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.

Conodonts are a group of extinct marine organisms widely used in Paleozoic and Triassic biostratigraphy. The genus Siphonodella comprises ozarkodinid conodonts from the latest Famennian to the middle Tournaisian. The phylogenetic model of this genus plays an important role in biostratigraphy. This study is based on the authors' and published data on 31 species of siphonodellids of cosmopolitan and East European groups. A new phylomorphogeny of Siphonodella is proposed on the basis of known mechanisms of morphological transformations of siphonodellid P1 elements during speciation processes, presence of interspecies transitional forms, and data on ontogeny. The main morphogenetic pathways in the evolution of siphonodellids are the following: formation and transformation of the rostrum, transformation of the platform outline, transformation of the platform ornamentation, transformation of the keel, and histological transformations. In the process of modeling the phylomorphogeny of Siphonodella, some taxonomic problems are solved: a new species S. thompfelli is described, a new variant of the division of S. lobata (Branson and Mehl) into morphotypes is proposed, the validity of S. plana (Huddle), S. praeplana Bardasheva, Bardashev, Weddige and Ziegler, and S. gladia Zhuravlev and Plotitsyn is clarified, and the knowledge of the morphospace of most representatives of the genus is extended. The phylogeny of siphonodellids provides a basis for the new phylogenetic zonal sequences based on the conodont phylogeneticzone concept.

Research subject. Upper Tournaisian clayey-siliceous-carbonate deposits of the Malaya Usa section (Polar Urals). Material and methods . The composition and structure of the deposits (50 samples, 51 thin sections) of the section were considered to determine the conditions of sedimentation; 13 samples were chemically disintegrated in 7–10% solution of acetic acid, and thin sections were studied for biostratigraphic analysis (conodonts, ostracods, and foraminifers); the carbon and oxygen isotopic composition of carbonate deposits (49 samples) were analyzed. A three-stage screening test of samples for isotopic-studies was carried out. Results . The Malaya Usa section comprises mainly micritic limestones with fossils silicified to various degree (wackstone to packstone). The features and composition of the sediments indicate sedimentation in low dynamic conditions below the wave base. Biostratigraphic analysis is based on the identification of foraminifers, as well as a few conodonts and ostracods. Foraminifers characterize the Eotextularia diversa–Dainella chomatica Zone, which is approximately comparable to the Upper typicus–anchoralis–latus conodont zonal interval and Entomoprimitia malinovkaensis‑Marginia tschigovae ostracode zones of the top of the Tournaisian. The isotope-geochemical data obtained from the section demonstrate a general lightening trend in the isotopic composition of δ13Ccarb up the section. This trend, along with the biostratigraphic data, mades it possible to compare the δ13Ccarb sequence of the Malaya Usa section with sequences from a number of geographically separated sections of the world (Belgium, South China, USA), which are considered as the Tournaisian isotope carbon event (TICE). Conclusions. The deposits of the studied section were probably formed under the conditions of a very gentle slope (ramp), where the material accumulated at the foot as a filling sequence in the southern part of the Korotaikha intrashelf paleodepression. Using the Malaya Usa section as an example, it is shown that not only the peak positive deviations of the TICE in the upper part of the isosticha Zone and in the lower part of the typicus Zone, but also the isotopic structure of the upper Tournaisian has a significant correlation potential. The use of the isotope-geochemical method as an auxiliary application to biostratigraphy made it possible to improve the age determination of the section from the interval of the Upper typicus–anchoralis–latus conodont zonal interval to the Upper typicus Zone of the standard conodont zonation.

Biostratigraphically constrained delta C-13(carb) variations in the Tournaisian successions of NE Europe (Komi Republic, Russia) are considered. Eight reference sections, characterizing platforms, slopes, and shelf depression facies, comprise the basis of this study. Of 412 studied samples of bulk carbonates, 268 samples passed all of the screening tests and were used to analyze the carbonate carbon isotope record. The established regional delta C-13(carb) data allowed the recognition of the distinct, terminal Famennian and mid-Tournaisian carbon isotope events and two minor positive shifts in the sulcata Zone and Upper duplicata-lowermost quadruplicata interval. The age of the isotope shifts is underpinned by conodont biostratigraphy. These isotope events and shifts are traced through the facies gradient from the shallow-water carbonate platform to deep-water shelf depression. Because the regional delta C-13(carb) variations match those known in different regions (Europe, South China, and North America), the recognized C-isotope shifts can be considered to be reliable isotope-stratigraphic markers for both regional and world-wide correlation.

