Frasnian-Famennian Boundary Research Articles

Diagenetic versus detrital origin of the magnetic susceptibility variations in some carbonate Frasnian–Famennian boundary sections from Northern Africa and Western Europe: implications for paleoenvironmental reconstructions

To provide a new insight into the diagenetic versus detrital origin of the magnetic susceptibility variations in ancient carbonate sequences, a study was conducted within four Frasnian–Famennian platform carbonate sections from Germany, France and Morocco. The study includes along-section magnetic susceptibility and carbonate content measurements complemented by analyses of magnetic hysteresis parameters, inorganic geochemistry and clay mineralogy. Our results show that the magnetic susceptibility evolution is dominantly controlled by the variations in the concentration of low-coercivity ferromagnetic magnetite grains and, to a lesser extent, of paramagnetic clays. In more detail, hysteresis ratios suggest the coexistence of two magnetite populations with significantly different grain size: (1) a dominantly coarse-grained detrital fraction including a mixture of multi-domain and single-domain particles (2) an authigenic fine-grained fraction composed of a mixture of single-domain and superparamagnetic particles. Despite a diagenetic imprint on the clay assemblages, no relationship is established between magnetic susceptibility and illite crystallinity, therefore discarding a noticeable distortion of primary within-section magnetic susceptibility evolution. The overall inherited character of the magnetic susceptibility fluctuations is corroborated by a significant correlation of magnetic susceptibility with terrigenous proxies (Zr, Th). The poorer correlation of magnetic susceptibility with the Fe content is consistent with the existence of a very fine-grained authigenic magnetite component that possibly induces a global magnetic susceptibility increase at the section scale, but no distortion of the within-section evolution. The magnetic susceptibility curves presented here provide a general record of climate-driven detrital influx and carbonate productivity through Frasnian–Lower Famennian times.

Devonian macrovertebrate assemblages and biogeography of East Gondwana (Australasia, Antarctica)

The biogeographic significance of Devonian macrovertebrate assemblages from East Gondwana is reviewed, with updates incorporating recent discoveries including new placoderms (antiarchs, groenlandaspid arthrodires, phyllolepids), sarcopterygians (mandageriine tristichopterids, actinistians, rhizodontids), and actinopterygians. Key taxa are illustrated by new specimens. Empirical patterns indicating range expansion from the area of origin for particular groups are analysed and discussed. The almost complete absence of armoured agnathans from the Devonian of Australia remains one of the most significant differences to Northern Hemisphere deposits, implying an isolating mechanism (oceanic or climatic barrier) between Gondwana and Laurussia during the Silurian–Early Devonian. Increasing faunal exchange with Asia during the Devonian may have been initiated with Pragian–Emsian faunal turnovers in South Chinese assemblages. A Gondwanan pattern in Devonian vertebrates persists into the Frasnian, until the Great Devonian Interchange at or near the Frasnian–Famennian boundary establishes strong Laurussian affinities in the early Famennian, when phyllolepid placoderms first appear in the Northern Hemisphere. New data documented on Early Devonian phyllolepids from Gondwana indicate an extended biostratigraphic range, and a diversity maximum in the Givetian–Frasnian. A late Famennian faunal exchange with Asia is suggested by sinolepid antiarchs, but requires corroboration from other fish groups, with conflicting Laurussian affinities indicated by new sarcopterygian and acanthodian data. An alternative explanation of physiological adaptation within sinolepids is discussed, as is the key question of tetrapod origins, for which the only Devonian examples outside Laurussia occur in Australia and China. The vertebrate biogeographic patterns are completely contrary to palaeogeographic reconstructions based on palaeomagnetic evidence.

New biostratigraphic subdivision at the Frasnian-Famennian boundary on the east European Platform

New biostratigraphic subdivision at the Frasnian-Famennian boundary on the east European Platform

Osmium isotopic evidence against an impact at the Frasnian-Famennian boundary

Two sections across the Frasnian-Famennian boundary were analyzed for Re and Os concentrations and ¹⁸⁷Os/¹⁸⁸Os ratios to evaluate evidence for a meteoritic input coincident with this boundary and its associated mass extinction. These sections are from a siltstone and shale sequence at Irish Gulf in New York, US and a calcareous shale and ferromanganese oxide sequence at La Serre in France. The Irish Gulf section, with an initial ¹⁸⁷Os/¹⁸⁸Os of about 0.49, does not show the characteristic meteoritic Os imprint with an ¹⁸⁷Os/¹⁸⁸Os of about 0.13. The integrity of the section with regard to Re and Os retention is indicated by the construction of an isochron with an age of 388 ± 41 Ma, consistent with independently determined ages for the Frasnian-Famennian boundary. Although the La Serre section, with Os concentrations as high as 33 ppb and Re concentrations ranging from 1.4 to 7.4 ppb, might be expected to show excellent evidence for a meteoritic contribution, the highly radiogenic isotopic composition (¹⁸⁷Os/¹⁸⁸Os 2.42-3.61) instead suggests recent massive Re loss or addition of radiogenic Os. These possibilities prevent the reconstruction of an initial ¹⁸⁷Os/¹⁸⁸Os value for the boundary for this site. Assuming reasonable Re concentrations prior to loss, the Os isotopic composition is inconsistent with a large meteoritic component. This inconsistency emphasizes that platinum group element (PGE) concentrations are not reliable indicators of extraterrestrial addition.

