Lampreys from the Jurassic of northeast China, and their implications for the history of the group.

As one of only two groups of living jawless Vertebrates, Lampreys (Petromyzontiformes) have an important place in our understanding of the history of the group. They have a unique feeding style, with a sucker mouth which they use to attach to their prey, before either detaching a chunk of tissue to be consumed or remaining attached and draining their host's blood. Fossil Lampreys are known from the Carboniferous, showing that they have been around for at least 360 million years, but unfortunately the post-Carboniferous fossil record of the group to-date comprises only two species from the Cretaceous Jehol Biota of China. These Jehol Lampreys are apparently little different from their modern relatives, implying that some significant changes had taken place between the Carboniferous and the Cretaceous, including a re-arranging of the arrangement of the keratinous teeth, the appearance of a worm-like ammocoete larval stage, the invasion of fresh-water environments, and the adoption of an anti-tropical distribution (i.e. being found outside the tropics in both hemispheres, but being absent from tropical areas). 

In a paper published in the journal Nature Communications on 31 October 2023, Feixiang Wu of the Key Laboratory of Vertebrate Evolution and Human Origins at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, Philippe Janvier of the Muséum national d’Histoire naturelle, and Chi Zhang, also of the Key Laboratory of Vertebrate Evolution and Human Origins at the , Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, describe two new species of Lamprey from the Middle Jurassic Yanliao Biota Lagerstätte of northeastern China, and discuss the implications of these for the evolution of Lampreys as a group.

Both of these Jurassic Lampreys have a feeding apparatus which includes well-developed movable biting plates on the tongue-like piston, something known in the living Pouched Lamprey, Geotria australis, which is found in New Zealand, Chile, Argentina, the Falkland Islands, South Georgia and the southwest and southeast corners of Australia, but which has not previously been seen in any fossil Lamprey. The discovery of this trait in Jurassic Lampreys has important implications for the history of the group, suggesting that this may be a lost ancestral trait in the group, not an advanced development in the Pouched Lamprey, as had previously been assumed.

Both new species are placed in a new genus, Yanliaomyzon, meaning 'Yanliao sucker' in reference to the Yanliao Biota and the feeding apparatus of Lampreys. Both members of the genus have oral discs with well-toothed anterior and lateral fields, the teeth on these fields are closely arranged, dorsally truncated, and spatulate in shape with the slightly concaved under-surface of the free edge protruding as a shallow blade. 

The first new species is named Yanliaomyzon occisor, where 'occisor' means 'killer' in reference to the presumed hunting habit of the species. The species is described from two specimens, the first being complete and coming from the Daxishan locality in Jianchang County, Liaoning Province, and the second comprising a head and the forepart of the body, and coming from the Nanshimen Village locality in Hebei Province. Both are from the Tiaojishan Formation, which is thought to be between 160 and 158.58 million years old. Yanliaomyzon occisor has a supraoral lamina which completely spans the lateral rims of the oral aperture, the central cusps of which are flanked immediately by two smaller projections. It has 16 circumoral teeth, and a tail which takes up 28% of its bodylength.

Jurassic Lamprey from the Yanliao Biota, China, Yanliaomyzon occisor (a) Photograph of holotype (IVPP V 15830); (b) Line drawing of the oral disc and dentition of (a); (c), (d) Paratype (IVPP V 18956B), photograph (c) and line drawing (d); (e) Restoration. Abbreviations: adf, ‘anterior dorsal fin’ (dorsal fin); af, anal fin fold; ba, branchial apparatus; ca, cloaca (anus); cot, circumoral teeth; da, dorsal aorta; dcf, dorsal lobe of caudal fin; dt, oral disc teeth; cf, caudal fin; e, eyes; dt, disc teeth; go, external gill openings; gp, gular pouch; ic, intestine contents; io, infraoral lamina; ll, longitudinal lingual lamina; ll.l, left longitudinal lingual lamina; ll.r, right longitudinal lingual lamina; lv, liver; ns, olfactory organ (nasal sac); oc, otic capsule; od, oral disc; of, oral fimbriae; op, oral papilla(e); paf, precloacal skin fold; pdf, ‘posterior dorsal fin’ (anterior part of caudal fin); pt, piston cartilage; so, supraoral lamina; tl, transverse lingual lamina; vcf, ventral lobe of caudal fin; V1?, ophthalmic ramus of trigeminal nerve? Wu et al. (2023).

The second new species is named Yanliaomyzon ingensdentes, where 'ingensdentes' means 'large teeth', in reference to the large cuspid laminae on the gouging piston of this species. This species is described from a complete specimen and a separate preserved oral disk, both from the Daohugou Beds at Wubaiding Village in Reshuitang County, Liaoning Province, a locality dated to about 163 million years ago. Yanliaomyzon ingensdentes has a supraoral lamina occupying roughly one-third of the rim of the oral aperture; as well as a transverse lingual lamina which almost equals the supraoral lamina in width. It has about 23 circumoral teeth, and a tail which makes up about 40% of its bodylength.

Jurassic Lamprey from the Yanliao Biota, China, Yanliaomyzon ingensdentes (f) Photograph of holotype (IVPP V 16715B), white arrow pointing to the skeletal relics in gut content; (g) Oral disc and dentition; (h) Restoration. Abbreviations: adf, ‘anterior dorsal fin’ (dorsal fin); af, anal fin fold; ba, branchial apparatus; ca, cloaca (anus); cot, circumoral teeth; da, dorsal aorta; dcf, dorsal lobe of caudal fin; dt, oral disc teeth; cf, caudal fin; e, eyes; dt, disc teeth; go, external gill openings; gp, gular pouch; ic, intestine contents; io, infraoral lamina; ll, longitudinal lingual lamina; ll.l, left longitudinal lingual lamina; ll.r, right longitudinal lingual lamina; lv, liver; ns, olfactory organ (nasal sac); oc, otic capsule; od, oral disc; of, oral fimbriae; op, oral papilla(e); paf, precloacal skin fold; pdf, ‘posterior dorsal fin’ (anterior part of caudal fin); pt, piston cartilage; so, supraoral lamina; tl, transverse lingual lamina; vcf, ventral lobe of caudal fin; V1?, ophthalmic ramus of trigeminal nerve? Wu et al. (2023).

Both species of Yanliaomyzon are large, with the complete specimen of Yanliaomyzon occisor measuring 642 mm; among extant Lampreys this is exceeded only by the Anadromous Sea Lamprey, Petromyzon marinus (maximum adult length 1200 mm), Pacific Lamprey, Entosphenus tridentatus (850 mm), Pouched Lamprey, Geotria australis (788 mm), and Arctic Lamprey, Lethenteron camtschaticum (790 mm). 

The most distinctive feature of these Lampreys is the extensively toothed oral disc and tongue-like piston, which is similar in morphology to that of the extant Poached Lamprey, a species with a Southern Hemisphere distribution, which is capable of delivering a powerful bite and removing large chunks of flesh from its prey. 

Feeding apparatus of Yanliaomyzon and the Pouched Lamprey, Geotria australis. (a)–(d) Oral disc and dentition of Yanliaomyzon ingensdentes, (a) Photograph (IVPP V 16716B) and (b) Line drawing; (c) Photograph (IVPP V 16716A), whitened with ammonium chloride, the white arrow pointing to the imprints of the wrinkles of the gular pouch; (d) Restoration. (e), (f) Oral disc and dentition of Yanliaomyzon occisor, (e) Photograph (IVPP V18956A), whitened with ammonium chloride; (f) Restoration; (g) Oral disc and dentition of Geotria australis. Abbreviations: cot, circumoral teeth; dt, oral disc teeth; gp, gular pouch; ic, intestine contents; io, infraoral lamina; ll, longitudinal lingual lamina; ll.r, right longitudinal lingual lamina; od, oral disc; of, oral fimbriae; op, oral papilla(e); so, supraoral lamina; tl, transverse lingual lamina. Wu et al. (2023).

