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The Dapingian (Arenig) siltstones and sandstones of the Tagoat Group, County Wexford, SE Ireland, contain a well-preserved and diverse brachiopod fauna including a new genus of alimbellid, Palaeotagoatia (type species: Orthis Bailyana Davidson) together with the plectorthid Ffynnonia costata (Bates) hibernica nov. subsp. Of the 13 forms documented, at least six are conspecific with brachiopods from the upper Arenig (Dapingian-lowest Darriwilian) Treiorwerth Formation on Anglesey and a further two are identified with species occurring in the older Dapingian Carmel Formation. The faunal province affinities of the fauna are with those assemblages assigned to the peri-insular and marginal Celtic province and which occupied positions within the Middle Ordovician Iapetus Ocean between the Laurentian and Baltic platform provinces. More precise correlation of Middle Ordovician units suggests a significant species richness during the Dapingian-earliest Darriwilian and signalling also an early development of the Celtic province. But the shallow-water siliciclastic facies associated with these islands may also have influenced the distribution of some elements of the Celtic brachiopods and promoted the prevalence of coarse-ribbed orthides, such as Paralenorthis.

Jawed polychaetes evolved and diversified extensively during the Ordovician. However, Ordovician polychaete jaws (scolecodonts) have remained poorly documented for many regions. This applies for the Prague Basin of peri-Gondwana, from where the previous study on Late Ordovician scolecodonts was published more than 70 years ago, with just two species preliminarily identified. The aim of the present paper was to fill this research gap and to study organic-walled microfossils from the boundary interval of the Králův Dvůr and Kosov formations (uppermost Katian and lowermost Hirnantian) at the Levín locality. As a result, a diverse assemblage of scolecodonts and chitinozoans was discovered. Chitinozoans are represented by at least 24 species from 15 genera, i.e., a relatively diverse assemblage whose species composition points toward the Ancyrochitina merga and Tanuchitina elongata biozones. The recovered jawed polychaete fauna contains at least 19 species from 14 genera. Taxa with labidognath and prionognath type maxillary apparatuses predominate in samples, whereas placognath and ctenognath taxa are relatively rare. A similar pattern is typical for the Laurentian samples but contrasts with the Baltic polychaete faunas. Polychaetaspids dominate in the Levín assemblage, followed by other families such as ramphoprionids, paulinitids, and atraktoprionids. The studied interval in the Levín section is represented by a succession of thin-bedded silty shales with diamictite beds, practically devoid of shelly fossils and with a variable degree of bioturbation. The deposits are interpreted as distal turbidites and debrites, reflecting sea-level changes driven by the growth and retreat of glacial ice and possibly also local tectonics. Reduced diversity and abundance of scolecodonts was recorded in the uppermost part of the Králův Dvůr Formation, which correlates with less bioturbation and finer silt fraction. The reported discovery shows wide geographical distribution and diversity of jawed polychaetes before and during the Hirnantian glaciation and mass extinction.

The Lower Ordovician Fezouata Shale is renowned for its exceptionally-preserved euarthropod fossils including numerous species of trilobites, some of which show remains of appendages and traces of the digestive system. Herein, we describe the early developmental stages of at least nine trilobite species from the Tremadocian strata of the Fezouata Shale, namely Platypeltoides magrebiensis, Nileus deynouxi, Symphysurus ebbestadi, Asaphellus sp., Megistaspis (Ekeraspis) hammondi, Orometopus sp., Anacheirurus adserai, Bavarilla zemmourensis, Indiligens sp., and several specimens of undetermined protaspides. This study considerably expands our knowledge of the development of early Ordovician trilobites. The preservation of appendages in the early stages of N. deynouxi and S. ebbestadi, and remains of the digestive tract in the latter species, suggests that some immature trilobites had similar morphology and anatomy as the adult individuals. Early developmental stages of Indiligens sp. might have fed and/or hidden on graptolites and demosponges. The extraordinarily large size of the early post-embryonic stages of P. magrebiensis, S. ebbestadi, Orometopus sp., Asaphellus sp., and undetermined protaspides suggests that these trilobites might have hatched from yolk-rich eggs. The presence of several trilobite species with notably large post-embryonic stages in the Fezouata Shale might be explained by seasonal or low productivity in the high-latitude margin of Gondwana.

