Silurian Research Articles

Evolution of volcanism and geodynamic settings in the Sakmara zone of the Southern Urals in the early Paleozoic

Research subject. Northern part of the Sakmara zone of the Southern Urals, represented by the Krakinsko-Mednogorsk paleovolcanic belt, including the Mednogorsk ore district and the Blavinsko-Komsomolskoye ore field. Aim. To reconstruct the paleogeodynamic settings during the Cambrian–Early Devonian in the Sakmara zone. Methods. An analysis of geological, petrogeochemical, structural-tectonical, and paleovolcanological data obtained by the authors and those reported in literature using conventional and new petrogeochemical diagrams. Results. The following stages of tectono-magmatic evolution in the Sakmara zone were distinguished: (1) Cambrian–Early Ordovician, continental rifting; (2) Early–Middle Ordovician, oceanic spreading; (3) Early Silurian, suboceanic rifting, serpentinite protrusions, edaphogenic breccias, siliceous rocks (starting a subduction zone); (4) Late Silurian–Early Devonian, island-arc basalt-rhyolite volcanism and sulfide deposits, volcanism of the shoshonitic and alkaline series, formation of the rear island arc. The Sakmara zone features no analogues of boninite volcanism (D1e2) typical of the West Magnitogorsk zone, which indicates the absence of signs of the volcanogenic strata of the Voznesensk–Prisakmar zone being pushed into the Sakmara structural zone. Conclusions. (1) In the Cambrian–Early Devonian, volcanogenic and volcanogenic-sedimentary formations and strata of the Sakmara zone formed a megacycle, starting with the processes of continental and oceanic rifting, which led to the subduction process and generation of the frontal and rear island arcs. (2) The volcanism in the Sakmara and Voznesensk–Prisakmar zones developed autonomously, without tectonic transfers from east to west. (3) The partial coincidence of the formation age of the Chanchar (D1lh–e1) complex of the Sakmara zone and the Turin complex (S2p–D1lh) of the Tagil zone suggests the presence of a reduced continuation of the western wing of the Tagil zone in the Sakmara zone.

Subduction retreat recorded in the southern Beishan orogenic collage of the Central Asian Orogenic Belt: New evidence from deep seismic reflection profiling

The crustal and upper mantle structure of the Beishan orogenic collage, which serves as the southern part of the middle Central Asian Orogenic Belt, provides crucial insights into the history of the multiple openings and closings of the Paleo-Asian Ocean during the Paleozoic. There is considerable dispute over the eventual closure position, timing, and subduction polarity of the Paleo-Asian Ocean, particularly in the southern Beishan orogenic collage. The main cause of these controversies is the lack of a high-resolution lithospheric structure in this area. In this study, we first present a 140-km-long, high-resolution seismic reflection profile taken across the northern Dunhuang Block and the southern Beishan orogenic collage. The seismic imaging provides new constraints on the structure of the lower crust, the Moho, and the upper mantle beneath the southernmost Central Asian Orogenic Belt. A subhorizontal reflector in the middle crust, two sets of north-dipping reflectors from the lower crust to the upper mantle, and several south-dipping reflectors in the upper crust in the northern part of the profile were imaged. Based on our study and other geological, chronological, and geophysical data, we propose that the two sequences of north-dipping reflectors from the lower crust to the upper mantle represent two stages of north-dipping subduction of the southern branch of the Paleo-Asian Ocean. The first stage in the southern Beishan orogenic collage is Late Silurian−Early Devonian, and the second stage is late Carboniferous−Early Permian. The two-stage subduction process gave rise to the Huaniushan arc and the subsequent Shibanshan arc, respectively. These findings provide new constraints on the controversial subduction polarity and the multistage amalgamation of the microcontinental blocks and arcs in the southern Central Asian Orogenic Belt.

