Early Cambrian Research Articles

Elliptical body fossils from the Fortunian(Early Cambrian) of Normandy (NW France)

Body fossils have been discovered in the Fortunian deposits of the Rozel Cape, in Normandy (NW France). The material consists of about 80 specimens preserved on a shale surface, recently observed at the base of a cliff at the Cap Rozel, in the Cotentin region. The fossils, centimetric in size, have an elliptical outline, with a peripheral bulge, generally without other conspicuous ornamentation, but showing sometimes concentric or radial lines possibly of taphonomic origins. In addition, these body fossils are preserved parallel to the bedding plane, locally rich in horizontal trace fossils (e.g. Archaeonassa Fenton & Fenton, 1937, Helminthoidichnites Fitch, 1850, Helminthopsis Heer, 1877) and also complex treptichinids burrows (e.g. Treptichnus pedum (Seilacher, 1955)) sometimes associated with microbial mats. The sedimentological characteristics of these deposits (ripple marks, syneresis cracks) correspond to a shallow marine shelf environment, with a variable hydrodynamism in the intertidal zone, low for surfaces showing elliptic fossils and syneresis cracks, higher for surfaces with ripple marks. These new discoveries unravel the potential of the Fortunian strata from Normandy and provide new information about the early Cambrian biocenoses.

Occurrence of Tannuolina in the early Cambrian siltstones of the Hazara Basin, North Pakistan, and its biostratigraphical significance

The tommotiids are an important group of Cambrian small shelly fossils, primarily retrieved from carbonate rock by acid dissolution. In this study, abundant isolated sclerites of Tannuolina are recovered from the siltstone-dominated upper part of the Hazira Formation in the eastern Hazara Basin, North Pakistan. This discovery of tommotiids preserved in the siltstones not only suggests their occurrence in a variety of sedimentary lithofacies, but also provides the opportunity to look for the sclerites or scleritomes (or even soft tissue) of tommotiids in much wider taphonomic windows. The sclerites include two morphs (i.e. mitral and sellate types). Through morphological comparison, they can be identified as Tannuolina zhangwentangi Qian and Bengtson, 1989. The large mitral and sellate sclerites (about 1 cm) illustrated in the present study manifest a relatively consistent morphology during size increase. The sellate sclerites may contain two sub-types, a larger sellate sclerite with sella on the sellate side and a smaller convex triangular sclerite without sella on either side. In the original scleritome, the two sub-types of sellate sclerites probably combine as a composite with the duplicatural side of the smaller one attached on the sella area of the large one. For the first time, T. zhangwentangi , previously unknown outside South China, has been recovered from the Indian subcontinent. The middle and upper part of the Hazira Formation exclusively bearing T. zhangwentangi can directly invite correlation with the S. flabelliformis – T. zhangwentangi Assemblage Zone of South China representing the uppermost Cambrian Stage 2. This new find not only signifies the utility of T. zhangwentangi for intercontinental biostratigraphical correlation, but also suggests that the Terreneuvian small shelly fossil (SSF) biostratigraphy between the Indian subcontinent and South China is consistent, comprising three (at least two) correlative SSF Assemblage Zones (Zone I, possibly Zone III, and Zone IV of South China). Additionally, our result may also support a relatively close palaeogeographical linkage between these two regions in the early Cambrian. Thematic collection: This article is part of the Advances in the Cambrian Explosion collection available at: https://www.lyellcollection.org/topic/collections/advances-in-the-cambrian-explosion

Influence of hydrothermal and upwelling events on organic matter accumulation in the gas-bearing lower Cambrian shales of the middle Yangtze Block, South China

