The correlation of the Silurian succession of Estonia with the global standard has long been considered reliable. However, new information, particularly on the distribution of microfossils, has changed our understanding of the Silurian stratigraphy in the region. Recent palaeontological and geochemical data suggest that: the lower part of the Juuru Regional Stage (RS) is of Hirnantian age; the age of the base of the Raikküla RS in terms of global chronostratigraphy remains problematic; the Aeronian–Telychian boundary correlates with a level in the middle of the Rumba Formation (Fm); the base of the Adavere RS is of latest Aeronian age; the former Riksu Fm is considered to be the proximal, older part of the Sõrve Fm; the traditional lower boundary of the Jaagarahu RS is diachronous, and the closest biostratigraphic horizon that could be used is the first appearance datum of Jeppssonia sagitta rhenana; the Wenlock–Ludlow boundary correlates with a level in the upper Rootsiküla RS; the base of the Paadla RS corresponds to a level in the upper Gorstian, in the lower(?) Phlebolepis ornata Vertebrate Biozone; the Sauvere and Himmiste beds of the Paadla Fm are of late Gorstian age, and the Uduvere Beds are of early Ludfordian age, corresponding to part of the Ancoradella ploeckensis Conodont Biozone; identifying the Ludlow–Přídolí boundary in the Estonian succession is problematic, lacking reliable criteria at present. With these amendments, we present an updated regional correlation scheme drawn on a regular time scale for the first time. A problem that needs to be addressed in the future is providing better biostratigraphic definitions for the bases of regional stages.

The fossil record of jaw-bearing polychaetes from South America is scarce, and only a few occurrences of Ordovician scolecodonts have been reported so far. Here, we document a monospecific jawed polychaete assemblage from the Darriwilian Lenodus crassus conodont zone of the San José Formation in the eastern Cordillera of Peru. The single species, Protarabellites luztejadae sp. nov., represents the oldest record of the family Ramphoprionidae worldwide and may point to the biogeographic origin of this group in Gondwana. The reconstructed jaw apparatus of P. luztejadae bears primitive characters such as the slightly enclosed myocoele, distinguishing it from the younger members of the family and providing insights into the evolutionary relationships with other labidognath families. The described assemblage is the first record of Ordovician scolecodonts from Peru, as well as from the northern part of the Central Andean Basin, and one of very few cases from where apparatus-based classification has been applied. However, the Palaeozoic scolecodonts in South America appear to be more common than generally thought. A review of poorly known regional literature shows that scolecodonts have been mentioned in various sites from Ordovician, Silurian, Devonian, and Carboniferous–Permian strata of Argentina, Bolivia, Brazil, Colombia, Peru, and Uruguay. This highlights the potential for future scolecodont studies in South America.

Abstract Mass extinctions in the early Palaeozoic have been attributed to global climate change and ocean anoxia with elevated phosphorus (P) proposed as a key driver. However, this hypothesis has lacked geochemical support due to the absence of proxies that can reconstruct changes in marine P availability. Focusing on the Late Ordovician Mass Extinction (LOME) and the Late Devonian Mass Extinction (LDME), we present carbonate-associated phosphate (CAP) data from seven globally distributed sections, providing a proxy record for seawater P variation across these events. Our data reveal short-lived, globally coherent P pulses that coincided with both events. These transient P surges align with biodiversity loss, widespread anoxia, and seawater temperature declines, suggesting a link between P flux, ocean anoxia, and global climate shifts, as supported by biogeochemical model results. These findings provide an empirical connection between brief marine P pulses and ecological crises during the LOME and LDME.

