Great Ordovician Biodiversification Event Research Articles

The Fezouata Shale Formation biota is typical for the high latitudes of the Early Ordovician—a quantitative approach

Abstract The Fezouata Shale Formation has dramatically impacted our understanding of Early Ordovician marine ecosystems before the great Ordovician biodiversification event (GOBE), thanks to the abundance and quality of exceptionally preserved animals within it. Systematic work has noted that the shelly fossil subassemblages of the Fezouata Shale biota are typical of open-marine deposits from the Lower Ordovician, but no studies have tested the quantitative validity of this statement. We extracted 491 occurrences of recalcitrant fossil genera from the Paleobiology Database to reconstruct 31 subassemblages to explore the paleoecology of the Fezouata Shale and other contemporary, high-latitude (66°S–90°S) deposits from the Lower Ordovician (485.4–470 Ma) and test the interpretation that the Fezouata Shale biota is typical for an Ordovician open-marine environment. Sørensen's dissimilarity metrics and Wilcoxon tests indicate that the subassemblages of the Tremadocian-aged lower Fezouata Shale are approximately 20% more heterogenous than the Floian-aged upper Fezouata Shale. Dissimilarity metrics and visualization suggest that while the lower Fezouata and upper Fezouata share faunal components, the two sections have distinct faunas. We find that the faunal composition of the lower Fezouata Shale is comparable with other Tremadocian-aged subassemblages from high latitudes, suggesting that it is typical for an Early Ordovician open-marine environment. We also find differences in faunal composition between Tremadocian- and Floian-aged deposits. Our results corroborate previous field-based and qualitative systematic studies that concluded that the shelly assemblages of the Fezouata Shale are comparable with those of other Lower Ordovician deposits from high latitudes. This establishes the first quantitative baseline for examining the composition and variability within the assemblages of the Fezouata Shale and will be key to future studies attempting to discern the degree to which it can inform our understanding of marine ecosystems just before the start of the GOBE.

Marine eutrophication within the Tarim Platform in sync with Middle to Late Ordovician climatic cooling

Previous work has proposed climatic cooling and atmosphere–ocean oxygenation as potential triggers for the Great Ordovician Biodiversification Event, with the suggestion of better oxygenated oceans during the Middle to Late Ordovician. However, recent studies have argued for spatial and temporal heterogeneity in marine redox state on several continents. Here we investigate a black-shale succession accumulated within the Tarim Platform via a combination of geochemical proxies to address these debates. Negative shifts in bulk nitrogen isotopes and synchronous increases in excess phosphorus suggest moderate-high marine primary production coinciding with the development of bottom-water anoxia, as indicated by enrichments in highly reactive iron and modest concentrations of redox-sensitive trace metals (Mo, U). Moreover, the occurrence of black shale correlates well with equivalent successions formed in deep-water marginal basins along several continents, including South China, North China, Laurentia and Baltica. This may suggest an expansion of marine anoxia in low-latitude zones of the late Darriwilian to early Sandbian oceans, probably as a result of enhanced upwelling in sync with climatic cooling. The extent and ultimate cause of marine anoxia requires further quantifying constraints at a global scale, which will enable potential links between global oceanic redox conditions and concurrent biotic changes to be evaluated in more detail. Supplementary material: Geochemical dataset for the Dawangou section (Table S1) is available at https://doi.org/10.6084/m9.figshare.c.7036552 Thematic collection: This article is part of the Chemical Evolution of the Mid-Paleozoic Earth System and Biotic Response collection available at: https://www.lyellcollection.org/topic/collections/chemical-evolution-of-the-mid-paleozoic-earth-system

