Cambrian Stage Research Articles

The bivalved arthropod Caudicaella aff. bispinata from the Heatherdale Shale (Cambrian Stage 3), South Australia

Sunellids are early Cambrian bivalved arthropods first recognized in South China that are important for understanding the early evolution of the group. However, their taxonomic affinity and distribution beyond South China is poorly known. Furthermore, the relationship between members of the Sunellidae and the globally dispersed Isoxys remains unclear. Recently, rare specimens of the sunellid Caudicaella aff. bispinata have been recovered from the lower Cambrian (Series 2, Stage 3) Heatherdale Shale, eastern Stansbury Basin, South Australia, representing the first record of the group from East Gondwana. Caudicaella aff. bispinata from Australia bears a semicircular carapace, a pair of anterodorsal sulci, anterior and posterior cardinal spines, as well as a distinct thick border or margin. It most closely resembles Caudicaella bispinata from the Shuijingtuo Formation in western Hubei Province, South China. The new discovery adds support for strong faunal similarity of the Konservat-Lagerstätten of South Australia and South China and extends the known geographical range of Cambrian sunellids to East Gondwana. The occurrence of Caudicaella bispinata in South China and Caudicaella aff. bispinata in East Gondwana strengthens the biostratigraphic correlation between those two palaeo-continents.

Microstructure and chancelloriid sclerites within a microbial buildup of the Wirrealpa Limestone (Cambrian Stage 4), South Australia

Well-preserved, distinctive columnar microbialites were discovered in the Wirrealpa Limestone (Stage 4, Series 2 of Cambrian) of South Australia. However, further studies are still required to unveil the microstructure and formation process of these microbialites. A microbial buildup from the Wirrealpa Limestone examined herein suggests that the microbialitic columns are composed of densely-distributed, polymorphic, and patched fabrics produced by microbes. These fabrics are inter-framed by packstone sediments that characterized by calcareous metazoan skeletons and cerebroid ooids. The patched fabrics can be categorized into three types: 1) laminated ‘patches’, 2) homogeneous microsparitic ‘patches’ (mesoclots), and 3) cemented ‘patches’ containing tightly-packed chancelloriid sclerites. There are two preservational conditions of chancelloriid sclerites. After death of the chancelloriid individuals (belonging to Chancelloria and rare Allonnia) that dwelled among or near the microbial reef, abundant sclerites were exfoliated, transported, and concentrated. Then, some of them were cemented by microbial communities, forming the third type ‘patches’, while others were accumulated in the packstones within reefal cavities and inter-framework space along with many skeletal fragments from trilobites and mollusks. Thus, this lower Cambrian thrombolite-stromatolite complex was formed within an ecosystem lacking archaeocyaths but dominated by microbes (reef builders) and chancelloriids (reef dwellers).

First multi-proxy chronostratigraphy of the lower Cambrian Byrd Group, Transantarctic Mountains and correlation within East Gondwana

Antarctica and Australia were sutured together at the equator during the major pulse of animal biodiversification associated with the Cambrian radiation. However, the lack of detailed systematic chemostratigraphic and biostratigraphic sampling of lower Cambrian sedimentary successions from Antarctica has significantly impeded precise age determination and correlation with Cambrian strata on other palaeocontinents. This study is the first to present integrated, simultaneously sampled biostratigraphic and chemostratigraphic (δ13C isotopes) data from the same measured stratigraphic sections through the lower Cambrian Byrd Group in the Transantarctic Mountains. Shelly fossil assemblages (brachiopods, tommotiids, molluscs, bradoriids, trilobites) from the Holyoake Range and Churchill Mountains facilitate direct correlation with the Dailyatia odyssei Zone of South Australia (Cambrian Stages 3–4), and trilobites provide strong correlation between the Starshot Formation and the Cymbric Vale Formation in western New South Wales. A new ID-TIMS radiometric date of 514.96 ± 0.16 Ma from a tuff in the lower Cymbric Vale Formation is similar to dates from tuff beds in the Third Plain Creek Member of the Mernmerna Formation in the Flinders Ranges, providing an important absolute-age tie point between these lower Cambrian successions. Chemostratigraphic data from the upper Shackleton Limestone in the Holyoake Range capture a negative δ13C excursion that can be correlated to negative values within the multi-peaked MICE (cycles V–VIII in Siberia). Integrated faunal and chemostratigraphic data indicate a Cambrian Stages 3–4 age, giving robust chronostratigraphic context for the upper Shackleton Limestone–Holyoake Formation–Starshot Formation succession for the first time, permitting reconstruction of the depositional history of the lower Cambrian of Antarctica and global correlation of Byrd Group strata.