Both species of Yanliaomyzon have gular pouches, a feature seen in 12 species of living Lamprey, as well as the Cretaceous Mesomyzon mengae, but unknown in other fossil Lampreys. This feature has been suggested to be connected to courtship displays in male Lampreys, or serving as an energy reserve during anadromous (sea-to-freshwater) migrations; seven of the extant species in which this feature is found undertake such migrations.

Also seen in both species of Yanliaomyzon is a long dorsal fin extending anteriorly until the level of the fourth gill pouch, and a a long precloacal skin fold, which extends anteriorly to the anterior branchial region.

A phylogenetic analysis recovered Yanliaomyzon as stem group Lampreys (i.e. more closely related to living Lampreys than to any other living group, but not decended from the last common ancestor of all living Lampreys). Notably, including Yanliaomyzon in the analysis led to Mesomyzon mengae also being recovered as a stem group Lamprey, where previous studies had recovered it as a member of the crown group. In this analysis all fossil Lampreys lie outside the crown group (the crown group comprises everything descended from the last common ancestor of all living members of a group), which now comprises only living species. 

In this new analysis, Geotria australis, the only member of the family Geotriidae, is recovered as the outgroup to all other Lampreys, with the genus Mordacia, with two species forming the family Mordaciidae, forming the sister group to the family Petromyzontidae (Northern Hemisphere Lampreys), which includes all other living Lampreys. This is another new interpretation, with previous analyses having suggested either that the Geotriidae and Mordaciidae are sister groups, on a separate branch to the Petromyzontidae, or that the Petromyzontidae and Geotriidae are sister groups, with the Mordaciidae being sister to the pair.

Time-calibrated phylogeny of the Cyclostomes and Lampreys. The time-tree is the all-compatible consensus tree summarized from the Bayesian total evidence dating analysis on the partitioned data. The node ages in the tree are the posterior medians, and the error bars at the nodes denote the 95% highest posterior density intervals. The shade of each circle represents the posterior probability of the corresponding clade. The colour of the branch represents the median relative evolutionary rate of the feeding mechanism characters at that branch. Abbreviations: C., Caspiomyzon; Cam., Cambrian; Carbon., Carboniferous; Dev., Devonian; En., Entosphenus; Eu., Eudontomyzon; G., Geotria; I., Ichthyomyzon; La., Lampetra; Le., Lethenteron; M., Mordacia; Ord., Ordovician; P., Petromyzon; Perm., Permian; Sil., Silurian; T., Tetrapleurodon; Y., Yanliaomyzon. Wu et al. (2023).

Yanliaomyzon occisor is the largest fossil Lamprey known to science, and would be large for a modern Lamprey. Among living Lampreys large size is associated with longer migrations, a wider range, larger clutches of eggs, and a greater tolerance for salt water. Many small Lamprey species do not feed at all after metamorphosing from their ammocoete larval stage. Based upon this, and the prevailance of anadromous migrations among Lampreys recovered as basal within the crown group by Wu et al.'s phylogenetic analysis, Yanliaomyzon occisor appears likely to have been an anadromous migratory species with a triphasic life cycle (this is known to have been the case in Mesomyzon mengae, a Cretaceous species found to be less closely related to the crown group than Yanliaomyzon occisor in the phylogenetic reconstruction).

The long dorsal fin and ribbon-like precloacal skin fold seen in both species of Yanliaomyzon also suggest that these Lampreys were powerful swimmers. Similar arrangements are seen in the European Eel, Anguilla vulgaris, and African Knifefish, Gymnarchus niloticus, both of with are capable of swimming against powerful currents, something likely to be useful in a Lamprey migrating upstream to reproduce.

Lampreys appear to have switched from a simple non-migratory life cycle lacking a separate larval stage to the modern three stage, anadromous migratory life cycle some time after the Carboniferous, and the discovery of Yanliaomyzon spp. strongly suggests that this had occurred by the Middle Jurassic. This change in lifestyle appears to have also been connected to a sharp increase in the body size of Lampreys, probably as a result of the interactions between Lampreys and a changing prey-community.

Lampreys first appeared in the Devonian, and have generally been assumed to have been either carnivorous or predatory from the outset. However, Wu et al. point out there is little evidence for such behaviour in Palaeozoic Lampreys, which are very small, lack an ammocoete larval stage, and have simply structured and tiny dentition and a small buccal cavity (the space where the glands which secrete anticoagulants are found in modern Lampreys). The oral disks of these Palaeozoic Lampreys were capable of attaching, but had little biting capacity. Furthermore, the majority of Palaeozoic Fish were covered with thick scales or amoured plates, which it is unlikely even a modern Lamprey could penetrate, and which it is highly unlikely that the much smaller and less well armed Palaeozoic Lampreys could have overcome. As an alternative, Wu et al. suggest that early Lampreys may have specialised in scraping Algae from the bodies of larger Animals, using their oral disks to stay attached when their hosts moved about. Adopting a specialist niche such as this would have enabled Lampreys to flourish in an environment where they faced competition from a large number of other Jawless Fish species, notably the Conodonts from which they are thought to have derived, and which were armed with similar feeding apparatus. This evolutionary jump could help to explain the rapid range expansion of Palaeozoic Lampreys, which were restricted to the southern polar region in the Devonian, but which had reached equatorial regions by the Late Carboniferous. 

Wu et al.'s phylogenetic reconstruction suggests that the ancestors of Mesomyzon mengae diverged from the ancestors of Yanliaomyzon spp. and modern Lampreys in the Early Jurassic, suggesting that more powerful oral disks associated with predation and parasitic behaviour had evolved by this point. This may have been linked to the rise of Teleost and Acipenseriform fish in the Early Jurassic, which typically have much thinner scales than the Gannoid Fish they supplanted, as well as the disappearance of potential competitors such as the Conodonts in the Permian and Triassic extinctions, which would have created new opportunities for Lampreys, leading to the development of more the specialized feeding apparatus and the increase in size seen in later members of the group. This increase in size would have facilitated the invasion of freshwater environments and the development of a migratory reproductive cycle.

The feeding apparatus and gut of Yanliaomyzon spp. appear similar to that of the modern carnivorous Pouched Lamprey, Geotria australis, indicating that this lifestyle had appeared by the Middle Jurassic. Wu et al. hypothesise that carnivory is the ancestral state for crown group Lampreys, and that parasitism arose as specialization derived from this, the reverse of the previously assumed scenario. The adaptation to a carnivorous lifestyle would have provided Lampreys with a high energy diet, enabling the evolution of larger body sizes, and longer migrations.

Modern Lampreys have an anti-tropical distribution, found in temperate and sub-arctic waters in both hemispheres, north and south of the 30° parallel and the 20°C isotherm. This preference for cool waters was also seen in the earliest Lampreys, and while the group have at times moved into more equatorial waters, this appears to have coincided with cooler intervals in the geological record. 

Timetree of the Petromyzontiformes projected with paleotemperature curve since the Devonian and biogeographic reconstructions of the group. Wu et al. (2023).

Lampreys were present in palaeoequatorial regions of Euramerica during the Late Carboniferous Ice Age, and all known Mesozoic fossils are restricted to temperate regions of the Northern Hemisphere. The size and morphology of these Mesozoic Lampreys suggests that they were probably stronger swimmers than even the most widespread current species, such as the Pouched Lamprey, Geotria australis, and Pacific Lamprey, Entosphenus tridentatus. Both of these predatory Lampreys are capable of following shoals of Fish for long distances, and reaching considerable depths; Pacific Lampreys are typically found at depths of 0-500 m, but the maximum depth at which the species has been recorded is 1485 m. A Lamprey capable of sustained swimming at such depths would be capable of migrating across the equator without ever having to enter warm water.

