Explore the vast range of titles available.

It is well-acknowledged that the northern margin of the Gondwana supercontinent was affected by a major magmatic event at late Cambrian (Furongian) to early Ordovician (Tremadocian–Floian) times. However, an accurate assessment of its extent, origin, and significance is partly hampered by the incomplete characterization of the numerous gneiss massifs exposed in the inner part of the Variscan belt, as some of them possibly represent dismembered and deformed Furongian–Lower Ordovician igneous bodies. In this study, we document the case of the “Cézarenque–Joyeuse” gneiss complex in the Cévennes parautochthon domain of the French Massif Central. The gneisses form decametre- to kilometre-thick concordant massifs interlayered within a pluri-kilometric sequence of mica- and quartz schists. They encompass two main petrological types: augen gneisses and albite gneisses, both typified by their blue and engulfed quartz grains with the augen facies differing by the presence of centimetre-sized pseudomorphs after K-feldspar and the local preservation of igneous textures. Whole-rock geochemistry highlights that many gneisses have magmatic ferrosilicic (acidic with anomalously high FeO t and low CaO) compositions while others are akin to greywackes. Collectively, it is inferred that the bulk of the Cézarenque–Joyeuse gneisses represents former rhyodacite lava flows or ignimbrites and associated epiclastic tuffs. Volumetrically subordinate, finer grained, and strongly silicic leucogneisses are interpreted as microgranite dykes originally intrusive within the volcanic edifices. LA–ICP–MS U–Pb dating of magmatic zircon grains extracted from an augen gneiss and a leucogneiss brackets the crystallization age of the silicic magmas between 486.1 ± 5.5 Ma and 483.0 ± 5.5 Ma which unambiguously ties the Cézarenque–Joyeuse gneisses to the Furongian–Lower Ordovician volcanic belt of SW Europe. Inherited zircon date distributions, Ti-in-zircon and zircon saturation thermometry demonstrate that they formed by melting at 750–820 °C of Ediacaran sediments. Zircon Eu/Eu* and Ce/Ce* systematics indicate that the melts were strongly reduced (fO 2 probably close to the values expected for the iron–wüstite buffer), possibly because they interacted during ascent with Lower Cambrian black shales. This would have enhanced Fe solubility in the melt phase and may explain the peculiar ferrosilicic signature displayed by many Furongian–Lower Ordovician igneous rocks in the northern Gondwana realm. We infer that crustal melting resulted from a combination of mantle-derived magma underplating in an intracontinental rift setting and anomalously elevated radiogenic heat production within the Ediacaran sedimentary sequences. Un important évènement magmatique a affecté la marge Nord du Gondwana de la fin du Cambrien (Furongien) au début de l’Ordovicien (Trémadocien–Floien). Afin de préciser son extension géographique et de mieux contraindre son origine et sa signification géodynamique, il est essentiel de caractériser finement les nombreux massifs de gneiss affleurant dans les zones internes de la chaîne Varisque, certains d’entre eux représentant probablement d’anciens corps magmatiques cambro-ordoviciens. Cet article présente de nouvelles données sur les gneiss dits de la « Cézarenque » ou de « Joyeuse » qui affleurent dans le domaine para-autochtone cévenol du sud du Massif Central français. Ces gneiss forment des massifs concordants dans la schistosité régionale, d’une épaisseur variant de la dizaine de mètres à plusieurs kilomètres, intercalés dans une épaisse séquence de mica- et quartz-schistes. Les deux types pétrographiques principaux sont un gneiss oeillé et un gneiss albitique, tous deux caractérisés par la présence de « phénocristaux » de quartz bleus d’origine magmatique, les gneiss oeillés se distinguant par leurs pseudomorphes centimétriques de feldspath potassique. Les compositions chimiques en roche totale démontrent que de nombreux gneiss ont des compositions magmatiques et se rattachent aux séries ferrosiliceuses (acides, anormalement riches en FeO t et pauvres en CaO). A contrario , certains gneiss albitiques ont des compositions qui les rapprochent de grauwackes. L’ensemble de ces observations suggère que les gneiss de Cézarenque–Joyeuse représentent d’anciennes coulées (pyroclastiques ou laviques) rhyodacitiques et leurs produits de remaniements proximaux. Un rare faciès de leucogneiss, subordonné en volume, correspondrait à d’anciens microgranites originellement intrusifs dans les édifices rhyodacitiques. Les datations U–Pb par LA–ICP–MS de grains de zircons extraits d’un gneiss oeillé et d’un leucogneiss indiquent que les magmas ont cristallisé entre 486,1 ± 5,5 et 483,0 ± 5,5 Ma, affiliant de fait les gneiss de Joyeuse–Cézarenque à la chaîne volcanique d’âge Furongien–Ordovicien Inférieur d’Europe de l’Ouest. La gamme d’âge des zircons hérités et la teneur en Ti des zircons magmatiques indiquent que les liquides proviennent de la fusion à 750–820 °C de sédiments d’âge Ediacarien. Les ratios Eu/Eu* et Ce/Ce* des grains de zircon démontrent que ces liquides étaient fortement réduits (fO 2 proche du tampon fer–wustite), potentiellement à la suite de leur interaction en route vers la surface avec les sédiments riches en matière organique du Cambrien Inférieur. Ce caractère réduit a pu augmenter la solubilité du fer dans les liquides et expliquerait de fait le caractère ferrosiliceux. Cet évènement de fusion partielle de la croûte continentale est vraisemblablement lié au sous-placage de magmas basiques dans un contexte de rift intracontinental et à la production de chaleur radiogénique anormalement élevée des séquences sédimentaires édiacariennes.