Geochemical and Sr-Nd isotopic implications for the petrogenesis of the late Silurian Shitoukengde mafic–ultramafic intrusion in the East Kunlun Orogen, NW China

The Shitoukengde mafic–ultramafic intrusion is the host of the second-largest sulfide deposit after the Xiarihamu Ni-Co deposit in the East Kunlun Orogenic Belt (EKOB), northern Tibet Plateau, China. Despite several studies, the age, petrogenesis, and the cause of low Ni-tenor for this intrusion remain poorly constrained. In this study, zircons separated from the pyroxenite at Shitoukengde yield a SHRIMP U-Pb age of 418.9 ± 3.1 Ma, corresponding to the widespread magmatism of the late Silurian to early Devonian in the EKOB. Whole-rock major and trace element compositions indicate that fractional crystallization played a key role in controlling the magma composition and element distribution within the intrusion. Mafic-ultramafic rocks of the intrusion, particularly peridotite, have highly variable and exceptionally elevated (87Sr/86Sr)i and negative ɛNd(t) values. Some samples from the Shitoukengde intrusion exhibit initial Sr-Nd isotope ratios that overlap with those from the Xiarihamu intrusion, while others (e.g., peridotite) display higher (87Sr/86Sr)i values than those observed in the latter. The unusual Sr-Nd isotopic compositions of the Shitoukengde intrusion could be attributed to the assimilation of Mg-rich carbonate within a deep-seated magma chamber. This contamination process facilitates the crystallization of olivine, consequently reducing Ni content in residual magma. Furthermore, the contamination of Mg-rich carbonate may promote oxygen fugacity and thus enhance the solubility of sulfur while restricting the sulfide saturation in the magma. We thus propose that the extensive contamination of Mg-rich carbonate is a key factor contributing to the relatively low Ni-tenor observed at Shitoukengde.

The southwesternmost record of late Silurian (Pridolian) early land plants of Gondwana.

The Silurian-Devonian transition played a crucial role in the development of early terrestrial ecosystems due to the rapid diversification of early vascular plants. However, records of Pridolian plants in western Gondwana are scarce, limited to outcrops located in southern Bolivia. In this contribution, an association of fossil plants housed in the Rinconada Formation is presented. This association corresponds to primitive fossil flora with reproductive structures and sterile axes linked to basal tracheophytes. The fossil assemblage is composed of Aberlemnia caledonica, Caia langii Cooksonia cf. cambrensis, C. paranensis, C. cf. pertoni, Hostinella sp, Cf. Isidrophyton sp, Salopella marcensis, Steganoteca striata, two morphotypes of doubtful taxonomy, and graptolites colonies. The association between flora remains and graptolites, represents a parautochthonous assemblage in an inner marine platform, dominated by gravity flows. This record has paleophytogeographic importance indicating the extension of the northwest Gondwana-southern Laurusia unit to more southern areas of Gondwana. This expansion would have been favored by the post-glacial climatic improvement of the Late Silurian, together with a great radiation capacity and environmental flexibility of the flora. Furthermore, the biochron is extended of three taxa (A. caledonica, C. paranensis and Cf. Isidrophyton sp) first known from the Lochkovian, to the Pridoli.

Alteration Zones as Exploration Parameters for Stratiform Pb-Zn-Ag-Ba Deposits in the Graz Paleozoic (Eastern Alps, Austria)

AbstractIn the Graz Paleozoic in the Eastern Alps (Austria), stratiform Pb-Zn-Ag-Ba deposits have been mined in the past. They are classified as sedimentary-exhalative (SEDEX) deposits and are hosted by polyphase deformed greenschist facies metasediments and volcanics of the Schönberg Formation (Schöckel nappe; Drauzug-Gurktal nappe system), which is upper Silurian to Lower Devonian in age. The stratiform ore horizons are dominated by galena, sphalerite, barite, pyrite, pyrrhotite, and magnetite and are accompanied by chalcopyrite, arsenopyrite, fahlore, pyrargyrite, tetradymite, cobaltite, ullmannite, breithauptite, electrum, and others. The project MRI_SEDEXPOT investigates these deposits to evaluate their exploration potential by focusing on hydrothermal alteration zones occurring in the footwall and hanging wall of the ore. Such alteration zones are globally used as exploration tools not only for SEDEX deposits. The investigated alteration zones show distinct mineralogical and geochemical characteristics. They contain Fe rich carbonates and chlorite, K‑ and Ba-feldspars, Ba bearing white mica, fluorapatite, REE and HFSE minerals. Disseminated sulphides are widespread, and albitization is typical below sulphide rich horizons. Geochemical profiles correlate well with the observed mineralogy. Ongoing investigations of these alteration zones should lead into defining proximity indicators to the ore and evaluation of the exploration potential of the stratiform Pb-Zn-Ba-Ag deposits of the Graz Paleozoic.