Crown group Lampreys were assumed to have arisen in the Southern Hemisphere between 280 and 220 million years ago, before the breakup of Pangea, and then Gondwana, leading to the anti-tropical distribution of the group. Wu et al.'s study suggests that the crown group is younger than previously thought, and probably arose in the Southern Hemisphere, with a subsequent migration of some groups back into the Northern Hemisphere. Wu et al. suggest that the crown group may have appeared around the end of the Cenomanian-Turonian Thermal Maximum, an event likely to have wiped out the stem group Lampreys, with the subsequent migration of Lampreys back into the Northern Hemisphere having occurred before the Palaeocene-Eocene-Thermal-Maximum, an event which would have excluded from lower latitudes. 

Crown group Northern Hemisphere Lampreys, Petromyzontidae, are thought to have arisen in the late Oligocene in western North America, and subsequently spread around the hemisphere following the development of ice caps in Greenland and the Arctic Sea, enabling them to spread to eastern North America and then Europe. The absence of such cold water close to the major landmasses of the Southern Hemisphere has led to the uneven species richness in the two hemispheres.

Wu et al. suggest that the morphology and lifestyle of Lampreys is not as conservative as previously thought, and that the group underwent a major evolutionary leap in the Jurassic, including a significant increase in size and swimming ability and the modification of the feeding apparatus into a more modern configuration. Crown group Lampreys are suggested to be much younger than previously hypothesized, and to have arisen in the Southern Hemisphere rather than the Northern Hemisphere, as previously thought.

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The Ba Moussa West Coral fauna, a new Early Carbiniferous Coral assemblage from central Morocco.

Mississippian rocks are common in the Moroccan Meseta. They have been studied and described by French geologists since the beginning of the twentieth century. The Mississippian stratigraphic successions are clearly different in the western and in the eastern parts of the Meseta. The succession was considered quite continuous from the Devonian to the Serpukhovian. However, sedimentation in the eastern part of the central Meseta (Azrou-Khenifra Basin) is more complicated. It took place in both a shallow-water carbonate platform and a deeper water flysch basin, within a tectonically active setting, involving movements of blocks, and transgressions and regressions that produced some gaps and unconformities. Sedimentation during the Tournaisian, early and mid Visean in the basin is regarded as being absent by some authors, whereas continuous or sporadic sedimentation during that time interval is suggested by others.

In a paper published in the Journal of Palaeogeography on 11 February 2020, Sergio Rodríguez of the Universidad Complutense de Madrid and the Instituto de Geociencias at the Consejo Superior de Investigaciones Científicas, Ian Somerville of the School of Earth Sciences at University College Dublin, Pedro Cózar, also of the Instituto de Geociencias at the Consejo Superior de Investigaciones Científicas, Javier Sanz-López of the Departmento de Geología at the Universidad de Oviedo, Ismael Coronado of the Institute of Paleobiology, Felipe González of the Departmento de Ciencias de la Tierra at the Universidad de Huelva, Ismail Said, also of the Universidad Complutense de Madrid, and Mohamed El Houicha of the Laboratoire de Géodynamique et Géomatique at the Université Chouaïb Doukkali, report the recent discovery of a relatively rich Mississippian (early Visean) Coral fauna in the southern part of the Azrou-Khenifra Basin, describe the Corals in detail and their host limestone rocks, and comment on their comparison and affinity with other coeval Coral assemblages in North Africa, Europe and southwest Asia. The microfossil content was also studied to enhance the biostratigraphic discussion and significance of the Coral fauna.

The beginning of Carboniferous sedimentation in the Khenifra region, which lies in the southern part of the Azrou-Khenifra Basin and contains the largest Mississippian outcrops in the eastern central Meseta, is usually considered to occur within the widely known late Visean transgression. However, two early Visean transgressions have been cited. The first one is imprecisely located as “to the north of Ba Moussa (point 1)”. The second one was equated with the base of V2b of mid Visean age.

In the southwestern margin of the Azrou-Khenifra Basin at Sidi Lamine and Tabainout, a thick shallow-water carbonate succession with basal Mississippian conglomerate and sandy limestone can be seen to rest unconformably on older (Ordovician) tilted siltstones and sandstones. A similar relationship is seen at the southeastern margin of the basin at Tiouinine where shallow water sandy limestones rest unconformably on red Ordovician sandstones.

(a) Location of Khenifra in central Morocco; (b) Geological map of Azrou-Khenifra Basin with Ba Moussa West coral fauna locality and other Coral localities mentioned in the text; (c) Simplified geological sketch map of Ba Moussa West area and the location of the studied limestone horizons BMW1 and BMW2. hV-Fm1, Lower Visean; hV-Fm2, Upper Visean. Rodríguez et al. (2020).

The eastern part of the Azrou-Khenifra Basin, northwest of Khenifra, is a region of mostly deep-water rythmic mudstones. However, recent field investigations at Ba Moussa West, northwest of a nappe folded as a north-south trending syncline, and approximately 3 km northwest of Khenifra city margins, have discovered two pale gray weathering limestone units within a thick dark gray siltstone and shale rhythmic succession. These limestones contain abundant corals that form the focus of this paper. The limestone units form two distinct parallel ridges, some 50m apart, and traceable laterally for over 200 m. They form prominent features on the landscape, compared to the subdued topography of the more easily eroded mudstones which encase the limestones. The beds dip steeply to the east (70°) and in places can be vertical. The two ridges expose respectively, 4.90m and 4.10m thicknesses of well-bedded limestones (with beds ranging typically from 10 to 40 cm thick) with thin dark gray shale interbeds.

(a) View looking south of limestone ridge (BMW1) about 5 m thick showing steeply dipping beds overlain and underlain by softer shales; (b) Limestone bed with large angular quartzite and sandstone lithoclasts (beside coin) succeeded by thin laminated sandy limestone and black shales, in turn overlain by bioclastic limestone rich in Corals; solitary Rugose Coral Siphonophyllia (black arrows) and Cerioid Tabulate Coral Turnacipora (white arrow), coin diameter is 2.5 cm; (c) Close-up view of richly bioclastic limestone bed with sharp base, showing abundant transverse sections of Siphonophyllia and Sychnoelasma (black arrows), hammer length is 40 cm; (d) Coarse-grained crinoidal limestone with longitudinal and transverse sections of Siphonophyllia khenifrense; (e) Thin section of rudstone at BMW1 showing bioclasts and lithoclasts. Abbreviations: br, brachiopod; bz, bryozoan; co, coral; cr, crinoid; gr, gastropod; st, sandstone; (f) Thin section of rudstone at BMW2 showing bioclasts and lithoclasts. Abbreviations: br, brachiopod; co, coral; cr, crinoid; sl, siltstone; st, sandstone. Rodríguez et al. (2020).

The limestones are variable in composition and texture, comprising coarse-grained, bioclastic and lithoclastic calcirudites, rich in crinoids, thick-shelled Brachiopods and relatively abundant Corals. The limestone beds consist of numerous sedimentary events. Some have sharp, erosive bases and show grading with laminated tops. Large angular lithoclasts of sandstone and siltstone (up to 20 cm in diameter) occur in some beds. Other limestones are buff weathered, fine-grained, laminated calcarenites. Under the petrological microscope two microfacies are differentiated. The first microfacies, which is less common, is a laminated Crinoidal wackestone-packstone containing small fragments of Crinoidal plates, Corals and Bryozoans. The second one, which is dominant, is a polymictic rudstone with fragmented Corals, Crinoids, Bryozoans, Brachiopods, Trilobites, Gastropods, Bivalves, Foraminifers and angular to subangular grains of quartzite sandstone and siltstone. The disposition of siliciclastic clasts and bioclasts is random in some beds, suggesting rapid sedimentation, but in some beds, most clasts are disposed mainly parallel to the stratification. The fragmentation of bioclasts is also variable.