The Early Paleozoic oceans were generally characterized by short trophic chains and simple ecological tiering dominated by suspension-feeding organisms. Howev-er, the Great Ordovician Biodiversification Event (GOBE) was responsible for the complexification of food webs, increasing depth and diversity of substrate ecospace utilization and increasing benthic competition for resources near the water-sub-strate interface. Daedalus is an enigmatic trace fossil that precedes this period of nearshore innovation and its disappearance is probably related to the escalation in tiering complexity, especially among benthic surficial feeders, that may have cut access to their main food sources. Daedalus producers were responsible for the oc-cupation of some of the earliest deep substrate tiers, probably feeding from particu-late organics and meiofauna using complex architectures for resource exploitation. They were responsible for characteristic ichnofabrics that can be found, especial-ly in Lower-to-Middle Ordovician sandstones, in the “Armorican Quartzite” and similar nearshore facies around Gondwana. Villuercas-Ibores-Jara and Naturtejo UNESCO Global Geopark show several geosites where the best-preserved forms of Daedalus can be found and where its crowded ichnofabrics can be followed in large-scale exposures. Indeed, these are mega-ichnosites of international paleontological relevance for understanding behavior and ecospace utilization of a characteristic trace fossil during the GOBE. This paleoecological significance meant that Daeda-lus became a symbol for a cross-border Interreg Project that connects the two UNE-SCO geoparks. The Armorican Quartzite Route is an 800 km-long road following the main geological structures and promoting sustainable tourism attractions along the way. The Bridge over the Armorican Quartzite has the goal of fostering sustainable development through common scientific and education tools, raising awareness of the GOBE as one of the most important events of biodiversification in the Earth history and a key element of the geological heritage of both UNESCO Geoparks.

