The composition of altered volcanic ash of the Late Ordovician Kinnekulle bed was studied in geological sections of the Baltic Paleobasin. The composition of altered ash varies with paleosea depth from northern Estonia to Lithuania. The ash bed in shallow shelf limestones contains an association of illite-smectite (I-S) and K-feldspar, with the K2O content ranging from 7.5 to 15.3%. The limestone in the transition zone between shallow- and deep-shelf environments contains I-S-dominated ash with K2O content from 6.0 to 7.5%. In the deep-shelf marlstone and shale, the volcanic ash bed consists of I-S and kaolinite with a K2O content ranging from 4.1 to 6.0%. This shows that authigenic silicates from volcanic ash were formed during the early sedimentary-diagenetic processes. The composition of the altered volcanic ash can be used as a paleoenvironmental indicator showing the pH of the seawater or porewater in sediments as well as the sedimentation rate.

Field evidence from the northern Cotentin Peninsula and regional data are used to construct a tectono-stratigraphic model for the Ordovician which characterizes basin development in the North Armorican Massif. In La Hague, 15 m of transgressive marine sandstone belongs to the Dapingian age Grès Armoricain Formation which onlaps lower Cambrian, rift-fill deposits via an unconformity. Approximately 450 m of overlying Darriwilian strata are dominated by shallow marine sandstone showing hummocky cross-stratification with subordinate shales containing trace and body fossils. Together, these facies support an interpretation of offshore shallow marine strata overlying a break-up unconformity. Regional analysis indicates the time gap at the unconformity is 20–40 Ma and formed from crustal upwarping, which was greatest in the north of the Armorican Massif. Dapingian strata (Grès Armoricain) thins irregularly to the north (0–94 m), interpreted to reflect passive onlap onto residual relief associated with the uplift and the initiation of thermal subsidence on the margin. The succeeding Darriwilian strata (Schistes de Beaumont to Grès de May) conversely display a steady thickening (161–623 m) to the north, the stratal patterns suggesting that from the Darriwilian onwards, the ocean basin to the north was firmly established as the main locus of subsidence on the continental margin.

Sedimentary rocks exposed at Dob’s Linn, Scotland, have significantly influenced our understanding of how life evolved over the Ordovician to Early Silurian. The current interpreted chronostratigraphic boundary between the Ordovician and Silurian periods is a Global Boundary Stratotype Section and Point (GSSP), calibrated to 443.8 ± 1.5 Ma (Hirnatian–Rhuddanian age), based on biostratigraphic markers, radioisotopic dates and statistical modelling. However, challenges arise due to tectonic disturbances, complex correlation issues and the lack of systematic dating in Ordovician–Silurian stratigraphic sections. Here, hundreds of zircon grains from three metabentonite ash horizons were dated using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). A subset of the grains were re-analyzed using Chemical Abrasion Isotope Dilution Thermal Ionization Mass Spectrometry (CA-ID-TIMS). We present a high-precision CA-ID-TIMS 238U-206Pb weighted mean date of 440.44 ± 0.55/0.56/0.72 Ma (±analytical/with tracer/with U-decay constant) for the Coronagraptus cyphus biozone. However, the study reports younger, and in certain cases, older LA-ICP-MS zircon dates within the Coronagraptus cyphus, Akidograptus ascensus and Dicellograptus anceps zones, suspected as being influenced by Pb loss and LA-ICP-MS matrix mismatch. The study reports concerns about the suitability of Dob’s Linn as a GSSP section and examines various LA-ICP-MS maximum depositional age (MDA) approaches, suggesting the use of the TuffZirc date and the youngest mode weighted mean (YMWM) as suitable MDA calculations consistent with CA-ID-TIMS results.

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.