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.

A poorly diverse trilobite assemblage is described from the Tremadocian strata (Tangissart Member of the Mousty Formation; Chevlipont Formation) of the Brabant Massif. These specimens represent so far the oldest trilobite record from Belgium. The recorded taxa, identified as Platypeltoides cf. croftii, Macropyge? sp., and Asaphidae indet., resemble contemporaneous trilobites from other parts of Avalonia (e.g., Wales, Shropshire) as well as some others from high-latitude Gondwana (Morocco). The nileid species P. cf. croftii suggests that the Tangissart Member of the Mousty Formation was likely deposited in deeper offshore environments on the open shelf of Avalonia. Morphological similarities between P. cf. croftii from the Brabant Massif, P. croftii from the UK, and P. magrebiensis from Morocco suggest close faunal interchange across the narrow Rheic Ocean.

New machaeridian material housed in the National Museum of Scotland alongside the type material held in the Natural History Museum London and the Sedgwick Museum of Earth Sciences, Cambridge, is documented here from the Ordovician of Girvan. The specimens are included in four taxa, three of these in open nomenclature. Syntypes and figured specimens of Plumulites scoticus by Nicholson and Etheridge (1879) and Withers (1926) are reviewed and the diagnosis emended. The description of these specimens follows standard terminology and we also introduce here a new terminology for Anterior Outer Shell Plate (AOSP). We also figure for the first time three unpublished letters from Mrs Elizabeth Gray to Thomas Henry Withers, where she criticises the illustrations of machaeridians published by Nicholson and Etheridge (1880) and Reed (1908), recognising issues identifying machaeridian plates and also clarifies a misunderstanding on some of the specimens described and illustrated in Reed (1908), being key for the distribution of Plumulites peachi. We assess the importance of machaeridians as part of the Lower Palaeozoic palaeobenthos and a key element of the palaeo-food chain. A review of the palaeogeographical distribution of the machaeridians is undertaken in the context of the Ordovician biodiversifications.

Ordovician strata exposed across the Cordillera Oriental and the Sierras Subandinas in northwestern Argentina were part of a large retroarc foreland basin developed along the Proto-Andean margin within the Central Andes in South America. A revised chitinozoan biostratigraphy along and across-strike for the Tremadocian, Floian, Dapingian, Katian and Hirnantian stages, calibrated with other fossil groups in the basin, allow pinpointing the most characteristic events that affected the basin fill testing global versus local controls in accommodation, and suggesting comparisons with other peri-Gondwanan records. According to the chitinozoan data, the glacially-related Ordovician deposits in northwestern Argentina are restricted to the Hirnantian, and unconformably overlie late Katian deposits. In the Caspalá area (Cordillera Oriental), an interval with synsedimentary deformation and reworked chitinozoans correlate with glacially-related deposits in other sites of the eastern part of the basin (Río Capillas and Mecoyita areas). A glacial waning stage is determined by a thin interval of organic‐rich black shales with sparse dropstones at the top of the Zapla Formation, containing Spinachitina oulebsiri associated with Desmochitina gr. minor, which together are typical latest Hirnantian components in other regions of Gondwana. Our study strengthens the foreland systems tract for the Ordovician Central Andean Basin with a volcanically fed interarc and foredeep depozone to the west (Puna region); a lower-accommodation forebulge depozone in the central area (mostly the Cordillera Oriental region); and a backbulge depozone (Sierras Subandinas and Sierras de Santa Bárbara) extending as far as the eastern Paraná Basin (reaching Paraguay and Brazil). Contemporaneous unconformities driven by global sea-level fluctuations were amplified or reduced due to deepening-narrowing or widening-shallowing, allowing contrasted accommodation, respectively associated to loading and relaxation. Ordovician chitinozoans from the Central Andean Basin indicate Northern, Western and peri-Gondwanan affinities, although locally some more cosmopolitan species described in Baltica, Avalonia and South China, are also recorded.

