Stromatoporoids are hypercalcified sponges and were major reef-building and reef-associated organisms in the middle Palaeozoic Era, but their skeletons are poorly preserved and continue to challenge interpretations of their original nature. In view of the very low amount of organic matter in the skeletons, fluorescence (FL) imaging has rarely been applied for the study of stromatoporoid skeletal structure. Since the Silurian rocks on Gotland are well preserved, the stromatoporoids there contain enough organic material to examine the microstructure of the skeletons using FL microscopy. FL highlights the presence of small amounts of organic matter in stromatoporoids by green-coloured light emission. By using this method, it is possible to visualize potentially original or near-original skeletal microstructures, which commonly appear extensively altered under transmitted light, particularly under cross-polarised light. Such alteration results from early-diagenetic neomorphism of stromatoporoid skeletons, in particular the formation of fabric-retentive irregular calcite (FRIC) crystals. FRIC permeates the entire skeleton, including its skeletal elements and cement-filled internal spaces. However, the FRIC formation did not redistribute the organic material in the skeleton, so that FL microscopy enables the visualization of the porosity of the galleries and, in certain genera, also the microporosity within the skeletons (microgalleries), demonstrating that many stromatoporoid skeletons originally had extremely high porosity and thus presumably also very high permeability. A systematic investigation of stromatoporoids from other periods and regions using FL microscopy could therefore not only provide valuable insights into the lifestyles of stromatoporoids, but also contribute to the systematics of this group, which remain problematic.

The closure timing of the Paleo-Tethyan Mianlue Ocean is crucial for undestanding plate tectonic reorganization and paleoenvironment changes. However, it remains highly debated, with estimates varying from the Late Permian to the Late Triassic due to insufficient geologic records within the suture zone. In this study, we investigate the Early Triassic inversion structures in the Huayingshan tectonic belt, located south of the Mianlue suture zone, to better constrain the collisional timing. Structural analysis of the Guanyin Gorge anticline in the southern HTB identifies positive inversion structures within the Lower Triassic Feixianguan Formation, as evidenced by decrease and eventually reversal in sense of displacements along the fault planes. The thicker strata and onlap geometries observed in the hanging walls relative to the footwalls provide evidence for syn-deposi-tional fault activity. Zircon U-Pb dating of tuff layers at Peijiawan constrain the timing of onset of the syndepositional reverse faulting between 250 and 249 Ma. Moreover, storm deposits are prevalently developed in the Feixianguan Formation and even more abundant within the fault-active interval. The contemporaneous development of similar inversion structures and storm deposits on tidal platform throughout the northern Sichuan Basin indicates a broader, regional-scale tectono-sedimentary episode driven by the regional tectonic transition from extension to compresion, which linked to complete consumption of the Paleotethyan Mianlue Ocean and onset of collision between the south China and north China blocks at 249 Ma. The collisional compression fuelled a sustained marine transgression, consequently amplifying storm sedimentation across the Upper Yangtze region.

Stromatoporoid faunas in the peri-Gondwanan regions during the Katian of the Late Ordovician exhibit a greater diversity of taxa compared with those from other contemporaneous continents and terranes. Geographical separation due to the numerous microplates comprising peri-Gondwana was a potential driver of this high diversity, but stromatoporoid faunas in many of these microplates have not been studied in detail because of their remote present-day locations. In this study, we report 12 genera and 16 species of stromatoporoids from the Upper Ordovician (Katian) Koumenzi Formation, exposed in the upper reaches of the Tianbao River, Qilian County, Qinghai Province, Northwest China, representing a unique occurrence in the North Qilian Mountains. This fauna includes Cystostroma inconstans Jeon n. sp., Pseudostylodictyon poshanense Ozaki, 1938, Labechia conglomerata Dong and Wang, 1984, Labechia shanhsiensis Yabe and Sugiyama, 1930 (Yabe and Sugiyama, 1930a), Labechia sp., Labechiella gondwanense Jeon in Jeon et al., 2022a, Radiostroma astroqilianus Jeon and Zhan n. sp., Aulacera arbuscula Jeon n. sp., Sinabeatricea crassicentra Jeon n. sp., Lophiostroma leptolamellatum Jeon n. sp., Clathrodictyon idense Webby and Banks, 1976, Clathrodictyon sp. cf. Cl. mammillatum (Schmidt, 1858), Ecclimadictyon tiewadianensis (Jiang et al., 2011), Ecclimadictyon nestori Webby, 1969, Plexodictyon qilianense Jeon n. sp., and Simplexodictyon uniplexum Jeon n. sp. In comparison with other peri-Gondwana regions and other adjacent areas, this fauna consists of a relatively high proportion of endemic taxa and some pandemic taxa, with species-level affinities most closely linking the North Qilian Mountains to North China, and to a lesser extent, South China. The occurrence of genera such as Radiostroma, Plexodictyon, and Simplexodictyon, previously restricted to Baltica and North China extends their known palaeogeographical ranges. The stromatoporoid assemblage of the North Qilian Mountains represents a transitional fauna between the Ordovician and Silurian, characterised by persistent labechiid dominance alongside the appearance of clathrodictyids and stromatoporellids, and the presence of both long-ranging Ordovician taxa and morphologically advanced genera, reflecting diversification trends also seen in the contemporaneous North China assemblage. Relatively high endemism of this stromatoporoid fauna is attributed to the island arc volcanic setting during the Late Ordovician, emphasizing the role of island arc setting in stromatoporoid diversification during the Ordovician. (c) 2025 Elsevier B.V. and Nanjing Institute of Geology and Palaeontology, CAS. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