Despite frequent use of conodonts as an archive for seawater Sr-87/Sr-86, available diagenetic screening tools are inconsistent predictors of primary seawater Sr-87/Sr-86 preservation. Here, we attempt to isolate variables affecting the preservation of seawater Sr-87/Sr-86 in conodonts. We present new Middle-Late Ordovician (similar to 470-450 Ma) conodont Sr-87/Sr-86 from mid-paleolatitude (30-45 degrees S) carbonate successions from central Sweden (Fj & auml;cka, K & aring;rg & auml;rde) and Estonia (Uuga Cliff, Viki borehole) which have low Conodont Alteration Index (CAI) values of 1-1.5. Coeval Sr-87/Sr-86 measurements from K & aring;rg & auml;rde and Estonian sections are offset from global seawater by similar to +2 x 10(-4) but preserve the overall structure of the seawater curve, suggesting minor diagenetic Sr exchange. Sr-87/Sr-86 measurements from the more argillaceous Fj & auml;cka section show extensive alteration with highly radiogenic values as much as similar to 6 x 10(-4) greater than global seawater and a stratigraphic trend that diverges from the seawater curve. This may be explained by highly radiogenic Sr contribution from the Kinnekulle K-bentonite and Fj & auml;cka shale units that occur at Fj & auml;cka section. We compiled 1164 conodont Sr-87/Sr-86 measurements spanning similar to 485-250 Ma and categorized measurements by host rock, sample preparation, paleolatitude, and CAI. Boxplots and Mann-Whitney U testing of residual values relative to the seawater curve (Sr-87/Sr-86(conodont) - Sr-87/Sr-86(seawater)) reveal that paleolatitude and associated differences in sedimentation rate affect preservation of seawater Sr-87/Sr-86 values in conodonts. The relative abundance of clay minerals did not significantly correlate with residual values for conodonts from mid- or low-latitude deposits. We recommend that future investigations of seawater Sr-87/Sr-86 records using conodont apatite target low-latitude settings with high sedimentation rate and CAI <= 2.

Numerous recent studies have revealed that the Ludfordian was an unstable part of the Silurian Period, linked with global climatic changes and glaciation, changes in ocean chemistry, and a distinct perturbation of the carbon cycle expressed by the largest carbon isotope excursion throughout the whole Phanerozoic, the Mid-Ludfordian carbon isotope excursion (MLCIE). These dramatic environmental changes were associated with faunal crises in conodont (Lau conodont Bioevent) and graptolite (Kozlowskii graptolite Bioevent) faunas. The response of other faunal groups to global environmental changes during the mid-Ludfordian has been less thoroughly documented. Here, we present an analysis of the taxonomic composition and stratigraphical distribution of jaw-bearing polychaete worms from the Ludfordian strata of the Prague Basin. Based on more than 5,000 diagnostic posterior maxillae, more than thirty species were recorded from the studied interval. The family Rhytiprionidae was recorded from the peri-Gondwanan region for the first time. During the MLCIE and the Mid-Ludfordian Glaciation interval, the jawed polychaete fauna was disturbed and reorganised but recovered relatively quickly, sooner than, e.g. lingulate brachiopods. The most distinct change was documented at the end of the Polygnathoides siluricus Zone marking the beginning of the glaciation. Approximately half of the species showed a continuous range through the event interval, twenty percent of species represented Lazarus taxa, six percent disappeared and 24 percent appeared during or after the event interval in the Kosov section. center dot Key words: scolecodonts, jawed polychaetes, Silurian, MLCIE, Lau and Kozlowskii bioevents, Prague Basin.

The mid-Silurian Mulde/lundgreni Event was characterized by a widespread biotic crisis among marine taxa (e.g., graptolites, conodonts), associated with a notable double-peaked positive carbon isotope excursion. These biotic and chemical signatures provide evidence that this time interval experienced significant environmental change. While there has been significant research on this critical interval within the Silurian, the oceanic redox conditions encompassing this event remain poorly constrained. Here, we utilize a multiproxy approach to investigate the paleoredox dynamics within two geographically distinct marine basins and paleocontinents spanning the Mulde/lundgreni Event: Abbott River, Twilight Creek (Arctic Canada, Laurentia), and the Priekule-20 core (Latvia, Baltica). We present new organic carbon isotopes (S13Corg), pyrite sulfur isotopes (S34Spyr), iron speciation, and trace metal data to reconstruct regional marine paleoredox conditions from three separate localities providing a more global view. Our geochemical records reveal synchronous positive excursions in S13Corg and S34Spyr across all sections, indicating increased burial of organic carbon and pyrite. Iron speciation data suggest persistently ferruginous anoxic bottom waters in Laurentia throughout the study interval. In contrast, bottom water redox conditions in Baltica were more variable, fluctuating from possibly oxic to ferruginous anoxic conditions during the Mulde carbon isotope excursion (CIE). Trace metal patterns include an initial enrichment followed by a reduction in all sections suggesting a drawdown during the first Mulde CIE peak, a renewed enrichment during biotic recovery, and a second decrease indicating another drawdown coinciding with the second Mulde CIE peak. Importantly, this two-step enrichment-drawdown pattern mirrors the double-peaked nature of the Mulde CIE, and thereby suggests globally dynamic bottom water conditions. Mo and U enrichment factors suggest differing degrees of restriction, but shared temporal trends still imply a global redox signal. These results support a model in which climate-driven sea-level fluctuations modulated marine deoxygenation, contributing to the timing and selectivity of extinctions among hemipelagic and planktonic taxa.