Distribution, geometry and palaeogeography of the Frasnian (Late Devonian) reef complexes of Banks Island, NWT, western arctic, Canada

Abstract Following the collapse of the >2000 km long Givetian (Middle Devonian) Inuitian/Ellesmere carbonate platform factory in arctic Canada, within the 0° to 10° equatorial palaeolatitudes north, the only Frasnian reefs in high arctic Canada retreated westwards, confined to northeastern Banks Island. These reefs, numbering well over 130, and dominated by corals and stromatoporoid sponges, were spread over c . 5000 km 2 , within the 220 m thick Mercy Bay Formation. Reefs were developed at four different stratigraphic levels (termed the A, B, C and D levels) during early and middle Frasnian time, periodically smothered by intervening siliciclastics during sea-level lowstands, and were finally buried by thick siliciclastic sands, silts and muds derived from the east during the late Frasnian. The Frasnian–Famennian boundary is masked within continental, deltaic facies bearing plant remains. The Banks reef and carbonate succession is preserved as horizontally stratified, in situ limestones, with the succeeding reefs backsteppping towards the east, in response to cyclic Frasnian sea-level rises. The Banks reefs were developed on the distal lobes of a megadelta, periodically covered by extensive lowland forests that stretched more than 2000 km west from Greenland and Ellesmere Island. Reefs are three-dimensionally preserved, unaffected by diagenesis, dolomitization, major tectonics, vegetative cover or glaciation, and thus display some of the finest, pristine Late Devonian reef complexes known. The reefs represent a range of geometries, from small circular to oval patch reefs a few tens of metres in diameter and <5–10 m thick, to larger prominent tabular (‘reef platform’ style) and domal reef structures >2 km across, or 300–400 m diameter, and 40–60 m thick. Coral and stromatoporoid-rich biostromes also formed a significant part of the reef seascape. Morphology of the reefs leads to an estimation of penecontemporaneous relief of 10–20 m above the prevailing sea floor, with reef-core facies generally dominated by stromatoporoid sponges, and flank facies by a variable mix of colonial tabulate and rugose corals, as well as stromatoporoids. Bryozoans and calcimicrobialites were rare, in contrast to other Late Devonian reefs such as those of the Canning Basin (Australia), or the Guangxi platform (South China); mudmound reef facies, such as seen in Belgium were limited.

Devonian Frasnian-Famennian transitional event deposits of Guangxi, South China and their possible tsunami origin

A suite of event deposits, isochronous and interrelated in origin, occurs widely near the Frasnian-Famennian boundary in Guangxi, South China. It is mainly distributed in the facies areas of the platform-margin slope and inter-platform rift-trough. The rudstone or calcirudite occur in the Liujing section of Hengxian and Luoxiu and Baqi sections of Xiangzhou in the facies area of platform-margin slope. The turbidites are observed in the sections of Yangdi and Baisha of Guilin, Du’an of Debao, Nandong and Sanli of Wuxuan, Xiangtian, Ma’anshan of Xiangzhou, Nayi of Chongzuo, Yunpan of Shanglin in the facies area of inter-platform rift-trough. The massive homogenites occur in sections of Mangchang, Luofu and Road from Nandan to Tian’e and Ma’anshan etc in the facies area of inter-platform rift-trough. Herein event deposits can be correlated in stratohorizon to the turbidite in the bottom of the Lower triangularis Zone in Hony railroad cut of Belgium, Devils Gate of Nevada, USA, Atrous of Morocco, South Urals and Fore-Kolyma of northeastern Siberia of Russia, and erosional discontinuities and brecciation in the Frasnian-Famennian boundary in South Polish-Moravian shelf. The event deposits could be caused by a violent tsunami related to bolide impacts into ocean.

Conodonts and radiolarians from pelagic cherts of the Frasnian–Famennian boundary interval at Bancheng, Guangxi, China: Global recognition of the upper Kellwasser event

The recognition of five standard conodont biozones and five radiolarian zones in the same set of samples from cherts within the Frasnian–Famennian (F–F) boundary interval at Bancheng, southeastern Guangxi, China allowed precise placement of the F–F boundary, and demonstrated the multi-episodic pattern of the F–F mass extinction. Frasnian pelagic radiolarian and conodont faunas had an acme in the lower Palmatolepis linguiformis Zone. In the upper Pa. linguiformis Zone, until the level of the upper Kellwasser extinction (Kw.), members of the radiolarian family Ceratoikiscidae and of the conodont Frasnian populations disappeared. Above the Kw, in the lower Palmatolepis triangularis Zone, new populations of the conodont Pa. triangularis gradually appeared and the survivor radiolarian genus Trilonche gradually re-appeared, both blooming in the lower part of the middle Pa. triangularis Zone. The Nehden extinction (Nd) in the uppermost middle Pa. triangularis Zone interrupted the recovery of the conodont and radiolarian faunas, but above this level the conodont species became more numerous, and the post-extinction radiolarian faunas (including Entactiniidae) recovered gradually.