Ordovician origin and subsequent diversification of the brown algae

Brown algae are the only group of heterokont protists exhibiting complex multicellularity. Since their origin, brown algae have adapted to various marine habitats, evolving diverse thallus morphologies and gamete types. However, the evolutionary processes behind these transitions remain unclear due to a lack of a robust phylogenetic framework and problems with time estimation. To address these issues, we employed plastid genome data from 138 species, including heterokont algae, red algae, and other red-derived algae. Based on a robust phylogeny and new interpretations of algal fossils, we estimated the geological times for brown algal origin and diversification. The results reveal that brown algae first evolved true multicellularity, with plasmodesmata and reproductive cell differentiation, during the late Ordovician Period (ca. 450 Ma), coinciding with a major diversification of marine fauna (the Great Ordovician Biodiversification Event) and a proliferation of multicellular green algae. Despite its early Paleozoic origin, the diversification of major orders within this brown algal clade accelerated only during the Mesozoic Era, coincident with both Pangea rifting and the diversification of other heterokont algae (e.g., diatoms), coccolithophores, and dinoflagellates, with their red algal-derived plastids. The transition from ancestral isogamy to oogamy was followed by three simultaneous reappearances of isogamy during the Cretaceous Period. These are concordant with a positive character correlation between parthenogenesis and isogamy. Our new brown algal timeline, combined with a knowledge of past environmental conditions, shed new light on brown algal diversification and the intertwined evolution of multicellularity and sexual reproduction.

Ordovician conodont biodiversity analysis from the Precordillera (Argentina): a new insight in the global context of the GOBE

ABSTRACT The rapid diversification of conodonts during the Ordovician has been extensively studied by several authors, each presenting distinct diversity curves for different paleocontinents. This study analyzes trends in conodont biodiversity in the Precordillera region, spanning from the late Tremadocian to the early Sandbian ages. A conodont dataset, incorporating species previously identified in sections across Central and Eastern Precordillera, is utillized. This contribution presents the conodont diversity curve for the Lower to early Upper Ordovician in the Precordillera for the first time. The resulting conodont biodiversity pattern reveals that the initial peak of diversification occurs during the O. intermedius Zone (middle-late Floian). A sharp decline in biodiversity is observed during the Dapingian. Finally, during the early Darriwilian, there is a rise in the diversification of conodont species, reaching a peak in the L. pseudoplaunus Zone. This pattern is also compared to curves from Baltica, South China, North China, and Tarim, showing similar trends among them. The variation in Lower-Middle Ordovician conodont diversity in the Precordillera may be linked to different pulses or events within the ‘GOBE’ (Great Ordovician Biodiversification Event).

Ecological novelty at the start of the Cambrian and Ordovician radiations of echinoderms

AbstractThe Cambrian and Ordovician radiations marked the origins of all major echinoderm clades and established their Phanerozoic ecological blueprint. Recent claims of modest innovation of early echinoderms and other animals suggest constraints on novelty during the origins of phyla. Here, we document the life‐habit richness, body size, tiering, habitat usage, mobility, diet and foraging habits of 366 Cambrian–Ordovician echinoderm genera across a time‐scaled phylogeny to identify the timing and impact of novelty. Most early echinoderms were sedentary, filter‐feeding microbivores, and their body sizes, diets and modes of locomotion were largely unchanged through time, despite major volatility in taxonomic composition. Cambrian echinoderms lived close to the seafloor, with stylophorans first evolving semi‐infaunality. Many Ordovician echinoderms lived farther from the seafloor, with more complex filter‐feeding organs and tiering structure, often using other organisms as substrates. Mobile carnivores and mass‐feeders emerge then, too, across diverse asterozoans and echinozoans. Most of these novelties coincide with the origins of individual clades during the early and late Cambrian (Terreneuvian/Series 2 and Furongian), and 10–20 million years pass before most become ecologically important. Life‐habit evolution within most taxa involved new variations in these traits, with crinoids and asterozoans better able to shift strategies. The Ordovician radiation records more new variability in pre‐existing Cambrian life habits rather than radical re‐invention, with important novelties (like mobile carnivory and mass‐feeding, increased height off the seafloor, and biotic substrates) becoming ecologically more impactful. Taxa with these strategies were those best able to capitalize on the newly heterogeneous Ordovician world.