Well preserved laminated structures occur within the Upper Ordovician of the Cellon section in the Carnic Alps (Austria), a world-famous reference section for Silurian conodont biostratigraphy. Microfacies from the Upper Ordovician Uqua Formation were characterised by using optical and scanning electron microscopy (SEM), an environmental scanning electron microscopy coupled with microanalyses (SEM/ESEM-EDX) and a confocal laser Raman microscopy. Ferruginous laminated structures overgrowing specific skeletal fragments occur in the lower part of the studied unit in the form of finely red-to greenish coatings composed of chamosite and goethite alternating with calcite bands. Laminae have arborescent to dendrolitic morphologies. Such morphologies suggest a biomediated genesis and the existence of a potential microbial factory acting in a nearby location from which coated material was later redeposited. These ferruginous coatings around organisms are not documented within the latest Ordovician Plöcken Formation at Cellon or in the coeval Wolayer Formation elsewhere.

There was a sudden increase in the diversity of bioclaustrations in the Sandbian (Late Ordovician) that continued somewhat more slowly in the Katian. The Sandbian was also the time when bioclaustrations became common, at least in Baltica. The major increase in the diversity of bioclaustrations in the Late Ordovician was an outcome of the GOBE, and we term it the Ordovician Bioclaustration Revolution. The Ordovician Bioerosion Revolution may partially be responsible for beginning of the Ordovician Bioclaustration Revolution in the Sandbian, as a number of these early bioclaustrations started their growth from initial borings. The diversification of bioclaustrations in the Sandbian involves mostly bryozoans and, to a lesser extent, brachiopods as hosts. The Katian increase in bioclaustration diversity involves mostly corals as the hosts and was likely unrelated or at least less influenced by the Ordovician Bioerosion Revolution. A new broadly conical bioclaustration, Kuckerichnus kirsimaei nov. cgen., nov. csp., is here described from the growth surfaces of hemispherical trepostome bryozoan colonies of Diplotrypa bicornis, Mesotrypa orientalis and Mesotrypa excentrica from the early Sandbian (Late Ordovician) of Estonia.

New caenagnathid dinosaur specimens from the upper Aguja Formation of West Texas comprise the most complete examples yet described from southern North America. Two individuals are represented. Osteohistology indicates that both were mature at their times of death. Although they share no overlapping elements, the two individuals are separated by overall body size, paleohabitat, and stratigraphic position and so they almost certainly represent different species. One individual consists of a partial hindlimb, but exhibits too few (3) characters to be phylogenetically informative and is, therefore, referred to an indeterminate caenagnathid – possibly one of two species previously recognized in coastal facies of the Aguja Formation. The second individual is more complete. It was recovered higher in section from more inland fluvial paleoenvironments and preserves fragmentary elements from throughout the postcranial skeleton. A histology-based growth model – the first for a caenagnathid – indicates that the second individual required at least five years to approach fully adult size. However, only 11 character states could be derived from its preserved remains. Phylogenetic analyses found this specimen deeply nested within Caenagnathidae, but were otherwise inconclusive and so it, too, is referred to an indeterminate caenagnathid. Nevertheless, several combined morphologies (e.g., absence of cervical epipophyses, dorsoventral depth of the hypapophysis on the second cervicodorsal vertebra, and greater trochanter of the femur only weakly separated from the femoral head) implies that the second individual represents an unknown species. One critical aspect of both specimens is that they provide significant new comparative material representing the sparse ‘southern’ caenagnathid record.