With onoing studies on the diversification of animals during the early Cambrian in the Yangtze Block, resource effects associated with environmental changes have started to be elucidated, in which regional sedimentary events might have played an important role. The gas-rich Yichang area in the southern Huangling anticline is a potential area for shale gas development and provides a more suitable target for unravelling the debates on organic matter (OM) accumulation in the Low Cambrian marine black shales. This study integrated elemental geochemical and mineralogy time-series analyses based on core samples drilled in Yichang area on the shallow-water platform to constrain the evolution of the depositional environments and consider the forcing function behind OM enrichment in the Shuijinguo Formation. Distinctive TOC and Cu/Al peaks within the black shale deposited in the Shuijingtuo Member I suggest intensification of bioproductivity level. The chemical index of alteration (CIA), regarded as an indicator of paleoclimatic variation, increased in the upsection, and together with increasing Sr/Cu and quartz/feldspar ratio values, suggest a trend toward warming conditions and a moderate degree of chemical weathering. The geochemical proxies for the degree of water mass restriction (Mo/TOC and Co × Mn) and paleoredox (MoEF, UEF, U/Th and Corg/P) indicate that the shallow-water shelf experienced weakly-to moderately-restricted water conditions under an anoxic/euxinic environment in the Shuijingtuo Member I and transited to a moderately-restricted water mass with suboxic conditions during the late Shuijingtuo and Shipai periods. A positive Eu anomaly within the Shuijingtuo Member I provided robust evidence of hydrothermal activity. We propose here for an opening of deposition conditions on the shallow-water platform which results in enhanced upwelling and hydrothermal activity (Si, Ba, et.), thereby ultimately disrupting the original nutrient cycling for the metabolism and prompting the OM accumulation in the epicontinental sea during the early Cambrian.

Fluctuations of continental chemical weathering control primary productivity and redox conditions during the Earliest Cambrian

The Ediacaran – Cambrian transition witnessed dramatic changes in the marine environment that are thought to have strongly influenced biological evolution. Previous studies have linked biological innovations to palaeo‐redox changes and suggested that increased productivity is the primary driver of the marine environment. However, the primary productivity and redox control mechanism still need to be clarified. Here, we present new organic carbon isotopes, total organic carbon contents, major elements, and trace and rare earth elements in drill‐core ZK107 from South China to reveal the continental chemical weathering, primary productivity, and redox control mechanism during the Earliest Cambrian (E‐C). Our studies have shown intense continental weathering, and primary productivity fluctuations that were supported by weathering (CIA, Rb/Al) and productivity (Babio, P) indicators. Additionally, the quantitative reconstructed productivity proxies (PP18, PP21, PP23) indicate a high primary productivity level in the E‐C ocean, surpassing that of the modern oceans. The strengthening of continental weathering mainly causes high primary productivity because it can alter the nutrient supply to the oceans. We suggest that changes in primary productivity closely associated with continental weathering are the leading cause of marine anoxia during the E‐C. This study establish a linkage between the land and marine system during the E‐C, confirming the terrestrial controls on the marine chemical changes and providing new insights into understanding the evolution of the Earth's surface changes during the Ediacaran – Cambrian transition.

The marine redox evolution and the formation model for the early Cambrian Gongxi-Tianzhu barite deposits in the South China Block

Late Ediacaran to early Cambrian is marked as a vital ore-formation interval of the Earth’s history, particularly for the bedded barite deposits hosted in cherts and shales. Although plenty of work has been carried out, controversies still exist, such as how the marine paleoenvironment changed in this critical interval, and where the origin of ore-formation materials (i.e., barium ions) was. To address these questions, here, we conducted systematic geochemical research on Gongxi-Tianzhu barite deposits in the South China Block, using redox proxies (MoEF-UEF, Mo-TOC, U-TOC patterns, and elemental ratios), REY patterns, and Sr isotopes. Redox proxies were used to reconstruct the paleoenvironment evolution in the Gongxi-Tianzhu region during the early Cambrian interval. Redox proxy results revealed that the bottom water was oxygen-depleted during most of the depositional time of the Niutitang Formation. The bedded and nodular barites in the Niutitang Formation were formed in the Sulfate-Methane Transition Zone (SMTZ) during the synsedimentary and early diagenetic processes. Formation of the barite ores depended greatly on the redox state variation of the bottom water and the barium remobilization in the sediments. The ore-formation event ceased when the supply of oxygen and sulfate from the upper water stopped, and the pre-stored Ba ions in the anoxic bottom seawater were exhausted and the SMTZ rose back into the water again. REY patterns and 87Sr/86Sr values recorded coeval seawater signals without hydrothermal activities during the studied interval. Based on the elemental and isotopic evidence, a three-stage redox-diagenetic model was established for the formation of the barite deposits in South China.