Life on the Edge: The Cambrian Marine Realm and Oxygenation

The beginning of the Phanerozoic saw two biological events that set the stage for all life that was to come: ( a) the Cambrian Explosion (the appearance of most marine invertebrate phyla) and ( b) the Great Ordovician Biodiversification Event (GOBE), the subsequent substantial accumulation of marine biodiversity. Here, we examine the current state of understanding of marine environments and ecosystems from the late Ediacaran through the Early Ordovician, which spans this biologically important interval. Through a compilation and review of the existing geochemical, mineralogical, sedimentological, and fossil records, we argue that this interval was one of sustained low and variable marine oxygen levels that both led to animal extinction and fostered biodiversification events throughout the Cambrian and Early Ordovician. Therefore, marine ecosystems of this interval existed on the edge—with enough oxygen to sustain them but with the perennial risk of environmental stressors that could overwhelm them. ▪ We review the current research on geochemistry and paleontology of the Cambrian and Early Ordovician periods. ▪ Low and oscillating oxygen levels in the marine realm promoted diversification and evolutionary innovation but also drove several extinction events. ▪ Taphonomic modes and marine authigenic pathways that were abundant in the Cambrian were supported by oceans that were persistently less oxygenated than today's oceans. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 52 is May 2024. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Reconstruction of the Yangtze Ramp during Floian to Darriwilian (Ordovician) in South China: Its Morphology, Controlling Factors and Significances

AbstractA robust stratigraphic framework and a coherent depositional ramp model for the Zitai, Dawan, Meitan and Ningkuo formations of Floian–Darriwilian age (Early–Middle Ordovician) in the Yangtze (Daoba, Xiangshuidong, Daling, Gudongkou and Honghuayuan sections) and Jiangnan regions (Nanba section) was created based on lithofacies and major element analysis. Three siliciclastic (LF1–3) and six carbonate (LF4–9) lithofacies are recognized representing sediments that were deposited in mixed siliciclastic and carbonate ramp environment. The intensity of mixed sedimentation and terrigenous input were evaluated using the elemental proxies Intensity of Mixed sedimentation (IM) and Aluminum Accumulation Rate (AlAR), as well as their mean values during certain time intervals. Mixed sediments are most well‐developed along the marginal Yangtze region, strongly impacted by recurrent influx of westerly derived terrigenous materials in response to global eustatic changes and regional tectonic movements, shaping the gently southeast‐dipping morphology. Regular terrigenous influx resulted in periods of enhanced primary productivity on the Yangtze Ramp as evidenced by matching biodiversity peaks in planktonic organisms, i.e., chitinozoans and acritarchs. Brachiopods and other shelly fauna were also able to proliferate as new niches developed along the gently dipping ramp floor with substrate changes. The biodiversification patterns suggest that terrigenous influx controlled in part by regional tectonics played a more important role than previously thought in the development of Great Ordovician Biodiversification Event in South China.

Stromatolites from the Majiagou Formation in the Ordos Basin, Northwestern China

Stromatolites declined remarkably in the Middle Ordovician, which has been speculatively related to the concomitant proliferation of metazoan reefs during the Great Ordovician Radiation Event, however, they are flourish and widely distributed in the Middle Ordovician (Darriwilian) Majiagou (Machiakou) Formation in the Ordos Basin, northwestern China. The stromatolites in the Ma-5 Member of the Majiagou Formation in the Ordos Basin exhibit significant morphological diversity, and are ready to be divided into wavy and domal stromatolites from the south and west part, columnar in the Northeast part, accompanied with rarely hemispheroid and conical ones. The stromatolites in the Ma-5 Member distribute in a subcircular ring belt, which can be subdivided into three sedimentary divisions, mostly near the boundary between the gypsum-bearing dolomite flat and gypsum salt lake, of which, the north-central division, with its thicker stromatolite-bearing strata and more complex stromatolite morphologies, was the most favorable for the establishment of stromatolites, where grazing animals were absent and burrowing animals few. Some calcified microorganisms were recovered from the stromatolites, which substantially increases their known diversity for this interval and suggestive of a severe undersampling of the middle Ordovician calcified cyanobacteria fossil record in the Ordos Basin.

