Michał Jakubowicz

Corals exhibit significant phenotypic flexibility in response to various environmental conditions. As a result, the presence of similar morphologies across modern and extinct corals is commonly independent from phylogenetic links. Among the recurring body plans, one particularly distinctive, ‘button coral’ morphotype is observed in the Palaeozoic, free-living, solitary rugose coral genus Microcyclus . This study focuses on Middle Devonian assemblages of the genus Microcyclus from two sites in southeastern Morocco: a carbonate-ramp succession exposed at Madène el Mrakib and a fore-reef succession of the Aferdou el Mrakib reef, which was affected by recurring mass flows of reef-derived debris. The study discusses the habitat preferences, life-history strategies, and potential degree of automobility of this genus based on analyses of their morphology and comparison with modern Scleractinia inhabiting similar environments. The ‘button coral’ morphology of Microcyclus , including the small, discoid corallite with protruded septa, and its inferred preference for habitats with low sedimentation rate point to the adaptation of the genus to living on the sediment surface in soft-bottom settings. Among modern free-living scleractinians, morphologically closest to Microcyclus appear to be Cycloseris boschmai and Cynarina macassarensis , the latter particularly notable for the presence of epitheca, a feature shared with the rugose corals. The morphological constraints indicate that Microcyclus was likely a lithosessile organism with the ability to excavate from a small amount of sediments.

The palaeogeographic configuration of the continental fragments and seaways that developed during the Devonian evolution of the Rheic Ocean remains insufficiently understood. One of the elusive elements is the palaeogeography of north-western Gondwana, and most notably the position of the Moroccan Meseta – the central part of the Moroccan Variscides, comprising a collage of blocks once located at the northern periphery of Gondwana. While some reconstructions place the Meseta as a distal, continuous segment of the Gondwana margin, others depict a very different scenario, envisaging that at some point the Meseta became separated from Gondwana by a wide oceanic basin. Here, we aim to better understand the Late Devonian position of the Meseta using a novel approach that combines two palaeoceanographic tracers: neodymium (Nd) and oxygen isotopes. These proxies, applied together on conodont apatite – an established archive of the composition (Nd and O isotopes) and temperature (O isotopes) of past seawater – provide new constraints on the pre-Variscan oceanography of the Gondwana margin. The analysed, uppermost Givetian-lower Famennian sections, which are representative of the Gondwana mainland (eastern Anti-Atlas) and the cratonward part of the Western Meseta (Middle Atlas) show similar, relatively unradiogenic εNd values. These signatures point to dominance of continental weathering-derived Nd sources in the epicontinental seas of northwestern Gondwana. The temporal trends observed in the studied sections also show notable similarities, which are primarily interpreted as reflecting variations in the continental-runoff vs. open-oceanic contributions to the local marine Nd isotope budget. These variations were controlled by changes in sea level, local tectonic movements, and the evolution of vascular plants on land. The distal, outboard margin of the Western Meseta exhibits less variable and more radiogenic εNd values, indicating a greater contribution from open-oceanic seawater. While the observed trends in oxygen isotope signatures are generally consistent with global records, the δ18O values are significantly lower than those reported from other parts of the Rheic realm. The most likely explanation for the observed 18O depletion is the increased role of freshwater input in the relatively high-latitude, semi-restricted epicontinental basins. Overall, the observed εNd–δ18O signatures are consistent with the location of Moroccan Meseta at the northern Gondwana margin. Some local variations in the isotope signals can be attributed to the semi-isolated nature of the studied basins, rather than to a presence of an extensive Late Devonian oceanic seaway between the Anti-Atlas and Meseta domains.This work was supported by the National Science Centre, Poland, grant No. 2022/47/ST10/00205.

Modern scleractinian corals can display high phenotypic plasticity, which reflects an interplay among various environmental controls, such as sediment input, water hydrodynamics or light intensity. In particular, the latter can strongly influence the morphology of coral species living across broad depth gradients. Light intensity was also a factor shaping the colonies of extinct Palaeozoic tabulate corals (Anthozoa: Tabulata). Based on gradual transitions in morphology observed in corals from the Middle Devonian (Givetian stage, ~ 385 Ma) mesophotic coral ecosystems (MCE) of the Aferdou el Mrakib reef (Anti-Atlas mountains, Morocco) and comparative material originating from different palaeobathymetric and biogeographical settings, we show that Devonian tabulate corals, such as Roseoporella and Alveolites, were characterised by high phenotypic plasticity and the ability to dramatically change their morphology depending on the inferred light conditions. Such a mechanism is similar to that observed in modern scleractinians, e.g. Porites sillimaniana. This recurring morphological theme suggests that Palaeozoic tabulate corals shared many functional characteristics of modern scleractinians.