Abstract Stromatoporoid sponge fossils were major diverse reef‐builders in the Palaeozoic Era; their taxonomic identification relies on thin sections examined under transmitted light microscopy, where vertical and transverse skeletal elements reveal diagnostic architectural features that vary with taxa. These elements typically appear darker than the cement‐filled internal spaces, allowing stromatoporoid taxa to be distinguished. However, stromatoporoid architecture is variable so that judgement of identification is not always unequivocal. Therefore, this study investigates the application of artificial intelligence (AI) to automate stromatoporoid identification, introducing a novel approach to streamline and standardise palaeontological taxonomy. For the first time, both vertical and transverse sections have been simultaneously analysed and integrated into an automated framework. High‐resolution images of thin sections from four well‐established Silurian genera, collected from the West Midlands and Shropshire counties, UK, were used to train supervised machine learning models. The images, captured using plane‐polarised transmitted light microscopy on thin sections, were digitally enhanced to increase contrast and eliminate background noise, ensuring that only skeletal features were used to inform the models. Despite variations in fossil preservation, section orientation and image quality, the AI models achieved classification accuracies of up to 96%. This demonstrates that stromatoporoid skeletal architecture is highly amenable to automated analysis, even under suboptimal conditions. The results represent a significant step forward in the application of AI to palaeontology, reducing reliance on manual identification and accelerating the classification process. Ultimately, this approach provides a new quantitative framework for evaluating stromatoporoid skeletal architecture, strengthening the scientific basis of their taxonomic interpretation and opening new avenues for analysing morphological diversity in hypercalcified sponges.

An assemblage of dome-shaped calcified fossils was discovered in resedimented deposits of the Monte Facito Formation (Middle Triassic, Ladinian) of the Southern Apennines, Italy. The fossils occur within a mass-transport deposit and are interpreted as having originated in a shallow-marine carbonate environment prior to downslope redeposition. In outcrop, they resemble stromatoporoids or calcareous algae and may have contributed to carbonate substrate development. The fossils consist of successive carbonate laminae separated by micritic to fine-grained carbonate layers. Internally, they display a porous fabric but lack chambers and diagnostic skeletal structures typical of stromatoporoids, other hypercalcified sponges, corals, or calcareous algae. Both the skeletal framework and part of the interlaminar carbonate have undergone extensive neomorphism. Energy Dispersive Spectroscopy analyses indicate a predominantly carbonate composition, whereas the interlaminar material contains Al and Si, suggesting a siliciclastic component. Cathodoluminescence analyses provided no significant additional information. Microbial crusts and possible bioerosional borings locally truncate both skeletal laminae and encrusting microbial fabrics. These features suggest post-mortem colonization by boring organisms, possibly including endolithic sponges, indicating prolonged exposure of the fossils on the seafloor before burial. The affinity of these enigmatic fossils remains unresolved, with hypercalcified sponges and calcareous algae representing the most plausible alternatives. Nevertheless, the associated microbial and bioerosional features indicate participation in a complex microbial/metazoan community that contributed to carbonate substrate production and ecological recovery during the rebuilding of shallow-marine ecosystems following the end-Permian mass extinction.