Cambrian stem-group ambulacrarians and the nature of the ancestral deuterostome

Deuterostomes are characterized by some of the most widely divergent body plans in the animal kingdom. These striking morphological differences have hindered efforts to predict ancestral characters, with the origin and earliest evolution of the group remaining ambiguous. Several iconic Cambrian fossils have been suggested to be early deuterostomes and hence could help elucidate ancestral character states. However, their phylogenetic relationships are controversial. Here, we describe new, exceptionally preserved specimens of the discoidal metazoan Rotadiscus grandis from the early Cambrian Chengjiang biota of China. These reveal a previously unknown double spiral structure, which we interpret as a chordate-like covering to a coelomopore, located adjacent to a horseshoe-shaped tentacle complex. The tentacles differ in key aspects from those seen in lophophorates and are instead more similar to the tentacular systems of extant pterobranchs and echinoderms. Thus, Rotadiscus exhibits a chimeric combination of ambulacrarian and chordate characters. Phylogenetic analyses recover Rotadiscus and closely related fossil taxa as stem ambulacrarians, filling a significant morphological gap in the deuterostome tree of life. These results allow us to reconstruct the ancestral body plans of major clades of deuterostomes, revealing that key traits of extant forms, such as a post-anal region, gill bars, and a U-shaped gut, evolved through convergence.

New trilobite assemblage from the lower Cambrian (upper Stage 4) of the Lake Zone, western Mongolia

AbstractThe western Mongolian Lake Zone was a Neoproterozoic to early Paleozoic volcanic arc where tuffs, lavas, fossiliferous siliciclastics, and carbonates accumulated during the early Cambrian. An uppermost Cambrian Series 2 (upper Stage 4) trilobite assemblage is described here from the Burgasutay Formation representing a continuous lower Cambrian succession at the Seer Ridge of the Great Lake Depression. The new assemblage is dominated by dorypygids and consists of 13 trilobite genera belonging to nine families includingCatinouyia heyunensisnew species. These fossils comprise the youngest and richest lower Cambrian trilobite assemblage in Mongolia. The composition of the Lake Zone fauna suggests its biogeographic affinity with the Siberian Platform and Altay-Sayan Foldbelt, but the presence of inouyiids also implies a connection of this region with East Gondwana.UUID:http://zoobank.org/fb0ebe23-144f-46ed-9543-7df9eea85fbc

Chemical Weathering Indices and their Use for Paleoclimatic Reconstructions (on the Example of the Vendian‒Lower Cambrian Section of Podolsk Transnistria)

The article provides an overview of various indices/indicators (Vogt, Parker, CIA, CIW, PIA, MIA, etc.) used in the study of weathering profiles/crusts and reconstruction of paleoclimatic environments of sedimentary sequence accumulation. Their possibilities are demonstrated by the example of Vendian-Lower Cambrian terrigenous deposits of Podolsk Transnistria (southwestern slope of the Ukrainian Shield). The distribution of the ba1 index values in this section indicates the presence of the material most transformed by the processes of chemical weathering in the mudstones of the Nagoryany Formation, the lower part of the Danylivka and the middle part of the Studenytsya formations. For mudstones of the Danylivka–Zbruch interval, the HM values are close to the HMPAAS. For rocks of the Yaryshiv–Nagoryany interval, the HM values are slightly higher, and for mudstones in the lower part of the Yaryshiv Formation, they are comparable to those inherent in continental clays of a hot tropical climate. The average SA index for mudstones of the entire section is 5.6 ± ± 0.7. Mudstones of the Hrushka–Nagoryany interval, where SA SAPAAS, are composed of more weathered material. The WIP values in the mudstones of the Mohyliv and Yaryshiv formations, as well as in the upper part of the Zbruch Formation, correspond to the interval of their values between PAAS and the average Archean granite. Clay rocks of other formations have WIP ≤ WIPPAAS values. The average CIA value for mudstones is 71 ± 4, which practically corresponds to the CIA value (70), which separates the sediments of cold/arid and warm/humid climates. Variations in the CIW index along the section are symbate with variations in the CIA. The vast majority of mudstones are characterized by PIA PIAPAAS values. The average CPA value is 91 ± 4, which is also typical for PAAS. The above and other data show that on the basis of a “direct” interpretation of the values of various indices of chemical weathering inherent in fine-grained clastic rocks, the paleoclimate that existed in the Vendian‒Early Cambrian on the territory of Podolsk Transnistria was rather moderate or warm humid. Comparison of the CIA values of mudstones with the values of this indicator for the particulate suspended matter of modern rivers suggests that the climate in the Vendian‒Early Cambrian resembled dry and humid subtropical or dry tropical with elements of humid.

