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The Pashyian Regional stage (horizon) is the main productive unit of the middle Devonian clastic succession of the South Tatar arch. This article presents, for the first time, maps of the lower and upper parts of the Pashyian, including data on sand-shale ratio, number of sand layers (reservoirs) and thickness, based on the analysis of logging data from 25,000 wells. The maps were created by spatial interpolation of Natural Neighbor and ArcGIS Pro software. The model of sedimentation of the Pashyian Regional stage reflects the interpretation of the plotted maps as well as the synthesis of the results of detailed core investigations (lithological, sedimentological, ichnotextural, petrophysical, etc.) and analysis of archive and published materials. The main points of the proposed model are as follows. The Pashyian sediments were formed in a marine basin, in an environment comparable to that of the middle shelf of modern seas – in an offshore zone dominated by current activity. The basin floor was a relatively flat plateau, on which sandy, silty and clay sediments were simultaneously accumulated. Sediments of all types accumulated during sea transgression. Sea regression caused erosion and destruction of the already formed sediments. Positive landforms of seabed relief, composed predominantly of sandy well-sorted material, comprised autochthonous underwater sand bars, formed by constant currents parallel to the bathymetric contour of the seabed. Underwater sand bars formed extensive systems nearly throughout the entire territory of the modern South Tatar arch. At the same time, allochthonous, poorly sorted, less mature sediments were formed in underwater troughs produced by transversal currents (directed from the shore towards the sea). The proposed model explains the consistent thickness of the Pashyian Regional stage, the mosaic distribution of sand bodies over the area, and the lens-like shape of the sand and siltstone reservoirs. The model can be extrapolated to other stratigraphic intervals of the Devonian clastic succession with similar sedimentological features.

This review summarizes research on the biostratigraphic application of Carboniferous marine and non-marine bivalves worldwide, with a focus on the southern margin of Laurussia and the Palaeotethys. Preliminary and established biostratigraphic zonations based on marine and non-marine bivalves are compiled and, if possible, correlated. Bivalve taxa of potential biostratigraphic significance are discussed, and possible limitations of bivalve zonations are outlined. Although marine and non-marine bivalves have not received primary attention for biostratigraphic purposes, a wealth of as yet unused data exists practically worldwide that can assist and complement the more favoured brachiopod, foraminiferid, ammonoid and conodont biozonations.

The Urzhumian (Wordian) and Severodvinian (Capitanian) reference section of the Monastery Ravine (Kazan Volga region, Russia), contains two siltstone lithofacies: (a) laminated and (b) massive. Depositional settings of the laminated siltstones can be interpreted as perennial to ephemeral lakes; depositional settings of the massive siltstones are interpreted as floodplains with soil cover and periodical silt influx in wet seasons. The bulk geochemistry of siliciclastics has revealed a high degree of weathering of both types of siltstone. The similar chemical composition, mineralogy, and grain size suggest the same provenance of silt material. Geochemical data indicate eastern source rocks, which are the Permian red beds of the Cis-Ural plains. The main genetic types of paleosols hosted in the massive siltstones in the studied section, include moderately developed Vertisols and Calcisols with drab-colored root traces, calcareous nodules, low chroma mottles, angular blocky peds and slickensides. These pedofeatures indicate in general a semi-arid climate with clear seasonality of rainfall. Major oxides of the paleosol matrix were used to estimate paleoprecipitation through the bases to alumina (ΣBases/Al) ratio and chemical index of alteration minus potassium (CIA-K) proxies. Estimates from Urzhumian paleosols indicate relatively low mean annual precipitation (MAP) of 390 mm/yr. The transition to the Severodvinian paleosols is marked by a noticeable MAP increase up to 777 mm/yr. This climate moistening coincides with changes in the morphology of paleosols from dolomitic with low redoximorphy to calcitic with gley horizons. The pedogenic carbonate nodules consisting of dolomicrite (5–10 μm) are very common in the Urzhumian paleosols and their morphology, micromorhology and isotopic features indicate the pedogenic origin of dolomite. Sedimentological and paleopedological features of the studied siltstones, as well as the location of the Volga-Ural basin in a mid-latitude position (25–32 °N) indicate a monsoonal character of precipitation during the Middle Permian.