A new morphological group of advanced Siphonodella with diagonal or/and longitudinal ribbed ornamentation on the upper side of the platform of the P1 elements was identified and described based on the author's paleontological material from the Polar Urals and published data from East European, Central, and Northeast Asian successions. The group includes two valid species Siphonodella nandongensis Li and Siphonodella vladimirovi Plotitsyn nom. nov. Due to the low representativeness of the paleontological material, eight taxa are described in an open nomenclature under the species names Siphonodella sp. A-H. Siphonodella vladimirovi is described as a nomen novum in accordance with the recommendations of the International code of zoological nomenclature (previously identified species appeared under the invalid species names Siphonodella diagonalica Pazukhin or Siphonodella zeravshanica Bardasheva). It is assumed that the species Siphonodella nandongensis Li, Siphonodella vladimirovi Plotitsyn, and Siphonodella sp. A-H came from morphologically similar species from the cosmopolitan group or as descendants of the East European group.

The middle–late Tournaisian (Hastarian–Ivorian) transition is marked by isotopic and sedimentological evidence of climatic cooling and glaciations accompanied by a sea level fall and changes in global ocean circulation. At this time, the extinction among conodonts was followed by its gradual recovery in the late Tournaisian–early Viséan. Siphonodellids, representing an important part of conodont assemblages in the early–middle Tournaisian, became extinct. This study aims to compare conodont diversity dynamics in Northeast Laurussia and Northeast Siberia. The materials used range in paleolatitudinal gradients from 20°N (the north of the Urals and Pechora Craton) up to 45°N (Northeast Siberia), and in paleoclimatic gradient from an equatorial to a warm subtropical climate. The middle and late Tournaisian conodont associations of these regions demonstrate a high similarity in taxonomic composition and diversity dynamics, which may suggest that the glaciation and the following decrease in temperature probably was not the direct cause of conodont extinction; instead, the changes in the food web implied by the carbon isotope compositions of conodont elements and host carbonates could have been the main driver of the crisis in conodont diversity

Ostracods from the Syadai Formation in the Polar Urals (sampling site at Syadeiyabtoyakha Creek) are represented by rare poorly preserved valves with a heavily corroded surface. Nevertheless, 14 genera in 4 orders and 11 families have been identified. A new genus Syadaikinia with the type species S. tumidosa sp. nov. and two new species such as Eurybolbina uralica sp. nov. and Baschkirina tesakovae sp. nov. are described. The age of the Syadai Formation is estimated as Silurian based on analysis of the stratigraphic distribution of ostracods and other fossils.

Traditionally, biostratigraphy of the terminal Famennian (Upper Devonian) and lower part of the Tournaisian (Mississippian) is based on phylogeny of the conodont genus Siphonodella Branson and Mehl. This study is focused on a Siphonodella species with a long taxonomical history. For the last three decades it was known as S. hassi Ji, 1985 and was used as a zonal index for the eponymic hassi Zone, used worldwide. However, this species faces several taxonomical problems. The main issue is the homonymy of S. hassi and S. cooperi hassi Thompson and Fellows, 1970. However, S. cooperi hassi is a subjective junior synonym of S. obsoleta Hass, 1959. The oldest junior synonym of S. hassi is S. wilberti Bardasheva, Bardashev, Weddige and Ziegler, 2004, and, according to a statement by the International Commission on Zoological Nomenclature, the latter represents the only available name for this species. An additional problem is the similar morphology of early ontogenetic stages of S. wilberti and S. quadruplicata Branson and Mehl, 1944, the youngest stratigraphically important siphonodellid species. In advanced ontogenetic stages, P-1 elements that possess two rostral ridges belong to S. wilberti only. The smaller P-1 elements having two or less rostral ridges comprise the S. wilberti-S. quadruplicata plexus. Maintenance of the wilberti Zone in the Tournaisian global zonation is suggested. (C) 2021 Elsevier Masson SAS. All rights reserved.

