Devonian-Carboniferous Boundary Research Articles

Biostratigraphy and facies around the D/C boundary interval of the Tuye-Darvar section, Eastern Alborz Range, NE Iran

The 1st order mass extinction at the Devonian/Carboniferous transition, known as the Hangenberg Crisis, is characterized by major transgressive/regressive cycles which led to widespread ocean anoxia during the Hangenberg Black Shale Event, as well as to a global major sea-level fall and the worldwide deposition of regressive Hangenberg Sandstone equivalents. The Devonian/Carboniferous transition at the Tuye-Darvar section in the eastern Alborz Range is studied in terms of conodont biostratigraphy, litho-, microfacies and sequence stratigraphy. In order to examine the biostratigraphical framework, forty conodont samples were systematically taken from the studied interval. Thirty-two conodont species belonging to ten genera led to the discrimination of twelve conodont zones, ranging from the Pseudopolygnathus granulosus Zone to the Scaliognathus anchoralis-Doliognathus latus Zone. Due to facies, the conodont record also exhibits some hiatuses. Field observations and sedimentological and microfacies studies led to the identification of thirteen facies types from sub-tidal environments to the fully marine environments, including seven microfacies types. The sediments deposited in a mixed carbonate-siliciclastic platform, revealed four third-order sequences. The Hangenberg Black Shale is not recorded in the Tuye-Darvar section as a result of depositional facies. In comparison with other studied Devonian/Carboniferous Boundary (DCB) sections of the central and northern Iran, the Tuye-Davar section suggests a tectonic position, which is most likely placed on a separate tectonic block.

Geochronological constraints on the Hangenberg Event of the latest Devonian in South China

The Hangenberg Event was one of the most significant mass extinctions accompanied by large-scale climate changes at the end of the Devonian period. Lack of precise and accurate geochronological constraints in the eastern Paleotethys has hindered a high-resolution global correlation of the Hangenberg Event. Here, we report the U–Pb dates of zircons from the bentonite samples intercalated with strata recording the Hangenberg Event in the Muhua region, Guizhou, South China, using both sensitive high-resolution ion microprobe (SHRIMP) and chemical abrasion - isotope dilution - thermal ionization mass spectrometry (CA-ID-TIMS) methods. A bentonite within the Hangenberg Event strata in the Muhua area was dated at 360.47 ± 0.68 Ma by CA-ID-TIMS, which is more precise than the results obtained by SHRIMP methods (361.5 ± 2.9 Ma and 359.8 ± 1.5 Ma). Thus, we propose that the Hangenberg Event took place at 360.47 ± 0.68 Ma in South China, based on the first precise CA-ID-TIMS zircon age from the Daposhang section. The age obtained from South China for the Devonian–Carboniferous boundary overlaps with that in Geologic Time Scale 2020, within the total uncertainties.

Devonian-Carboniferous boundary beds in the Kuz-Yama section (Northern Cis-Urals) - Conodonts, ostracods, and carbon isotope record