Age-related cognitive decline is associated with metabolic, vascular, and inflammatory changes, making it challenging to distinguish primary causes from secondary (downstream) effects. This study demonstrates that brain aging follows a ...Understanding the key drivers of brain aging is essential for effective prevention and treatment of neurodegenerative diseases. Here, we integrate human brain and physiological data to investigate underlying mechanisms. Functional MRI analyses across four ...

Chitinozoan investigations on the late Middle to early Late Ordovician in South China are limited. Documented chitinozoan occurrences are mainly from the Miaopo Formation on the Upper Yangtze Platform. The present study reports new data from the Miaopo Formation at the Zhenjin section in the Yichang area. In total, 13 genera and 33 species are recognised, and three new species are described: Spinachitina? coronifera sp. nov., Lagenochitina yichangella sp. nov. and Pellichitina confragosa sp. nov. The Baltoscandian index species, Laufeldochitina striata, is documented in the lowermost part of the formation. This is the first report of this species in South China. The L. striata Biozone is suggested for the basal part of the formation due to the presence of the eponymous species. The index species of the Laufeldochitina stentor Biozone, the Armoricochitina granulifera and Cyathochitina megacalix subzones adopted in the Jieling section, are also observed in the Zhenjin section. However, according to the new data obtained at Zhenjin, the first appearance datum of C. megacalix and A. granulifera coincides with that of L. striata. Therefore, the C. megacalix Subzone is kept but moved to the L. striata Biozone. Armoricochitina granulifera has stable occurrences in almost the entire Miaopo Formation, corresponding to the Nemagraptus gracilis graptolite biozone. It is slightly older than Baltic records but could be useful for recognising this time interval in South China.

The end of the Ordovician witnessed major perturbations in the ecosystem, seriously affecting global marine biodiversity. Nevertheless, some marine organism groups and their crisis -bound palaeogeographic distribution are still understudied. Among the outliers are eunicid polychaetes, even though they flourished and diversified extensively during the Ordovician. A collection of seven genera of jaw-bearing polychaetes, including the new ramphoprionid genus Spitiprion Tonarov & aacute;, Suttner, & Hints, with type new species of Spitiprion khannai Tonarov & aacute;, Suttner, & Hints, is described here from Katian (Upper Ordovician) deposits of Spiti, northern India. The new species is preserved as isolated maxillae and a jaw cluster, and 3D models of the maxillary apparatus are reconstructed based on submicron-CT. Along with the scolecodonts, a low -diversity assemblage of chitinozoans was recovered, comprising five genera. The most common chitinozoan species are Acanthochitina cf. cancellata and Spinachitina suecica .

The oxygen isotope ratio 18 O/ 16 O (expressed as a δ 18 O VSMOW value) in marine sedimentary rocks has increased by ~8‰ from the early Paleozoic to modern times. Interpretation of this trend is hindered by ambiguities in the temperature of formation of the carbonate, the δ 18 O seawater , and the effects of postdepositional diagenesis. Carbonate clumped isotope measurements, a temperature proxy, offer constraints on this problem. This thermometer is thermodynamically controlled in cases where carbonate achieves an equilibrium internal distribution of isotopes and is independent of the δ 18 O of the water from which the carbonate grew; therefore, it has a relatively rigorous chemical–physics foundation and can be applied to settings where the δ 18 O of the water is not known. We apply this technique to an exceptionally well-preserved Ordovician carbonate record from the Baltic Basin and present a framework for interpreting clumped isotope results and for reconstructing past δ 18 O seawater . We find that the seawater in the Ordovician had lower δ 18 O seawater values than previously estimated, highlighting the need to reassess climate records based on oxygen-isotopes, particularly where interpretations are based on assumptions regarding either the δ 18 O seawater or the temperature of deposition or diagenesis. We argue that an increase in δ 18 O seawater contributed to the long-term rise in the δ 18 O of marine sedimentary rocks since the early Paleozoic. This rise might have been driven by a change in the proportion of high- versus low-temperature water–rock interaction in the earth’s hydrosphere as a whole.