A new radiodont from the lower Cambrian (Series 2 Stage 3) Chengjiang Lagerstätte, South China informs the evolution of feeding structures in radiodonts

Radiodonts, a diverse clade of early Palaeozoic stem-group euarthropods, have provided critical information for understanding the evolution of this phylum, and were important constituents of marine ecosystems at this time. The well-known Chengjiang Lagerstätte (Cambrian Stage 3, c. 518 Ma) has yielded the highest known diversity of radiodonts of any Cambrian Konservat-Lagerstätten, and represents a crucial deposit for radiodont research. One important but generally overlooked Chengjiang radiodont taxon, previously identified as Anomalocaris sp. or Radiodont C, is herein designated as the type species of a new monotypic genus, Shucaris ankylosskelos gen. et sp. nov., based on dozens of specimens. Shucaris is distinctive for its combination of several characters, including the presence of two pairs of endites on its proximal-most claw podomere, posteriorly inward curved endites on claw podomeres 2–6, and, most strikingly, the coexistence of gnathobase-like structures and an oral cone. New material of Ramskoeldia consimilis records the first known oral cone in this species, confirming that oral cones and gnathobase-like structures co-occur in multiple radiodont taxa. Phylogenetic analyses retrieve Shucaris as either an early diverging member of Anomalocarididae or as sister to the clade Anomalocarididae + Amplectobeluidae. Moreover, our phylogenetic analyses also support the divergence between hurdiid and non-hurdiid radiodonts. https://zoobank.org/urn:lsid:zoobank.org:pub:6989C81B-06B2-4AED-8297-3B200A6F8D69

Marine redox and nutrient dynamics linked to the Cambrian radiation of animals

Abstract The early Cambrian witnessed an increase in metazoan ecosystem complexity, likely linked to enhanced oxygen and nutrient availability. However, while improved stratigraphic and geochemical records suggest that the Cambrian explosion occurred under highly dynamic redox conditions, mechanistic links to nutrient availability and early Cambrian evolutionary innovations are poorly constrained. Here, we report paleoredox and nutrient data for two drill cores documenting late Cambrian Stage 2 to Stage 3 (ca. 522–514 Ma) strata from the Yangtze block, South China. The development of water-column euxinia during Cambrian Stage 2 led to extensive recycling of P, fueling elevated primary production and hence an increase in atmospheric and shallow-marine oxygen concentrations. The resulting expansion of oxygenated shelf area promoted sedimentary P retention and, in combination with a diminished supply of P from upwelling, drove a transition to oligotrophic shallow-marine conditions during Cambrian Stage 3. Reduced primary production and limited water-column oxygen consumption allowed for the stabilization of oxygenated continental shelf habitats that supported a burgeoning biotic complexity.

Cadmium isotope evidence for near-modern bio-productivity in the early Cambrian ocean

The Ediacaran–Cambrian transition was one of the major transitions in Earth's history, with mass extinctions and radiation of animals. This event has been linked with perturbations in ocean geochemistry, including redox conditions and the oxygenation of Earth's surface environments. Here we present a highly resolved cadmium isotope record (δ114Cd) from shales deposited in a deep-water environment covering the Late Ediacaran to Cambrian Stage 4. Our δ114Cd data vary from −0.70 to +0.35 ‰ and demonstrate the importance of local redox-dependent burial phases on bulk sediment Cd isotope compositions, which are superimposed on global-scale changes in the Cd cycle. The heaviest Cd isotope composition recorded in the early Cambrian Liuchapo Formation is similar to the modern deep ocean, implying a broad similarity in global Cd cycling, perhaps involving similar organic-Cd burial rates. The provision of bio-limiting nutrients by continental weathering with rising oxygen concentrations in Earth's surface environments could have expanded the habitable area of the ocean and contributed to animal diversification.