Three-dimensional morphology of the biramous appendages in Isoxys from the early Cambrian of South China, and its implications for early euarthropod evolution.

Early euarthropod evolution involved a major transition from lobopodian-like taxa to organisms featuring a segmented, well-sclerotized trunk (arthrodization) and limbs (arthropodization). However, the precise origin of a completely arthrodized trunk and arthropodized ventral biramous appendages remain controversial, as well as the early onset of anterior-posterior limb differentiation in stem-group euarthropods. New fossil material and micro-computed tomography inform the detailed morphology of the arthropodized biramous appendages in the carapace-bearing euarthropod Isoxys curvirostratus from the early Cambrian Chengjiang biota. In addition to well-developed grasping frontal appendages, I. curvirostratus possesses two batches of morphologically and functionally distinct biramous limbs. The first batch consists of four pairs of short cephalic appendages with robust endites with a feeding function, whereas the second batch has more elongate trunk appendages for locomotion. Critically, our new material shows that the trunk of I. curvirostratus was not arthrodized. The results of our phylogenetic analyses recover isoxyids as some of the earliest branching sclerotized euarthropods, and strengthens the hypothesis that arthropodized biramous appendages evolved before full body arthrodization.

Geological characteristics of the southern segment of the Late Sinian—Early Cambrian Deyang—Anyue rift trough in Sichuan Basin, SW China

Based on the latest drilling, seismic and field outcrop data, the geological characteristics (e.g. strata, development and sedimentary evolution) of the southern segment of the Late Sinian—Early Cambrian Deyang—Anyue rift trough in the Sichuan Basin are analyzed. First, the strata in the southern segment are complete. The first to second members of Dengying Formation (Deng 1 + Deng 2) are found with relatively stable thickness (400–550 m), and the third to fourth members (Deng 3 + Deng 4) show great thickness difference between the marginal trough and the inner trough, which is up to 250 m. The Cambrian Maidiping Formation and Qiongzhusi Formation in southern Sichuan Basin are relatively thin, with the thickness changing greatly and frequently. Second, the Deyang—Anyue rift trough extended southward during the Deng 4 period, affecting southern Sichuan Basin. Compared to the middle and northern segments of the rift trough, the southern segment is generally wide, gentle and shallow, with multiple steps, and alternating uplifts and sags, which are distributed in finger shape. Third, the Deng 1 + Deng 2 in southern Sichuan Basin records the dominance of carbonate platform and unobvious sedimentary differentiation, and the Deng 4 exhibits obvious sedimentary differentiation, namely, basin—slope—secondary slope—slope—secondary slope—platform margin—restricted platform, from the inner trough to the marginal trough. Fourth, the rift trough in southern Sichuan Basin has evolved in four stages: stabilization of Deng 1—Deng 2, initialization of Deng 3—Deng 4, filling of Maidiping—Qiongzhusi, and extinction of Canglangpu Formation.