The Devonian-Carboniferous boundary interval is characterized by biotic crisis, global transgressive-regressive sequences and carbon cycle change, probably caused by climatic changes and volcanic activity. This interval attracts the attention of researchers worldwide in relation to selecting a new Devonian-Carboniferous boundary level. However the Northern Cis-Urals region remains poorly studied in this respect. This study focuses on the Kuz-Yama key section (Northern Cis-Urals) and contributes to our knowledge of the regional expression of the Devonian-Carboniferous boundary in northeastern Europe. The study of the Kuz-Yama section (Verkhnyaya Pechora Trough) involved an examination of the calciturbidites, hemipelagic shales and carbonates that constitute the poorly constrained uppermost Famennian to lower Tournaisian deep-water succession. The conodont and ostracod associations are poorly diversified. The presence of Polygnathus zikmundovae, in association with a positive δ13Ccarb excursion (up to 3.9 ‰), marks the base of the Late praesulcata conodont Zone (=base of the Protognathodus kockeli conodont Zone) in the lower part of the succession. The Devonian-Carboniferous boundary is indicated by the appearance of Siphonodella sulcata, that coincides with a positive shift in the decoupling of δ13Ccarb and δ13Ccon values, probably caused by a global decrease in CO2 associated with the regional crisis within pelagic ecosystems. The sulcata conodont Zone, bounded by the first occurrence of Siphonodella sulcata at the bottom and Siphonodella bransoni at the top of the section, contains Tournaisian ostracods. The ostracods compose mixed associations of the shallow-water Pseudoleperditia venulosa-Coryellina alba-Cribroconcha primaris ostracod Zone and the deep-water Armilla uralica ostracode Zone. The co-occurrence of shallow-water and deep-water conodont and ostracod taxa in the Tournaisian part of the section enhances the correlation of the shallow-water and deep-water conodont and ostracod zonations. The base of the shallow-water Siphonodella ludmilae conodont Zone roughly correlates with the base of the deep-water bransoni Zone. The base of Armilla uralica deep-water ostracod Zone in the Kuz-Yama section lies within the Pseudoleperditia venulosa-Coryellina alba-Cribroconcha primaris shallow-water ostracod Zone. The following sequence of events can be recognised in the Kuz-Yama section: first occurrence of Polygnathus zikmundovae Zhuravlev (approximately corresponding to the base of the Protognathodus kockeli Zone), positive shift in the inorganic δ13C (= Hangenberg carbon isotope excursion), latest Famennian regression, negative shift in the δ13C of conodont elements just below the base of the sulcata Zone, entry of the Tournaisian transgression, positive shift in decoupled inorganic and conodont δ13C near the base of sulcata Zone, positive shift in inorganic δ13C in the sulcata Zone, appearance of the Tournaisian ostracods, first occurrences of the conodonts Siphonodella bransoni Ji and S. ludmilae Zhuravlev & Plotitsyn (= base of the bransoni Zone). In general, this sequence corresponds to that identified around the Devonian-Carboniferous boundary in other regions.

Devonian–Carboniferous transition in various facies of Northeast Laurussia (North Urals)

This study analyzed 14 stratigraphic sections exposing the Devonian–Carboniferous boundary interval in the North Urals, representing different facies from shallow-water carbonate platform to bathyal, in order to assess the possibility of recognizing regional levels corresponding to the base of the conodont zones S. sulcata and Protognathodus kockeli. The base of the S. sulcata Zone is regionally recognized by the first appearances of the conodonts Siphonodella sulcata (Huddle), S. semichatovae Kononova and Lipnjagov and Patrognathus crassus Kononova and Migdisova, roughly correlated with the first appearance of the foraminifer Tournayellina pseudobeata Reitlinger and Kulagina, and with the first appearances of the ostracods Cribroconcha primaris Kochetkova, Chamishaella grekoffi Tchizhova, Glyptolichvinella spiralis (Jones and Kirkby), Armilla uralica Sobolev, Calcarofera ex gr. media Schornikov, Compositocostata cumina Sobolev, Spinoalacia tschigovae Sobolev, Richterina latior Rabien. The base of the S. sulcata Zone occurs in the interval between two positive inorganic carbon isotope excursions. The base of the Protognathodus kockeli Zone is marked by the first appearance of Protognathodus kockeli (Bischoff) in a very narrow facies belt on the slope of the carbonate platform, recognized regionally by the onset of the last phase of the positive inorganic carbon isotope excursion, the beginning of the transgression after the terminal Famennian regression, the beginning of the unilocular foraminifera Bisphaera bloom and probably the appearance of the ostracods Monoceratina sp. 1. The level corresponding to the base of Pr. kockeli Zone is poorly supported biostratigraphically, but is characterized by clear isotope-stratigraphic and cyclostratigraphic data, although the eustatic signal may be obscured in places by local tectonics.

Anomalous facies of the Kinnesswood Formation in the Cumbraes Basin: a late Famennian playa complex in SW Scotland

Among the important geological attributes of the small Cumbraes Basin in the southwestern off-shore Midland Valley of Scotland, the 4 km-long, fragmented but well-exposed section through the Devonian–Carboniferous boundary sequence in southern Great Cumbrae island ranks highly. In a continuous short stretch of the south and west coasts, large portions of this sequence are superbly preserved in tide-washed strandline surfaces and in old post-glacial sea cliff. This enhanced exposure leaves no doubt that the outcrops belong to two, immediately successive, members of the Kinnesswood Formation – the lower the Doughend Sandstone, the upper the Foul Port Mudstone – and that they provide some of the best available exposures on which to base description and interpretation. The facies of the fluviatile Doughend Sandstone shares many characteristics with the underlying Kelly Burn Sandstone of the Upper Old Red Sandstone, whereas that of the Foul Port Mudstone is distinct and unusual, attributed to local tectonics, influenced by movements on older and more major fractures such as the Highland Boundary Fault, with sediment accumulation taking place in a playa-lake complex of considerable extent.