Spatiotemporal distribution and morphological diversity of the Cambrian Wiwaxia: New insights from South China

Wiwaxia is a well-known Cambrian animal and a relative of annelids and molluscs, characterized by its densely arranged, imbricated dorsal sclerites. Despite its prevalence in the fossil record, the scarcity of articulated specimens leading to persistent uncertainties regarding its morphological diversities and evolutionary history has fueled ongoing investigations. This study presents an additional articulated specimen and isolated sclerites from the Cambrian Stage 4 Douposi Formation in Yunnan Province, South China, representing a new species—Wiwaxia douposiensis sp. nov. The newly described materials, originating from a new Cambrian Konservat-Lagerstätte, exhibit typical Burgess Shale-type preservation. They unveil a distinctive two-order rib pattern and aspect ratios of sclerites. The temporal and spatial distribution of Wiwaxia suggests a potential origination centered in South China during the Cambrian Age 3, with an expanding pattern from shallow to deep waters and from warm-humid to dry-cold climates during the Cambrian Age 4 to Drumian. Statistical and phylogenetic analyses highlight the dual aspects of conservative aspect ratios in sclerites, and morphological variations observed at the species levels within Wiwaxia.

Links between hydrographic restriction, redox conditions, and organic matter accumulation in the Early Cambrian intrashelf basin, South China

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.

Sedimentological and Geochemical Characterization of the Early Cambrian Eastern Yunnan, Southwestern China

The accurate reconstruction of the early Cambrian paleoclimate and paleoceanographic conditions on the Yangtze Plate is crucial for understanding the ancient environment during the Cambrian Explosion. It is also a key factor in understanding the ecological habits of organisms during the Cambrian Explosion. The study utilized field outcrops, thin section analysis, and major and trace elements to investigate the sedimentary environment, provenance, paleoweathering, and paleoclimate of the Lower Cambrian Hongjingshao (HJS) Formation (Cambrian Stage 3, ~515 Ma) in the Yangtze Basin, eastern Yunnan, SW China. The HJS sandstones are composed of 10 lithofacies, including massive and weakly bedded gravel supported by coarse sandstone (Gm), trough cross-bedded sandstone (St), planar cross-bedded sandstone (Sp), ripple cross-laminated sandstone (Sr), horizontal bedded sandstone (Sh), scour-fill sandstone (Se), massive sandstone (Sm), fine to medium sandstone with thin bed muddy siltstone (Fl), muddy siltstone (Fsc), and mudstone (Fm). On the basis of these lithofacies, channel fill and over-bank deposits in delta and shallow shelf depositional environments are suggested for HJS Formation. The major elements-based provenance discriminant function and mineral composition indicate that felsic rocks from the recycled orogen and continental block are the main sediment source terrane for the HJS sandstones of the study area. CIA, PIA, and CIW values range from 71.29 to 93.72, indicating an intermediate to intense chemical weathering and semiarid to humid climate conditions in Cambrian Stage 3. The research findings have clarified the paleoclimate and paleoceanographic environment of the Early Cambrian in the Yangtze region, which is of significant importance for understanding the early biological and ecological marine environment in the study area.

Trilobites of the Cranbrook Lagerstätte (Eager Formation, Cambrian Stage 4), British Columbia

Abstract The Eager Formation (Cambrian Stage 4) of the Cranbrook area, British Columbia, contains abundant, sometimes beautifully preserved, trilobites and rare non-biomineralized taxa. Trilobites were first reported just over a century ago but have received little research attention, resulting in uncertainty in the number and identity of species within the assemblage and ambiguity in the age of the fauna. The trilobites of the Eager Formation in the Cranbrook area are described herein based largely upon material collected in 2015. A minimum of eight (and perhaps up to 11) trilobite species are recognized. The four species of olenelloids (Olenellus santuccii Webster n. sp., Olenellus? schofieldi, Mesonacis eagerensis, and Wanneria cranbrookense Webster n. sp.) are by far the most abundant elements of the fauna. At least two, and perhaps as many as five, species of dorypygid are present, as are two species of “ptychoparioids”. Paucity and poor preservational quality of specimens mean that the various dorypygid and “ptychoparioid” species are left in open nomenclature. Trilobite diversity in the Cranbrook Lagerstätte is comparable to that within other Lagerstätten from Cambrian Stage 4 (Series 2) of Laurentia. The diversity and abundance of trilobites, combined with biostratinomic and trace fossil data, suggest that the assemblage is autochthonous and/or parautochthonous, and that the local environment was at least periodically able to support a “typical” benthic trilobite community. The age of the Cranbrook Lagerstätte is constrained to lie within the middle Dyeran Stage of Laurentia, within what was a substantial stratigraphic gap in the distribution of Burgess Shale-type Lagerstätten. UUID: http://zoobank.org/5beab9df-6b6a-4d6d-95e8-57057cd47a66