Ordovician rocks are widely distributed in the Baltoscandian region as well as in Poland, Belarus, Ukraine and Moldova. The Ordovician studies in this area were initiated in the 19th century in the outcrop belt in northern Estonia. The strata are well accessible here and fossils and sedimentary structures are excellently preserved. Further south, in other countries, the succession is lying progressively deeper (up to 2500 m) in the subsurface, except for limited exposure in Ukraine. The thickness of the Ordovician within the Estonian outcrop area reaches about 100 m but exceeds 200 m in several parts of the subsurface area. Since the 1960s, several Ordovician correlation charts have been compiled for this area. Recent developments in the stratigraphy are summarised in the volume ‘A Global Synthesis of the Ordovician System: Part 1’ (Geological Society, London, Special Publication, 532). The system of bio-, litho- and chronostratigraphic units is highly detailed in the area. The regional stages defined in Estonia were introduced for the western part of the East European Platform in the 1980s. The correlation of the regional succession to the global stratigraphic standard is generally well constrained, although it still needs to be refined in some details. A novel element of the stratigraphic standard, the isotopic zones, is based on secular variations of stable carbon isotopic composition of bulk carbonates and allows amendments to the correlation of strata. The application of a regular timescale is based on a well-dated system of biostratigraphic marker levels that were traced into the Baltic Palaeobasin and further to the south. The dated boundaries were tied to the regional succession mainly based on the correlation of conodont, chitinozoan and graptolite zones, but also using chemostratigraphic events. Correlation of formations to the chronostratigraphic standard in ten subregions (North and Central Estonia, South Estonia together with West Latvia and West Lithuania, Kaliningrad Region, East Latvia, Central Lithuania, East Lithuania together with northwestern Belarus, southwestern Belarus, West Volyn and Podillya together with East Volyn and Moldova) is summarised in an emended correlation chart. Development of the subregional correlation charts was well coordinated before the 1990s and the charts were based on a unified nomenclature of lithostratigraphic units for major facies zones that crossed the national borders. Trends in the development of nomenclature and correlation of formations have been different in different countries after 1991. This resulted in increasing differences in nomenclature and rank of lithostratigraphic units in subregions and led to an increase of the number of subregions. The development towards a more detailed stratigraphic classification in Lithuania has elevated the rank of many former units (several formations are now ranked as superformations, etc.). In 2011, a completely new system of formations and members replaced the formerly applied standard in the Kaliningrad Region. The climatic history of the region presented in papers of the last decades is modified in the light of the newest results of isotope-geochemical studies on Baltoscandian sections, which do not support the idea of gradual warming throughout the Middle and Late Ordovician in the region. The global cooling trend was also influencing Baltica despite the continental drift towards the lower latitudes. The richly fossiliferous regional succession has been extensively studied, but analyses with a broader view have been sparse. According to the general understanding, backed by data on different fossil groups, the main origination episodes in the early Darriwilian and Darriwilian–Sandbian transition led to the peak of regional diversity in the early Sandbian. Remarkable extinction events known from the early Darriwilian, early Sandbian and early Katian are expressed to a different degree in different fossil groups. The major extinction event in the latest Katian–Hirnantian, which impacted all major invertebrate groups, has been ascribed to the Hirnantian glaciation, the related glacioeustatic sea-level fall and the repeated rapid rearrangement of facies. A recovery that started in the latest Ordovician was relatively slow. Significant spatial differences in the dynamics of biodiversity within the eastern Baltic area and between this area and Scandinavia are considered partly due to uneven data coverage.