Organic carbon isotope stratigraphy of Devonian-Carboniferous boundary sections in the Rhenish Mountains

Abstract The boundary between the Devonian and the Carboniferous has been drawn using ammonoids, conodonts and miospores, while geochemical data have only rarely been used. The sampling of eight sections (Aprath, Apricke, Effenberg, Wocklum, Stockum, Drewer WJ, Drewer WA, Marsberg) at the northern margin of the Rhenish Mountains demonstrates the potential of the isotope ratio of organic carbon to regionally correlate sections in different facies areas. With the help of the carbon isotopes, a distinct stratigraphic succession is documented, which is characterised by several positive and negative excursions; this succession was recorded in all examined sections. The carbon isotopes can therefore make a significant contribution to the stratigraphic subdivision of sections and complement the biostratigraphic methods.

Resolving complex stratigraphic architecture across the Burlington shelf and identifying the Devonian-Carboniferous (Hangenberg) and Kinderhookian-Osagean (Tournaisian) boundary biogeochemical events in the type area of the Mississippian subsystem

The tristate area of Iowa, Illinois, and Missouri contains some of the best-exposed Mississippian strata in the world, including the type area for the Mississippian subsystem, across a broad carbonate platform known as the Burlington shelf. Strata have been mapped as thinnest along the central middle shelf and thickening both up-ramp and down-ramp, forming a complex dumbbell-like stratigraphic pattern rather than a simple clinoform geometry thinning into the basin. Additionally, two significant hiatuses at the Devonian-Carboniferous boundary and Kinderhookian-Osagean boundary greatly complicate stratigraphic correlations across the region. As a result, the precise temporal relationships between strata deposited across the region remain uncertain. Two large biogeochemical events occurred during this interval that provide facies-independent chronostratigraphic tools: the Hangenberg event, which marks the Devonian-Carboniferous boundary, and the Kinderhookian-Osagean boundary event. To target these events, we collected 66 conodont samples and 1005 carbonate carbon isotope samples from three cores and three outcrops and integrated the results with existing data from key facies/depth transitions across the Burlington shelf. Our new data demonstrate a complex relationship among complementary stratigraphic thicknesses, where the Devonian-Carboniferous boundary interval is thin or absent in the up-ramp inner-shelf setting and preserved in a significantly expanded interval in the central to distal middle-shelf deposits of southeast Iowa and northeast Missouri. However, the overlying Kinderhookian-Osagean boundary interval is not preserved in this down-ramp setting but is preserved in significantly expanded strata in the up-ramp inner-shelf setting of central Iowa.

Carbon Isotope Stratigraphy across the Devonian–Carboniferous Boundary in the East Paleo-Tethys Realm, Tibet, China

The Devonian–Carboniferous boundary is one of the most important turning points in geological history, marked by the Late Paleozoic Ice Age and Late Devonian extinction. This study investigates the carbon isotope stratigraphy across the Devonian–Carboniferous boundary in Lhasa block, Tibet, China, which was part of the Gondwana continent during that time. The carbon isotope curves show a significant negative excursion trend, consistent with those of the South China block and other regions on the Laurentia continent. This global negative shift may be attributed to the burial of significant amounts of 12C-rich organic matter in strata, a consequence of the Late Devonian extinction event. Based on the carbon isotope curve and stratigraphic data of the Lhasa block, this study determined, for the first time, the specific horizon of the Devonian–Carboniferous boundary in Tibet, which is located between grayish white bioclastic limestone and yellowish-brown sandy limestone in the upper part of the Chaguoluoma Formation (D1C1ĉ). These findings provide a new reference for the international stratigraphic community to reconsider the position of GSSP on the Devonian–Carboniferous boundary, as well as study the Late Devonian mass extinction and Late Paleozoic glaciation.