Impact of global climate cooling on Ordovician marine biodiversity

Global cooling has been proposed as a driver of the Great Ordovician Biodiversification Event, the largest radiation of Phanerozoic marine animal Life. Yet, mechanistic understanding of the underlying pathways is lacking and other possible causes are debated. Here we couple a global climate model with a macroecological model to reconstruct global biodiversity patterns during the Ordovician. In our simulations, an inverted latitudinal biodiversity gradient characterizes the late Cambrian and Early Ordovician when climate was much warmer than today. During the Mid-Late Ordovician, climate cooling simultaneously permits the development of a modern latitudinal biodiversity gradient and an increase in global biodiversity. This increase is a consequence of the ecophysiological limitations to marine Life and is robust to uncertainties in both proxy-derived temperature reconstructions and organism physiology. First-order model-data agreement suggests that the most conspicuous rise in biodiversity over Earth’s history – the Great Ordovician Biodiversification Event – was primarily driven by global cooling.

Skeletal abundance of Upper Ordovician coral reefs, Lourdes Formation, western Newfoundland

Reefs experience profound evolutionary and ecological changes through the Early Paleozoic, with the rise and fall of the lower Cambrian archaeocyathan reef and the later advent of other sponge and coral reefs. The Upper Ordovician Lourdes Formation of the Long Point Group, western Newfoundland, preserves small coral bioherms in a series of fault blocks along Long Point on the Port au Port Peninsula. These bioherms, preserved in the Black Duck Member 3, contain tabulate corals, bryozoans, and stromatoporoids adding to their framework. The overlying Beach Point Member preserves deeper water, storm-dominated deposition, and bioherms are absent from this setting. An analysis of skeletal abundance through the point counting of thin sections from samples within and outside the reefs reveal a mosaic of skeletal abundance across these settings. The bioherms and associated channel deposits preserve the highest skeletal abundance whereas the settings just outside of the reefs and in facies overlying the reefs preserve notably lower skeletal abundance. This suggests that reef settings harbored a higher diversity and abundance of skeletonizing organisms, more so than environments adjacent to but outside of the reefs. Though variations in depositional processes can affect the abundance of skeletal material through physical processes, the storm-dominated facies of the Beach Point Member generally contain more abundant oncoids, peloids, and intraclasts than skeletal material. Even when examining broadly similar facies within and outside of reefs, this difference in animal abundance remains. Examining Upper Ordovician coral reefs of the Lourdes Formation reveals that the Great Ordovician Biodiversification Event resulted in an increase in skeletal abundance from their earlier reef counterparts, but that the abundance of skeletal animals is driven by particular environmental conditions in reef settings, such as supersaturation of CaCO3, wave activity fostering local increases in oxygen, and warm temperatures. In this way, reefs provided some of the most suitable places for skeletal organisms to thrive in the Early Paleozoic.

Editorial Preface to Special Issue: The radiations within the Great Ordovician Biodiversification Event

In order to improve our understanding of the biodiversification of marine life that took place during the early Palaeozoic, we present this special issue focused on the Ordovician radiations. The Great Ordovician Biodiversification Event (GOBE) is a conceptual term that is today largely used to refer to the most significant increase of marine biodiversity of the Phanerozoic that occurred during the Ordovician Period between 485 and 444 Ma. Some authors, focusing on taxonomic diversity counts of selected groups, understand the GOBE to be related to a single dramatic biodiversification event of short duration in the Darriwilian Stage of the Middle Ordovician Series between 470 and 455 Ma, whereas others follow the more traditional view and consider the Ordovician biodiversification as an aggregation of radiation events capturing a large and complex increase of taxonomic diversity but also ecological complexity of marine organisms covering the entire Ordovician. This special issue features 16 selected papers that provide different perspectives on these Ordovician biodiversification events, illustrating a variety of radiations occurring during the Ordovician Period. Several papers focus on the available biodiversity datasets and their biases, and the difficulty to distinguish and interpret the various regional and global scales. It becomes clear that the Cambrian and Ordovician radiations are artificially separated by a late Cambrian ‘Furongian Biodiversity Gap’ and that a single long-term early Palaeozoic radiation is more and more visible. A few papers provide additional data on this crucial interval across the Cambrian-Ordovician boundary displaying a much higher ecological complexity than previously assumed. Both the diversification of the plankton and the evolution of nektonic groups during the Ordovician are investigated, in papers including Computational Fluid Dynamic simulations. The virtual special issue closes with papers documenting the development of reefs during the Late Ordovician.