Upper Marianian (Cambrian Series 2) trilobites from the Totanés–Noez area (Central Iberian Zone, Toledo province, Spain): systematics and intercontinental correlation

AbstractMarianian (Cambrian Series 2) trilobites from the area between Totanés and Noez (Central Iberian Zone) are described and their biostratigraphical significance is discussed. The trilobites come from eight localities and can be divided into two main assemblages: the first is characterised by Serrodiscus bellimarginatus, Chelediscus cf. garzoni, Atops calanus, Pseudatops reticulatus and indeterminate polymeroid trilobites. The second assemblage consists of Serrodiscus bellimarginatus, Triangulaspis cf. fusca, Andalusiana palaciosi n. sp., Termierella totanesensis n. sp. and Acanthomicmacca sp. Both fossil associations indicate a late Marianian age in the regional stratigraphic scheme for the Cambrian of the Iberian Peninsula, as Serrodiscus, Chelediscus and Pseudatops have been recorded from the upper Marianian substage, while Triangulaspis, Andalusiana, Termierella and Acanthomicmacca are indicative of the middle to upper Marianian. The taxa considered here have regional and intercontinental correlation potential. The presence of the globally distributed genus Serrodiscus allows correlation in the Cambrian Series 2. The first assemblage, composed of Serrodiscus, Chelediscus, Atops and Pseudatops, has been reported from the Ossa-Morena Zone, Avalonia, Laurentia, Baltica and Siberia, improving the correlation between these regions. The second assemblage of Andalusiana, Termierella, Triangulaspis and Serrodiscus is comparable to other assemblages from the Ossa-Morena Zone, the Iberian Chains and Morocco, with Triangulaspis also reported from Newfoundland and Siberia. In addition, the proposal of the ISCS regarding the co-occurrence of Hebediscus, Calodiscus, Serrodiscus and Triangulaspis as a potential marker for the base of the Cambrian Stage 4 may imply that the upper Marianian of Iberia could be approximately correlated with the base of this stage.

Revisiting Ediacaran to early Cambrian depositional history of western North China: did it remain passive until the mid-Paleozoic?

Whether the North China Block (NCB) remained in extension from its cratonization to the mid-Paleozoic is questionable. Here, we conduct a synthesis of zircon U–Pb data of Statherian–Ordovician sandstones to provide a historical review of provenance changes in the western NCB through time. In contrast to typical NCB basement sources characterized by c. 2.7–1.8 Ga ages with spectral peaks at c. 2.5 and 1.9 Ga during much of c. 1.8–0.45 Ga, Ediacaran to Cambrian Stage 3 succussions contain abundant zircons with Meso- to Neoproterozoic ages. The exotic provenance, further verified by southeastward palaeo-flow, implies sources from the western Bainaimiao arc terrane (BAT), where basement rocks with suitable ages exist. Hence, the BAT should have evolved at the NCB margin before c. 0.56 Ga, but after rifting of the NCB (until c. 773 Ma). This event led to a craton-wide stratigraphic break during the Mesoproterozoic–Ediacaran. The presence of 521–515 Ma detrital zircons in Cambrian Stage 3 strata indicates subduction onset of the Palaeo-Asian Ocean before c. 515 Ma, coincident with the Precambrian–Cambrian boundary paraconformity. A sequence of depositional shifts triggered by the tectonic activities of the BAT unveils a complicated plate reorganization history of the northern NCB, contesting the view that the NCB remained passive from the Statherian. Supplementary material : Figures S1, S2 and Tables S1 and S2 are available at https://doi.org/10.6084/m9.figshare.c.7040743

Life in the Cambrian shallows: Exceptionally preserved arthropod and mollusk microfossils from the early Cambrian of Sweden