Multiple diachronous “Black Seas” mimic global ocean anoxia during the latest Devonian

Abstract The Devonian-Carboniferous boundary (DCB) Hangenberg extinction in Europe (ca. 359 Ma) and interval of black shale deposition has been considered a global anoxic event, the cause of which remains debated. However, the lack of a prominent global carbon isotope excursion and the duration of anoxia (3–4 m.y. at least) is inconsistent with other global anoxic events (e.g., Mesozoic ocean anoxic events) throughout the Phanerozoic. We examined geochemical and biostratigraphic data along with radiometric dating from purported Hangenberg event sections across the DCB in western Canada and demonstrate that the onset of anoxic pulses in Laurentian seas started before 363 Ma, diachronous with those in Baltica (Europe). Thus, DCB black shales do not record a single global anoxic event. We suggest instead that black shales near the DCB record multiple, but diachronous, Black Sea–like basins around the globe, promoted by the Late Devonian tectonic configuration. This drove a gradual biotic crisis in the form of regional punctuated benthic faunal extinction events.

The ammonoids from the Gattendorfia Limestone of Gattendorf (Devonian–Carboniferous boundary; Upper Franconia, Germany)

The early Tournaisian (Early Carboniferous; Mississippian) ammonoids from the classical abandoned limestone quarry of Gattendorf (Upper Franconia) are revised, using the historical collections as well as so far undescribed material. The ammonoid assemblage is composed of prionoceratid ammonoids of the six genera Mimimitoceras, Paragattendorfia, Stockumites, Acutimitoceras, Gattendorfia and Gattenpleura, which indicate a stratigraphic position near the Devonian–Carboniferous boundary in the earliest Carboniferous. The new species Stockumites hofensis sp. nov. and S. nonaginta sp. nov. are described.

A fish for Phoebe: a new actinopterygian from the Upper Carboniferous Coal Measures of Saddleworth, Greater Manchester, UK, and a revision of Kansasiella eatoni

Abstract The origin of crown Actinopterygii has been dated to near the Devonian-Carboniferous boundary, but poor phylogenetic signal occludes the origin of modern actinopterygian biodiversity. Scarcity of comparative endoskeletal anatomy from this period likely contributes to the uncertainty. For example, only a handful of neurocrania have been described from the Carboniferous period despite an abundance of fossil taxa. Here we present a new actinopterygian, Phoebeannaia mossae gen. et sp. nov., from the Bashkirian Coal Measures of Saddleworth, Greater Manchester, UK, represented by an exceptionally preserved neurocranium and associated dermal bones. Morphological similarities to the Virgilian Kansasiella eatoni prompted comparative study and construction of a new neurocranial character matrix. Subsequent systematic analysis enabled identification of a cluster of similar Late Palaeozoic neurocrania, exploration of trends in braincase evolution across time, and investigation of potential causes of cladistic instability. Our neurocranial dataset reliably places this new specimen on the neopterygian stem near Kansasiella, but it struggles to incorporate extant taxa such as Polypteriformes, leading to uncertainty in branching pattern and inferred neurocranial transformation series. This work triples the number of complete Carboniferous neurocrania described with modern microcomputed tomography and provides a framework for future testing of large-scale hypotheses regarding the diversification and origin of ray-finned fishes.

Uranium and molybdenum isotope evidence for globally extensive marine euxinia on continental margins and in epicontinental seas during the Devonian-Carboniferous Hangenberg Crisis