Benthic response to the strong Silurian climatic fluctuations—implications from Gotland (Sweden)

Climatically the Silurian was a most unstable period, as it is shows several strong δ13C excursions, with the Lau excursion being the strongest short-lived positive δ13C excursion of the entire Phanerozoic. The causes of these excursions remain a topic of debate. In order to determine how benthic communities responded to these climatic perturbations, a microfacies analysis of carbonate rocks from the Silurian period (Late Telychian to the uppermost Ludfordian) of Gotland was conducted. The development of reefs was significantly enhanced during periods characterized by carbon isotope excursions, compared to time intervals without. In periods of elevated δ13C values (≥ 3 ‰) the abundance of bryozoans and microbes increases, and in some areas, they form reefs (bryostromatolites), while corals and stromatoporoids were the main reef builders in periods of lower isotope ratios (0–3 ‰). Furthermore, a significant increase of phosphatic components, gypsum pseudomorphs and the microproblematicum Palaeomicrocodium is observed in times of increased isotope ratios. A model is proposed that explains the changes in the benthic communities over time. It is based on previously published models and explains the geochemical processes from coastal areas to the open ocean. Changes are assumed from a humid climate in times without isotope excursions to an arid climate in times of isotope excursions. It emphasizes processes that may have led to the strong δ13C shifts during the Silurian period, and includes the phosphorus cycle in sea water as an important factor in the development of the benthic communities.

The Silurian and the Devonian in the Svoge Unit, Western Bulgaria – a brief overview

Five stops, containing Silurian–Devonian strata of the Svoge Unit (Western Bulgaria) are described. At Stop 1 near the village of Tsarichina, Upper Devonian flysch (Katina Formation) can be observed, which ends the Paleozoic marine sedimentation in the area. The other two Devonian formations are observable at stop 2 near the village of Vlado Trichkov. These are the black shales of the Ogradishte Formation and the light shales of the Romcha Formation. At the village of Tseretsel (stop 3), all Silurian units are observed: the bedded cherts (lydites) of the Saltar Formation, the graptolite-bearing black shales of the Mala Reka Formation, and the grey to gray-green laminated shales of the Yabukov Dol Formation. The boundaries of the Silurian System, based on graptolites, were established at the next two stops, respectively at stop 4 the Silurian/ Devonian boundary and at stop 5 the Ordovician/Silurian boundary.

Improved pore structure characterization and classification of strong diagenesis sandstones by data-mining analytics in Tazhong area, Tarim Basin.

The Silurian system in Tazhong area is characterized by extensive, low-abundance lithological reservoirs with strong diagenesis, resulting in significant heterogeneity. The complex pore structure in this area significantly impacts fluid control, making accurate characterization and classification of pore structures crucial for understanding reservoir properties and their influence on oil and gas distribution. Based on 314 Mercury Injection Capillary Pressure (MICP) samples in combination with core slices and thin casting slices observation, a pipeline of characterization and classification scheme by data-mining analytics of strong diagenesis sandstone pore structure types in the study zone is established, and the characteristics of different pore structures are clarified. According to the pore structure parameter abstracted by MICP data compression and variable analysis based on hierarchical clustering and principal component analysis (PCA) analysis, the variables are reasonably evaluated and screened, and the screened variables can be divided into three groups: mean pore throat radius-maximum pore throat radius-median pore throat radius-pore throat diameter mean variable group, microscopic mean coefficient variable group, and median pressure displacement pressure-relative sorting coefficient variable group. The combination of classification schemes analysed by decision tree model and linear discriminant analysis (LDA) model was determined. In the two-dimensional projection diagram of LDA model, a relatively obvious distribution of low displacement pressure, middle displacement pressure and high displacement pressure was obtained, and three distribution lines were nearly parallel. Based on the relevant information, 6 combined classification schemes suitable for final pore structure modelling were determined verified by microscopic observation. The correct characterization and classification of pore structure can be applied to the prediction of pore type, which can be used to improve the prediction of oil and gas distribution and oil and gas recovery in the future.

A new species of Mucophyllum rugose coral encrusted by bryozoans, tentaculoid tubeworms, and tabulates from the upper Silurian of Saaremaa, Estonia

A new species of Mucophyllum rugose coral encrusted by bryozoans, tentaculoid tubeworms, and tabulates from the upper Silurian of Saaremaa, Estonia

The making of a nucleic acid sensor at the dawn of jawed vertebrate evolution.