Nonmarine biostratigraphic/biochronologic schemes have been created for all or parts of the late Carboniferous–Middle Triassic using palynomorphs, megafossil plants, conchostracans, blattoid insects, tetrapod footprints and tetrapod body fossils, and these provide varied temporal resolution. Cross correlation of the nonmarine biochronologies to the Standard Global Chronostratigraphic Scale has been achieved in some parts of the late Carboniferous–Middle Triassic in locations where nonmarine and marine strata are intercalated, the nonmarine strata produce biochronologically significant fossils and the marine strata yield fusulinids, conodonts and/or ammonoids. Other cross correlations have been aided by magnetostratigraphy, chemostratigraphy and a growing database of radioisotopic ages. A synthetic nonmarine biochronology for the late Carboniferous–Middle Triassic based on all available nonmarine index fossils, integrated with the Standard Global Chronostratigraphic Scale, is presented here. The focus is on the nonmarine biostratigraphy/biochronology of blattoid insects, conchostracans, branchiosaurid amphibians, tetrapod footprints and tetrapod body fossils within the biochronological framework of land-vertebrate faunachrons. Correlation to the Standard Global Chronostratigraphic Scale presented here is divided into seven time intervals: Pennsylvanian, Carboniferous–Permian boundary, Cisuralian, Guadalupian, Lopingian, Permian–Triassic boundary and Early to Middle Triassic. The insects, conchostracans and branchiosaurs provide robust nonmarine correlations in the Pennsylvanian–Cisuralian, and the footprints and tetrapod body fossils provide robust correlations of varied precision within the entire Pennsylvanian–Middle Triassic. Radioisotopic ages are currently the strongest basis for cross correlation of the nonmarine biostratigraphy/biochronology to the Standard Global Chronostratigraphic Scale, particularly for the Pennsylvanian–Cisuralian. Chemostratigraphy and magnetostratigraphy thus far provide only limited links of nomarine and marine chronologies. Improvements in the nonmarine-marine correlations of late Paleozoic–Triassic Pangea require better alpha taxonomy and stratigraphic precision for the nonmarine fossil record integrated with more reliable radioisotopic ages and more extensive chemostratigraphic and magnetostratigraphic datasets.

A burrow previously assigned to the ichnotaxon Palaeophycus insignis is redescribed, and a new ichnotaxon Olongichnus solodukhoi n. igen. n. isp is erected for this flattened, slightly curved, multi-oriented, smooth burrow that has a thick lining with internal and external very fine-grained mucuous layers. It shows subrectangular to oblong form in cross section. O. solodukhoi is collected from the lowermost part of the Member D (Seryi Kamen Member, Pechishchi Beds) of the late Kazanian (middle Permian) stratotype section (East European Platform). It occurs in yellowish grey dolostone formed within open shallow-marine environments. O. solodukhoi records combined feeding and dwelling behaviours of a suspension-feeding organism. It could be produced by burrowing organisms with a simple flattened morphology, such as flatworms or polychaetes; however, we favour elongate and ultra-elongate bivalves as the potential tracemakers of the reported burrows.

Large nonmarine bivalves, Permianaia gusevi gen. et sp. nov. (family Naiaditidae Scarlato et Starobogatov, 1979) from the terminal Permian (Zhukovian Regional Stage, Vyatkian Stage) of Vyazniki-1 and Sokovka localities (Vladimir Region) are described. The new genus is characterized by the Unio-like (oval rectangular), elongated, posteriorly expanded shell, duplivincular ligament, and absence of teeth in the hinge. This combination of characters distinguishes the genus Permianaia gen. nov. from nonmarine Naiaditidae and from Unio-like genera of Late Permian nonmarine bivalves, Palaeomutela and Palaeanodonta. Permianaia gusevi sp. nov. does not occur in the older Permian beds and, consequently, can be regarded as a biomarker of the terminal Permian deposits, the Zhukovian Regional Stage.

Data on global geographical and stratigraphic distribution of Permian nonmarine bivalves, which have not included in generalizing works are considered. The continental Permian beds of Euramerican, Angarian, Cathaysian, and Gondwanan paleozoogeographic realms are compared with reference to the nonmarine bivalve fauna from the Late Kungurian to Permian–Triassic boundary. It is concluded that the family Palaeomutelidae Lahusen, 1897 and also the genera Anadontella Betekhtina, 1987, Prilukiella Plotnikov, 1945, and Concinella Pogorevitsch in Betekhtina, 1966 are of the greatest significance for correlation of particular sections.