The limestones can be regarded as proximal debris flow and multistorey high-density turbidite bodies, with numerous event beds, deposited in a prevailing succession of distal turbidite beds. Thus, the coral assemblage is allochthonous and may have been transported far from its original depositional shelf setting.

The two limestone horizons (BMW1 and BMW2) were sampled and corals were collected. Samples from BMW1 contain almost entire Brachiopods and Corals, whereas in BMW2 most bioclasts are completely broken and very few Coral specimens are identifiable at generic or specific level. The coral assemblage is relatively rich, but their diversity is quite low (5 genera and 7 species). The assemblage comprises solitary Rugose Corals and Tabulate colonies. Many corals are well preserved and nearly complete, missing only the apexes and showing sometimes compressed calices when they show few skeletal elements and are filled with muddy sediment. However, others are completely fragmented or crushed or have lost much of their dissepimentaria. Fifty specimens were collected, of which 38 have been definitively identified.

Thin sections of samples were studied to describe the microfossil content. Owing to the brecciated character of many beds, including boulders of large size, only the fine-grained limestones yield Foraminifers. Assemblages are relatively abundant in those fine-grained limestones, although specimens are commonly crushed, and diversity is limited to a few genera. Assemblages from BMW1 are slightly richer than BMW2, although this may be the result of more intense sampling and sectioning.

A large sample from limestone BMW1 (3.8 kg weight) was etched with 8%–10% buffered formic acid solution, following the standard technique to avoid damaging. The low abundance of Conodont elements includes one complete P1 element and six broken elements with upper surface damaged and a few with surface dissolution, which could be in relation to significant transport and resedimentation of elements. The colour of Conodonts shows values of 4.5 to 5 for the alteration index. Reworking of Conodonts may be causing a higher colour alteration index value, but small recrystallised apatite surface is observed in Conodonts. Some specimens preserve a smooth surface, but etched surfaces with pits are often discerned. It suggests short heating on proximity to an igneous intrusion.

Conodonts from samples of BMW1. (a)–(b) Fragment of element of Kladognathus sp., DGO 15624, and detail of the face where breakage shows a lamellar inner structure and small apatite crystal 2–3 μm in size interpreted as recystallized and, later, slight dissolution; (c)–(e) Aboral and oral views of Mestognathus cf. beckmanni, DGO 15625, and detail of the margin of the platform with a strong dissolution located on the ornamentation of ridges and carina causing the inversion of surface relief; (f) Oral view of Polygnathus lobatus with pits due to dissolution of the Conodont surface, DGO 15622; (g) Gnathodus pseudosemiglaber, DGO 15623; (h)–(i) Oral and aboral views of Polygnathus inornatus, DGO 15621. Conodonts are stored in the Museum of Geology of the University of Oviedo. Rodríguez et al. (2020).

The allochthonous shales embedding the limestone horizons were sampled for palynomorphs. A total of 12 shale samples were crushed and dissolved following the classical extraction techniques. After complete removal of carbonate and silicate minerals, the organic remains were oxidized with Fuming Schulze solution and mounted in slides for microscope analysis. Palynomorphs recovered from shales are dominated by phytoclasts and, in minor proportions, by spores, whereas organic-walled marine microphytoplankton and amorphous organic matter are virtually absent. The reduced number of spores and their irregular state of preservation precluded further taxonomic identification. The large proportion of equidimensional to lath-shaped phytoclasts and the absence of marine components may be explained by the intense reworking and effective dilution associated to low-density turbidity currents. The brownish-black to black colour of spores and phytoclasts points to a thermal alteration index which essentially agrees with the colour alteration index values observed for conodonts from the limestone sample.

The Coral assemblage from Ba Moussa West contains a new species of Siphonophyllia, with other solitary Rugose Corals, such as Sychnoelasma urbanowitschi, Cravenia lamellata, Cravenia tela, and Cravenia rhytoides. Colonial Tabulate Corals recorded include Turnacipora megastoma, and Pleurosiphonella crustosa. Themost abundant specimens collected belong to the genus Siphonophyllia (20) and Turnacipora (7). Most other species are represented only by three specimens or less.

The assemblage is similar to that described from lower Visean (Arundian) Moel Hiraddug Formation in North Wales, UK. In both regions the large Siphonophylliid Corals represent the dominant component in dark gray bioclastic limestone and shale lithofacies, in which colonial Rugose Corals are absent. However, the Ba Moussa succession has a lower diversity Coral assemblage and the specimens are not as well preserved. This may be explained by the sedimentological setting at Ba Moussa, with the Corals occurring in graded limestone beds containing large exotic clasts, interpreted as debris flow and proximal turbidite deposits.

The stratigraphic range of Sychnoelasma urbanowitschi, and the three species of Cravenia (Cravenia lamellata, Cravenia tela, and Cravenia rhytoides) is very restricted, typically diagnostic of the early Visean throughout Western Europe. Turnacipora megastoma occurs also, typically in the early Visean.

The Ba Moussa West assemblage has similarities with Tafilalt in Eastern Morocco, where a richer early Visean solitary Rugose assemblage is recorded including Cravenia, Siphonophyllia and Sychnoelasma, but where colonial Rugose genera are also absent. Similar assemblages containing dominant Cyathopsids plus Sychnoelasma, Pleurosiphonella and Micheliniids have been reported in Canada and United States, and in Mid-Asia.

The Ba Moussa limestone beds are clearly older than other Mississippian sections in the Khenifra area, as confirmed by the associated Foraminifers and Conodonts. Coral assemblages from Tabainout and Sidi Lamine, 20 km and 30 km respectively, further west of Ba Moussa West, at the western margin of the Azrou-Khenifra Basin, contain fasciculate and massive colonial Rugose Coral genera (Siphonodendron and Lithostrotion) of late Visean (Asbian) age. Both sections have basal transgressive deposits with in situ shallow-water limestones containing ooids and Calcareous Algae. At Tiouinine, 8 km southeast of Khenifra on the eastern margin of the basin, very rich and diverse late Visean (Brigantian) Coral assemblages form a reefal tract. The early Visean age of the Ba Moussa West limestone correlates with the early Visean age of the transgressive point 1, located to the north of Ba Moussa.

The assemblage in samples from BMW1 contains the Foraminifers Earlandia vulgaris, Earlandia elegans, Endothyra spp., Endothyra similis, Endolaxina sp., Endothyranopsis (Eosinopsis) sp., Eosparastaffella sp., Eosparastaffella concinna, Eosparastaffella evoluta, Eosparastaffella interiecta, Eosparastaffella macdermoti, Eosparastaffella aff. macdermoti, Eosparastaffella ovalis, Eosparastaffella simplex, Eosparastaffella tumida subsp. 1, Eosparastaffella vdovenkoae, Eotextularia diversa, Granuliferella sp., Globoendothyra sp., Lapparentidiscus sp.,? Lituotubella sp., Mediocris mediocris, Mediocris ovalis, Mediocris aff. ovalis, Omphalotis sp., Pseudoplanoendothyra sp., Septabrunsiina sp., Septaglomospiranella sp., Spinobrunsiina sp., Spinolaxina sp., Tetrataxis sp. and Urbanella (Brenckleites) fragilis. The Algospongia recorded are very common Kamaena delicata and Palaeoberesella lahoseni, as well as Stacheoides spissa and Exvotarisella sp.