Self and nonself discrimination is fundamental to immunity. However, it remains largely enigmatic how the mechanisms of distinguishing nonself from self originated. As an intracellular nucleic acid sensor, protein kinase R (PKR) recognizes double-stranded RNA (dsRNA) and represents a crucial component of antiviral innate immunity. Here, we combine phylogenomic and functional analyses to show that PKR proteins probably originated from a preexisting kinase protein through acquiring dsRNA binding domains at least before the last common ancestor of jawed vertebrates during or before the Silurian period. The function of PKR appears to be conserved across jawed vertebrates. Moreover, we repurpose a protein closely related to PKR proteins into a putative dsRNA sensor, recapturing the making of PKR. Our study illustrates how a nucleic acid sensor might have originated via molecular tinkering with preexisting proteins and provides insights into the origins of innate immunity.

Novel pterygotid sea scorpions from the Silurian and Devonian of Gondwana

Sea scorpions (eurypterids) are a group of extinct aquatic chelicerates that have a fossil record spanning the Ordovician through to the end Permian extinction. Due to their size and preservational potential, eurypterids have been well-documented in European and North American deposits, with less evidence from Gondwana. There is particularly limited evidence of the group from the Australian fossil record. To rectify this, we report new examples of pterygotid eurypterids from the late Silurian Wallace Shale and Early Devonian Merrimerriwa Formation, Mulga Downs Group in New South Wales. These specimens are assigned to Pterygotus sp. (Wallace Shale) and Jaekelopterus c.f. rhenaniae (Merrimerriwa Formation, Mulga Downs Group). The material is used to explore the extensive distribution and dispersal of these large, pelagic eurypterids and consider the position of the animals in their respective paleoecosystems. We conclude that these new records evidence the diversity of larger arthropods in Australian deposits and that ongoing documentation of these rare fossils is required to further understand the Paleozoic of Australia.

Solid bitumen Rhenium-Osmium (Re–Os) isotope geochronology and existing problems: Sampled of Sinian-Cambrian gas reservoir in Sichuan Basin, China

Understanding the key timings related to petroleum evolution is crucial for optimizing exploration targets and assessing oil/gas resources, attract petroleum geologists’ attention worldwide. Recently, hydrocarbon (oil and bitumen) Re–Os isotope dating has been innovatively applied to constrain the timing related to oil/gas generation, however, the resulting Re–Os isochron ages can be complex and challenging to interpret. This study utilizes various geochemical and geochronological data from Sinian to Cambrian natural gas reservoirs in the Sichuan Basin to reconstruct the hydrocarbon evolution process and discuss the significance of different bitumen Re–Os dating results. The gas accumulation in the Sinian-Cambrian reservoirs experienced four stages of evolution: (1) initial oil generation during the Ordovician to Silurian periods, (2) secondary oil generation during the Triassic period, (3) gas generation through thermal cracking of liquid oil from the Jurassic to Cretaceous periods, and (4) gas reservoir redistribution since the late Cretaceous. The Re–Os dates (ca. 485 Ma) of low maturity and biodegraded bitumen from the western Sichuan Basin record the oil generation during the Ordovician before the Caledonian tectonic event. The Re–Os dates (ca.184–128 Ma) of highly mature bitumen associated with MVT Pb–Zn deposits in northern Sichuan Basin provide insights into both liquid oil-cracking and thermochemical sulfate reduction (TSR) processes. The complex Re–Os dates (ca.414 Ma, ca.154 Ma) of highly mature bitumen from the central Sichuan Basin may represent different periods related to either oil or gas generation. Future studies should explore the genetic type, maturity, thermal cracking, or TSR degrees of bitumen to better understand the significance of Re–Os dates.

Amoebozoan testate amoebae illuminate the diversity of heterotrophs and the complexity of ecosystems throughout geological time