Four high-precision U-Pb CA-IDTIMS dates for the Roadian (lower Guadalupian) of 274.0 ± 0.12 and 273.1 ± 0.1 Ma and for the Wuchiapingian (lower Lopingian) of 260.16 ± 0.39 and 258.14 ± 0.20 Ma were obtained from Permian tuff beds in the periphery of the Okhotsk Massif, N-E Russia. The beds are well constrained within the local lithostratigraphic and regional biostratigraphic framework. Besides regional bivalves and brachiopod faunas, the ammonoid Sverdrupites harkeri occurs close to the recovered Roadian tuffs. This ammonoid provides direct correlation of the tuffs with the Kazanian Stage on the Russian Platform and the Roadian Stage in the Canadian Arctic, where the conodont index species of the Roadian Stage Jinogondolella nankingensis gracilis and Sverdrupites harkeri and other Roadian ammonoids are documented from the Assistant Formation. Our review of the Kungurian-Roadian-Wordian successions in the main well-studied regions suggests that the base of the Roadian Stage should be extended down to approximately 277 Ma and the Roadian-Wordian boundary should be placed at approximately 271 Ma. The new calibration of the Roadian Stage reduces the duration of the enormously long Kungurian Stage from about 10 Myr approximately to 6.0–6.5 Myr. If the proposed calibration is correct, the duration of Guadalupian Series would be almost twice as great as in the previous estimates. Our results also suggest a possible correlation of the P3 alpine glacial event in the Eastern Australia with the Wordian and possible uppermost Roadian Stages. This P3 event corresponds with the disappearance of ammonoids and conodonts in the upper Roadian and Wordian in high-latitudes regions, i.e. N-E Russia, Russian and Canadian Arctic and Australia, perhaps due to extreme cooling of these areas. The new radioisotopic ages in the Okhotsk Massif and surrounding regions improve the numerical calibration of the palynological zonation in the Eastern Australia and provide a direct correlation of the regional biostratigraphic brachiopod and bivalve zonation of N-E Russia with the International Geologic Time Scale. This zonation becomes a solid and reliable regional correlation tool within the vast territory from Taymyr and East Siberia to entire N-E Russia and Transbaikalia.

The Siberian Traps volcanism is widely considered the main cause of the end-Permian mass extinction, the greatest biological crisis in the Earth history. While the extinction is interpreted as catastrophic and sudden with estimates of duration of approximately 35–40 thousand years from marine strata in South China, various lines of evidence have emerged for a more complex, prolonged, and diachronous extinction pattern. We present here the results of a multidisciplinary study of the Permian-Triassic continental transition in the Kuznetsk Basin, Russia. The region is proximal to the Siberian Traps LIP and the detrimental effects of the flood basalt volcanism in the Kuznetsk Basin may have been of similar scale as in the main area of the Siberian Traps distribution (Tunguska and Taymyr regions). Whereas earlier work has placed the Permian-Triassic boundary position between the coal-bearing Tailugan Formation and the volcanoclastic Maltsev Formation, here we revised the traditional model using three independent methods: radioisotopic CA-IDTIMS U-Pb zircon ages, δ13Corg isotope values and paleomagnetic proxies. The regional extinction of the humid-dominated forest flora (cordaites) and the aridity-induced biotic turnover in the Kuznetsk Basin occurred 820 kyr earlier than the end-Permian extinction event recorded in South China at 251.94 Ma. The biota in Kuznetsk Basin at the turnover subsequently diversified (with some exceptions) across the Permian-Triassic transition. By compiling a large taxonomic database, we find that marine and terrestrial biotic diversity in Siberia progressively increased from the beginning of the Permian up to the middle Roadian (early Guadalupian global glacial event). After that time, the diversity at the species and generic level progressively and slowly declined towards the aforementioned latest Changhsingian (252.76 Ma) biotic turnover. Starting from this time, the biota rapidly diversified in the latest Changhsingian and Early-Middle Triassic. We suggest that the Permian-Triassic mass extinction mostly occurred in the tropics and subtropics due to the strong climatic warming, which was relatively low in late Changhsingian and gradually but quickly extends in the latest Changhsingian to an abnormally high temperature and extremely low oxygenated water in the oceans that was deadly for most marine animals. The warm climate shift poleward during Permian-Triassic transition in the middle-high latitudes caused the replacement (turnover) of the humid-related biotas by the dry climate-related and more diverse communities, which continued to expand throughout the Triassic in both marine and terrestrial habitats. The pattern of the Permian-Triassic event in both marine and terrestrial habitats was more intricate in terms of extinction, turnover, and diversity of biota within the different climatic zones and environmental habitats than has been generally considered.