The end-Devonian Hangenberg Crisis was one of the biggest Phanerozoic mass extinctions. However, the mechanism(s) that triggered this event is still debated. In this study, multiple geochemical paleoredox proxies (redox-sensitive trace metals [e.g., Mo, U, Re, V] and isotope systems [Mo, U, S, C]) were applied to the Exshaw Formation black shales to infer ocean redox conditions during the Hangenberg Crisis. The measured δ13Corg values generally decrease upsection in the Exshaw Formation. The Exshaw black shales have increasing maturity levels from east to west in the Western Canada Sedimentary Basin. Large sulfur isotope fractionations (–15‰ to 65‰) between original seawater sulfate and pyrite is best explained by microbial sulfate reduction during deposition and early diagenesis rather than thermochemical sulfate reduction. Precise Re-Os ages previously reported for the Exshaw Formation suggest that metal geochemical signatures in these shales, including overmature shales, were minimally affected by hydrocarbon maturation and reflected depositional conditions. Both Sr/Ba and TS/TOC ratios (the latter only from immature shales affected minimally by hydrocarbon generation) suggest predominantly brackish and marine settings with significant water exchange between the basin and open ocean. The TOC contents, redox-sensitive trace metal concentrations (Mo, U, Re), and Fe speciation indicate local bottom waters ranged from suboxic (with sulfidic pore waters) to euxinic. The authigenic δ98Mo and δ238U values for the Exshaw shales range from 0.3‰ to 1.1‰, and from –0.23‰ to 0.39‰, respectively. The δ98Mo of the Exshaw Formation may have been offset to lower values than coeval seawater because of a local particulate Fe-Mn shuttle and/or local weakly euxinic bottom waters. Two different correlation patterns (positive correlation and no correlation) of δ238U and U enrichment factors (EF) were observed for Exshaw black shales deposited from locally euxinic bottom waters. The positive correlation group samples (−0.13‰ to 0.23‰) suggest U isotope fractionations (0.4‰ to 0.8‰) between sediments and seawater like those observed in modern euxinic basins. Higher δ238U values (>0.2‰) in the no correlation group suggest even larger U isotope offsets (0.8–1.2‰) from seawater, possibly associated with removal of U into organic floccule layers during deposition. Applying the above Mo and U isotope fractionations to the Exshaw shales, global seawater δ98Mo and δ238U at the Devonian-Carboniferous boundary (DCB) may have been in the range of 1.4‰ to 1.9‰ and −0.9‰ to −0.5‰, respectively. A Mo and U isotope mass balance model suggests a large areal extent of euxinic seafloor (6–10%) at the DCB, which could be pervasive along continental margins and in epicontinental seas during transgression. Therefore, our data support expanded ocean euxinia at the DCB as an important contributor to the Hangenberg Crisis.

Mercury isotope evidence for recurrent photic-zone euxinia triggered by enhanced terrestrial nutrient inputs during the Late Devonian mass extinction

Widespread oceanic anoxia marked by globally extensive deposition of organic-rich black shale during the Late Devonian was a major factor in the mass extinctions at the Frasnian-Famennian (FFB, ∼372 million years ago) and Devonian-Carboniferous boundaries (DCB, ∼359 million years ago), although the triggers for these deoxygenation events are still under debate. Here, we apply a novel paleoredox proxy, Hg isotopes, to investigate Late Devonian ocean redox variation and its causes. We found no Hg enrichments in North America across either the FFB or DCB, thus arguing against the hypothesis of global-scale volcanism as the trigger for Late Devonian environmental and biotic crises. Gradual negative shifts of both mass-independent fractionation (Δ199Hg) and mass-dependent fractionation (δ202Hg) occurred between the FFB and DCB, suggesting a progressive increase of Hg inputs associated with terrestrial organic matter. Moreover, multiple abrupt negative excursions of Δ199Hg (down to −0.19‰) along with concurrent positive shifts of δ202Hg occurred just above the FFB and across the DCB, providing strong evidence for recurrent photic-zone euxinia (PZE) that was preceded by increasing terrestrial inputs in the epicontinental seas of North America. We suggest that the increase of terrestrial inputs of nutrients, probably via expansion of vascular land plants, stimulated marine primary productivity and eventually PZE, which may have been a key kill mechanism for the Late Devonian mass extinction.

Latest Devonian–Early Mississippian conodont biostratigraphy in the Naqing section, Guizhou, South China

The Naqing section in Luodian (Guizhou, South China) contains a deep-water carbonate-dominated succession of the Late Devonian through the Permian. In this study, 72 conodonts species/subspecies of 11 genera were obtained from the lower part of this section including. Seven middle to late Famennian and four early to late Tournaisian conodont biozones were recognized as, in ascending order, Palmatolepis marginifera marginifera Zone, Palmatolepis marginifera utahensis Zone, Polygnathus granulosus Zone, Palmatolepis rugosa trachytera Zone, Polygnathus styriacus Zone, Palmatolepis gracilis manca Zone, Palmatolepis rugosa rugosa Zone, Siphonodella isosticha Zone, Gnathodus punctatus Zone, Gnathodus typicus-Gnathodus cuneiformis Zone, and Gnathodus typicus-Protognathodus cordiformis Zone. Several key conodont biozones across the Devonian–Carboniferous Boundary (DCB) interval found elsewhere are not discovered in the Naqing section, likely due to a conodont-barren interval in the siliciclastic facies. The middle Famennian–late Tournaisian conodont biozones illustrated in this study provide a robust chronostratigraphic framework for further research on biotic and paleo-environmental events during the Late Devonian through Early Mississippian critical greenhouse-icehouse climate transition.