Heterotrophic protists are vital in Earth's ecosystems, influencing carbon and nutrient cycles and occupying key positions in food webs as microbial predators. Fossils and molecular data suggest the emergence of predatory microeukaryotes and the transition to a eukaryote-rich marine environment by 800 million years ago (Ma). Neoproterozoic vase-shaped microfossils (VSMs) linked to Arcellinida testate amoebae represent the oldest evidence of heterotrophic microeukaryotes. This study explores the phylogenetic relationship and divergence times of modern Arcellinida and related taxa using a relaxed molecular clock approach. We estimate the origin of nodes leading to extant members of the Arcellinida Order to have happened during the latest Mesoproterozoic and Neoproterozoic (1054 to 661 Ma), while the divergence of extant infraorders postdates the Silurian. Our results demonstrate that at least one major heterotrophic eukaryote lineage originated during the Neoproterozoic. A putative radiation of eukaryotic groups (e.g., Arcellinida) during the early-Neoproterozoic sustained by favorable ecological and environmental conditions may have contributed to eukaryotic life endurance during the Cryogenian severe ice ages. Moreover, we infer that Arcellinida most likely already inhabited terrestrial habitats during the Neoproterozoic, coexisting with terrestrial Fungi and green algae, before land plant radiation. The most recent extant Arcellinida groups diverged during the Silurian Period, alongside other taxa within Fungi and flowering plants. These findings shed light on heterotrophic microeukaryotes' evolutionary history and ecological significance in Earth's ecosystems, using testate amoebae as a proxy.

Improved pore structure prediction based on a stacking machine learning model for low-permeability reservoir in Tazhong area, Tarim Basin

Stacking is an ensemble machine learning method designed to improve overall performance by combining the predictions of multiple base learners. The core idea of Stacking is to use the output of different base learners as input and make final predictions through a meta-learner, allowing the model to benefit from the strengths of various base learners and adapt better to complex data patterns. Predicting pore structures in low permeability is a sophisticated nonlinear modeling issue affected by multiple factors, including sedimentary and diagenetic effects on the structure. An individual model to predict pore structure is not satisfactory. In this paper, we applied a fusion of random forest and Adaboost models as base learners, which emerges as the cornerstone of this research, showcasing remarkable versatility and integration of diverse learner strengths. The Silurian system in Tazhong area features a vast expanse of lithological reservoirs with low abundance and high heterogeneity. Capillary pressure plays a crucial role in the distribution of these less permeable reservoirs by pore structure type influence. Aiming to predict the pore structure for the low-permeability reservoir in the field, which is characterized by low abundance and high heterogeneity, the study strategically integrates geological theory into specialized databases and harnesses machine learning methods. The resulting machine learning modeling dataset leverages geological factors and logging response characteristics to strengthen the ensemble machine learning stacking model. This sophisticated model not only achieves exceptional accuracy but also undergoes rigorous validation by analyzing predicted pore structure types alongside production data. The model's average accuracy was 79.4%, which practically hit the test set's accuracy threshold, meaning that the model was in an optimal state. In essence, the proficiency of the stacking model in predicting pore structure types serves as a transformative force, offering significant advancements in hydrocarbon distribution predictions and paving the way for more streamlined and effective future EOR activities.

The oldest ‘millipede’-plant association? Age, paleoenvironments and sources of the Silurian lake sediments at Kerrera, Argyll and Bute, Scotland

ABSTRACT On Kerrera island, poorly dated braided stream conglomerates and sandstones are interbedded with finer lake siltstones and shales, with land plants, fish and arthropods, including ‘millipedes’. Crystallization dates for minerals in the overlying Lorne Plateau Lave are older than 425 Ma (Upper Silurian Ludfordian), yet palynological data are supposed to give an early Devonian (419 Ma) age. Detrital zircon U/Pb dates of sediments enclosing the fish beds give two clusters of dates, one Proterozoic and the other Silurian. The most concordant youngest single grain date of 427.0 ± 4.5 Ma (0% discordance) gives the maximum age of the Fish beds and is consistent that the radiometric dates of the overlying rapidly erupted lavas. This maximum age is slightly older than that of the ‘millipede’-bearing Cowie fish bed with maximum age of 414.3 ± 7.1 Ma, which make the Kerrera ‘millipedes’ the oldest so far dated.

Structure, Evolution, and Mitochondrial Genome Analysis of Mussel Species (Bivalvia, Mytilidae).