Early Permian continental deposits include a large number of localities containing anthracosiid-like non-marine bivalves traditionally assigned to Anthraconaia Trueman and Weir, 1946, Palaeanodonta Amalitzky, 1895, and Palaeomutela Amalitzky, 1892. In most cases, these classifications are only tentative due to insufficient preservation in which the shells are missing their main characteristics: the ligament and the hinge. Non-marine bivalves from the Early Permian Upper Pospelovka Subformation of Russky Island (South Primorye, Far East Russia), described here as Hypoanthraconaia gen. nov., differ morphologically from the above genera by a set of external features including the initial shell, the mode of intersection of the growth lines with the dorsal margin, and the details of the sculpture. Hypoanthraconaia gen. nov. shows the most external similarity with “atypical” anthracosiid-like morphotypes of Anthraconaia that are widespread in the Late Pennsylvanian and Early Permian of eastern North America, and the Stephanian and Early Permian Lower Rotliegend of northwestern Europe. On this basis, the new genus is conventionally assigned to the family Naiaditidae Scarlato and Starobogatov, 1979.

The genus Verneuilunio Starobogatov, 1987 (with the type species Naiadites verneuili Amalitzky, 1892) was distinguished from the genus Palaeanodonta Amalitzky, 1895 based on the differences in the hinge line, which were recognized using published data. Both genera were included in the family Palaeanodontidae Modell, 1964, which the author regarded as a subjective synonym of the family Palaeomutelidae Lahusen, 1897 (=Palaeomutelidae Weir, 1969). Revision of the collection of V.P. Amalitzky performed by the authors has shown that the initial diagnosis of the genus contains certain inaccuracy complicating identification and assignment to higher-rank taxa. The present study improves the diagnosis of the genus Verneuilunio and provides a detailed description of its type species Naiadites verneuili Amalitzy, 1892. Based on the presence of the duplivincular, slightly amphidet ligament, the genus Verneuilunio is referred to the family Naiaditidae Scarlato et Starobogatov, 1979. This character sharply distinguishes this genus from other co-occurring Unio-like genera of Late Permian nonmarine bivalves, i.e., Palaeomutela Amalitzky, 1892, Palaeanodonta Amalitzky, 1895, Oligodontella Gusev, 1963, and Opokiella Plotnikov, 1949. Verneuilunio is most similar in appearance to atypical Anthraconaia Trueman et Weir, 1946, which is characterized by Unio-like outline and widespread in the Upper Carboniferous. Data processing of morphometric parameters of Verneuilunio verneuili Amalitzky, 1892 and A. pruvosti (Tschernyshew, 1931), which is most similar to it in external outline has revealed statistically significant distinctions in elongation of the posterior shell end. According to available data, the geographical range of Verneuilunio is restricted to the central part of the East European Platform and its stratigraphic range falls in the lower substage of the Severodvinian Stage (=Early Capitanian).

This paper presents the preliminary results of investigations of carbonate nodules from red paleosols of the reference sections of the Middle and Upper Permian of the Kazan Volga region. The main genetic types of paleosols are calcic gleysols and gleyed vertisols, the pedogenic nodules are composed of dolomite and calcite. Scanning electron microscopy and spectroscopic and isotopic methods of analysis show that the nodules retained their original mineral composition and were formed with the participation of soil microorganisms under conditions of contrasting seasonality in an arid climate. In the vicinity of the Kiama–Illawarra paleomagnetic boundary, the Urzhumian paleosols show a transition in the composition of pedogenic nodules from dolomite to calcite. It is suggested that this transition was caused by the humidization of climate, which is supported by lithological and isotopic data.