(a) Eotextularia diversa, BMW1, Pc4367; (b) Latiendothyranopsis sp., BMW2; (c) Omphalotis sp., BMW1, Pc4364; (d) Eoparastaffella tumida, BMW1, Pc4364. (e) Granuliferella sp., BMW1, Pc4364; (f) Mediocris aff. ovalis, BMW1, Pc4366; (g) Eoparastaffella simplex, BMW1, Pc4366; (h) Eoparastaffella ex gr. simplex (Eoparastaffella tumida subsp. 1), BMW1, Pc4367; (i) Eoparastaffella aff. concinna, BMW1, Pc4365; (j) Eoparastaffella evoluta, BMW2; (k) Eoparastaffella vdovenkoae, BMW1, Pc4366; (l) Eoparastaffella macdermoti, BMW1, Pc4364; (m) Eoparastaffella ovalis, BMW1, Pc4367; (n) Endolaxina sp., BMW1, Pc4367; (o) Pseudoplanoendothyra sp., BMW1, Pc4364; (p) Endothyranopsis (Eosynopsis) sp., BMW1, Pc4364. Scale bar same for all figures. Rodríguez et al. (2020).

The assemblage is characterized by a high diversity in Eoparastaffella species, and in particular, the first species with pointed periphery in the last whorl, Eoparastaffella tumida subsp. 1 and Eoparastaffella ex gr. simplex. Although the marker for the base of the MFZ9, as well as the marker for the base of the Visean, Eoparastaffella tumida subsp. 1 is derived from Eoparastaffella simplex from the basal levels of the MFZ9, and thus, the assemblages can be attributed to the base of the Visean. It is noteworthy for the occurrence of Eoparastaffella concinna and Eoparastaffella evoluta, also derived from Eoparastaffella simplex in more advances stages of the MFZ9.

The foraminiferal assemblage recorded in BMW2 is composed of Earlandia minor, Earlandia vulgaris, Endothyra spp., Endothyra ex gr. bowmani, Endothyra prisca, Endothyra similis, Eotextularia diversa, 'Glomospira' sp., Eoparastaffella sp., Eoparastaffella concinna, Eoparastaffella interiecta, Eoparastaffella macdermoti, Eoparastaffella simplex, Eoparastaffella tumida subsp. 1, Eoparastaffella vdovenkoae, Mediocris mediocris, Latiendothyranopsis sp., Omphalotis sp., Plectogyranopsis sp., and Pseudoplanoendothyra sp. This assemblage also contains the pointed and slender Eoparastaffella, including Eoparastaffella. concinna, which is a more evolved form than the ancestral stock of pointed Eoparastaffella. In consequence, the assemblage is also assigned to an advanced stage in the MFZ9. The Algospongia recorded in those levels contain Palaeoberesella lahoseni, Kamaena delicata, Issinella sp., and Exvotarisella sp.

The Conodont fauna studied in samples from BMW1 includes Polygnathus inornatus, Polygnathus lobatus (that is usually related with the first species), and a fragment of Polygnathus sp. These taxa were usually described in the early to mid Tournaisian Siphonodella–Polygnathus inornatus Assemblage Zone in the British Isles. However, it has been indicated that Polygnathus inornatus ranged up to the upper Tournaisian Gnathodus typicus Conodont Zone in Cornwall (UK). Polygnathus inornatus have been reported in the upper Tournaisian Scaliognathus anchoralis Zone of the Moravia-Silesia and the Dinant-Namur basins, and in the earliest Visean, just at the first occurrence of Pseudognathodus homopunctatus in the Belgian area. A late Tournaisian to early Visean age is supported by the occurrences of one P1 element of Gnathodus pseudosemiglaber, one P1 fragment of Mestognathus sp. and one P2 element probably corresponding to Kladognathus sp. The fragment of Mestognathus sp. shows dissolution of the carina and ornamentation of the platform, and the blade and the dorsal part of the platform are broken. The parapet area is close to that described in Mestognathus praebeckmanni. The secondary keel seems to be formed with a basal groove, as in Mestognathus beckmanni, but the specimen is broken. The first occurrence of Mestognathus beckmanni was indicated just below the lower boundary of the Visean Stage at the Global Boundary Stratotype Section in the Pengchong section, South China and in a few localities of Western Europe, although it is often recorded in Visean beds. The early Visean Pseudognathodus homopunctatus species is lacking in Rodríguez et al.'s sample.

The new species of Siphonophyllia is named Siphonophyllia khenifrense, which refers to the town of Khenifra within the Azrou-Khenifra Basin in Morocco. Seventeen whole specimens were recovered, all from Ba Moussa West, as well as 29 transverse sections and 15 longitudinal sections.

The whole specimens are cylindrical Corallites between 20 mm and 40 mm in alar diameter and recorded fragments are up to 20 cm long, often without calice. The dissepimentarium is often abraded. The outer wall is thin.

Siphonophyllia khenifrense. (a)–(c) Holotype DPM BMW1-1: (a) DPM BMW1-1A, transverse section., (b) DPM BMW1-1B, transverse section., (c) longitudinal sections; (d)–(e) DPM BMW1-6: (d) transverse section, (e) longitudinal sections; (f)–(g) DPM BMW1-20: (f) longitudinal sections, (g) transverse section.; (h) DPM BMW2-16, transverse section; (i) DPM BMW2-4, transverse section; (j) Wall microstructure in Siphonophyllia khenifrense, DPM BMW1-1, L, Lamellae; (k) Septal microstructure in Siphonophyllia khenifrense, DPM BMW2-16, Gr, Granular axial septum; F, Fibronormal middle zone; L, Lamellar external zone. Black arrows indicate the position of the cardinal septum. Corals are housed in the Geodinamica, Estratigrafía y Paleontología Department of the Universidad Complutense de Madrid. Rodríguez et al. (2020).

The tabularium diameter varies from 17 mm in immature stage to 31 mm in adult stage. The tabularium is wide, 3/5 to more than 4/5 Corallite diameter; the variation in tabularium width is a function of the age of the specimen (immature vs mature Corallite) and variation in the width of the dissepimentarium, which although generally narrow, can also be variably preserved. The number of major septa ranges commonly between 42 and 61, but up to 68 may be present. The septa are long, almost reaching the axis in immature stage but withdrawn from the centre in mature adult stage. They are straight to slightly flexuous in the tabularium, thinning axially and straight to sinuous in the dissepimentarium. Major septa are strongly thickened in the tabularium but are thin in the dissepimentarium; septa can be slightly thicker in cardinal quadrants and thinner in counter quadrants. The minor septa are also thickened where they penetrate slightly into the tabularium, but not as thick as majors; in the dissepimentarium they are thin. They are variable in length, from 1/4 to 1/3 length of majors. The cardinal septum is slightly shorter in most mature Corallites and located in a closed small cardinal fossula. It is often flanked by two major septa which are shorter than the others. Counter septum is inconspicuous, but shorter in late adult stages.

The dissepimentarium is narrow (typically 1/10 to 1/5 Corallite diameter) and mainly composed of interseptal regular dissepiments. The dissepiments are more irregular in the external part of the dissepimentarium, with occasional lonsdaleoid dissepiments. Typically 3 to 6 rows of slightly angular concentric dissepiments are present in the dissepimentarium. In longitudinal section, the dissepiments are small and elongate. They are declined to the tabularium from 60° to 70°.

The tabulae are mostly complete flat domes with some splitting; horizontal, medially sagging and convex tabulae can be present, sloping down peripherally to prominent gutters. They are relatively widely spaced numbering between 6 and 12 each centimetre.