The Durness Group of NW Scotland records deposition on the Laurentian margin from the basal Miaolingian (Cambrian, 509 Ma) to the Dapingian–Darriwilian boundary interval (Middle Ordovician, 470.3–468.9 Ma). The 930 m thick succession of peritidal and subtidal carbonates was deposited on the Scottish promontory, a nearly 120° deflection in the Palaeozoic continental margin between the Appalachian and Greenland sectors. These sediments were deposited as part of the Great American Carbonate Bank, a non-uniformitarian, continent-scale carbonate platform developed on the peneplaned craton. Measurement and description of a bed-by-bed composite section through the Durness Group provide a high-resolution reference framework that integrates conodont biostratigraphy, chemostratigraphy and sequence stratigraphy, including correlation with the Sauk megasequence and its subdivisions. The Sauk II–Sauk III sequence boundary marks the base of the group. The top of the group is faulted against rocks of the Moine thrust zone, generated by the Scandian orogeny, but sedimentation was probably terminated by the earlier Grampian arc–continent collision at 470–469 Ma. The highly mature quartz arenites of the underlying Ardvreck Group (Cambrian Series 2) indicate that there was no source-to-sink depositional continuity from the Hebridean foreland to the Dalradian Supergroup, which has coeval clastic sedimentary rocks of contrasting composition.

During the Early Ordovician Epoch, the Mediterranean brachiopod Province was extensive in the higher-latitude sectors of the globe in the Southern Hemisphere. The latter was much occupied by the massive continent of Gondwana, which stretched from north of the Equator S-wards to cover the South Pole. The Mediterranean Province can be separated into two groups: Group 1, the higher-latitude fauna dominated by large linguliform brachiopods; and Group 2, which is more diverse, particularly in orthides. The large linguliform brachiopod faunas are particularly well known in southern Europe (France, Spain and Bohemia) and North Africa, and the second group in Avalonia, Chile and Argentina. The province is different from, but merges with, more diverse contemporary faunas in the lower latitudes of Gondwana to its north, although the latter contrast with other lower-latitude faunal provinces in South China, Laurentia, Siberia and elsewhere. Since the Rheic Ocean between Avalonia and Gondwana was relatively narrow during the Early Ordovician Epoch, the Avalonian brachiopods were integral parts of the Mediterranean Province, but only until end of the Dapingian Age. This paper focuses on the earlier phases of the Mediterranean Province, although the province continued until near the end of the Ordovician Period. Intermediate-latitude Baltica and some other faunas are included in new principal components and other analyses in order to compare them with the Mediterranean Province faunas. Radiation was very significant for many brachiopod taxa during the period, with first appearances of the Plectambonitoidea (Taffiidae), several orthide families (Euorthisinidae, Tarfayidae and Anamalorthidae) and the earliest endopunctate orthide, the dalmanelloid Lipanorthis.

The Fezouata Shale Formation in present-day Morocco is a site of exceptional fossil preservation from the Lower Ordovician that provides a unique view of animal life before one of the most important radiation events in Earth's history, the great Ordovician biodiversification event (GOBE). Previous work on the fossil diversity of the Fezouata Shale has suggested that there are faunistic differences between the two major intervals with exceptional preservation and that the overall shelly biota of the Fezouata Shale is comparable to other Lower Ordovician sites that reflect open-marine conditions. In this study, we make the first comprehensive quantitative comparison between the Fezouata Shale Formation and other high-latitude Early Ordovician sites based on their shelly fossil biotas with publicly available fossil occurrence information from the Paleobiology Database. We find that the fossil subassemblages of the stratigraphically older lower Fezouata Shale are more heterogeneous than those of the younger upper Fezouata Shale. The fossil biota preserved in the lower Fezouata Shale is most similar to those found in other high-latitude deposits from the Lower Ordovician. We also find that there are differences in faunal composition between Tremadocian- and Floian-aged deposits. Our work provides the first quantitative support for faunistic differences between the lower and upper Fezouata Shale Formation and indicate that the lower Fezouata Shale conventional fossil biota is typical for the Tremadocian, further contextualizing the ecology of the polar regions before the GOBE as informed by this major site of exceptional fossil preservation. 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.