Source of gold and ore-forming processes in the Zarshuran gold deposit, NW Iran: Insights from in situ elemental and sulfur isotopic compositions of pyrite, fluid inclusions, and O−H isotopes

The Zarshuran gold deposit (155 t Au, average grade: 2.63 g/t), NW Iran, provides a new paradigm for understanding the multicomponent ore-forming processes and metallogeny of gold during the evolution of hydrothermal fluids. It is characterized by auriferous quartz veins and gold coexisting with disseminated Fe-As-S sulfide minerals that are hosted in a sequence of Early Cambrian metasedimentary rocks. A complex paragenesis is defined by five sulfide stages: pre-ore stage pyrite (Py0), early ore-stage pyrite (Py1 and Py2), middle ore-stage pyrite (Py3a, Py3b, and Py3c), late ore-stage pyrite (Py4), and post ore-stage pyrite (Py5). Py0 with framboidal texture is characterized by relatively low concentrations of As, Au, Cu, and Sb, and has a broad range of negative δ34S values from −28.2 to −3.7 ‰, indicating microbial reduction of marine sulfate. Py1 and Py2 show porous texture with As-rich bright bands also having high Au (mean: 18.4 ppm). Similarly, narrow ranges in δ34S from −3.6 to +3.2 ‰ likely suggest a deep-seated magmatic sulfur source. In the middle ore-stage, Py3a cryptocrystalline cores have a range of positive δ34S (+5.4 to +26.7 ‰) values and high concentration of As(-Au), whereas later well-crystallized outer rim Py3c is Co(-Ni) rich, has a much more restricted δ34S range (−2.9 to +1.4 ‰), indicating an evolution of fluid composition from As-rich to Co-rich. Most of the gold (mean: 20.2 ppm) and trace elements (Hg, Ag, Sb, and Tl) were deposited in Py4, whose δ34S values (+5.4 to +26.7 ‰) show that ore fluid sulfur in this stage was mainly sourced from a mixture of magmatic sulfur with minor input of reduced sedimentary rocks. Low gold and trace element content with slightly negative δ34S values (–8.5 to –4.2 ‰) of Py5 show that they were likely formed from a metal-poor oxidizing fluid.The mineralizing fluid system can be described as carbonic-aqueous with low to moderate salinity (3.2–15.1 wt% NaCl equiv.) and medium temperature of 285 to 317 °C (early ore-stage) and 255 to 290 °C (late ore-stage), which suggests that phase separation was responsible for gold precipitation during late ore-stage As-Hg-Sb sulfide veins. The δ18Ofluid ranges from −7.8 to +4.2 ‰, and the δD values for fluid inclusions in mineral range from −105 to −65 ‰, suggesting involvement of meteoric water during late- to post ore-stages. Our results indicate that the Zarshuran is a distal disseminated gold deposit formed during southward subduction of the Proto-Tethys oceanic lithosphere beneath the northern margin of Gondwanan terranes through the early Cambrian.

Sedimentological and Petrographical Characterization of the Cambrian Abbottabad Formation in Kamsar Section, Muzaffarabad Area: Implications for Proto-Tethys Ocean Evolution

The current sedimentological and petrographical research of the Abbottabad Formation has been carried out in order to understand the formation and evolution of the Proto-Tethys Ocean during the Cambrian on the northern margin of the Indian Plate. The Muzaffarabad region is located east of the Upper Indus Basin and the southern part of the Hazara Kashmir Syntaxis. The geological history of the region varies from the Precambrian to the recent period. The Cambrian Abbottabad Formation is well exposed along the Hazara Kashmir Syntaxis at the core of the 500-m-thick Muzaffarabad anticline. The Abbottabad Formation is an unconformity-bounded allo-stratigraphic unit. It has an unconformable lower contact with the Late Precambrian Dogra Formation and an unconformable upper contact with the Paleocene Hangu Formation. The Abbottabad Formation has been divided into four lithofacies, from bottom to top, namely, thinly interbedded dolomite and shale, cherty-stromatolitic dolomite, oxidized limonitic-brecciated zone, and quartzite, with significant lithological changes. Petrographic studies revealed four types of dolomites: fine crystalline dolomite (Dol. I), dolomitic cryptocrystalline chert (Dol. II), algal mat-stromatolitic dolomite (Dol. III), and intraclastic-dolo-grain stone (Dol. IV). The mineral composition of dolostone was analyzed using X-ray diffraction (XRD) and found to be consistent with previous petrographic studies. The dolomite mineral content decreased from base to top, while chert increased towards the top. Elemental weight percentages through energy dispersive X-ray (EDX) analysis show different elements constitute the minerals found in the dolostone, as confirmed by petrographic and XRD analysis. Using outcrop data, facies information, and geochemical data, a modified depositional model of the Abbottabad Formation was developed. During the Early Cambrian period, the formation was deposited in a shallow subtidal to supratidal setting of the Proto-Tethys Ocean. The top of this deposit marks the Cambrian–Paleocene boundary. Because of the progressively coarsening outcrop sequences, this formation seems to be at the very top of the Proto-Tethys Ocean’s shallow marine system.