The wall microstructure is microlamellar, as well as the septal stereoplasm and thickenings of tabulae and dissepiments. The septal mesoplasm is granulofibrous with incipient development of microtrabeculae. The tabulae and dissepiments are microgranular.

At least four transgressive phases have been differentiated in the Azrou-Kenifra Basin which were related with fault activity and resedimentation on the margins of tectonic blocks. The early Visean Corals at Ba Moussa West are the oldest occurrence in this basin, and are an important fauna differentiated from the commonly described faunas in late Visean beds of the western margin of the basin at Sidi Lamine and Tabainout, as well as in the northern part of the basin at Adarouch.

The early Visean age in the MFZ9 is older than the previously considered age for North Ba Moussa point 1 (Zone 11 or equivalent MFZ10), in spite of Foraminifer species that was based on their zonal correlation, Earlandia vulgaris and Eotextularia diversa, are also occurring in samples from BMW1 and BMW2 (assigned here to the MFZ9).

The Ba Moussa West succession is a resedimented body of shale, siltstone and limestone with early Visean microfossils and Corals, indicating that the probable age of sedimentation was very close to that of skeletal growth of the components. The corals and microfossils correspond to shallow-water taxa dwelling on a neighbouring sedimentary relief. The coralline assemblage shows a distinctive dominance of solitary rugosans, the absence of colonial Rugosans and occurrence of colonial Tabulate Corals. Moreover, the solitary forms are dominated by Siphonophyllia khenifrense and Sychnoelasma urbanowitschi, and the Tabulate Coral Turnacipora megastoma. A similar association of Siphonophyllia aff. garwoodi and Sychnoelasma urbanowitschi is known from the early Visean of the Laval syncline in Normandy (north France), although with colonial Rugosans there (Solenodendron spp.). This colonial genus is not recorded in the Azrou-Khenifra Basin first until the late Visean.

This colonial genus is not recorded in the Azrou-Khenifra Basin first until the late Visean. However, none of the seven listed key taxa of this subzone are recorded in Morocco, although the genera Siphonophyllia, Cravenia and Sychnoelasma are present. Perhaps of greater significance though, is that whereas Siphonophyllia hawbankense is only recorded in the underlying upper Tournaisian RC4ß1 subzone, a new taxon Siphonophyllia hawbankense subsp. A which starts in this subzone, extends into RC4ß2 subzone. The strong possibility exists though, that this corresponds to the small Siphonophyllia urbanowitschi of Ba Moussa, which represents the transition to larger typical forms in RC5 Zone.

The Ba Moussa West Coral fauna, although quite restricted in its diversity, nevertheless, contains typical elements of the Western European Coral province (which includes North Africa and Nova Scotia). In particular, the dominance of solitary Rugosa and Tabulate Corals is a feature of the early Visean assemblages which are recognised in northwest Europe: Normandy (north France), southern Belgium, southwest Province, North Wales, Craven Lowlands and South Cumbria (Great Britain), and Dublin Basin (Ireland). Similar early Visean faunas with solitary rugosans are known in the eastern part of the Anti-Atlas region at Tafilalt in eastern Morocco and in the Béchar Basin in Algeria. The late Tournaisian to early Visean Rugose Coral fauna from Tafilalt is richer than that from Ba Moussa. It is dominated by solitary genera, both undissepimented (Sychnoelasma, Cravenia) and dissepimented (Bifossularia, Cyathoclisia, Clisiophyllum, Siphonophyllia, Palaeosmilia, Amygdalophyllum), and is lacking colonial Rugosans.

Palaeogeographic distribution of the Coral taxa recorded in Ba Moussa West in the Palaeotethys region and around Laurentia and Baltica. (s) Siphonophyllia, (u) Sychnoelasma urbanowitschi, (c) Cravenia, (t) Turnacipora, (p) Pleurosiphonella. (1) Ba Moussa West, (2) Tafilalt, (3) Midcontinent, (4) Western Interior, (5) Canadian Rockies, (6) Carnic Alps, (7) Western Europe, (8) Eastern Europe, (9) Moscow Basin, (10) Ural Mountains, (11) Tian-Shan (Northwest China), (12) Turkey, (13) Transcaucasia, (14) Iran, (15) Himalaya, (16) South China. Rodríguez et al. (2020).

It was previously considered that since the Azrou-Khenifra Basin only had late Visean and younger Coral assemblages, so too the Jerada Basin in northeast Morocco, they were isolated from other marine basins in the early Visean. Connections among the Azrou-Khenifra Basin, northwest Europe, Tafilalt, and other Saharian basins in Algeria (Béchar Basin) were open from the Asbian and Brigantian (late Visean). The Ba Moussa West Corals, Foraminifers and Conodonts suggest that marine seaways were available for migrations between the Azrou-Khenifra Basin and other regions from the early Visean. Similar early Visean faunas with solitary Rugosans are known in the eastern part of the Anti-Atlas region at Tafilalt, in eastern Morocco and in the Béchar Basin in Algeria. The marine connections between northwest Europe and the southern part of the Azrou-Khenifra Basin is supported by similar early Visean assemblages recognized in northwest Europe with abundant solitary Rugose and Tabulate Corals, but with colonial Rugosans: Normandy (north France), southern Belgium, southwest Province, North Wales, Craven Lowlands and South Cumbria (Great Britain), and Dublin Basin in Ireland.

In relation to the tabulate corals, the Tabulate Turnacipora megastoma in the Ba Moussa West assemblage was also known from Central Saharian basins, but also from the Chadian-Arundian (early Visean) locations in northwest Europe (UK, Ireland, France, Germany?). The occurrence of Pleurosiphonella crustosa is the first report in North Africa and suggests marine connection with the Urals. It was first described from the upper Tournaisian of Transcaucasia and its age range extends here slightly into the early Visean. The dispersion between southwest Asia (Armenia, Taurides and Alborz) and the Azrou-Khenifra Basin, via Tafilalt, Béchar and Sinai, is poorly established. Some solitary Rugosans (Siphonophyllia) are common to all areas, but others, such as Kueichouphyllum and the colonial form Eokoninkocarinia, indicative of Asiatic affinity are clearly absent in Morocco.

A new early Visean Coral assemblage has been discovered transported in the rhythmic facies deposits of the southern part of the Azrou-Khenifra Basin, northwest of Khenifra, Morroco. The Ba Moussa West coral fauna includes the new species Siphonophyllia khenifrense, as well as Sychnoelasma urbanowitschi, Cravenia lamellata, Cravenia tela, Cravenia rhytoides, Turnacipora megastoma and Pleurosiphonella crustosa. The early Visean age of the Coral assemblage is supported by microfossil data, which confirms a previous hypothesis that indicated a first transgression during the early Visean in the Carboniferous of the Meseta. The allochthonous coral assemblage was recovered from coarse-grained proximal limestone debris flow and turbidite beds within a fault-bounded rhythmic unit in the eastern part of the basin. No evidence remains of the former early Visean shallow-water platform from which the Corals were derived. All other in situ platform carbonate rocks around the southern margin of the Azrou-Khenifra Basin are of late Visean (Asbian–Brigantian) age. The early Visean Ba Moussa West Coral fauna can be compared with that from the Saharian basins of southeast Morocco and Algeria. Most of the genera and species in the Ba Moussa West assemblage are identical to those in Western Europe, indicating possible marine connections. The new Rugose species described, Siphonophyllia khenifrense, is probably endemic to North Africa. Its ecological niche in northwest Europe was occupied by Siphonophyllia cylindrica or Siphonophyllia aff. garwoodi.