This paper reviews the foraminiferal biostratigraphy of the Devonian–Carboniferous (D–C) boundary beds in Russia and Western Kazakhstan. The regional records of the zonal successions of Russia are summarised from sections and boreholes of the Volga-Urals Subregion of the East European Platform (Syzran no. 401 Borehole, Melekesskaya no.1 Borehole); Timan-Pechora Province of the East European Platform (Kamenka Section); South Urals (Sikaza, Zigan, Ryauzyak and Dzerzhinka sections); Middle Urals (Pershino Section); and West Siberia, Vagai-Ishim Basin (Borehole no. 1 Kurgan-Uspenskaya), and the zonal successions of Western Kazakhstan are summarised from the Mugodzhary Mountains. Three foraminiferal biozones can be defined in the interval comprising the latest Devonian and earliest Carboniferous: Quasiendothyra kobeitusana Zone, the Tournayellina pseudobeata – remnant Quasiendothyra Zone, and the Earlandia minima Zone. The D–C boundary interval is situated within the Tournayellina pseudobeata –remnant Quasiendothyra Zone. A correlation with the ammonoid and conodont zonations is proposed, which is useful for correlating deep-water and shallow-water successions.

Research subject. Regional manifestations of the Dasberg eustatic event in the shelf and bathyal Pai-Khoi successions. The event appears in the Lower-Middle expansa zones interval (Upper Devonian, Famennian).Aim. To evaluate the manifestations of the event in the realm of transition from the shallow-water shoal succession of the Pai-Khoi carbonate parautokhtone towards the deep-water (bathyal) successions of the Kara shale allokhtone.Materials and methods. A number of successions comprising different facies and located in different parts of Pai-Khoi were studied: the Silova-Yakha River section and Tal’beyshor Creek section (south-western Pai-Khoi), the Lymbad’yakha section and the Peschanaya River section (northern Pai-Khoi). The interpretation of facies and the reconstruction of transgression-regression couplets were conducted based on the previously developed models of shoal and bathyal sedimentation. The stratigraphic framework comprised data on conodonts, transgression acmes, and carbonate carbon isotopic record.Results. The Pai-Khoi successions comprising Lower–Middle expansa zones demonstrate four transgression-regression cycles. The transgression acme of the third cycle marks the Dasberg eustatic event. The absence of anoxia is characteristic of this event in the region under consideration. The carbonate carbon isotope record of the Silova-Yakha River section shows a structure similar to that of North American successions. Variations in δ13Cкарб were likely to be caused by climate changes and perturbations of the global oceanic circulation.Conclusions. The stratigraphic interval comprising Lower–Middle expansa or Lower–Upper expansa (expansa s. l.) zones is detected clearly in different facies. A more detailed subdivision and correlation in the region under consideration is possible on the basis of evaluating manifestations of the Dasberg eustatic event: a characteristic eustatic succession and variations of the carbonate carbon isotopic composition.

Conodonts were the Palaeozoic and Triassic extinct group of the marine animals possessing discussed affinities. The only mineralized parts of conodonts are tooth-like elements composing feeding apparatus. The conodont elements are consisted of complex fluorapatite-protein nanocomposites. The mineral component of conodont elements is represented by apatite-(CaF) with minor content of CO3 ions, Na, and Sr. Organic matter is composed of protein(s), consisting less than 4% of a conodont element. The study material comprises conodont elements of the middle Frasnian genera Youngquistognathus and Mehlina, and latest Famennian-Tournaisian species: Polygnathus parapetus Druce, Hindeodus crassidentatus (Branson et Mehl), and Ligonodina sp. Conodont elements were used for analysis of composition of organic matter and apatite matrix, and organic carbon isotope values. Studied conodont elements are characterized by quite high Sr/Ca values in the albid tissue ranging from 0.002 up to 0.016 that is specific for marine animals of low trophic level. Low δ13Corg values in conodont elements ranging in wide interval from −30.4 up to −22.5‰ also suggest low trophic level of conodonts. Thus, data obtained advocate that conodonts were marine consumers of low trophic level feeding on phytoplankton or/and zooplankton. This conclusion promises possibilities to revise functional morphology of conodont elements and apparatuses, and elucidates significant biochemical differences between conodonts and marine vertebrates.

Журавлев А.В., Плотицын А.Н., Груздев Д

New conodont species of the genus Polygnathus from the Upper Famennian of the Urals and Cis-Urals are described. Po. tsygankoi sp. nov. is distributed from the expansa‑praesulcata conodont zones. This species occupies an intermediate position in the phylogenetic lineage Polygnathus atasuensis–Po. tsygankoi sp. nov.–Po. lobatus sensu Kononova. Po. serriformis sp. nov. is known from the praesulcata Zone. Evolutionary relationships of Po. serriformis sp. nov. are unknown.