The timing of metazoan reef recovery after the end-Ordovician mass extinction remains poorly constrained in many regions. In South China, the earliest post-Ordovician metazoan reef has been previously considered middle Aeronian in age. Here, we report a newly discovered coral reef from the Xiangshuyuan Formation at the Dulin section, Guizhou Province. Integrated carbon isotope stratigraphy and biostratigraphic data indicate that the small patch reef occurs immediately above the peak of the early Aeronian carbon isotope excursion and within the range of the conodont Ozarkodina obesa and virgianid brachiopod concentration beds. These data constrain the reef to the early Aeronian (Llandovery), making it the earliest known post-Ordovician metazoan reef in South China. Comparison with reef recovery in Baltica and Laurentia, this case provides important evidence that rebuilding of metazoan reef ecosystems was geographically heterogeneous.

Reconstructing early stromatoporoid diversification across Gondwana and peri-Gondwanan terranes is hindered by incomplete and weakly constrained records — particularly those from Sibumasu Terrane despite its close palaeobiogeographical affinity with North China, where the first major radiation of labechiid stromatoporoids occurred. The timing of the first stromatoporoids in Sibumasu has remained uncertain, maintaining a long-recognised temporal and palaeogeographical gap between their earliest occurrences in South China and the subsequent diversification that took place in North China. Here, we provide the first systematic description of Middle Ordovician (Darriwilian) stromatoporoids from Thailand, documenting five genera and six species (two new and one in open nomenclature) from the unnamed Darriwilian strata in Satun (southern Thailand) and the upper Darriwilian unit of the Tha Manao Formation in Kanchanaburi (western Thailand), comprising Rosenella woyuensis Ozaki, 1938, Cystostroma satunense sp. nov., Labechia variabilis Yabe and Sugiyama, 1930a, Labechiella regularis (Yabe and Sugiyama, 1930a), Labechiella sp., and Thamnobeatricea kanchanaburiensis sp. nov. Integration of the new taxonomic data with conodont biostratigraphy indicates that, to our knowledge, stromatoporoids first appeared in Sibumasu by the early Darriwilian, represented by the Histiodella holodentata–Tangshanodus tangshanensis Conodont Biozone, earlier than their first occurrence in North China. This occurrence indicates that stromatoporoids had already expanded across peri-Gondwanan terranes by the early Middle Ordovician and partially narrows the long-recognised temporal gap between the earliest Early Ordovician records and their later Middle Ordovician diversification. Relatively higher affinities between Sibumasu and North China compared with other continents and terranes support a shallow marine connection across peri-Gondwana during the Middle Ordovician. These results help fill a long-standing regional gap in stromatoporoid research and provide a clearer temporal framework for evaluating early palaeobiogeographical relationships among peri-Gondwanan terranes.

The Middle Darriwilian Isotopic Carbon Excursion (MDICE) is a globally recognized Ordovician carbon isotope event, but its records in the South China Block (SCB) are commonly incomplete due to regional stratigraphic hiatus. This study presents an integrated analysis of conodont biostratigraphy, carbon isotope stratigraphy, and microfacies analysis from a stratigraphically continuous succession spanning the Kuniutan to lower Pagoda formations (Darriwilian to early Katian) at the Mayang section, Upper Yangtze Platform, SCB. Microfacies data indicate a shallowing event during the deposition of the uppermost Kuniutan to lowermost Datianba formations (upper Pygodus serra Biozone, late Darriwilian), yet the absence of subaerial exposure features supports continuous sedimentation. Carbon isotope data reveal a positive excursion of similar to 1 parts per thousand within the Kuniutan Formation, characterized by a sustained high-value interval from the Eoplacognathus suecicus to Pygodus serra biozones and a shortened falling limb, interpreted as an apparent complete record of the MDICE. A second, larger positive excursion (2 parts per thousand-3 parts per thousand) near the top of the section, initiating in the upper Baltoniodus alobatus Biozone and peaking in the lower Hamarodus brevirameus Biozone, is correlated with the early Katian Guttenberg Isotopic Carbon Excursion (GICE) or its regional equivalent. Global comparison confirms the MDICE as a broadly correlative perturbation of the Ordovician carbon cycle, while pronounced regional differences in its expression highlight the importance of depositional and palaeogeographic controls.