Abstract Burgess Shale–type (BST) Lagerstätten record an exceptional variety of Cambrian soft-bodied fauna, yet these deposits are typically restricted to outboard depositional settings >1000 km from the paleocoastline. For shallow, well-oxygenated shelf environments, our knowledge of non-mineralized animals (the majority of diversity) is severely limited, giving rise to substantial bias in our perception of Cambrian biotas. An alternate means of detecting soft-bodied Cambrian fauna, independent of paleobathymetry, is to use acid maceration to extract microscopic organic remains of non-mineralized animals, known as “small carbonaceous fossils” (SCFs). Here, a hitherto unknown diversity of Cambrian arthropod and mollusk remains are reported from shallow-marine sediments (Cambrian Stage 3 Mickwitzia Sandstone, Sweden). These microfossils comprise a variety of arthropod cuticles preserving sub-micron-scale anatomy alongside abundant radular mouthparts from mollusks—among the oldest known arthropod and molluscan SCFs on record. Significantly, at least three distinct types of fossil radula are identifiable (uniseriate, distichous, and polystichous forms), revealing that substantial diversification of the basic molluscan radula had already taken place by the early Cambrian. These cryptic elements of the biota—otherwise undetectable in such deposits—offer novel insights into Cambrian primary consumers as well as aspects of the fauna that are absent from deeper-water BST deposits.

Early Cambrian hyoliths from the Brigus Formation of Avalonian Newfoundland

The Lower Cambrian (Placentian–Branchian Series) of Avalonian southeastern Newfoundland contains one of the most diverse and best-preserved assemblages of Early Cambrian hyoliths from the North American continent. Primary type, topotype and hypotype specimens from the Brigus Formation collected in the late nineteenth and early twentieth centuries are herein emended or newly recognized as Nevadotheca excellens, Levinoconus florencei gen. et sp. nov., Brigoconus greavesi gen. et sp. nov., Poutinella crispenae gen. et sp. nov., Toutonella chaddockae gen. et sp. nov., and Holmitheca quadricostata. Other less complete material is assigned to Aimitus sp., Dorsolinevitus? sp., Grantitheca? sp., Tizilites spp. 1 and 2, Hyolithidan spp. 1–8, and Opercula A–E. Biogeographical connections of the Avalonian species to Baltica, Laurentia, East Gondwanan South Australia, and the Siberian Platform are reinforced. The temporal distribution for most of these taxa remains unchanged except for Tizilites and Dorsolinevitus, which were previously known only from either the top of Cambrian Stage 4 or the Lower–Middle Ordovician, respectively. John M. Malinky [jmalinky@sbcglobal.net], Department of Physical Science, San Diego City College, 1313 Park Boulevard, San Diego CA 92101 (retired), Gerd Geyer [gerd.geyer@uni-wuerzburg.de], Lehrstuhl für Geodynamik und Geomaterialforschung, Institut für Geographie und Geologie, Bayerische Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany.

Shadows of the past: An Ediacara-style taphonomic window in advent of Cambrian bioturbators

One of the hypotheses, explaining a sudden vanishing of the Ediacaran biota at the Ediacaran-Cambrian transition, correlates this phenomenon with a disappearance of specific taphonomical factors providing fossilization of soft-bodied organisms (‘Cheshire Cat’ hypothesis). The terminal Ediacaran-Lower Cambrian fossil record does reveal a progressive development of bioturbation and associated decrease of microbial mats, which played a crucial role in the late Precambrian ecosystems. But the tempo of the evolution of organism-sediment interactions significantly varied between different zones of marine basins. Hence, if the ‘Cheshire Cat’ hypothesis is correct, the sedimentary environments barely (if any) colonized by the burrowing eumetazoans might provide the unique interplay of conditions for habitat and Ediacara-style preservation of soft-bodied organisms even in the early Cambrian. Herein we present the new carbon isotope data calibrating the Terreneuvian record in the Ura Uplift of South Siberia and also report fossilized soft-bodied organisms morphologically-close to the Ediacaran holdfasts preserved in association with microbially-induced sedimentary structures in the Cambrian Stage 2 carbonate-siliciclastic Nokhtuisk Formation. Although the late Terreneuvian age, this marginal marine succession is poorly bioturbated and hosts the soft-bodied fossils in the intervals undisturbed by the burrowing organisms. Our study demonstrates the possibility of Ediacara-style preservation of soft-bodied organisms in specific habitat and taphonomic conditions until being replaced by the Cambrian endobenthos.