Redox-sensitive trace metal hyper-enrichment in Tremadocian Alum Shale (graptolite argillite) in northwestern Estonia, Baltic Palaeobasin

The Alum Shale Formation, a widespread metal-rich black shale in Baltoscandia, was deposited during the Cambrian–Ordovician transition. Given that it straddles a period of alternating extinction/recovery events following the Cambrian Explosion and prior to the Great Ordovician Biodiversification Event, the Alum Shale in Scandinavia has been extensively studied to assess the effect of oceanic redox variations on these biotic events. However, there is an on-going debate about the depositional environment and the prevailing redox state. An Early Ordovician (Tremadocian) equivalent of the Scandinavian Alum Shale, locally known as graptolite argillite, can be found in Estonia and NW Russia, in the easternmost extension of the ‘Alum Shale Sea’. In this contribution, we studied the redox-sensitive metal distribution and accumulation mechanisms in graptolite argillite successions in NW Estonia. Our aim was to provide better constraints on redox and depositional environments in the Baltic Palaeobasin. Our findings reveal that despite a consistent mineral and major elemental composition, V, Mo, Re and U contents vary systematically within the stratigraphy of the up to 7-m-thick black shale unit, with element contents reaching up to hyper-enrichment at the base (Mo > 200 mg/kg, U > 100 mg/kg and V > 1000 mg/kg) followed by a declining trend towards the upper part of the succession. This implies that temporal changes in redox conditions or sedimentation rates controlled trace element accumulation in the graptolite argillite. Although the redox-sensitive trace elements are hyper-enriched (particularly Mo), which has previously been interpreted to reflect euxinic depositional conditions, the V/Al vs. U/Al and V/Al vs. Mo/Al ratios indicate a non-euxinic anoxic depositional environment within a perennial oxygen minimum zone for the graptolite argillite. Further evidence of an unrestricted, relatively open marine depositional environment possibly influenced by upwelling is suggested by Cd/Mo and Co × Mn relationships. These results are supported by UEF and MoEF ratios that follow a seawater enrichment trend. The observed redox-sensitive element contents could be explained via diffusion-controlled uptake owing to the low sedimentation rates of the graptolite argillite (3–5 mm/kyr). These results highlight considerable redox variability within the Baltic Palaeobasin underscoring the need for a careful and critical reassessment of global redox interpretations based on Alum Shale data and emphasise the importance of using multiple independent redox and depositional environment proxies to determine palaeoredox conditions.

Microfacies Analysis of Mixed Siliciclastic-Carbonate Deposits in the Early-Middle Ordovician Meitan Formation in the Upper Yangtze Platform in SW China: Implications for Sea-Level Changes during the GOBE