The early and late Aeronian carbon isotope excursions (CIEs) are important chemostratigraphic markers for regional and global correlation of the lower Silurian carbonate successions. Although both excursions have been recognized in the South China Block (SCB), their spatial heterogeneity and underlying controls remain insufficiently constrained. Here, new carbon isotope (delta C-13(carb)) and conodont data from the Tunping, Dongkala, and Yangshishan sections are used to investigate the variations of these Aeronian CIEs in the SCB. The early Aeronian CIE was identified at Dongkala and Yangshishan, and the late Aeronian CIE was identified at Tunping and Dongkala. Furthermore, a minor positive excursion was recognized together with the virgianid brachiopod coquinas and biostratigraphically diagnostic conodonts Ozarkodina obesa and Ozarkodina wangzhunia at Dongkala. Regional correlation of Aeronian carbon isotope records indicates that the maximum values of the early Aeronian CIE were relatively stable (similar to 2.4-2.7 parts per thousand) across the platform, whereas those of the late Aeronian CIE show a pronounced basinward decline, from 3.5 to 3.8 parts per thousand in the platform-interior settings to 2.6 parts per thousand at the platform-margin. This contrast is interpreted to reflect differences in water mass circulation related to the development of platform-margin reefs. During the late Aeronian, widespread reefs along the platform margin are here interpreted to have restricted water mass circulation between the platform-interior and the open ocean, enhancing residence time of surface water with high delta C-13(carb) values in the platform-interior settings. In contrast, the absence of reef barriers during the early Aeronian is interpreted to have allowed more effective water mass mixing, producing a more homogeneous delta C-13(carb) signature across the basin. These results highlight the critical role of local palaeoenvironmental factors, specifically carbonate platform morphology and water circulation, in modulating the stratigraphic expression of global carbon cycle perturbations.

Sponges of the Lower Greensand Group (LGS) are well preserved and occur in sediments of a sandy matrix. Abundant in the Faringdon Sponge Gravel Member (FSG), these sponges, mostly Calcareans, are found in Oxfordshire, with notable preservation at Little Coxwell quarries. This study provides descriptions of common species following the updated Porifera classification and recent sponge taxonomy research, illustrated with specimens from the Natural History Museum, London (NHM), British Geological Survey (BGS), and Natural History Museum Basel (NMB) collections. The following taxa are recorded and described: 1) Calcareans: Barroisia anastomosans (Parkinson, 1822), Barroisia clavata (Keeping, 1883), Barroisia irregularis (Hinde, 1884), Dehukia crassa (de Fromentel, 1861), (Elasmoierea]faringdonensis (Mantell, 1854), (Elasmoierea] mantelli (Hinde, 1884), Peronidella gillieroni (de Loriol, 1869), Peronidella prolifera (Hinde, 1884), Peronidella ramosa (Roemer, 1839), Oculospongia dilatate (Roemer, 1864), Tremospongia pulvinaria (Goldfuss, 1826), Raphidonema contortum (Hinde, 1884), Raphidonema porcatum (Sharpe, 1854), Raphidonemafarringdonensis (Sharpe, 1854), Raphidonema macropora (Sharpe, 1854), Raphidonema pustulatum (Hinde, 1884), Endostoma foraminosa (Goldfuss, 1826); and 2) Hexactinellids: Lonsda contortuplicata (Lonsdale, 1849). Key findings include the identification of Tethyan biogeographic affinities and ecological adaptations that highlight the role of these sponges in early reef-like systems. By refining species descriptions and linking them to broader Cretaceous ecosystems, this work enhances understanding of sponge biodiversity, evolutionary strategies, and their contributions to carbonate platform development during periods of environmental change. Crown Copyright (c) 2025 Published by Elsevier Ltd on behalf of The Geologists' Association. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

This study presents the first integrated carbon isotope stratigraphy and conodont biostratigraphy from the Rhuddanian to Aeronian (Llandovery, early Silurian) carbonate successions at the continuously exposed Kuaizishan and Yangdeng sections in Guizhou Province, South China Block (SCB). At the Kuaizishan section, a positive carbon isotope excursion (CIE) is recorded within the Xiangshuyuan Formation, along with the biostratigraphically diagnostic conodont Ozarkodina obesa. At the Yangdeng section, the Shiniulan Formation records both the falling limb of a positive CIE and a complete CIE, associated with Ozarkodina wangzhunia and Ozarkodina parahassi. Based on the biostratigraphic data, the CIE in the Xiangshuyuan Formation and falling limb in the Shiniulan Formation are interpreted as the early Aeronian CIE (Ozarkodina obesa Biozone), while the complete CIE in the Shiniulan Formation is attributed to the late Aeronian CIE (Ozarkodina guizhouensis Biozone). These results suggest that the Xiangshuyuan Formation spans from the middle Rhuddanian to earliest middle Aeronian, and the Shiniulan Formation from the early to late Aeronian. A composite carbon isotope curve from both sections provides a regional reference for correlating early Silurian carbonates within the SCB. The early Aeronian CIE shows comparable patterns across Laurentia, Baltica, and the SCB, indicating its global extent. In contrast, the late Aeronian CIE exhibits reduced expression towards the basin in both the Michigan Basin of Laurentia and the Upper Yangtze region of the SCB. Together, these two Aeronian CIEs highlight their global significance and stratigraphic utility. Furthermore, carbon isotope stratigraphy indicates that the widely distributed metazoan reefs in the Shiniulan Formation (the SCB) and the Minier Formation (Laurentia) are approximately coeval, providing evidence for a global-scale recovery of reef environments following the endOrdovician mass extinction, which was completed by the late Aeronian Age.