The Arenig Fawr area of North Wales constitutes the type area for the British Lower to Middle Ordovician Arenig Series and is complemented by sections in the Carmarthen and Whitland areas of South Wales. We describe chitinozoan assemblages from both areas in order to aid correlation of the Arenig Series in its type region with the global Ordovician series and stages. Chitinozoans recorded from Arenig Fawr provide permissive rather than conclusive evidence but suggest that the Henllan Ash Member correlates with the upper Floian Stage Slice Fl3 or lower Dapingian Stage Slice Dp1. Better results were obtained from South Wales where six chitinozoan assemblages are distinguished, ranging in age from late Tremadocian to middle Darriwilian (early Llanvirn). Most species are known from South China, Gondwana and/or Baltica where there are controls on ranges. They show that much of the lower Arenig (Moridunian) succession in South Wales correlates with the upper Floian Stage (Fl3). Correlatives of the lower and middle Floian Stage (Fl1, Fl2), if present, must be represented by the Ogof Hên Formation and lowest Carmarthen Formation. Chitinozoan assemblages from the upper Arenig Series (Fennian Stage) are more readily correlated with Gondwanan biozones and indicate correlation of the Fennian Stage with the Dapingian and lower Darriwilian (Dw1) stages. The middle Arenig Whitlandian Stage is constrained in South Wales to an interval from the uppermost Floian Stage to the basal Dapingian Stage, resulting in an inferred increased rate of sediment accumulation.

Evidence from the earliest-known crinoids (Tremadocian, Early Ordovician), called protocrinoids, is used to hypothesize initial steps by which elements of the calyx evolved. Protocrinoid calyces are composed of extraxial primary and surrounding secondary plates (both of which have epispires along their sutures) that are unlike those of more crownward fossil and extant crinoids in which equivalent calycinal plating is strongly organized. These reductions inspired several schemes by which to name the plates in these calyces. However, the primary-secondary systems seen in protocrinoids first appeared among Cambrian stem radial echinoderms, with primaries representing centers around which secondaries were sequentially added during ontogeny. Therefore, the protocrinoid calyx represents an intermediate condition between earliest echinoderms and crownward crinoids. Position and ontogeny indicate certain primaries remained as loss of secondaries occurred, resulting in abutting of primaries into the conjoined alternating circlets characteristic of crinoids. This transformative event included suppression of secondary plating and modification or, more commonly, elimination of respiratory structures. These data indicate subradial calyx plate terminology does not correspond with most common usage, but rather, supports an alternative redefinition of these traditional expressions. Extension and adoral growth of fixed rays during calyx ontogeny preceded conjoined primaries in earliest crinoids. Restriction with modification or elimination of calyx respiratory structures also accompanied this modification. Phylogenetic analyses strongly support crinoid origination from early pentaradiate echinoderms, separate from blastozoans. Accordingly, all Tremadocian crinoids express a distinctive aggregate of plesiomorphic and apomorphic commonalities; all branch early within the crinoid clade, separate from traditional subclass-level clades. Nevertheless, each taxon within this assemblage expresses at least one diagnostic apomorphy of camerate, cladid, or disparid clades.

Experimental burial of polychaete (Nereis) and crustacean (Crangon) carcasses in kaolinite, calcite, quartz, and montmorillonite demonstrates a marked effect of sediment mineralogy on the stabilization of nonbiomineralized integuments, the first step in producing carbonaceous compression fossils and Burgess Shale–type (BST) preservation. The greatest positive effect was with Nereis buried in kaolinite, and the greatest negative effect was with Nereis buried in montmorillonite, a morphological trend paralleled by levels of preserved protein. Similar but more attenuated effects were observed with Crangon. The complex interplay of original histology and sediment mineralogy controls system pH, oxygen content, and major ion concentrations, all of which are likely to feed back on the preservation potential of particular substrates in particular environments. The particular susceptibility of Nereis to both diagenetically enhanced preservation and diagenetically enhanced decomposition most likely derives from the relative lability of its collagenous cuticle vs. the inherently more recalcitrant cuticle of Crangon. We propose a mechanism of secondary, sediment-induced taphonomic tanning to account for instances of enhanced preservation. In light of the marked effects of sediment mineralogy on fossilization, the Cambrian to Early Ordovician taphonomic window for BST preservation is potentially related to a coincident interval of glauconite-prone seas.