Fluid evolution and petroleum accumulation in the precambrian gas reservoirs of the Sichuan Basin, SW China

Deep marine carbonates comprise one of the most significant reservoir types globally. Reconstruction of Precambrian oil reservoirs and the complex tectonism that affected Precambrian carbonate reservoirs demonstrate the complexity of the natural gas enrichment mechanism. A comparison between the fluid evolution of the well-preserved reservoirs in the central Sichuan Basin and the poorly preserved reservoirs in the southeastern Sichuan Basin offers insights into the factors that control the preservation of ancient reservoirs. We conducted geochemical analysis of the multistage cementation sequence, Raman quantitative analysis on fluid (gas) inclusions, and rare earth element (REE) analysis of diagenetic fluid environments. Hydrocarbon migration can cause variations in the diagenetic environment. Quantitative Raman measurements of the trapping pressure, trapping temperature, and composition of fluid (gas) inclusions are also useful in defining periods of gas accumulation. In situ U–Pb dating results of dolomite veins indicate that extensive dolomitization in the Sichuan Basin began during the early Cambrian. In the central Sichuan Basin, reservoir pressure evolved from normal during the Middle Jurassic to strongly overpressured during the mid-Cretaceous. Overpressure generated before Himalayan tectonism appears to be positively correlated with gas preservation. Overpressure within inclusions bearing hydrogen sulfide indicates that thermochemical sulfate reduction (TSR) does not produce intense dissolution that increases the porosity of carbonate reservoirs and reduces pore pressure. Poorly preserved reservoirs were intruded by exogenous and hydrothermal fluids but remained normally pressured. Secondary methane-rich gas inclusions record normal pressure conditions during the early Paleogene.

A new red alga preserved with possible reproductive bodies from the 518‐million‐year‐old Qingjiang biota

AbstractMacroalgae have been a key ecological component of marine ecosystems since the Proterozoic period and are common fossil forms in Cambrian Burgess Shale‐type Lagerstätten. However, in most cases, it is difficult to place these early fossil algae into modern groups because little distinctive morphology is preserved. Here, we describe a new form of macroalgae, Qingjiangthallus cystocarpium gen. & sp. nov., from the Qingjiang biota of South China. The new taxon is represented by 546 specimens remarkably preserved with characteristics that allow a phylogenetic placement into crown groups of red algae. Centimeter‐sized thalli resemble members of the extant Rhodymeniophycidae (a subclass of the class Florideophyceae), and hence suggest a florideophycean affinity, which indicates that ahnfeltiophycidaen and rhodymeniophycidaen algae may have diverged at least 518 Ma, accordant with estimations of molecular studies. The presence of possible cystocarps on Qingjiangthallus thalli suggests that evolutionary innovation of a triphasic life cycle in red algae may have occurred no later than the Early Cambrian. The branching patterns and branch width of Qingjiangthallus are consistent with the coarsely dichotomously branched morphogroup, which was previously present in the Ediacaran, Ordovician, and afterward, but absent in the Cambrian.