Devonian stromatoporoid collections have been added to the Natural History Museum, London (UK) for over 100 years. The characteristics and systematic position of these specimens, however, have received little attention. In this study, 297 Devonian stromatoporoid specimens comprising material documented by H. A. Nicholson from the UK, Germany, United States, and Canada, plus specimens described by E. A. Ripper from Australia, were re-examined. Overall, 50 species belonging to 29 genera were systematically redescribed based on recent progress, mainly including Actinostroma, Petridiostroma, Stictostroma, Pseudotrupetostroma, and Parallelopora. Three-dimensional reconstructions reveal stromatoporoid architectural patterns, crucial for enhancing understanding and revision of stromatoporoid identification. This study underscores the significance of three-dimensional reconstruction in taxonomic research on stromatoporoids. The NHMUK material is combined with data from publications and the Paleobiology Database (PBDB) to perform a network analysis of the global occurrence of Devonian stromatoporoids at the generic level; this reveals a close relationship of the global stromatoporoid fauna during the Early Devonian, indicating a widespread distribution, despite this interval being regarded as a time of global stromatoporoid contraction. The Middle Devonian assemblage shows a much higher cosmopolitan occurrence in the context of the subsequent Eifelian-Givetian global stromatoporoid proliferation, consistent with the known pattern from other studies of Middle Devonian stromatoporoids. Overall, the NHMUK collections are a valuable resource to help understand the global occurrence of Devonian stromatoporoids.

Hypercalcified sponges are poriferans with a calcareous skeleton secreted on and in the soft tissue.Living examples,and fossils of some such sponges in Mesozoic and Cenozoic strata,contain sponge spicules and can be classified within modern poriferan groups of the Classes Demospongiae and Calcarea,which are otherwise almost entirely soft-bodied.However,other fossil forms,largely Palaeozoic archae-ocyaths,stromatoporoids and chaetetids,lack spicules,so their classification relies on the calcareous skel-eton alone.Because of these discrepancies,although the hypercalcified skeleton is useful for low-level taxonomy in fossils,it has no proven phyletic value,so the systematic position of non-spiculate forms is problematic.Thus the hypercalcified skeleton has for many years been considered a grade of organisation of the skeleton,and the terms archaeocyath-grade,stromatoporoid-grade,chaetetid-grade,sphinctozoan-grade and inozoan-grade are applicable.Nevertheless,archaeocyaths have also been separated as a class,by sponge researchers,creating a quandary about their taxonomic status.Two older classification terms are redundant:sclerosponges(previously a class of all hypercalcified sponges)and pharetronids(previously a subgroup now divided into sphinctozoans and inozoans).Pharetronids are polyphyletic within the Demo-spongiae and Calcarea.Hypercalcified sponges'history began with archaeocyaths(early-mid Cambrian).Then,prominence of stromatoporoid-grade in the mid-Palaeozoic,and chaetetid-grade in the Carboniferous,was followed by a sparse record in both groups for much of the Permian while sphinctozoan-and inozoan-grades expanded.The Mesozoic has a good record of sphinctozoans,inozoans,stromatoporoids and chaetetids up to the end-Cretaceous.Cenozoic forms are uncommon but 19 genera of modern-day demosponges and calcarean sponges encompass all five grades,versus the total modern sponge diversity of 680 genera.Hypercalcification is diverse in modern sponges,involving aragonite,high-Mg and low-Mg calcite;ancient groups reflect this range in their variation of preservation(including widespread diagenetic alteration)that makes under-standing of hypercalcification mechanisms problematic.Presence of hypercalcified sponges from Early Cambrian to modern times,with short breaks associated with extinction events,demonstrates that hypercalcification was an iterative evolutionary feature.For example,the stromatoporoid-grade appeared in Early to Mid-Ordovician and continued through geological history to modern representatives,albeit with taxa turnover through time.Stromatoporoids are traditionally viewed as becoming extinct at the end-Devonian Hangenberg event,but because they form a grade,rather than a proven phyletic group,discussion of the extinction of stromatoporoids as a group has little meaning;it is more appropriate to consider that certain sponge taxa,possessing stromatoporoid-grade skeletons,became extinct.Rare stromatoporoid-grade taxa in Lower Carboniferous strata support such a view.Although their polyphyletic nature was recognized for Mesozoic and Cenozoic forms,the 2015 Treatise on hypercalcified sponges treats stromatoporoids and archaeocyaths as distinct groups.Modern hypercalcified sponges are sponge taxa that just happened to hypercalcify.Thus fossil hypercalcified sponges may be best considered as a system of hypercalcification across a complex and varied skeletal morphospace within Classes Demo-spongiae and Calcarea,alongside the evolutionary history of the phylum Porifera,to aid understanding of their changes in time.