During the Early-Middle Ordovician, the Upper Yangtze Platform experienced extensive development of a distinctive set of mixed siliciclastic-carbonate deposits known as the Meitan Formation. To conduct a comprehensive study of the relationship between mixed sedimentation and sea-level changes, the Hailong section, situated at the southwest margin of the Upper Yangtze Platform in SW China, was selected as the study area due to its typical mixed sedimentary sequence. This section was effectively compared with sections in other regions. Clustering analysis of the point-count groups from the Honghuayuan and Meitan Formations revealed nine microfacies that developed during this period. Such a diverse range of microfacies provides the evidence of frequent sea-level changes in the Yangtze Platform throughout this period. Specifically, two sea-level rises were identified during the early TS.2b and early TS.3b, with the early TS.2b event occurring globally. Furthermore, four sea-level falls were observed in the late TS.2b, TS.2c, TS.3a, and late TS.3b periods. The late TS.2b sea-level fall was globally significant. From TS.2c onwards, distinct variations in sea-level changes among the Yangtze Platform, North China, Baltoscandia, Australia, and North America emerged due to alterations in the paleogeographic pattern. By comparing the sea-level curves in the Upper Yangtze Platform with the diversity curves of chitinozoans, acritarchs, and brachiopods, it became evident that environmental changes played a crucial role in the Great Ordovician Biodiversification Event (GOBE), especially during the Floian. The rising sea level and increased detrital materials fostered the development of diverse habitats, prompting organisms to adapt to varying environments. In general, rising sea levels favored increased brachiopod diversity, while falling sea levels favored enhanced planktonic diversity among chitinous and acritarch species. By shedding new light on the relationship between Ordovician sea-level changes and biodiversification in the Yangtze region, this study offers a fresh perspective on the subject from the microfacies analysis.

Radiodont frontal appendages from the Fezouata Biota (Morocco) reveal high diversity and ecological adaptations to suspension-feeding during the Early Ordovician

IntroductionThe Early Ordovician Fezouata Shale Formation (485–475Ma, Morocco) is a critical source of evidence for the unfolding Great Ordovician Biodiversification Event (GOBE), the largest radiation in animal diversity during the Paleozoic. The Fezouata Shale preserves abundant remains of ancient marine organisms, including hundreds of specimens of radiodonts, a diverse and globally distributed group of stem lineage arthropods that first appeared as raptorial predators during the Cambrian Explosion.MethodsIn this work, we study 121 radiodont frontal appendages from the Fezouata Shale. Frontal appendages are the most commonly preserved body parts of radiodonts, and their well-preserved anatomical characters are crucial for describing taxonomic diversity at the species level, while also providing essential data on mode of life, paleoecology, and feeding behaviour.ResultsOur data allow for a systematic review of suspension-feeding Hurdiidae radiodonts from Fezouata. The genus Pseudoangustidontus is recognised as a radiodont and ascribed to Hurdiidae, and a new second species of this genus is identified, Pseudoangustidontus izdigua sp. nov. Aegirocassis benmoulai is also reviewed and its diagnosis amended with new details of differentiated endites in this appendage. The morphological similarity between both genera allows us to erect Aegirocassisinae subfam. nov., which groups together the suspension-feeding hurdiids of the Fezouata Shale.DiscussionSuspension-feeding radiodont appendages are more abundant than those of sediment sifting or raptorial radiodonts, with the Fezouata Shale showing the highest diversity of suspension-feeding radiodonts in the history of the group. This dominance and diversity of frontal filter-feeding appendages follows the “Ordovician Plankton Revolution”, which started in the upper Cambrian and saw a huge radiation in plankton diversity.

Progressive marine oxygenation and climatic cooling at the height of the Great Ordovician Biodiversification Event

Progressive marine oxygenation and climatic cooling at the height of the Great Ordovician Biodiversification Event

A new pustulose bivalve from the Late Ordovician of the Prague Basin (Czech Republic) and remarks on the diversification of pteriomorphids

The new genus Alena Polechová, Zicha and Rak, with its type species Alena pustulosa nov. gen., nov. sp., is described from the Sandbian Letná Formation of the Prague Basin (Czech Republic) as a new Late Ordovician bivalve with pustules. Alena pustulosa nov. gen., nov. sp. appears to be an endobyssate bivalve based on its functional morphology. It is placed within Cyrtodontida (Pteriomorphia) and included in the diversified Modiolopsis draboviensis Association from the Letná Formation, which comprises eleven bivalve species. The bivalve association from the Letná Formation is formed predominantly by pteriomorphids (now six species out of eleven) and shows that pteriomorphian groups also radiated strongly in shallow-water sands and silts in mid-latitude areas. All main groups of Pteriomorphia are already known from the Early Ordovician and diversified rapidly during the Ordovician. The Gondwana and peri-Gondwana margins with two Early Ordovician radiation centers (Central Australia and Northwestern Argentina) played important roles in their early diversification and dispersion to other palaeocontinents. The function of the sculpture in bivalves is discussed with an emphasis on the Ordovician bivalves. The ornamentation in bivalves serves several purposes such as supporting the stable position of the shell in a substrate, helping in burrowing, strengthening of shell, and protecting against predators. In the Ordovician bivalves, the main function of the sculpture is to stabilise the shell in the substrate.