Biostratigraphy of the Devonian–Lower Carboniferous Deposits of the Bambui-Olingda Subzone (South Muya Ridge, Western Transbaikalia)

The results of biostratigraphic studies of the Devonian–lower Carboniferous deposits of the Bambui-Olingda subzone of the Vitimkan-Tsipinsk zone of the Baikal-Vitim fold system are presented. These deposits are isolated from the composition of the Riphean and Vendian–Cambrian formations. Their age was revised on the basis of numerous findings of fauna and flora. The carbonate Bambui and terrigenous Chulegma formations and the terrigenous-carbonate Kadalin stratum are assigned to the Devonian. The volcanogenic-terrigenous Yendekt sequence is dated as late Devonian (Famennian)–early Carboniferous (Tournaisian), while the terrigenous Ognenskaya and Amatkanskaya formations are dated as early Carboniferous (Tournaisian and Visean). Significant changes have been made to the Paleozoic stratigraphy scheme of the subzone. Paleolandscape reconstructions for the Bambui-Olingda basin and coastal land areas in the Devonian–early Carboniferous are proposed. The Devonian carbonate complex was formed in calm conditions of a shallow warm shelf sea. A change in the paleolandscape situation occurred at the Devonian–Carboniferous boundary. Sedimentation of significant thicknesses continued under the conditions of a deepening open shelf-slope paleobasin adjacent to the volcanic zone.

New Pitus and Eristophyton-type woods from the Tournaisian of Queensland, Australia: taxonomic, biogeographic, and palaeoclimatic implications

The fossil record of arborescent lignophytes shows an increasing anatomical diversity during the Tournaisian (360-347 Mya), suggesting a morpho-anatomical diversification following the extinction of the progymnosperm Archaeopteris Dawson, 1871 at the Devonian-Carboniferous boundary. This view has been partly constructed on recent investigations of Early Carboniferous deposits in eastern Australia. In this paper, we describe new silicified wood remains from the Tournaisian of the Burdekin Basin, northeastern Queensland, Australia, that are anatomically close to the genera Pitus Zalessky, 1911 and Eristophyton Gordon, 1935. While uncertain, the taxonomic affinities of these wood remains have several implications for future studies of Early Carboniferous arborescent lignophytes. First, the taxonomy of Eristophyton and Pitus has become increasingly problematic as reports of fossil wood assigned to these genera have extended beyond Laurussia. Second, Eastern Gondwanan and Laurussian floras display a convergent pattern of diversification from the Devonian, with the probable presence of a diversity of lignophyte trees in the Early Carboniferous. Third, convergence between Laurussia and Australia with respect to wood anatomy and discrete growth rings may be consistent with past suggestions of a monsoonal circulation during the Early Carboniferous.

Uppermost Devonian–Lower Carboniferous conodonts from the Indert Formation in the Shine Jinst area, South Mongolia

In southwestern Mongolia, conodonts from the Indert Formation at its type locality in Shine Jinst were investigated to improve regional and global correlation of the uppermost Devonian–lowermost Carboniferous. The abundance and diversity of the conodont fauna is quite low in general, whereas representatives of Siphonodella, Protognathodus, Pseudopolygnathus, Polygnathus and Bispathodus have been collected from the Indert-Uul section. Five conodont biointervals, from the Bi. costatus-Pr. kockeli Interregnum (ckI) to the Siphonodella sandbergi Zone, could be recognized from the top Heermorit Member and the lower part of the Shombon Member, while the middle and upper parts of the Shombon Member cannot be zoned based on the conodonts. Absence of index fossils Si. sulcata, Pr. kuehni and Pr. kockeli hampered precise recognition of the Devonian–Carboniferous boundary (DCB) at Indert-Uul, but this transition is well delineated by typical uppermost Famennian and Tournaisian conodont assemblages. The DCB is tentatively placed at the base of the Shombon Member, consistent with the lithological boundary and the resuming of carbonate production during the DCB transitional interval.