Early Cambrian high pressure/low temperature metamorphism in the southeastern Tarim craton in response to circum-Gondwana cold subduction

The circum-Gondwana subduction initiated by the early Cambrian has been suggested to reflect the establishment of the modern plate tectonics. The metamorphic rocks with low thermobaric (T/P) ratios indicative of cold subduction in the present tectonic regime have not been well investigated. To better understand the circum-Gondwana subduction and to test its possible link with the emergence of the modern plate tectonics, this study focused on blueschist-facies metamorphic rocks in the Altyn Tagh of the southeastern Tarim craton. Mineral assemblage and chemistry, phase equilibrium modelling, and quartz-in-garnet Raman elastic geobarometry reveal that the zoisite blueschist and glaucophane (Gln)-bearing quartz schist in northern Altyn Tagh were metamorphosed to lawsonite to epidote blueschist-facies at 520–545 °C and 16–19 kbar. It reflects high-pressure (HP)/low temperature (LT) metamorphism with low T/P ratios of <300 °C/GPa and thermal gradients of <10 °C/km. These blueschist-facies metamorphic rocks underwent rapid decompression starting at P-T conditions of <495 °C and <9.6 kbar during exhumation. Ar-Ar geochronology records paragonite Ar-Ar plateau ages of 520–506 Ma for the zoisite blueschist samples and phengite Ar-Ar plateau ages of 522–516 Ma for the Gln-bearing quartz schist samples, suggesting that the peak HP/LT metamorphism occurred prior to ca. 522 Ma. Based on new results and available data from the major Gondwana blocks, cold subduction was suggested to profoundly operate along circum-Gondwana in the early Cambrian after the amalgamation of Gondwana. The extensive circum-Gondwana subduction represents the earliest global cold subduction in Earth’s history associated with the establishment of the modern plate tectonics, as directly recorded by the studied early Cambrian blueschist-facies metamorphic rocks and a dramatic drop in the mean T/P of metamorphism since the early Paleozoic.

The Cambrian cirratuliform Iotuba denotes an early annelid radiation.

The principal animal lineages (phyla) diverged in the Cambrian, but most diversity at lower taxonomic ranks arose more gradually over the subsequent 500 Myr. Annelid worms seem to exemplify this pattern, based on molecular analyses and the fossil record: Cambrian Burgess Shale-type deposits host a single, early-diverging crown-group annelid alongside a morphologically and taxonomically conservative stem group; the polychaete sub-classes diverge in the Ordovician; and many orders and families are first documented in Carboniferous Lagerstätten. Fifteen new fossils of the 'phoronid' Iotuba (=Eophoronis) chengjiangensis from the early Cambrian Chengjiang Lagerstätte challenge this picture. A chaetal cephalic cage surrounds a retractile head with branchial plates, affiliating Iotuba with the derived polychaete families 'Flabelligeridae' and Acrocirridae. Unless this similarity represents profound convergent evolution, this relationship would pull back the origin of the nested crown groups of Cirratuliformia, Sedentaria and Pleistoannelida by tens of millions of years-indicating a dramatic unseen origin of modern annelid diversity in the heat of the Cambrian 'explosion'.

Copper and Zinc isotopes trace the evolution of the Ediacara-Early Cambrian paleo-ocean redox condition in the Tarim Basin, China