Expanded marine anoxia at the Cambrian-Ordovician transition: Evidence from lime mudstone I/Ca and δ238U signatures of the GSSP in western Newfoundland, Canada

The early Paleozoic witnessed two spectacular radiations of marine organisms—the Cambrian (Є) Explosion and the Great Ordovician (O) Biodiversification Event. However, the period between them was characterized by a marine biodiversity plateau attributed to recurring biocrises. In the current study, we present the I/Ca records from slope lime mudstones of the Green Point Formation in western Newfoundland, along with a three-sink U-isotope mass balance model and previously reported limestone δ238Ucarb signals of the same interval, to further explore oceanic redox conditions at the Є-O boundary. The I/Ca ratios of the lime mudstones, deposited along the eastern Laurentian (western Iapetan) continental slope, exhibit a narrow range between 0.02 and 0.33 μmol/mol. Poor correlations between the I/Ca values and their [Sr], δ18O, Mn/Sr, Fe/Sr, Mg/Ca, and δ13Corg counterparts, together with near-micritic textures of the limestones, argue against significant influences of postdepositional alterations on the I/Ca signatures. The iodine-depleted lime mudstones, with I/Ca values well below the Proterozoic Eon baseline I/Ca ratios (∼0.5–1 μmol/mol), suggest the presence of shallow marine oxic-anoxic interfaces along the regional continental margin. Substantially low limestone I/Ca ratios (<0.5 μmol/mol) and dysoxic to anoxic depositional conditions have also been reported from several other age-equivalent sections deposited along the shelf and slope of ancient Iapetan and Laurentian continental margins. As a result, seawaters surrounding the Iapetan and Laurentian continental margins at the Є-O transition might have been commonly poorly oxygenated with shallow oxyclines or expanded oxygen minimum zones in the shelf and/or slope areas. This interpretation is further supported by our three-sink U-isotopemass balance modeling, which predicts widespread marine anoxia at the Є-O boundary with anoxic to euxinic water covering 1.0–21.1% of the ocean floor, significantly higher than the modern day (∼0.2%). Furthermore, the wide range of the oceanic δ238UOC (−1.22 to −0.25‰) values, estimated from the limestone δ238Ucarb signals, might reflect oceanic redox oscillations during this period. However, the δ238UOC fluctuations could also be attributed to variable accumulations of the 238U-enriched authigenic U phases during early diagenesis. Overall, evidence from the lime mudstone I/Ca ratios and the estimated extent of marine anoxia at the Є-O boundary in this study aligns with earlier viewpoints that the slowness of marine biodiversity accumulation during the late Cambrian and the Early Ordovician was linked to widespread oceanic anoxia.

Swimming and feeding in the Ordovician trilobite Microparia speciosa shed light on the early history of nektonic life habits

Computational Fluid Dynamic simulations (CFD) show that the Ordovician trilobite Microparia speciosa had a high stability in the horizontal plane (parallel to the flow currents), suggesting that it could be very stable in the water column when it was enrolled, for hovering or to get stabilization when the current disturbed the swimming. This indicates a new way to use enrolment in trilobites during the Ordovician radiation, not only for protection against predators or the environment but also as a hydrodynamic tool to maintain stability within the water column. Metachronal propulsion was likely used by Microparia speciosa for swimming and, in a high-viscosity scenario, also for feeding. This is suggesting given that the vortex below Microparia speciosa takes particles directly to the mouth, and taken more particles when the trilobite is orientated in the way of the current (and no countercurrent). This suggests a passive feeding strategy for better energy saving when the trilobite is hovering in water column.