Temperature-driven heterochrony as a main evolutionary response to climate changes in conodonts.

Can we predict the evolutionary response of organisms to climate changes? The direction of greatest intraspecific phenotypic variance is thought to correspond to an 'evolutionary line of least resistance', i.e. a taxon's phenotype is expected to evolve along that general direction, if not constrained otherwise. In particular, heterochrony, whereby the timing or rate of developmental processes are modified, has often been invoked to describe evolutionary trajectories and it may be advantageous to organisms when rapid adaptation is critical. Yet, to date, little is known empirically as to which covariation patterns, whether static allometry, as measured in adult forms only, or ontogenetic allometry, the basis for heterochrony, may be prevalent in what circumstances. Here, we quantify the morphology of segminiplanate conodont elements during two distinct time intervals separated by more than 130 Myr: the Devonian-Carboniferous boundary and the Carnian-Norian boundary (Late Triassic). We evidence that the corresponding species share similar patterns of intraspecific static allometry. Yet, during both crises, conodont evolution was decoupled from this common evolutionary line of least resistance. Instead, it followed heterochrony-like trajectories that furthermore appear as driven by ocean temperature. This may have implications for our interpretation of conodonts' and past marine ecosystems' response to environmental perturbations.

Coprolite diversity reveals a cryptic ecosystem in an early Tournaisian lake in East Greenland: Implications for ecosystem recovery after the end-Devonian extinction

The early Tournaisian (Carboniferous) stage represents a key episode in the evolution of vertebrates. It follows the end-Devonian Hangenberg extinction event, which led to a major perturbation to both terrestrial and aquatic vertebrate ecosystems, and resulted in a significant restructuring of assemblages. However, few faunal associations of this age have been described, and our understanding of faunal turnover across the Devonian-Carboniferous boundary remains poor. In this paper, we present an analysis of coprolite material from early Tournaisian lacustrine facies at Celsius Bjerg on Ymer Ø in East Greenland, which overlies the world-famous latest Devonian tetrapod-bearing localities. Fifty-five coprolite specimens (defined as a single coprolite or a piece of shale containing coprolites) were analysed using propagation phase-contrast synchrotron microtomography (PPC-SRμCT). Through a study of external morphology, shape and size combined with information about internal structures, we categorise coprolite morphotypes, and interpret their origin. Notably, we identify a greater number of coprolite morphotypes compared to vertebrate taxa known from skeletal material, indicating the existence of a cryptic ecosystem that has not yet been recovered as body fossils. Vertebrate diversity in the immediate aftermath of the end-Devonian extinction is inferred to have been higher than expected, and might have included transient faunal elements within an open system, perhaps involving marine basin connections. Our results show that coprolites offer an alternative fossil data source, revealing diversity that is otherwise not always captured by the skeletal record.

The Palaeozoic genusPsephodus(Chondrichthyes, Cochliodontiformes) and the transition from teeth to tooth plates in holocephalians

Remains of the chondrichthyanPsephodusMorris and Roberts, 1862consist mainly of isolated tooth plates. The genus has a range from the Late Devonian (Famennian) to the Late Mississippian (Serpukhovian). The most complete specimen is a partially articulated set of teeth and tooth plates of the type species,P. magnus, from the early Serpukhovian of East Kilbride, Lanarkshire, Scotland. A row of minute nodes is present along the basal margins of theLophodus-like andHelodus-like teeth of the East Kilbride specimen. This distinguishes these teeth from typicalLophodus-like andHelodus-like teeth. As inHelodus simplex, some specimens ofPsephodusdisplay varying degrees of fusion of teeth from the same labiolingually oriented file into tooth plates. However, unlikeHelodus simplex, some specimens ofPsephodusdisplay fusion of mesiodistally separated tooth plates, each representing a fused tooth file, into a larger tooth plate.Psephodus, which crosses the Devonian–Carboniferous boundary, may be ancestral to other Mississippian holocephalians, such asCochliodusorChondrenchelys, in which dentitions consist of a few tooth plates with only few, or no, separate teeth. An unpublished watercolour by Agassiz's artist Dinkel depicts five tooth plates, which can be designated as syntypes ofP. magnus.A lectotype forPsephodus crenulatusis designated.