Compared to the MSC phosphorite, the MTC phosphorite has higher ∑REY and TOC, and these parameters have a positive relationship. MTC phosphorite has REY patterns resembling those of contemporary organic matter. Furthermore, dolomite cement has a higher ∑REY than dolomite in the phosphorus-bearing dolostone. Additionally, pyrites are located on the surface of fluorapatite in the Zhijin phosphorites. It is reasonable to suggest that the REY was released into the pore water owing to the anaerobic oxidation of organic matter at the interface between seawater and sediment, resulting in the REY enrichment of Zhijin phosphorites. [ABSTRACT FROM AUTHOR]

[Display omitted] • Direct evidence of biological interactions in the early Cambrian. • Brachiopods dominated a vertically stratified benthic community. • In situ preservation of different phyla that fill multiple ecological niches. • An original benthic community preserved in life position from the Guanshan Biota. The Cambrian radiation is characterized by the emergence of diverse bilaterian animal phyla and the establishment of complex marine ecosystems. The Guanshan Biota records an unusual ecological transition from trilobite- to brachiopod-dominated communities during Cambrian Stage 4. This community transition is accompanied by direct evidence of in situ biological interactions such as durophagous predation and kleptoparasitism. Here we describe new material from the Guanshan biota, focusing on an association of palaeoscolecidomorphs and brachiopods with parasitic tube worms that occur on micro-bedding planes. The bedding plane assemblages are dominated by the organophosphatic brachiopod Neobolus wulongqingensis encrusted with kleptoparasitic tube-dwelling worms, along with infaunal palaeoscolecidans. Taphonomic and sedimentological evidence indicates that these specimens are commonly preserved in life position, and thus the association between individuals represent potential biological interactions. This case study reveals that ecosystems during the early Cambrian exhibited a well-developed system of tiering and a complex trophic network, easily distinguished from the simple communities typical of precursor deposits in the Ediacaran. Brachiopods forming extremely dense concentrations on the sea floor are effectively acting as ecosystem engineers, not only to stabilize the soft-substrate seafloor, but also act as an alternative substrate for the oldest empirically demonstrated kleptoparasites. The in situ biological interactions preserved in the Guanshan Biota are critical for filling gaps in our knowledge of ecosystem complexity in the Cambrian. [ABSTRACT FROM AUTHOR]

Copyright of Acta Sedimentologica Sinica is the property of Acta Sedimentologica Sinica Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

During the Late Neoproterozoic and Early Cambrian, a series of paleogeographic and paleoecological events occurred associated with deglaciations and the evolution of life. The appearance of fossils representing diverse phyla, novel body plans and complex ecologies in Ediacaran and Cambrian sedimentary successions has sparked diverse hypotheses about potential drivers for the radiation of early animals during this interval. Recently, new macroscopic fossils of carbonaceous compressions with unique features have been found in Anning, Yunnan, China. The fossils’ bodies are ribbon-shaped and bilaterally symmetric, with dense longitudinal features and transversal features. The fossils occur 0.68 m below a bentonite interlayer which has been dated 535.2 ± 1.7 Ma by Ri-xiang Zhu and his team in 2009. The relatively simply morphology of these fossils, coupled with a lack of preserved internal structures challenge efforts taxonomically identify the precursor organism and definitively ally it to a living group. However, the symmetry and unusual features of the body are analogous to members of Platyzoa. The presence of ribbon-shaped fossils in the Zhongyicun Member in Anning indicates that these organisms were at least a locally significant component of Cambrian seafloor ecosystems, and may hold important implications for our understanding of the early evolution of Bilateria.

The early Cambrian witnessed profound environmental changes and biological evolution in Earth’ history. During this period, organic-rich shales were widely distributed over almost the entire Yangtze Block. However, the dominant factor that drove the significant accumulation of organic matter (OM) remains controversial and is still debated. Here, we analyzed TOC, organic carbon isotopes, iron speciation, major and trace elements for the lower Cambrian Niutitang Formation in the upper slope Meiziwan section, to investigate the dominant factor controlling OM accumulation. High contents of TOC and Baxs reveal an OM-enriched feature of the Niutitang Formation, and the coupled relationship between them suggest a strong production control on OM accumulation at Meiziwan. Meanwhile, negative relationships between TOC and chemical index of alteration (CIA) values as well as Al contents suggest that influence of chemical weathering and terrestrial input on OM accumulation were limited. Fairly low CoEF × MnEF values provide strong evidence that the deposition of organic-rich shales was under the control of oceanic upwelling event. The upwelling event would bring nutrient-rich deep waters into surface water, stimulating phytoplankton bloom and primary productivity in surface water and facilitating OM enrichment. Meanwhile, enhanced accumulation of OM would have promoted subsequent bacterial sulfate reduction, leading to the occurrence of occasional euxinia (evidenced by iron speciation and redox-sensitive trace element data) and promoting preservation of OM. Taken together, our results shed light on the critical role of oceanic upwelling on the marine primary productivity on the earliest Cambrian Yangtze Platform.

Copyright of Journal of Northeast Petroleum University is the property of Journal of Northeast Petroleum University Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The chemical composition of sponge spicules was studied based on the debatable fossil early Cambrian sponge Lenica unica Gorjansky, 1977 from the Siberian Platform. The study has shown that the skeleton of this giant sponge is built of large monaxon megascleres that are a natural carbonate–silica–organic compound with a complicated chemical composition. The data we obtained confirm that the earliest early Cambrian sponges formed skeletal elements of a mixed carbonate–biosilica composition. The authors suggest that this skeleton composition was the initial for all spiculate sponges. [ABSTRACT FROM AUTHOR]