Caryocaridids are a unique representative of the pelagic arthropod group during the Ordovician and stand out from other arthropods (ostracods, trilobites, etc.) for their remarkable pelagic abilities. Herein, we report on a species of caryocaridids, Soomicaris cedarbergensis, from the Lower Ordovician in northwestern Xinjiang, NW China, which shows the rare enrolled carapaces with the evidence of cuticular ultrastructure preserved. These caryocaridid specimens from Xinjiang provides the substantial evidence for the presence of caryocaridids in the Central Asian Orogenic Belt. This discovery suggests that S. cedarbergensis appeared as early as the Early Ordovician (late Tremadocian) and persisted until the end-Ordovician (Hirnantian) and is the longest-ranging species of known caryocaridids. The cuticle of the carapace in S. cedarbergensis is preserved in carbonate-fluorapatite, which can be divided into three mineralized lamellae (outer, middle, and inner). The outer and inner lamellae both consist of three layers, which seem to correspond to the epicuticle, exocuticle, and endocuticle of extant crustacean carapaces, respectively. The particular ultrastructure of the carapace of Ordovician caryocaridids (thin cuticle; thickened inner lamella cuticle; and large, complex oxygen supply system) probably represents an adaptation to the pelagic lifestyle during the Ordovician plankton revolution. Caryocaridids are a unique representative of pelagic arthropods from the Ordovician period. They are typically found as flattened carapaces in mudstones and shales. This study reports on a species of caryocaridids, Soomicaris cedarbergensis, discovered in the Lower Ordovician of northwestern Xinjiang, NW China. The species shows the rare enrolled carapaces with a preserved cuticular ultrastructure. These specimens of caryocaridids from Xinjiang are the first reported in the Yili Block, and provide the substantial evidence that the paleogeographic distribution of caryocaridid phyllocarids could extend to the Central Asian Orogenic Belt. This species existed from the late Tremadocian until the end of the Ordovician (Hirnantian), making it the longest-ranging known species of caryocaridids. The carapace cuticle of S. cedarbergensis is composed of carbonate-fluorapatite and can be divided into three mineralized lamellae: outer, middle, and inner. The outer and inner lamellae each consist of three layers that correspond to the epicuticle, exocuticle, and endocuticle of extant crustacean carapaces. Moreover, the polygonal reticulation structure of the carapace in archaeostracans appears to be similar in shape and size to the hemolymph sinuses of leptostracans. This unique ultrastructure of the carapace cuticle in caryocaridids is believed to be better suited for a pelagic lifestyle.

The origins and age distribution of the Himalayan high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks are critical for understanding the pre-Himalayan history. Although the protoliths to the UHP Tso Morari eclogites in Ladakh, NW Himalaya are believed to be the Permian Panjal volcanics, the geochronological evidence is absent. Here, we demonstrate that the protoliths of the UHP Tso Morari Complex formed in a continental rift setting at the Indian margin associated with the northern East Gondwana during the Early Paleozoic. Zircon U–Pb dates from eight gneisses and one garnet amphibolite indicate the Early Paleozoic bimodal magmatism of 493–476 Ma, which could be associated with the separation of South China from North India. Except for arc-related eclogites found in the Nidar ophiolite, the eclogites and amphibolites are rift-related, exhibiting enriched light rare earth elements and high concentrations of incompatible elements, along with evidence for crustal contamination. Our findings support the previously reported diversity in the sources and ages of the protoliths of the Himalayan HP–UHP metamorphic rocks along the orogen.