Based on the nucleotide sequences of the mitochondrial genome (mitogenome) of specimens taken from two mussel species (Arcuatula senhousia and Mytilus coruscus), an investigation was performed by means of the complex approaches of the genomics, molecular phylogenetics, and evolutionary genetics. The mitogenome structure of studied mussels, like in many other invertebrates, appears to be much more variable than in vertebrates and includes changing gene order, duplications, and deletions, which were most frequent for tRNA genes; the mussel species' mitogenomes also have variable sizes. The results demonstrate some of the very important properties of protein polypeptides, such as hydrophobicity and its determination by the purine and pyrimidine nucleotide ratio. This fact might indirectly indicate the necessity of purifying natural selection for the support of polypeptide functionality. However, in accordance with the widely accepted and logical concept of natural cutoff selection for organisms living in nature, which explains its action against deleterious nucleotide substitutions in the nonsynonymous codons (mutations) and its holding of the active (effective) macromolecules of the polypeptides in a population, we were unable to get unambiguous evidence in favor of this concept in the current paper. Here, the phylogeny and systematics of mussel species from one of the largest taxons of bivalve mollusks are studied, the family known as Mytilidae. The phylogeny for Mytilidae (order Mytilida), which currently has no consensus in terms of systematics, is reconstructed using a data matrix of 26-27 mitogenomes. Initially, a set of 100 sequences from GenBank were downloaded and checked for their gender: whether they were female (F) or male (M) in origin. Our analysis of the new data confirms the known drastic differences between the F/M mitogenome lines in mussels. Phylogenetic reconstructions of the F-lines were performed using the combined set of genetic markers, reconstructing only protein-coding genes (PCGs), only rRNA + tRNA genes, and all genes. Additionally, the analysis includes the usage of nucleotide sequences composed of other data matrices, such as 20-68 mitogenome sequences. The time of divergence from MRCA, estimated via BEAST2, for Mytilidae is close to 293 Mya, suggesting that they originate in the Silurian Period. From all these data, a consensus for the phylogeny of the subfamily of Mytilinae and its systematics is suggested. In particular, the long-debated argument on mussel systematics was resolved as to whether Mytilidae, and the subfamily of Mytilinae, are monophyletic. The topology signal, which was strongly resolved in this paper and in the literature, has refuted the theory regarding the monophyly of Mytilinae.

Middle Silurian–Middle Devonian Magmatic Rocks in the Eastern Segment of the Northern Margin of the North China Craton: Implications for Regional Tectonics

This paper presents a detailed study including LA-ICP-MS zircon U-Pb dating, geochemical, zircon Hf isotope, and whole rock Sr-Nd isotope analysis of magmatic rocks from the Yitong County, Jilin Province, NE China. These data are used to better constrain the Middle Silurian–Middle Devonian tectonic evolution in the eastern segment of the northern margin of the North China Craton (NCC). Zircon U-Pb dating results show that the Ximangzhang tonalite formed in the Late Silurian (425 ± 6 Ma); the basalt, andesite, and metamorphic olivine-bearing basalt in the Fangniugou volcanic rocks formed in the Middle Silurian (428 ± 6.6 Ma) and Middle Devonian (388.4 ± 3.9 Ma, and 384.1 ± 4.9 Ma). The Late Silurian tonalites are characterized by high SiO2 and Na2O and low K2O, MgO, FeOT, and TiO2, with an A/CNK ratio of 0.91–1.00, characteristic of calc-alkaline I-type granite. They are enriched in Rb, Ba, Th, U, and K, and depleted in Nb, Sr, P, and Ti, with positive εNd(t) (+0.35) and εHf(t) (+0.44 to +6.31) values, suggesting that they mainly originated from the partial melting of Meso–Neoproterozoic accretionary lower crustal material (basalt). The Middle Silurian basalts are characterized by low SiO2, P2O5, TiO2, and Na2O and high Al2O3, FeOT, and K2O, enriched in Rb, Ba, Th, U, and K and depleted in Nb, Ta, Sr, P, and Ti, indicative of shoshonitic basalt. The Late Silurian tonalites have positive εNd(t) (+4.91 to +6.18) values and primarily originated from depleted mantle magmas metasomatized by subduction fluids, supplemented by a small amount of subducted sediments and crustal materials. The Middle Devonian volcanic rocks exhibit low SiO2, TiO2, and Na2O and high K2O, and MgO, enriched in Rb, K, and LREEs and depleted in Nb, Ta, Sr, and HREEs, characteristic of shoshonitic volcanic rocks. Their εNd(t) (+2.11 to +3.77) and εHf(t) (+5.90 to +11.73) values are positive. These characteristics indicate that the Middle Devonian volcanic rocks primarily originated from depleted mantle magmas metasomatized by subduction fluids, with the addition of crustal materials or subducted sediments during their formation. Based on regional geological data, it is believed that the study area underwent the following evolutionary stages during the Silurian–Devonian period: (1) active continental margin stage of southward subduction of the Paleo–Asian Ocean (PAO) (443–419 Ma); (2) arc-continent collision stage (419–405 Ma); (3) post-collision extension stage (404–375 Ma); (4) active continental margin stage, with the PAO plate subducting southward once again (375–360 Ma).