Bivalves assigned to the genus Palaeanodonta Amalitzky, 1895 comprise more than 30 species and are widespread in malacofaunas of the Permian Period in both Eurasia and Gondwana. The history of the establishment of Palaeanodonta, changes in the opinion of the author of the genus concerning its composition and type species are considered. Relationships of Palaeanodonta with the genus Naiadites Dawson, 1860, which was initially regarded as its senior synonym, and with Anthraconaia Trueman et Weir, 1946, which shows superficial similarity, are analyzed. Certain characters, such as the external opisthodetic ligament, reduced pseudotaxodont hinge, crossed lamellar comarginal and radial shell microstructure, resulted in the conclusion that the type species of the genus, Unio castor Eichwald, 1860, the species close to it, i.e., P. longissima (Netschajew) and P. rhomboidea (Netschajew), and also the species included in the group Palaeanodonta fischeri (Amal.) (P. subcastor (Amal.), P. okensis (Amal.), P. parallela (Amal.), P. obunca (Netschajew), P. amalitzkyi (Silantiev)), as well as the group Palaeanodonta dubia (Amal.) (P. umbonata (Amal.), P. sibirzevi (Amal.), P. indeterminata (Amal.), P. monstrum (Amal.)) belong to the genus Palaeomutela Amalitzky, 1892. The species listed are similar in the reduced pseudotaxodont hinge apparatus, which is characterized by a narrow hinge area and few small teeth, which are usually at most ten in number, and occasionally fewer down to complete disappearance. This character is the basis for the recognition of the subgenus Palaeanodonta Amalitzky, 1895 within the genus Palaeomutela Amalitzky, 1892. The diagnoses of the genus, subgenera, and their species composition are provided.

Nonmarine bivalves from the Permian of the Indian Gondwana Coal Measures are described for the first time. Six new bivalve genera, Gondwanaiadites, Bakulia, Gangamya, Raniganjelia, Gondwanadontella, and Indonellina, and 13 new species of four families from the Raniganj Formation of the Raniganj, Jharia, and North Karanpura basins of the Damodar valley are established. Morphological characters used in the descriptions of the Indian fauna only represented by external molds and imprints of shells are reviewed. The position of the Indian fauna of nonmarine bivalves in the modern system of Bivalves is considered. The stratigraphic distribution of nonmarine bivalves and use for correlation of coal beds are analyzed. All of new genera closely resemble the genera from the Kuznetsk Coalfield, where the stratigraphic scale for Angaraland is established. Three stratigraphic intervals of the Raniganj Formation are recognized based on the species composition of bivalves. They are found useful for interbasin correlation, despite a low morphological and taxonomic diversity. The lower interval is characterized by Gondwanaiadites olgae sp. nov. and Bakulia damodarensis sp. nov. The middle interval has an interesting assemblage of Raniganjelia, Gondwanadontella, and Indonellina.. The upper interval is characterized by Gangamya jhariaensis and Gangamya elongata. The assemblage under study is dated Kazanian–Urzhumian. The bivalve fauna from the “water body facies” of Indian Gondwana was presumably more saltwater than that of the Kuznetsk Basin.

The Permian–Triassic boundary marks the greatest mass extinction in Earth's history. In order to understand the real causes of this severe extinction event, multidisciplinary investigations around the globe are required. Here, the terrestrial Permian–Triassic boundary sections in the Vladimir region, Central Russia, were sampled bed-by-bed for conchostracan study. In the Early Triassic intervals the following taxa were recognized for the first time: Cornia germari (Beyrich, 1857), Euestheria gutta (Lutkevitch, 1937), Magniestheria mangaliensis (Jones, 1862), Palaeolimnadiopsis vilujensis Varentsov, 1955, and Rossolimnadiopsis Novozhilov, 1958. The wide distribution of C. germari demonstrates their high value for biostratigraphy, since this species was also reported from the Lower Buntsandstein Subgroup in the Germanic Basin as well as from Early Triassic deposits in Hungary, Greenland and Siberia. The assumption of an Early Triassic age of the studied sections is also supported by associated Tupilakosaurus bone fragments, which point to the Tupilakosaurus wetlugensis Zone in the earliest Triassic.