The timing and tectonic evolution of the closure of the Paleo-Tethys Ocean remain controversial, with dates ranging from Late Permian time to the Jurassic. Field mapping shows that the regional Kaixinling unconformity lies between the Upper Permian-Middle Triassic Nayixiong Formation and Middle-Upper Triassic Jiapila Formation in the North Qiangtang terrane of China. It is characterized by similar to 50-cm-thick weathering crust and abrupt changes in stratigraphy, lithofacies, and biological realms in the Kaixinling area. Zircon U-Pb dating of interlayered tuffs and siliceous dolomite accurately constrained formation of the Kaixinling unconformity to 243-239 Ma (Anisian-Ladinian). Detrital zircon grains from the Nayixiong and Jiapila formations show unimodal peak ages of 256 Ma and 302 Ma, respectively, which indicate provenance mainly from the northern magmatic arc. The absence of PanAfrican ages rules out the basement source of the North Qiangtang terrane. These data, integrated with contemporaneous paleolatitude data from the North Qiangtang terrane, suggest that the development of the Kaixinling unconformity was associated with the tectonic transition from the Jinshajiang oceanic subduction-related retroarc foreland basin to the collision-related intracontinental foreland basin in the North Qiangtang. Thus, development of the unconformity marked the closure of the Jinshajiang Ocean at 243 Ma. Our results further highlight diachronous suturing of the Paleo-Tethys Ocean from the Ailaoshan Ocean in Early Triassic time to the Jinshajiang Ocean in Middle Triassic time, which signified the broader-scale reconstruction of the Pangea supercontinent.

The Middle Devonian witnessed the expansion of global coral-stromatoporoid reefs, but knowledge of reefs derives mainly from Laurussia and adjacent regions. Although South China Block (SCB) is renowned for reef proliferation in the eastern Palaeotethys, reef growth and their global implications remain under-explored. Using data from 353 references and 6731 thin sections, this study provides a comprehensive summary regarding reef ecosystems of SCB, plus comparison and integration with global records. Two main findings are outlined: (1) Reef growth. Three well-differentiated ecological associations and six high-resolution developing stages are recognised, and the mid-Givetian marked the peak of reef-building. The transition from epicontinental seas to rifting platforms reflects reef expansion from extensive open marine biostromes to barrier reef–lagoon communities. Reef growth was driven primarily by climate, temperature, and sea-level changes/tectonic activities, facilitating carbonate precipitation and extensive shallow-water habitats. (2) Global implications. Compared with major reef regions, the significance of SCB is noted in three aspects, including abundant fossil materials, unusual palaeogeographic location (eastern Palaeotethys), and exceptional reef pattern (broad inland sea to platforms and intra-platform basins). Based on the newly refined reef database, the global spatial distribution of Givetian reefs indicates SCB as a reef-building hotspot in eastern Palaeotethys. Moreover, the new temporal trajectory implies the reef peak occurred earlier in the Middle Devonian, supporting Givetian as an acme of Phanerozoic reef-building. Overall, this study highlights the underestimated role of SCB in understanding global Givetian reef expansion, Phanerozoic reef evolution, and the intricate interplay of local/global factors controlling reef growth.