The final part of the Ordovician Period was characterised by major perturbations in climate, environments and ecosystems, as proved by numerous studies. These changes resulted in one of the biggest extinctions in Earth’s history. Most research in early Palaeozoic biogeography is based on data derived from extensively studied localities in North America and Europe, and much less information is available from other parts of the world. Here, we present the first results of our study on Katian/Hirnantian scolecodonts from the Spiti region, India. This area was part of the Tethyan Himalaya of the Gondwana palaeocontinent, located at low palaeolatitudes. Various representatives of algae, bryozoans, corals, cephalopods, conodonts, ostracods and other groups have already been described from this region. Palaeontological data and facies analysis indicate shallow-water conditions within the subtropical–tropical realm. In addition, the specific carbon isotope (δ13Ccarb) trend suggests that late Ordovician carbonate deposits in the region occurred during the pre-Hirnantian global warming interval, the so-called Boda Event. The migration pathways of bryozoan communities have shown that those from Spiti were very similar to the faunas of Laurentia, Baltica, Siberia and southern China during the early Late Ordovician. It is assumed that taxa originating in Laurentia, crossed the Tornquist Sea from Baltica to Avalonia, then to southern Europe, and subsequently extended along the shelf of northern Gondwana, including India and Australia. The organic-walled microfossils of the Spiti region have previously received much less attention than the shelly faunas and conodonts. In the present study, we have focused on scolecodonts – the jaws of polychaete worms, a group that has been abundant since the Cambrian, playing an important role in various marine ecosystems. Polychaetes are a vital part of the food chain and play an essential role in organic matter decomposition and nutrient cycling processes within sediments. The jawed polychaetes are also known for their high resistance to ecological stress. In Spiti, scolecodonts are mainly found in silty limestones containing 60–70% of carbonate minerals. The majority of scolecodonts were recovered from the lithological units 8 and 13 of the studied section, representing well-stabilised shallow-shelf carbonate environments close to the top of a transgressive system tract in otherwise relatively nearshore to shoreline settings. Our results show that at least seven jawed polychaete genera are present in the assemblage. A new endemic ramphoprionid species belonging to the genus Megaramphoprion has been recorded. Other taxa include representatives of Oenonites, Mochtyella, Vistulella, Atraktoprion, Xanioprion and paulinitids, which occur in both scolecodont-rich units. The same genera occur in coeval strata in Baltica and Laurentia. On the other hand, some genera such as Pistoprion and Kalloprion are missing in the Spiti samples and also in other Gondwanan collections. In Laurentia, the family Hadoprionidae is already present in the Late Ordovician, but in Gondwana it appears in the late Silurian. Paulinitids are common in late Katian and Hirnantian strata (Amorphognathus ordovicicus conodont Zone) in Laurentia and Gondwana, as well as in Baltica. Similarly to coeval Laurentian polychaete faunas, labidognath and prionognath taxa outnumber the species with a placognath-type jaw apparatus. The distinct and abundant genus Pteropelta in Baltica and Laurentia has so far not been recorded in the latest Ordovician of Gondwana. In addition to scolecodonts, the studied organic-walled microfossil assemblage contains chitinozoans belonging to the genera Acanthochitina, Conochitina, Spinachitina, and possibly Tanuchitina. The diversity is lower than presented by previous authors and reported from other regions.

We report the discovery of a rich assemblage of latest Katian and Hirnantian scolecodonts (poly -chaete jaws) from a new Ordovician-Silurian boundary outcrop in the Reinu quarry, northern Estonia. The recovered polychaete fauna contains at least 40 species attributed to 11 families. Many common taxa appear to range from the latest Katian to the Hirnantian and into the Rhuddanian, indicating the resilience of jawed polychaetes to the end-Ordovician mass extinction. The Reinu assemblage is similar to the coeval faunas known from other Baltic sec tions, as well as from Anticosti Island, Canada, although with some specific features. The study revealed the highest abundance of scolecodonts in the Ordovician of Baltica, with ca 5400 maxil lae per kilogram of rock recorded in the Siuge Member of the arina Formation, Porkuni Regional Stage, early Hirnantian.

Jawed polychaetes evolved and diversified extensively during the Ordovician. However, Ordovician poly-chaete 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 Kraluv Dvur and Kosov formations (uppermost Katian and lowermost Hirnantian) at the Levin local-ity. 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 prionog-nath 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 poly-chaete faunas. Polychaetaspids dominate in the Levin assemblage, followed by other families such as ramphoprionids, paulinitids, and atraktoprionids. The studied interval in the Levin 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 tecton-ics. Reduced diversity and abundance of scolecodonts was recorded in the uppermost part of the Kraluv Dvur 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.(c) 2023 Elsevier Masson SAS. All rights reserved.