The shift of matground ecosystems to bioturbator-free settings was investigated in the Ordovician wave-dominated marine strata of the Shirgesht Formation in Central Iran. Ten ichnofabrics are recognized in shoreface–offshore and barrier-island sedimentary facies, representing a proximal-to-distal depositional trend along the studied profile. In the offshore settings, intensive burrowing on several tiers and bioirrigation, referred to the Thalassinoides (Th), Cruziana–Skolithos–Rosselia (CrSkRo), crowded Trichophycus (CT) and Cruziana–Helminthopsis (CrHe) ichnofabrics, prevented the development of matgrounds. As a result of the Ordovician radiation, progressive ecospace utilization by suspension-, deposit- and detritus-feeders, as represented by the Skolithos (Sk), Rosselia–Skolithos (RoSk), crowded Rosselia (CR) and Conichnus–Skolithos (CoSk) ichnofabrics, is regarded as a key factor for sediment mixing and reworking and thus disruption of the matgrounds in shoreface settings. However, the restriction on microbial growth was reduced in lower-offshore / shelf or tidal-flat and back-barrier settings, where Chondrites–Planolites (ChPl) and Planolites (Pl) ichnofabrics dominate. This restricted microbial growth resulted from physico-chemical stresses on infaunal communities, as shown by the low ichnodiversity, scattered burrowing, absence of complex tiering, and prevalence of simple feeding strategies. This study suggests that following the early Palaeozoic evolutionary radiations, a considerable increase in abundance of detritus-feeders, deposit-feeders, suspension-feeders, predators, vagile bilaterian metazoans and grazers in shallow-water benthic communities caused mat-building microorganisms to migrate into lower-offshore / shelf, tidal-flat and back-barrier settings, where colonization by burrowing organisms was delayed.

Graptolites have been collected from sections through Lower Ordovician strata in northern Iran. At the Saluk Mountains, in the Kopet–Dagh region, mudrocks yielded fragmentary tubaria of Rhabdinopora sp. cf. R. flabelliformis, indicating the presence of lower Tremadocian strata there; stratigraphically, they lie between two limestone beds with the euconodont Cordylodus lindstromi. At Simeh–Kuh in the eastern Alborz Mountains (Semnan Province), upper Tremadocian – lower Floian strata include laminated dark mudstones that contain restricted graptolite faunas, mainly of small declined didymograptids; these are thought to represent incursions of plankton during periods of marine highstands. The lower major flooding surface in Simeh–Kuh coincides with an invasion of the graptolite biofacies and an incursion of Hunnegraptus? sp.; the second major flooding surface is associated with an incursion of Baltograptus geometricus. They were most probably synchronous with those in the lower part of the Hunnegraptus copiosus Biozone and at the base of the Cymatograptus protobalticus Biozone in the of the Tøyen Shale Formation succession of Västergötland, Scandinavia, suggesting that observed characters of sedimentation were eustatically controlled.

Guided by conodont biostratigraphy and unconformities observed in the field, stable carbon isotopic analysis (δ13Ccarb) was performed on 210 samples from Lower–Middle Ordovician (Tremadocian to Darriwilian) sections and wells in the Tarim Basin, NW China. The δ13C trend in the Tarim Basin sections has three distinct characteristics: (1) from the Tremadocian to the Floian, a positive shift from −1.9 ‰ to −0.2 ‰ is observed near the boundary between the Penglaiba Formation and the Yingshan Formation; (2) from the Floian to the Dapingian, a positive shift in δ13C from −3 ‰ to −0.7 ‰ occurred under large-scale sea-level rise and a change in the sedimentary environment from a restricted platform to an open platform. Changes in the conodont type are also observed in the Tabei region; and (3) from the Dapingian to the Darriwilian, δ13C first decreased and then increased, showing a negative shift at the Dapingian–Darriwilian boundary. During the Floian, δ13C decreased in the study area, while it first decreased and then increased in other regions, which may reflect local sea-level movements in response to isostatic crustal movements. Two types of positive shift were identified at the Floian–Dapingian boundary, which likely show the effects of local factors, including a disconformity, dolomitization, and platform restriction, superimposed on the global signal of the carbon isotope. Some conodont zonations and recurrent negative excursions in Tremadocian, Floian and Dapingian stages appear to be truncated by unconformities, which are accompanied by short-term subaerial exposure due to sea-level fall and local tectonic uplift.