The globally-recognised Homerian (Silurian) carbon isotope excursion (CIE) occurs in the Much Wenlock Limestone Formation and Gorsley Limestone of the May Hill and Gorsley inliers of the southern part of the Midland Platform (eastern Avalonia), England. This CIE is associated with eustatic sea-level fluctuations and time-specific facies, and is documented in limestone facies formed in a tropical shallow marine setting, that crop out at the key localities of Hobbs Quarry, Hobbs Lane, and Linton Quarry. Carbon isotope trends and values from these localities identify the falling limbs of the first and second peaks of the Homerian CIE at Hobbs Quarry and Hobbs Lane, respectively, and the low between these peaks at Linton Quarry. The identification and correlation of these parts of the Homerian CIE, alongside the correlation of lithofacies, bentonites and sea-level changes, indicate that at May Hill, the Much Wenlock Limestone Formation began near the top of Silurian stage slice Ho1 and ended near the top of Ho3 (i.e. middle to end Homerian), as is the case for the inner part of the Midland Platform to the north of the study area. Lithofacies of the Gorsley Limestone (Gorsley Inlier), alongside carbon isotope values, correlate to the middle of the Much Wenlock Limestone Formation, as developed at May Hill and across the inner part of the Midland Platform. This indicates that the unconformity at the top of the Gorsley Limestone omits the latest Homerian limestones and second peak of the Homerian CIE, as well as the overlying Gorstian. Furthermore, the uplift, subaerial exposure, and weathering of pyrite-rich sediments in the Gorsley area, to form the Gorsley High, may have supplied iron into the surrounding marine environment, and resulted in the localised development of the latest Homerian ferruginous crinoidal grainstones of the neighbouring May Hill Inlier and Ledbury Hills. These results demonstrate that the CIE and associated strata in the study sites are consistent with regional trends, and therefore add to the global database of these mid-Silurian stratigraphic changes.

Stromatoporoid sponges were important reef-builders during the middle Paleozoic, yet their early history and integration into reef ecosystems remain poorly understood. Here, we report Lophiostroma leizunia Jeon sp. nov., the oldest known stromatoporoid from upper Tremadocian to lower Floian (~480 My-old) strata of South China. L. leizunia formed complex reef structures, playing crucial roles in framework construction and binding other components, including calcimicrobes, lithistid sponges, stalked echinoderms, and Calathium. This discovery pushes back the fossil record of stromatoporoids and the reefs that they formed by approximately 20 My, advancing the onset of the Great Ordovician Biodiversification Event in reef evolution. L. leizunia unusually constructed its skeleton using fluorapatite-a feature previously unknown in sponges. This establishes Porifera as the first metazoan phylum known to have utilized all three principal biominerals: silica, calcium carbonate, and calcium phosphate. The presence of phosphatic skeletons in this early stromatoporoid expands our understanding of biomineralization capabilities in early animals and suggests that the genetic toolkit for diverse biomineralization strategies may have been present in early sponges. The unique combination of the earliest known reef-building stromatoporoids and their phosphatic skeletal composition provides insights into the evolutionary dynamics of biomineralization and the rise of metazoan-dominated reef ecosystems during a critical period of Earth's history.

Since early Palaeozoic time, stalked crinoids provided a unique substrate for attachment of various epi-and endobiontic organisms (sclerobionts), which therefore had access to suspended food particles and did not compete with benthic filter-feeding organisms on the sea floor below them. However, many epibiontic colonizers were partly embedded by the crinoid stereom, so the syn vivo (during life of the host) biotic interactions may be uniquely preserved in the fossil record. Numerous crinoid-sclerobiont associations and their symbiotic relationships are reported in literature; however, the previous studies were mainly devoted to specific epibiontic colonizers. Here, we present a comprehensive study of the whole assemblage of colonizers on Lower Devonian (Pragian) crinoid stems from the famous locality at Hamar Laghdad, Morocco, providing a uniquely broad picture of crinoid-sclerobiont biocenosis and symbiosis. We show that more than half of the Schyschcatocrinus pluricolumnals were colonized syn vivo. The most abundant symbionts are: Tremichnus pits-producing epibionts (found in 43% of host crinoids), different bioclaustrations and stems possessing galls/swellings. In addition, some crinoids, tabulate and rugose corals are embedded by the hosts' stereom. In comparison to crinoid-sclerobiont associations from other sites, those from Hamar Laghdad show the most severe, so far recorded, syn vivo infestation by parasitic Tremichnus-producing epibionts. The picture emerging from our data and previous observations from another site (Tala n'Taleb, south-western Morocco) shows that Early Devonian crinoid biocoenoses of the southern Rheic Ocean realm contained abundant ectoparasitic epibionts, which are interpreted to have easily infected the hosts forming such dense populations. Additionally, a single Ferestromatopora stromatoporoid found encrusting a pluricolumnal is the oldest record of this genus, also demonstrating a wider geographic range than previously known during the Early Devonian, at a time when stromatoporoids were globally impoverished.