The microfossil determinations provide greater precision in the age dating of the Ba Moussa West limestones. The foraminiferal assemblages from BMW1 can be attributed to the lowermost Visean (MFZ9). Similarly, the Conodont fauna recovered from the same beds, although sparse, suggests a late Tournaisian to early Visean age.

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Methylmercury poisoning as a possible cause of the end-Devonian Mass Extinction.

The end-Devonian was a time of significant changes in the global climate and biosphere, including the biodiversity crisis known as the Hangenberg Event. This event occurred roughly. 13.5 million years after the Frasnian-Famennian Mass Extinction, and was linked with globally widespread deposition of the anoxic Hangenberg Black Shale. The Hangenberg Extinction (with 50% marine genera loss) significantly affected the pelagic realm, especially Ammonoids, Conodonts, many Vertebrates, and benthic reef biotas, such as Trilobites and Ostracods, and had an ecological impact similar to the end-Ordovician Mass Extinction. Moreover, a drastic reduction of Phytoplankton diversity is also observed at the Devonian/Carboniferous boundary. Deposition of the Hangenberg Black Shale was a short-term event that lasted between about 50 and 190 thousand years, while the extended crisis interval encompassed a time span of one to several hundred thousand years. The postulated factors responsible for this global event, such as high productivity and anoxia, a calcification crisis caused by ocean acidification, perturbation of the global carbon cycle, glacio-eustatic sea-level changes driven by orbital forcing, volcanic and hydrothermal activity, and evolution of Land Plants, are still vividly discussed. In fact, extensive volcanism has been implicated in all ‘Big Five’ mass extinctions and other biotic crises in the Phanerozoic, including the Hangenberg Crisis. As the main source of mercury in the geological past was volcanic and submarine hydrothermal activity, and mercury anomalies in the sedimentary record have recently been used as a proxy for volcanic activity in relation to global events and palaeoenvironmental perturbations, including for the Devonian/Carboniferous boundary from different palaeogeographical domains.

In a paper published in the journal Scientific Reports on 30 April 2020, Michał Rakociński, Leszek Marynowski, and Agnieszka Pisarzowska of the Faculty of Natural Sciences at the University of Silesia in Katowice, Jacek Bełdowski and Grzegorz Siedlewicz of the Institute of Oceanology of the Polish Academy of Sciences, Michał Zatoń, also of the Faculty of Natural Sciences at the University of Silesia in Katowice, Maria Cristina Perri and Claudia Spalletta of the Department of Biological, Geological and Environmental Sciences at the University of Bologna, and Hans Peter Schönlaub of the Commission for Geosciences of the Austrian Academy of Sciences, report very large, anomalous mercury spikes in two marine Devonian/Carboniferous successions of the Carnic Alps, supporting volcanism as the driving mechanism (ultimate cause) of the Hangenberg Event. Furthermore, They also detected methylmercury, a strong neurotoxin that bioaccumulates in the food chain, in sedimentary rocks for the first time. Thus, Rakociński et al. claim that volcanic-driven methylmercury poisoning in otherwise anoxic seas could be an another proximate (direct) kill mechanism of the end-Devonian Hangenberg extinction.

Rakociński et al. examined two successions of deep-water, pelagic sedimentary rocks, encompassing the uppermost Devonian and Devonian/Carboniferous boundary intervals: Kronhofgraben (Austria) and Plan di Zermula A (Italy) in the Carnic Alps. The Kronhofgraben and Plan di Zermula A sections consist of organic-rich Hangenberg Black Shale and micritic limestone.

Late Devonian (360 million years ago) palaeogeographic map. showing the studied localities and the location of prominent areas of Late Devonian magmatism and associated volcanism, as well as (Al) giant mercury deposits reactivated by Variscan magmatic and tectonic activity in Almadén (Spain). Rakociński et al. (2020).

The Kronhofgraben section in the central Carnic Alps of Austria is situated in a gorge of the Aßnitz Creek, about 7 km east of Plöckenpass and 1 km northwest of the Kronhof Törl pass at the Austrian–Italian border. The Devonian/Carboniferous boundary beds crop out in the eastern side of the Kronhofgraben gorge at an altitude of 1390 m. The Plan di Zermula A section in the southern Carnic Alps of Italy appears on the western slope of the Mount Zermula massif, along the road from Paularo to Stua di Ramaz. Grey limestones and black shales represent the studied interval in both sections. The Hangenberg Black Shale horizon is assigned to the upper part of the Bispathodus ultimus Conodont Biozone (equivallent to the Middle-Upper Siphonodella praesulcata zones) in Kronhofgraben (40 cm thick) and in Plan di Zermula A (15 cm thick) is underlain by Cephalopod limestones of the lower part of the Bispathodus ultimus Zone (equivallent to the Upper Apsotreta  expansa- Lower Siphonodella praesulcata zones). The first carbonate bed above the Hangenberg Black Shale belongs to the Siphonodella sulcata Zone (equivallent to the Protognathodus kockeli Zone).

The Devonian/Carboniferous boundary in both sections is situated directly above the Hangenberg Black Shale. The Devonian/Carboniferous boundary may be somewhat problematic and needs redefinition (caused by problems with discrimination of Siphonodella sulcata from its supposed ancestor Siphonodella praesulcata). The new criterion for definition of the base of the Carboniferous System proposed by the Working Group on the boundary is: identification of the base of the Protognathodus kockeli Zone, beginning of radiation and top of major regression (top of Hangenberg Black Shale) and end of mass extinction. In the limestone overlying the Hangenberg Black Shale, Conodonts of the species Protognathodus kockeli were found in both sections. Therefore, the position of the Devonian/Carboniferous boundary did not changed in comparison to previous studies.

The Devonian/Carboniferous boundary successions in the Plan de Zermula A and Kronhofgraben were deposited in deeper palaeoenvironment. In the late Devonian, Carnic Alps represented the northern tips of Gondwana and belonged to the Gondwana-derived Bosnian–Noric Terrane accreted to the intra-Alpine Mediterranean terrane during the Carboniferous. The investigated rocks outcropped in the Carnic Alps reflected strong thermal alteration.

The Hangenberg Black Shale intervals in the sections investigated display extremely high mercury values, with maxima of 20216 and 9758 parts per billion in Kronhofgraben and Plan di Zermula, respectively. The Hangenberg Black Shale from the Plan di Zermula A section contains mercury anomalies that are roughly 13–100 times higher than the 100 parts per billion background, whereas in the Kronhofgraben section the anomalies are roughly 12–84 times higher than the background values.

Interestingly, significant concentrations of methylmercury were found in the whole Kronhofgraben section, where methylmercury is in the range 13–348 picograms per gram, dry weight. Additionally, we found 55 picograms per gram, dry weight of methylmercury in the Novchomok section in Uzbekistan and 72.72 picograms per gram, dry weight of methylmercury sampled from the uppermost Devonian part of the Woodford Shale from the Arbuckle Anticline in Oklahoma, USA. Traces of methylmercury were also found in the Hangenberg Black Shale interval at Kowala Quarry, Poland (20.66 picograms per gram, dry weight of methylmercury).

In comparison to methylmercury levels found in modern sediments (reaching from 1000 to 700000 picograms per gram, dry weight in polluted basins), those detected in sedimentary rocks studied, are relatively low. However, the original amounts of methylmercury in the investigated sediments would have been higher but impoverished during diagenesis. The mercury enrichments are observed in organic-rich Hangenberg equivalent intervals such as Kronhofgraben (from 0.51 to 13.28% total organic carbon) and Plan di Zermula A (from 0.7 to 12.53% total organic carbon). The values of the mercury/total organic carbon ratio in the Hangenberg Black Shale at Kronhofgraben range from 815 to 8096.5 (parts per billion/%), while the background samples show a range from 387.5 to 985 (parts per billion/%). In Plan di Zermula A, the values of mercury/total organic carbon ratios in the Hangenberg Black Shale range from 779 to 3269 (parts per billion/%) and are higher than those from the background samples (ranging from 84.5 to 676.8 parts per billion/% mercury/total organic carbon).