Nektaspids are Palaeozoic non-biomineralized euarthropods that were at the peak of their diversity during the Cambrian Period. Post-Cambrian nektaspids are a low-diversity group with only a few species described so far. Here we describe Tariccoia tazagurtensis, a new species of small-bodied nektaspid from the Lower Ordovician Fezouata Shale of Morocco. The new species differs from the type (and only other known) species from the Ordovician strata of Sardinia (Italy), Tariccoia arrusensis, in possessing more pointed genal angles, a cephalon with marginal rim, a pygidium with anterior margin curved forwards, a rounded posterior margin, and longer and more curved thoracic tergites. The two specimens of T. tazagurtensis sp. nov. show remains of digestive glands that are comparable to those seen in the Cambrian nektaspid Naraoia. The rare occurrence of T. tazagurtensis sp. nov. in the Fezouata Shale and the distribution of other liwiids suggest that these liwiids were originally minor members of open-marine communities during the Cambrian Period, and migrated into colder brackish or restricted seas during the Ordovician Period.

Investigation of the Fezouata Shale has added to our knowledge on the initial diversification of metazoans. These Lower Ordovician deposits yielded abundant and diverse remains of cuticularized to lightly sclerotized organisms, in addition to numerous soft tissues. Described fossilized soft parts recovered from the Fezouata Shale belong mainly to arthropods. Soft tissues in echinoderms, a main component of the Fezouata Biota, remain largely unexplored. Here, we show that soft tissue-like impressions previously reported in eocrinoid and somasteroid echinoderms from this formation, are the results of modern weathering and fossil preparation that involved the use of latex molds: they do not reflect any original (soft) anatomical features of these organisms. These two examples suggest that reports of putative soft parts, especially in taxa with no current representatives, need to be thoroughly and critically evaluated.

Mixed-layer illite-smectite samples from the Ordovician and Silurian K-bentonites of the Baltic Basin and the Baltic Shield (Norway, Sweden, Denmark, Poland and Estonia) were dated by K-Ar on several grain fractions and were studied by X-ray diffraction (XRD), both on oriented and random preparations, in order to reveal the conditions of smectite illitization in the area. Authigenic K-feldspar was also dated. The geographic pattern of the degree of illitization (% smectite in illite-smectite measured by XRD) is consistent with other indicators of palaeotemperatures (acritarchs, conodont alteration index, vitrinite reflectance, apatite fission track ages). It reveals the highest maximum palaeotemperatures (up to at least 200ºC) along the Norwegian and the German-Polish branches of the Caledonides and the lowest palaeotemperatures (120ºC) in the central part of the studied area. The distribution of K-Ar ages is not well correlated with this pattern, revealing a zone of older ages (Lower Devonian-Lower Carboniferous) between Denmark and Estonia, and areas of younger ages (Upper Devonian to Carboniferous/Permian boundary) to the north and south of this zone. The zone of older ages is interpreted as the result of illitization induced by a thermal event in front of the Caledonian orogenic belt (migration of hot metamorphic fluids?). The areas of younger ages are considered as representing deep burial illitization under a thick Silurian-Carboniferous sedimentary cover, perhaps augmented by a tectonic load. The K-Ar dates invalidate the hypothesis of a long-lasting low-temperature illitization as the mechanism of formation of the Estonian Palaeozoic illite-smectite. The ammonium content of illite-smectite from the Baltic K-bentonites reflects the proximity of organic-rich source rocks that underwent thermal alteration at the time of illite crystallization.

Ion probe data for zircons from tuffs within the Llfynant flags (Arenig) and the Serw Formation (lower Llanvirn) of north Wales have been revised using better statistical methods for separating detrital ages, making allowance for recently-found variability in radiogenic 206Pb/238U in the reference zircon SL13, and testing the sensitivity of the ages to the secondary ion discrimination slope. The revised ages are options of 469.2 ± 2.1 (σ) or 472.9 ± 2.9 Ma for the Llfynant flags dependent on mixture modelling, and 465.3 ± 1.4 Ma for the Serw Formation. All ages are within error of previous SHRIMP results and the Serw age now has the same numerical value as a previous MSID age for the same sample. It is shown that an MSID age of 483 ± 0.5 Ma with interpreted Pb loss for a late Tremadoc bentonite is dependent on the correction for common Pb, and that a slightly more radiogenic choice for the common Pb composition places nearly all data on Concordia. The latter would indicate that the bentonite might contain two zircon populations: inherited grains at 482 Ma and tuff magmatic grains at 473 Ma, which is more compatible with the SHRIMP Arenig result. Interpretations of other MSID zircon ages from the Ordovician are also sensitive to choice of common Pb, and raise the likelihood that many multigrain ages might be too old owing to admixture with slightly older inherited zircon. A supposed 1–2% technical bias of SHRIMP 206Pb/238U ages relative to MSID is refuted.