The earliest named stromatolite Cryptozoon Hall, 1884 (Late Cambrian, ca. 490 Ma, eastern New York State), was recently re-interpreted as an interlayered microbial mat and non-spiculate (keratosan) sponge deposit. This "classic stromatolite" is prominent in a fundamental debate concerning the significance or even existence of non-spiculate sponges in carbonate rocks from the Neoproterozoic (Tonian) onwards. Cryptozoon has three types of microbially-induced carbonate layers: clotted-pelletoidal micrite with microbial filaments, clotted-pelletoidal micrite with vesicular structure, and dense microcrystalline laminae. A fourth, stratiform to patchy fabric comprises suspect sponges. Using contextual fabric analysis, elemental mapping, cathodoluminescence, fluid inclusions, electron backscatter diffraction, U-Pb dating, and burial history, the sponge interpretation is denied. Neither a distinct sponge body outline nor a canal system is identifiable. Instead, the suspect fabric is secondary in origin, and best explained as a product of Carboniferous (Mississippian) deep burial alteration associated with basement reactivation. Key petrographic observations include heterogenous recrystallization via aggrading Ostwald ripening with interfingering reaction fronts typical for partially miscible fluids, a granoblastic calcite texture (incipient metamorphism), and subsequent hypidioblastic white mica (arguably Carboniferous/Permian, Alleghenian orogeny). Topotype Cryptozoon is a stromatolite altered to sub-greenschist metacarbonate. The published Tonian to Phanerozoic record of interpreted non-spiculate sponges requires reassessment.

Stromatoporoids are common shallow marine hypercalcified sponges in two major episodes with distinctive skeletal architectures: 1) Palaeozoic: Early to Middle Ordovician, to Late Devonian; and 2) Mesozoic: Late Triassic to Cretaceous and rare Cenozoic, but not confirmed in Permian and earlier Triassic strata. Stromatoporoids appeared in Early to Middle Ordovician strata, important in buildups from late Middle Ordovician metazoan expansions (Great Ordovician Biodiversification Event). Throughout the Palaeozoic, some stromatoporoid taxa occur across several palaeocontinents, and, if they are the same biological taxa, presumably migrated as larvae across oceans, implying biotic resilience. Palaeozoic stromatoporoids suffered 5 events of decline; Event 1): end-Ordovician Mass Extinction; surviving forms are more typical of the Silurian, marking change of abundance from labechiid to clathrodictyid forms. Event 2): late Silurian to Early Devonian contraction: stromatoporoids became scarce with low generic diversity, presumably related to global sea-level fall. Intra-Silurian extinction events principally affecting conodonts and graptolites, associated with positive carbon isotope excursions, seem not to have affected stromatoporoids, likely because of their shallow marine benthic habit, contrasting pelagic oceanic planktonic and nektonic fauna influenced by oceanographic changes. Expansion to their late Early to Middle Devonian (Eifelian and Givetian) acme, as one of the Phanerozoic’s major global reef systems, was likely linked to global sea-level rise, when epeiric seas expanded, but followed by Event 3): end-Givetian extinction, likely related to sea-level fall; Event 4): Frasnian-Famennian (F-F) extinction; and Event 5): end-Devonian (Hangenberg Event) extinction; 4 and 5 may be related to cooling, anoxia and potentially, magmatism. The apparent stratigraphic gap between end-Devonian and Triassic occurrence is normally interpreted as extinction of Palaeozoic stromatoporoids, but rare Carboniferous examples in England, Russia, USA and Japan prove survival in shallow marine environments. An interpretation that stromatoporoid-grade sponges lost ability to calcify is unlikely, because chaetetid hypercalcified sponges expanded and built reefs in the Carboniferous. Important is those skeletal architectures of hypercalcified sponges, such as stromatoporoids and chaetetids, are regarded as ‘grades of organisation’ of the skeleton, lacking phyletic value; living stromatoporoid- and chaetetid-grade sponges occur in the Demosponge and Calcarea sponge classes based on spicules. This implies that extinction of sponge taxa that just happened to have been stromatoporoid-grade hypercalcifiers may explain stromatoporoid loss in the end-Devonian, and may point to an unpreserved crisis in non-calcifying Porifera, noting a poor sponge record in end-Devonian strata. Having also survived the end-Permian and end-Triassic extinctions, sponges with ability to produce stromatoporoid-grade skeletons expanded again in the Jurassic, together with sphinctozoan and inozoan grades, then survived the K-Pg extinction although are rare after the Cretaceous. Stromatoporoids seem to be more abundant during calcite seas times, so there may be both an oceanographic chemical control on their development and bias in preservation towards calcite rather than aragonite mineralogy. Overall, the hypercalcifying ability of sponges was not lost throughout their Phanerozoic history; thus, stromatoporoids and other hypercalcified sponges are preserved evidence of resilience of sponges in Earth history, contrasting other celebrated reef-building forms, such as tabulate and rugose corals, and rudist bivalves, that died out.