Over the past decades, neodymium (Nd) isotopes have received considerable attention in paleoceanography as a tool for reconstructing past seawater circulation, local weathering inputs, and sea‐level change. In this study, we have investigated the Nd isotope composition of a shallow‐water Serpukhovian (Carboniferous) carbonate succession to explore icehouse cyclicity pattern and seawater dynamics on the Karatau carbonate platform in southern Kazakhstan. The cyclic succession formed in response to glacio‐eustasy and composed of subtidal and intertidal limestones displays a large variation in ε_Nd(326 Ma) values from −1.6 to +4.3, corresponding to differences in the isotopic composition of two seawater masses present in the Uralian–Turkestan Ocean during the Serpukhovian: (a) highly radiogenic deep waters and (b) less radiogenic surface waters. The Nd isotope excursions within the icehouse cycles are more complex than simple transgressive‐regressive cycles. They probably reflect a temporal pattern of the sub‐Milankovitch climatic perturbations during Carboniferous interglacial intervals. The episodic appearance of rich brachiopod communities was forced by the inflow of highly radiogenic, nutrient‐enriched waters, presumably driven by upwelling. Nd isotope analyses of cyclic intertidal and subtidal carbonates have great potential to produce high‐resolution records of seawater dynamics on shallow‐water carbonate platforms.

Microconchid and anticalyptraeid tubeworms from the Middle Devonian of Morocco are described for the first time. The assemblage, coming from the locality of Madène el Mrakib (Mader Basin, eastern Anti-Atlas), consists of one microconchid species, Palaeoconchus sanctacrucensis, and one new anticalyptraeid species, Anticalyptraea madenensis. Both species encrusted uppermost Eifelian/lowermost Givetian brachiopod shells. P. sanctacrucensis was hitherto only known from the upper Emsian–upper Eifelian of the Holy Cross Mountains, Poland. Thus, its presence in the shelf habitats of northern Gondwana implies its migration from the southern margin of Laurussia during the transgression associated with the late Eifelian Kačák Episode. The encrustation patterns of the brachiopod shells by the Moroccan microconchids indicate that the brachiopods only served as favourable hard substrate for colonisation by the microconchid larvae. However, the occupation of different valves of the same brachiopod by microconchids and Anticalyptraea may imply that both encrusters potentially avoided direct competition for suspended food particles. Characteristics and position of Oichnus borings present on a few microconchids and anticalyptraeids indicate that they were not necessarily formed syn vivo and produced by predatory organisms. It is likely that they were drilled by some tiny suspension-feeders and may represent non-predatory domichnia.urn:lsid:zoobank.org:act:AA095519-5E68-4EDA-B331-E3944F813412

In recent years, Nd isotopes have seen increasingly common use in studies of hydrocarbon seeps. Given the distinct Nd isotope signature of mafic igneous rocks, particular emphasis in these investigations has been on reconstructing former interactions between the seeping fluids and volcanic materials. The results of our case studies on ancient seeps underlain by mafic volcanic bodies, including Cretaceous seep carbonates of the Outer Carpathians (Czech Republic) and Basque-Cantabrian Basin (Spain), as well as Eocene seeps of the Cascadia convergent margin (Washington) consistently document their significant enrichment in volcanogenic Nd, reflected in their increased Nd isotope ratios (εNd values). The extent of this 143Nd-enrichment varies depending on the geological context of given seeps, most notably the εNd signatures and thickness of the volcanics and overlying sedimentary piles, and the Nd isotope signal of background local pore waters. The highest εNd values are observed in seep carbonates very shallowly underlain by thick mafic volcanics: the Cretaceous seep of the Carpathians and Eocene seeps of Cascadia. For the former, the εNd values are up to 7.5 units higher than the signature of coeval non-seep pore water, whereas for the latter the εNd values are as high as +1.9, close to the highest value ever recorded for seawater. More moderate 143Nd-enrichment typifies the Cretaceous seeps of Spain, for which the volcanic intrusions were emplaced at considerable depths below the seafloor. In such cases, the Nd isotope signature of the fluid-volcanic interactions was partially obscured by subsequent interactions between the fluids and the overlying sediments. Rather than focusing solely on exploring the new geochemical tool, the primary aim of our studies was to address broader questions regarding the tectonic architecture and geological history of the sedimentary basins that host given seep deposits. For the Eocene seeps of Washington, the Nd isotope data served to document interactions between the methane-rich fluids and the volcanic terrane of Siletzia, which underlies the Cascadia forearc; these results placed important stratigraphic and structural constraints on the activation and earliest history of convergence in Cascadia, following Siletzia accretion. For the studied Cretaceous seeps, all hosted by early, sediment-covered rifts, the studies demonstrated that Nd isotopes offer a valuable new tool of deconvolving methane fluxes from different organic matter alteration pathways for the very complex, sedimentary-magmatic systems of incipient rifts. At the same time, these studies emphasized important limitations of Sr isotopes, the system most commonly used to document interactions between the seeping fluids and igneous rocks. Because of the much higher Sr/Nd ratios observed in pore waters than in igneous rocks, the potential of Sr isotopes to record fluid-volcanic interactions is considerably lower than that of Nd isotopes. Thus, broader use of Nd isotopes can assist in identifying potential volcanogenic fluid endmembers for the numerous sedimented rifts for which evidence for magmatic involvement in the fluid expulsion remains equivocal.This work was supported by the National Science Centre, Poland, grant No. 2016/23/D/ST10/00444