Olistostromes with calcareous olistoliths are rare components in the Ordovician successions in NW Europe and North America, having been described from only a small number of localities. One of the best exposed, but least known, is in the Garn Formation in coastal outcrops in Anglesey in northwestern Wales. Here, in the graptolite-bearing shales of the Garn Formation, there are numerous limestone olistoliths that are derived from an otherwise unknown ‘ghost’ formation whose original depositional site remains an enigma. These olistoliths contain a Baltoscandian type of conodont fauna that is otherwise unknown in Wales and England. It represents the Baltoniodus variabilis Subzone of the Amorphognathus tvaerensis Zone. Similar, but slightly older, conodont faunas are recorded from olistoliths in the Tweeddale Member of the Shinnel Formation in southern Scotland and in probable olistoliths of the Cobbs Arm Limestone in northeasternmost Newfoundland. Approximately coeval conodont faunas are present in calcareous olistoliths in the Woods Hollow Shale of West Texas and the Womble Shale in the Ouachita Mountains of Arkansas, USA. Lithological and conodont evidence indicates that the calcareous olistoliths were derived from carbonate sediments deposited in relatively shallow water. It is concluded that the study of ‘ghost’ formation olistoliths may provide otherwise unavailable but important data bearing on the marine depositional history of a particular region.

Aglaspidid euarthropods are usually preserved as flattened carapaces. We here describe a new species and genus, Gogglops ensifer, from the Ordovician Shaanxi Province, China, which shows the best three-dimensional (3D) preservation yet discovered. We show that the cuticle was originally composed of calcium carbonate, except for the visual surfaces of the eyes which were probably originally organic but have been selectively phosphatized. A review is given of the problems pertaining to the original nature of the aglaspidid cuticle. Gogglops is probably closely related to certain poorly known aglaspidid genera from Ordovician deposits of the Siberian Platform. Phylogenetic analysis indicates that the new genus is closely related to Aglaspis itself, and securely nested within the clade Aglaspidida.

The lower Palaeozoic marine succession of NE Vietnam accumulated on the South China plate. Despite historical works dating to French colonial times, the stratigraphy and palaeontology of the succession is poorly constrained. Chief amongst the lower Palaeozoic lithostratigraphical divisions is the Than Sa Formation, a c. 1200 m thick succession of clastic rocks of Cambrian and early Ordovician age. Newly collected graptolites (including Rhabdinopora? sp. and Tetragraptus approximatus) from the upper part of the formation identify strata assignable to the Tremadocian and Floian stages of the Lower Ordovician. The same succession also probably records the Cambrian–Ordovician boundary. Our analysis identifies one or more intervals of graptolite-bearing laminated mudstones in the upper part of the Than Sa Formation that may be widespread in NE Vietnam during the early Tremadocian.

The Los Sombreros Formation represents the western continental margin slope deposits of the Argentine Precordillera, a sub-terrane accreted to Gondwana as part of the Cuyania Terrane in early Palaeozoic times. The age of these gravity-driven deposits is controversial and, therefore, a precise biostratigraphic scheme is essential to reveal the evolution of the continental margin. New conodont samplings along with sedimentological and structural analysis carried out in the Los Sombreros Formation in the La Invernada Range provide clues to its depositional framework. The sedimentary succession is made up of dominantly calciturbidites, carbonate breccias and conglomerates, along with mudstones that represent the pelagic/hemipelagic background sedimentation. It displays hectometric to outcrop-scale slump folds with variable hinge-line orientations and pinch-and-swell structures, evidencing soft-sediment deformation, consistent with a slope to base-of-slope setting. Three limestone samples from this succession include conodonts referable to the pandemic Hirsutodontus simplex Subzone of the Cordylodus intermedius Zone (upper Furongian, Cambrian) and from the Macerodus dianae Zone (upper Tremadocian, Ordovician), implying that a slope connected the shallow-water shelf with a deep-water (oceanic) basin at least since late Cambrian times. The conodont faunas show affinities to coeval assemblages from outer shelf and slope environments around Laurentia yet they are not conclusive to postulate a geographic origin for the Precordillera. The thermal alteration of the conodonts is consistent with sedimentary burial and nappe stacking in this sector of the Precordillera.