Taxonomy, biostratigraphy and paleobiogeography of Strenuaeva (Trilobita) from the Marianian (Cambrian Series 2) of Iberia

Problems surrounding the identification and systematics of taxa belonging to the trilobite family Ellipsocephalidae have been discussed for several decades. The ellipsocephalid genus Strenuaeva is revised herein on the basis of material from Spain, including the type material of the identified species together with newly collected specimens from the Ossa-Morena Zone and the Iberian Chains. Two species are recognized as valid for these regions: S. sampelayoi and S. incondita. The species ‘S. melendezi’ and ‘Ellipsostrenua alanisiana’ from Spain, as well as ‘S. marocana’ from Morocco, are considered junior synonyms of S. sampelayoi. Previous assignment of the Spanish species to Issafeniella is rejected. The abundant available material of S. sampelayoi made it possible to evaluate the taphonomical role in the preservation of some characters and to recognize intraspecific variability similar to that described in S. inflata from Baltica, reinforcing its assignment to Strenuaeva. Biostratigraphically, Strenuaeva ranges from uppermost Cambrian Stage 3 to uppermost Cambrian Stage 4. In Iberia, it is restricted to the middle Marianian in the Ossa-Morena Zone, whereas it is known from the lowermost middle Marianian to the lowermost upper Marianian in the Iberian Chains. Strenuaeva is known from Baltica (Scandinavia and Holy Cross Mountains, Poland), Iberia (Spain), Morocco and, possibly, western Avalonia (Newfoundland), hence being a characteristic genus of the Acadobaltic faunal Province.

Variable depositional environments and organic matter enrichment of Early Cambrian shales in the Middle Yangtze region, South China

Organic-rich shales of the Lower Cambrian Shuijingtuo Formation (SJT) in the Middle Yangtze region (South China) represent an important target for shale gas exploration and development in China. The lower section of the SJT mainly consisting of organic-rich black shale deposits has been regarded to be a favorable target for shale gas exploration and received substantial attentions, while the upper section of the SJT primarily consisting of limestone, silty shale and black shale deposits has been rarely studied. Most recently, a major breakthrough of shale gas exploration and development has been made in the upper section shale deposits. In order to better understand the redox evolution of sedimentary environment throughout the whole Early Cambrian time and organic matter (OM) enrichment mechanism of the SJT shale deposits, geochemical investigations, including analyses of total organic carbon (TOC), as well as major and trace element compositions, were conducted on the upper section of the SJT shales from Well EYY1. Our results showed that the upper section of the SJT shale deposits was accumulated during the Cambrian Stage 3. The TOC values of the studied SJT shale samples ranged from 0.34 to 3.65 wt%. The lower part of the upper section of the SJT shale, which exhibited relatively high TOC contents (∼2.0 wt%), was deposited under anoxic bottom-water conditions during the early Cambrian Stage 3, while the upper part, showing relatively low TOC content (1.0 wt%), was deposited under suboxic bottom-water conditions with intermittent episodes of anoxic bottom-water conditions during the middle–late Cambrian Stage 3. The palaeo-productivity of the SJT shale was estimated to be generally low, and displayed little variation. The OM enrichment mechanism of the SJT shale deposits underwent dynamic changes due to relative sea-level changes in the Yangtze Ocean. The OM enrichment of the early and late Cambrian Stage 3 shale deposits was primarily controlled by reducing conditions, while that of the middle Cambrian Stage 3 shale deposits may have been influenced by episodic upwelling activities.

U–Pb zircon ages from tuffaceous beds in the Terreneuvian to Cambrian Series 2 sections of Avalonian southern New Brunswick, Canada: new constraints on chronostratigraphic correlations and the Cambrian time scale