New finds and revision of available collections of nonmarine bivalves provided grounds for development of a zonal scale for terrestrial sequences of the Permian System based on species belonging to the genus Palaeomutela Amalitzky, 1891, which are characterized by regular changes in the structure of the shell hinge. The scale includes two parallel zonal successions that are based on the stratigraphic distribution and evolutionary trends of two morphological lineages of the genus. The zonal succession based on development of the P. umbonata group (dwellers of mobile waters and silty-psammitic substrates) includes 11 range zones: stegocephalum, ovatiformis, umbonata, quadriangularis, krotowi, wohrmani, numerosa, ulemensis, keyserlingi, curiosa, golubevi. The zonal succession based on development of the P. castor group (dwellers of calm waters and silty-pelitic substrates) includes eight range zones: larae, castor, olgae, doratioformis, marposadica, fischeri, obunca, amalitzkyi. The proposed zonal units are correlated with scales based on ostracod, fish, and tetrapod fossils. New species Palaeomutela golubevi sp. nov. and P. amalitzkyi sp. nov. are described with the extended diagnosis of the genus Palaeomutela.

The microstructure of aragonitic and calcitic shells of the genus Palaeomutela Amalitzky, 1891 is examined. The aragonitic shell consists of three main layers, each is distinguished by certain crossed lamellar microstructure: comarginal, radial, and complex. As aragonite is recrystallized into pelitic calcite, microstructural shell features are preserved. Many species of Palaeomutela from localities of different age display the same microstructural pattern, which is possible to regard as a character of generic rank.

The area, which occupies the right bank of the Volga River from the mouth of its tributary, Sviyaga River, to the southern border of Tatarstan, and the right bank of the Kama River from its mouth to the northeastern border of Tatarstan, is considered to warrant the possible creation of a national geopark. The area is characterized by well-defined limits, and it occupies 2,080 km2 that is commensurate with an area of many successfully functioning geoparks. It has more than 150 years of history of comprehensive geological studies carried out by prominent Russian and foreign geologists. A significant number of geosites (29) of different types and significance levels accessible to visitors are revealed there. The majority of the geosites are of interest first of all in terms of stratigraphy and palaeontology as well as geomorphology, tectonics, petrology, hydrogeology, and history of geology and mining. These geosites allow study of the history of the Earth's organic world in the Middle-Upper Permian, geological events, and processes that occurred in this region, as well as landform evolution. This area is of aesthetic value owing to the variety and beauty of its natural landscapes. It also contains abundant nongeological natural, historical, and cultural heritage sites attractive for tourists. It is also characterized by a high level of economic development, well-developed infrastructure, and the existence of officially protected sites that occupy a fifth of the territory. The area is of high educational significance as well. It has, for a long time, been used as a geological training ground for students of the Kazan State University, Russia. Moreover, the scientific and methodical basis for seven educational-excursion routes including visiting particularly interesting geological, historical, and cultural sites of the planned geopark has been already developed. The comprehensive evaluation of this area shows that it meets all necessary criteria for geopark creation.

Tentative biostratigraphic correlations, based on marine faunas, have been made by various workers between Ufimian and Kazanian sequences in their type areas in the Volga–Urals region of Russia and Roadian and Wordian sequences in their type area in Texas, United States. Unfortunately, palynological correlation between the Russian and United States sequences is not possible, due to lack of data from the latter. However, detailed palynological data are available from rocks of Roadian and Wordian age in the Canadian Arctic Archipelago, and therefore indirect correlations are possible. Palynomorph assemblages from the Canadian Arctic and other circumpolar areas, such as the southern Barents Sea and Greenland, are different from those of the Ufimian and Kazanian Russian sequences in their type areas. This is likely to be the result of variations in the parent flora in response to significant paleoclimatic differences. For example, the climate of the Volga–Urals region in Late Permian times was probably hot and arid, whereas that of the Canadian Arctic, Barents Sea, and Greenland was cooler and probably more humid.

208 Detailed descriptions and new biostratigraphic, paleomagnetic, geochemical, chemostratigraphic, and sedimentological data on the most important type and reference sections of the Ufimian, Kazanian, Urzhumian, and Severodvinian stages are presented. The guidebook is intended for geologists and paleontologists who study Permian stratigraphy, paleontology and mineral resources, and for students and teachers of fieldwork techniques. The publication was supported by the Russian Foundation for Basic Research and the Open Joint Stock Company ‘Tatneft’.