This contribution deals with the record of the Ordovician genus Pygodus in Cuyania, Argentina. Several classical sections have been sampled for conodonts in the Precordillera and the San Rafael Block, involving diverse sedimentary deposits: coarse clastic rocks with carbonate cement, carbonate beds intercalated in fine clastic deposits, green shale, and black carbonate deposits. The recovered species of this genus are Pygodus lunnensis Zhang, P. anitae Bergström, P. serra (Hadding), P. protoanserinus Zhang and P. anserinus Lamont & Lindström. These key conodonts indicate middle to upper Darriwilian – lower Sandbian age for the bearing strata. The species P. protoanserinus is recorded for the first time from the Precordillera. Detailed observations were made on specimens of P. anitae, P. lunnensis and P. protoanserinus; the two last species are described for the first time from Cuyania.

Compound orchocladine sponges are unusual in the Early Palaeozoic. In Europe, silicified material of Late Ordovician age has hitherto been referred to as Aulocopium aurantium Oswald, 1847 and the invalid Aulocopium compositium Conwentz, 1905. An examination of new material has resulted in the recognition of a new genus, Hydraspongia, with two new species, H. polycephala and H. erecta, and a third new species, Perissocoelia megahabra, to which most specimens can now be assigned. These taxa form part of rich erratic sponge assemblages, which originate from unknown source areas in the Baltic, and have been collected in northern and western Europe from fluvial sandy deposits of the Eridanos River system, which drained the Baltic area from the Middle Miocene to Early Pleistocene.

One of the few and most complete records of the MDICE (Middle Darriwilian Isotope Carbon Excursion) is herein documented from Baltoscandia. Based on a core section penetrating the condensed Lower–Middle Ordovician succession (~46 m) on the island of Öland, southeastern Sweden, we provide an integrated scheme for carbon isotope chemostratigraphy (313 samples) and conodont biostratigraphy (29 samples) for this period. The carbonate succession in the Tingskullen core records 12 conodont zones and 6 subzones, including the Oepikodus evae, Trapezognathus diprion, Baltoniodus triangularis, B. navis, B. norrlandicus, Lenodus antivariabilis, L. variabilis, Yangtzeplacognathus crassus, Eoplacognathus pseudoplanus (Microzarkodina hagetiana and Microzarkodina ozarkodella subzones), E. suecicus, Pygodus serra (E. foliaceus, E. reclinatus, E. robustus and E. lindstroemi subzones) and Pygodus anserinus zones in ascending order. The δ13Ccarb record reveals an apparently complete record of the MDICE, including a rising limb, a well-defined peak and a falling limb. The anomaly covers a thickness of c. 27 m in the core and spans the Eoplacognathus pseudoplanus, E. suecicus, Pygodus serra and P. anserinus conodont zones. Combined with the new, detailed conodont biostratigraphy, the MDICE in the Tingskullen core can be used for detailed correlation with successions from Baltica, North America, the Argentine Precordillera, South China and North China.

Altered volcanic ash interbeds (bentonites) in the upper Katian of Baltoscandia indicate significant volcanic activity in neighbouring tectonically active areas. Katian bentonites in the East Baltic can be reliably correlated using sanidine phenocryst composition. Ratios of immobile trace elements TiO2, Nb, Zr and Th to Al2O3 enable extension of the correlations to Scandinavia, where late diagenetic alterations could have caused recrystallization of sanidine phenocrysts. At least seven volcanic eruptions were recognized in Baltoscandian sections. Several bentonites found in deep-sea sediments are absent in shallow-sea sediments, indicating extensive breaks in sedimentation and erosion during late Katian and Hirnantian times. The areal distribution pattern of Katian bentonites in Baltoscandia indicates a volcanic source from the north or northwest (present-day orientation) from the margins of the Iapetus Palaeo-Ocean. Signatures of ultra-high-pressure metamorphism in the Seve Nappe (Central Sweden) and intrusions in the Helgeland Nappe Complex in Central Norway have been proposed as potential sources of the magmas that generated the volcanic ashes deposited in the East Baltic in Katian times. Geochemical similarities between Baltoscandian and Dob's Linn bentonites from southern Scotland suggest a common volcanic source in Katian times.