Abstract Avalonian sections in the Saint John area, southern New Brunswick, have long contributed to global understanding of Cambrian chronostratigraphy. A tuffaceous bed in the Ratcliffe Brook Formation (RBF) in the Somerset Street section dated at c. 531 Ma has traditionally been considered to post-date small shelly fossils attributed to the Watsonella crosbyi Zone in the Hanford Brook section. A fine-grained tuffaceous bed approximately 8 m stratigraphically lower in the Somerset Street section yields a chemical abrasion isotope dilution–thermal ionization mass spectrometry zircon age of 532.3 ± 0.3 Ma; a tuffaceous carbonate unit in the lower RBF in Hanford Brook gives an age of 531.5 ± 0.3 Ma. Crystal and crystal-lithic tuff beds near the top of the RBF yield ages of 520.3 ± 0.3 Ma (in Hanford Brook) and 519.1 ± 0.3 Ma (in Ratcliffe Brook). The new ages confirm the correlation between the Somerset Street and Hanford Brook sections based on acritarchs and make the association of small shelly fossils in the Hanford Brook section younger than 531 Ma. This result is relevant to ongoing discussions on the age of the base of undefined Cambrian Stage 2. The radiometric ages also support a young age for the upper part of the RBF, perhaps extending into Epoch 2.

U–Pb zircon ages from tuffaceous beds in the Terreneuvian to Cambrian Series 2 sections of Avalonian southern New Brunswick, Canada: new constraints on chronostratigraphic correlations and the Cambrian time scale

Abstract Avalonian sections in the Saint John area, southern New Brunswick, have long contributed to global understanding of Cambrian chronostratigraphy. A tuffaceous bed in the Ratcliffe Brook Formation (RBF) in the Somerset Street section dated at c. 531 Ma has traditionally been considered to post-date small shelly fossils attributed to the Watsonella crosbyi Zone in the Hanford Brook section. A fine-grained tuffaceous bed approximately 8 m stratigraphically lower in the Somerset Street section yields a chemical abrasion isotope dilution–thermal ionization mass spectrometry zircon age of 532.3 ± 0.3 Ma; a tuffaceous carbonate unit in the lower RBF in Hanford Brook gives an age of 531.5 ± 0.3 Ma. Crystal and crystal-lithic tuff beds near the top of the RBF yield ages of 520.3 ± 0.3 Ma (in Hanford Brook) and 519.1 ± 0.3 Ma (in Ratcliffe Brook). The new ages confirm the correlation between the Somerset Street and Hanford Brook sections based on acritarchs and make the association of small shelly fossils in the Hanford Brook section younger than 531 Ma. This result is relevant to ongoing discussions on the age of the base of undefined Cambrian Stage 2. The radiometric ages also support a young age for the upper part of the RBF, perhaps extending into Epoch 2.

Trilobites of Thailand's Cambrian–Ordovician Tarutao Group and their geological setting

AbstractTuff‐bearing upper Cambrian to lowermost Ordovician strata on Ko Tarutao island, Satun province, southernmost peninsular Thailand, contain a rich trilobite fauna relevant to global biostratigraphy, peri‐Gondwanan palaeogeography and shifting evolutionary mode. This area of Sibumasu, a lower Palaeozoic marginal Gondwanan terrane, is shown to have been closely associated with Australia, North China (Sino‐Korea) and other continental fragments from the supercontinent's northern equatorial sector, including South China at that time. Shared faunas also suggest a Kazakhstani and Laurentian association. Collections from eight sections yielded 10 newly discovered species and one new genus from ancient shoreface and inner shelf siliciclastic deposits. With the new taxa and revision of taxa known previously, we refine the age of the upper two formations of the Tarutao Group to the middle of Cambrian Stage 10, and lower–middle Tremadocian. Two biozones are erected for Sibumasu: the Eosaukia buravasi Zone, encompassing all Cambrian sections from Ko Tarutao, and the Asaphellus charoenmiti Zone, encompassing the Tremadocian fauna discussed herein. The new genus is Tarutaoia and new species are Tsinania sirindhornae, Pseudokoldinioidia maneekuti, Pagodia? uhleini, Asaphellus charoenmiti, Tarutaoia techawani, Jiia talowaois, Caznaia imsamuti, Anderssonella undulata, Lophosaukia nuchanongi and Corbinia perforata. Other taxa reported for the first time from Tarutao are Mansuyia? sp., Parakoldinioidia callosa Qian, Pseudagnostus sp., Homagnostus sp., Haniwa mucronata Shergold, Haniwa sosanensis? Kobayashi, Lichengia simplex Shergold, Pacootasaukia sp., Wuhuia? sp., Plethopeltella sp., Apatokephalus sp., Akoldinioidia sp. 1 and Koldinioidia sp.