Volcanic and hydrothermal activities are considered to be the main sources of elevated mercury in sedimentary. Besides mercury delivery to the atmosphere by volcanic activity, other processes can produce mercury spikes in the sedimentary record, including widespread wildfires, terrestrial input, magmatic emplacement or thermogenic processes related to bolide impact rocks. Additionally, some authors have suggested that mercury enrichments can be sulphide-hosted in euxinic (high sulphur/low oxygen) facies, and high mercury spikes not necessary would be connected with volcanic activity. However, in such a case, the Hg enrichments would be well-correlated with total sulphur, which is not observed in our sections. Although extensive wildfires on land were confirmed during the Hangenberg event, based on the co-occurrence of charcoal and high concentrations of polycyclic aromatic hydrocarbons in sedimentary rocks, these, however, could have also been induced by volcanism, as evidenced by the co-occurrence of charcoals and ash layers. No conclusive evidence for bolide impact at the Devonian/Carboniferous boundary has been detected thus far. In fact, at the Devonian/Carboniferous boundary, volcanic activity has frequently been documented, mainly on the basis of the presence of ash layers below, above and within the Hangenberg Black Shale (e.g. in the Holy Cross Mountains, Iberian Pyrite Belt, and Rhenish Massif), mercury spikes, as well as the presence of abnormal or strongly altered spores (tetrads), which could reflect the mutagenic effect of regional acidification caused by explosive volcanism. The most plausible sources of very large amounts of mercury during the end-Devonian interval are the massive Magdalen silicic large igneous province and the Siberian (Yakutsk–Viluy) and/or the Kola–Dnieper large igneous provinces; however, the interval also overlaps with formation of the Almaden mercury deposit (last mineralisation pulse episodes), which constitutes one of the largest geochemical anomalies on Earth and coincided with the first phase of the Variscan Orogeny (mountain-building episode associated with the formation of the supercontinent of Pangea), as considered for the Hangenberg Crisis. According to current knowledge, three large igneous provinces encompass the Late Devonian interval (380–360 million years ago): Yakutsk-Viluy (Siberia; continental type with an area of 0.8 million km²), Kola-Dnieper (Baltica; continental type with area of 3 million km²) and Magdalen (Laurussia, continental-silic type). Moreover, Rakociński et al. cannot exclude other additional mercury sources, for instance connected with explosive eruptions which could overlap with large igneous province activity. Mercury has a strong affinity to organic matter and to a minor extent can also be associated with sulphides and clay minerals; therefore, mercury is normalized to total organic carbon content. Importantly, the mercury spikes in Rakociński et al.'s sections are also evident when normalized to total organic carbon content, which can be interpreted as an effect of increased input of mercury to the basins independently of the potential influence of reducing depositional conditions. The mercury vs. aluminium oxide correlation in the investigated successions is very weak, indicating no correlation of mercury with the clay fraction. However, mercury exhibits a good correlation with molybdenum in the all sections. This could indicate that some mercury was associated with sulphides as a result of its intensified precipitation in a sulphide-rich (euxinic) water column. In the sections investigated, mercury vs. total sulphur correlation is very weak, which does not confirm sulphides as host of mercury. However, the mercury vs. total organic carbon correlation in the Devonian/Carboniferous boundary at Novchomok section is very low, which confirm that mercury enrichments are facies independent and thus are indicative of volcanic activity during this time. For the Kronhofgraben and Plan di Zermula A sections this correlation is good, suggesting possible different sources of this element. However, as already emphasised, there are a number of lines of evidence for volcanic and hydrothermal activities, as well as widespread wildfires, during this time allowing for a firm statement that increased mercury input to the basins was connected with diverse volcanic activities and related combustion of biomass on land. Moreover, the Hangenberg Event took place during an interglacial period; therefore, some mercury could have originated from permafrost melting but even if this process had taken place, mercury would have previously accumulated in the permafrost as a result of volcanic or pyrogenic processes. To summarise, based on all the available data, Rakociński et al. state that the main sources of mercury were volcanism and related hydrothermal activities. In fact, volcanic processes are main sources of mercury in atmosphere.

Schematic model of deposition, mercury sources and mercury methylation during the Hangenberg Event. Rakociński et al. (2020).

The organic form of mercury (methylmercury) is a strong neurotoxin that is bioconcentrated in aquatic food chains and is able to cross the blood–brain barrier; thus, this form of mercury is much more toxic to living organisms than inorganic mercury. In modern environments, methylmercury is generated predominantly by anaerobic microorganisms, such as sulphate-reducing Bacteria (e.g., Geobacter sulfurreducens). Despite widespread mercury pollution, annual emissions of mercury have recently been higher from natural sources than anthropogenic ones, constituting as much as 70% of all mercury emissions. However, the concentrations of mercury detected in all the end-Devonian sections are surprisingly high, similar to the present-day mercury concentrations found in highly polluted basins, e.g., some parts of the Baltic Sea. The mercury concentrations of up to 20 000 parts per billion in Kronhofgraben and 1000–10 000 parts per billion in the Plan di Zermula A, and mercury spikes determined in Germany, south Vietnam, the Czech Republic and south China sections suggest, that global mercury concentrations were highly elevated during the Hangenberg event. This finding implies that, during favorable sedimentary conditions, very high concentrations of methylmercury can be produced on the global scale. In the investigated samples Rakociński et al. measured relatively minor amounts of methylmercury in comparison with the methylmercury levels in modern sediments. In polluted basins, concentrations of methylmercury vary from 1000 to 700 000 picograms per gram, dry weight of methylmercury and are much higher relative to total methylmercury concentration from Rakociński et al.'s sections. However, the original amounts of methylmercury in the investigated sediments would have been higher, assuming large enrichment of total mercury in anomalous samples. It is very probable that methylmercury could have been demethylated during diagenesis as a result of the common diagenetic process of demethylation, which is influenced by temperature. Because of the strong thermal alteration of the investigated rocks, the occurrence of demethylation seems to be very likely.

Therefore, regardless of the mercury source, its high level in the end-Devonian water column, subsequent trapping in sediment and biomethylation to the more toxic methylmercury form by anaerobic Bacteria, would have had an additional devastating impact on aquatic life during the Hangenberg Event. This can be produced under conditions of extended anoxia/euxinia during this time and the occurrence of rich sulphate-reducing Bacteria communities which can change mercury to its methyl form. Additionally, blooms of Green Algal phototrophs (prasinophytes) during black shale events would have contributed, mostly indirectly, to methylmercury production. However, indisputable evidence for Bacterial mercury methylation is the occurrence of notable concentrations of methylmercury in the sediments investigated and the similarities in the distributions of mercury and methylmercury in the Kronhofgraben section.

Observation of modern marine environments has confirmed that methylmercury is highly toxic to animals at higher trophic levels (such as Fish, Birds and Mammals). In this light it seems to be evident that severe extinction of marine and nonmarine Fish and Tetrapods, as well as pelagic Conodont Animals, during the Hangenberg event may also have resulted from methylmercury poisoning that could have affected different aquatic habitats. Although the effect of methylmercury on benthic invertebrates is regarded as minimal, these organisms were significantly affected by concomitant, globally widespread anoxia. Such anoxia asphyxiation–methylmercury poisoning may have also been kill mechanisms in other mass extinctions, but this should be tested by searching for traces of methylmercury in other sedimentary rocks.

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