Evolutionary and ecologic controls on benthos distribution from an upper Cambrian incised estuarine valley: Implications for the early colonization of marginal-marine settings

Estuarine deposits of the Furongian Pico de Halcón Member of Argentina contain trace and body fossils that allow assessing the role of evolutionary and environmental factors on early colonization of marginal-marine environments. The outer region of the estuary was characterized by a subtidal sandbody flanked by tidal flats. Vertical burrows of suspension feeders and probably passive predators are abundant in the high-energy areas of the outer estuary. Horizontal trilobite and vermiform trace fossils record the activities of mobile deposit, detritus, and suspension feeders in more protected settings. Arthropod trackways, vertical burrows, and horizontal vermiform burrows have been recorded in middle estuarine deposits. The occurrence of trackways attests the ability of some late Cambrian marine arthropods to periodically migrate inland following tidal channels or via salt-water wedges. No trace and body fossils have been found in inner valley areas. Outer and middle estuarine deposits contain the trilobite Neoparabolina frequens argentina. Its record in brackish-water settings is envisaged as a result of its generalist nature and ability (physiologic or behavioral) to cope with a wide array of conditions, flexible resource requirements, and successful spreading.Although trace-fossil distribution along the salinity gradient reflects the basics of the “brackish-water model”, the remarkably low ichnodiversity in the middle estuary and the lack of trace fossils in the inner estuary are interpreted as reflecting evolutionary controls. Teichichnus assemblages, forming monospecific occurrences or with Planolites, in fine-grained, heterolithic facies seem to be remarkably persistent through geologic time. The studied trace fossils yield insights on attempts of some clades to expand into stressful brackish-water settings early in their evolutionary histories. In addition, comparison of the environmental tolerance of the Furongian benthos with other occurrences in younger early Paleozoic estuarine settings allows to track early steps of the colonization of brackish-water environments as it developed during the Ordovician Radiation.

Coupled sedimentary and δ13C records of western Thailand and South China from Middle to Late Ordovician: Sea-level and climate changes prior to the Hirnantian glaciation

The Middle to Late Ordovician sedimentary records are of great interest because they potentially contain critical signatures about triggers of the Great Ordovician Biodiversification Event and the Late Ordovician Mass Extinction. A long climate cooling has been proposed by some scholars, which may have initiated in Middle Ordovician and eventually culminated in the Late Ordovician (Hirnantian) glaciation; however, the origin, frequency, magnitude and duration of this cooling-glaciation event are still in debate. A sedimentary and multi-stratigraphic study of the Middle to Late Ordovician transition records may help reveal the evolutionary history of this great climate change. Here we document and correlate the sedimentary facies, conodont biostratigraphy and carbon isotope chemostratigraphy of latest Darriwilian through early Katian limestone successions in the Kanchanaburi area of western Thailand and in South China. This correlation reveals two remarkably cross-regional transgression events and their following sea-level highstand periods. The first transgression happened in the latest Darriwilian, leading to the deposition of the Tha Manao Formation of restricted carbonate facies at the Nautiloid Site Geopark in Thailand, Sibumasu Terrane, and the Miaopo and chert-rich Hulo formations of black shale facies in South China. The second transgression was initiated in the earliest Katian, forming the extensively distributed open-marine carbonates of the Pa Kae and Pagoda formations in platform settings, and the debrite-rich, deep-water carbonate of the Yenwashan Formation in slope settings. We also document coupled changes in depositional facies and carbon isotopic compositions, showing that the rising limb of the Guttenberg Isotopic Carbon Excursion (GICE) co-occurred with the second transgression period. The South China and Thailand Sibumasu records are comparable to those of many North American successions, indicating a presumably global sea-level rise in the early Katian, which may have been caused by the waning of the potential Darriwilian–Sandbian icecaps.