The Ediacaran-early Cambrian transition is a critical period in the Earth's evolutionary history. A series of important geological and biological events are likely related to environmental change. Ocean oxygenation levels during the early Cambrian are still controversial and barely constrained although abundant geochemical studies have been conducted. The black shale widely developed in the Early Cambrian can record paleo-environmental information of contemporaneous seawater. The early Cambrian Yuertusi Formation black shale in the Luntan 3 well from the Tarim Basin was studied in terms of Cu and Zn isotopic composition and redox sensitive trace element to improve the knowledge of the extent of ocean oxygenation during the Ediacaran-Early Cambrian. The Yuertusi Formation is divided into four stages according to lithology variation and records of Cu and Zn isotopes and redox sensitive elements, indicating the fluctuation of ocean redox condition. The total organic carbon (TOC) varied widely from 0.10% to 18.25% with an average of 6.34%. Enrichment of Mo, U and V elements which have a synergistic relationship with TOC and redox proxies (ratios of U/Th and V/Cr, EF(Mo)-EF(U)) of the four stages in the Yuertusi Formation, indicate an anoxic even euxinic deposition environment, especially in stages Ⅰ and Ⅲ. Most of the Cu isotope values, lower than the modern seawater values, also indicate that the ocean state was anoxic and did not reach the current ocean oxidation level, which shows the same characteristics as the other profiles in different areas. The variation in the Cu isotope ranges from −3.10‰ to +1.23‰, and the Zn isotope ranges from 0.09‰ to 1.46‰. Relatively large Cu and Zn isotopic fractionation in stages Ⅰ and Ⅲ occur with an intense reduction environment, which is attributed to the combination of sulfide deposits that are eventually immobilized in sediments. Under the overall background of an anoxic environment, the four stages show a cycle of relative anoxic-weak oxidation-anoxic-weak oxidation, which is inferred to be connected with the transgressive-regressive cycle and weathering enhancement variation based on Ce anomaly, Mo/Toc ratios and C stable isotope data. The Early Cambrian transgression provided an anoxic even euxinic environment conducive to organic matter preservation for the development of black shale during stage Ⅰ. With the sea level fluctuation and weathering enhancement, the sea retreated gradually, productivity gradually increased, and the marine environment was relatively oxidized. The increasing trend of the whole Cu and Zn isotope values from bottom to top in the Yuertusi Formation is interpreted to be gradual relative oxidizing.

Thecate stem medusozoans (Cnidaria) from the early Cambrian Chengjiang biota

AbstractCnidarians are phylogenetically located near the base of the ‘tree of animals’, and their early evolution had a profound impact on the rise of bilaterians. However, the early diversity and phylogeny of this ‘lowly’ metazoan clade has hitherto been enigmatic. Fortunately, cnidarian fossils from the early Cambrian could provide key insights into their evolutionary history. Here, based on a scrutiny of the purported hyolith Burithes yunnanensis Hou et al. from the early Cambrian Chengjiang biota in South China, we reveal that this species shows characters distinct from those typical of hyoliths, not least a funnel‐shaped gastrovascular system with a single opening, a whorl of tentacles surrounding the mouth, and the lack of an operculum. These characters suggest a great deviation from the original definition of the genus Burithes, and a closer affinity with cnidarians. We therefore reassign the material to a new genus: Palaeoconotuba. Bayesian inference of phylogeny based on new anatomical traits identifies a new clade, including Palaeoconotuba and Cambrorhytium, as a stem group of sessile medusozoan cnidarians that are united by the synapomorphies of developing an organic conical theca and a funnel‐like gastrovascular system. This study unveils a stem lineage of medusozoans that evolved a lifelong conical theca in the early Cambrian.

An early Cambrian polyp reveals a potential anemone‐like ancestor for medusozoan cnidarians

AbstractCnidarians 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 &amp; 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.

Chuaria Walcott from the early Cambrian Qingjiang biota: a taxon persisted for billions of years

As a globally distributed genus, Chuaria has an extremely long temporal range from the late Paleoproterozoic to the Cambrian Miaolingian but it is still absent in the pre-Miaolingian Cambrian. Here, we describe new specimens of Chuaria from the Qingjiang biota (black shale of the Cambrian Stage 3 Shuijingtuo Formation, ca. 518 Ma) in Hubei Province, South China, and review its tempo-spatial distribution worldwide. Chuaria from the Qingjiang biota is preserved as carbonaceous compressions or impressions that are discoidal or subdiscoidal in outline and range from 1.40 mm to 5.60 mm in diameter, it is interpreted as a spheroidal and thick-walled eukaryote, and its presence fills the early Cambrian gap in the fossil record. Chuaria remained consistent in morphology and survived harsh environmental perturbations throughout its evolutionary history. Its earliest record was restricted in a small region of Columbia supercontinent; thereafter its records were across a range of continents and became worldwide during the time of Rodinia supercontinent.