Zinc isotopic evidence for enhanced continental weathering and organic carbon burial in the Early Silurian

The Late Ordovician and Early Silurian periods experienced glaciation and mass extinctions. However, debates exist regarding the factors that initiated the glaciation and the subsequent delay in biotic recovery. To uncover the relationships between paleoclimate, marine redox states, and biotic recovery in the Early Silurian, temporal variations in Zinc (Zn) isotopic compositions along with other multi-geochemical proxies were analyzed for a carbonate-dominated section in South China. A stratigraphic δ66Zn trend was found in studied section, indicating a widespread disturbance to the marine Zn cycle. A significant negative shift in δ66Zn by up to ∼0.7 ‰ was observed immediately following the Hirnantian Glaciation and HICE, which can be attributed to a combination of reduced burial efficiency of organic carbon, fast weathering of large igneous provinces and increased riverine Zn input. This was followed by a ∼ 0.4 ‰ rise in δ66Zn values, alongside a sharp decrease in δ13C values, indicating an increase in sulfide precipitation (e.g., ZnS, FeS) due to expansions of euxinia. Subsequently, a notable rise in δ66Zn values by up to ∼1.1 ‰ is interpreted as an increase in organic carbon burial, given the concurrent variation of δ66Zn and δ13C during this period. The findings suggest that the increase in organic carbon burial, a reduction in reverse weathering, or a combination of them contributed to the glaciation in Silurian. Furthermore, the weathering-induced high primary productivity led to anoxic water conditions in the latest Hirnantian and Rhuddanian. This prolonged marine anoxia in seawater thus caused the deposition of organic-rich sediments and hindered biotic recovery in the Rhuddanian.

Palaeoscolecids from the Ludlow Series of Leintwardine, Herefordshire (UK): the latest occurrence of palaeoscolecids in the fossil record

AbstractThe documentation of cuticular micro‐ornament is vital for the taxonomic assignment of palaeoscolecids: vermiform lower Palaeozoic ecdysozoans interpreted as stem‐group priapulans or early diverging panarthropods. This is due to the absence of the character‐rich proboscis and tail hooks in palaeoscolecid material not from Burgess Shale‐type Konservat‐Lagerstätten. Here, the cuticular micro‐ornamentation of palaeoscolecids from the upper Silurian (Ludlow) fauna of Leintwardine (Herefordshire, England), is described using scanning electron microscopy and reflectance transformation imaging. This material is taxonomically unstable because it was included in an effective wastebasket genus (Protoscolex) long before these imaging techniques were developed. The Leintwardine material is shown to be most closely comparable to a palaeoscolecid from the Darriwilian (Middle Ordovician) of the Builth–Llandrindod inlier, Powys, Wales, and is transferred accordingly to Radnorscolex Botting et al. as Radnorscolex latus (Bather). The Leintwardine fauna represents the uppermost stratigraphic occurrence of palaeoscolecids, constrained to the Saetograptus leintwardinensis Zone (lower Ludfordian), and the comparatively sparse Silurian palaeoscolecid record is subsequently discussed. It is hypothesized that palaeoscolecids may have become extinct during the mid‐Ludfordian Lau Event, the onset of which is recorded in the biozone immediately above the Leintwardine fauna (Bohemograptus Zone). Finally, the British palaeoscolecid fauna is summarized, including a new record from the Dapingian (Middle Ordovician) of Carmarthenshire, South Wales.

Jaymoytius: A vegetarian Silurian lamprey?

Jamoytius kerwoodi was a primitive, eel‐like jawless fish that lived in the Early Silurian period (444–433 myr) and lived over a stagnant bottom environment, at the transition from a marine‐influenced, probably brackish‐water, deep‐water basin to a shallower water, less saline and probably freshwater basin. Jamoytius lacked teeth and was most likely a free‐living herbivore grazing on floating plants, as were possibly many of its euphanaropid relatives. The palaeoenvironment in which it lived compares well with that of living lampreys, especially as their ectoparasitic mode of life may have evolved from ancestral grazers.