Record of climate-driven morphological changes in 376 Ma Devonian fossils

The Lower and Upper Kellwasser horizons represent two anoxic events that mark the mass extinction at the Frasnian-Famennian (F-F) boundary. Among other groups, conodont animals were severely affected, but the genus Palmatolepis survived with a complete turnover at the F-F boundary. Here the fine morphological variations of the genus Palmatolepis and the sea-surface temperature evolution are quantified in two F-F boundary sections using morphometrics and oxygen isotopic composition of apatite, respectively. In accordance with other F-F sections, the isotope records show two positive excursions of ~1‰ during the Lower and Upper Kellwasser anoxic events. The conodont shape and the oxygen isotopic composition of the genus Palmatolepis are significantly correlated within the Frasnian and Famennian Stages, suggesting a strong environmental influence on the morphology of the feeding apparatus of the conodont animal. We propose that the morphological differences are linked to changes in the trophic position of Palmatolepis: enhanced organic carbon burial, which is supported by global positive carbon isotope excursions in inorganic and organic carbon during both the Lower and Upper Kellwasser events altered the primary biomass production and thus, the subsequent nutrient supply to higher trophic levels. While the carbon and oxygen isotopic shifts are of similar amplitude during Lower and Upper Kellwasser events, the variation of the shape of Palmatolepis during the Upper Kellwasser, i.e., the F-F boundary, is more pronounced than during the Lower Kellwasser.

Conodont biostratigraphy of the Chattanooga Shale, Middle and Upper Devonian, southern Appalachian basin, eastern United States

The Chattanooga Shale of the southern Appalachian Basin contains a diverse conodont fauna of the high Givetian, Frasnian, and Famennian. The predominantly fine-grained strata were deposited in an offshore setting where depositional packages are separated by unconformities. Conodonts allow regional and global correlation of these strata, recognition of the Frasnian-Famennian boundary, and narrow biostratigraphic constraint of two Frasnian ash beds, MN Zone 8 for the Belpre Ash and upper MN Zone 13 for the Center Hill Ash. Three new Frasnian palmatolepid conodonts are described in open nomenclature, and the holotype ofPalmatolepis regularisCooper is reillustrated.

Ostracod and conodont faunal changes across the Frasnian-Famennian (Devonian) boundary at Els Castells, Spanish central Pyrenees

Diverse conodont and silicified ostracod assemblages were found in the Spanish Pyrenees (Els Castells section), in the Frasnian/Famennian boundary beds (late rhenana and/or linguiformis to late triangularis zones), in strata below and above the well-known Kellwasser Extinction Event. Many of the ostracods studied here are conspicuous elements of the “Thuringian Mega-Assemblage”, and show maximum affinities with faunas from the southeastern Cantabrian Mountains (Spain), eastern Thuringia and the Harz (Germany). The composition of the faunas, however, is not uniform through the Els Castells section. A rather sharp break exists, roughly coinciding with the Frasnian/Famennian boundary. The break is characterized by the disappearance of many ostracods typical of the “Thuringian Mega-Assemblage”. This correlates with an important change in the conodont faunas related to the Kellwasser Event. The Frasnian (late rhenana and/or linguiformis zones) palmatolepid-polygnathid biofacies is followed in Famennian strata (middle and late triangularis zones) by a palmatolepid-icriodid biofacies. The icriodid maximum in the earliest Famennian rocks of the Pyrenees correlates with the “Icriodid Peak” described elsewhere. These changes depend on the age and palaeoenvironmental conditions.

Re–Os depositional ages and seawater Os estimates for the Frasnian–Famennian boundary: Implications for weathering rates, land plant evolution, and extinction mechanisms

Four TOC-rich shale intervals spanning the Frasnian–Famennian (F–F) boundary were recovered in a drillcore (West Valley NX-1) from western New York (USA) and radiometrically dated using Re–Os. Two of the black shale intervals (WVC785 from ∼ 2.9 m below, and WVC754 from ∼ 6.4 m above the F–F boundary, respectively) yielded statistically overlapping ages with uncertainties of < 1.1%. An interpolated age and associated graphically determined uncertainty of 372.4 ± 3.8 Ma provides new absolute age constraints on the F–F boundary. This date is ∼ 4.1 Ma younger than the latest proposed F–F boundary age of 376.1 Ma obtained by interpolation of U–Pb dates from volcanic zircon [Kaufmann, B., 2006. Calibrating the Devonian Time Scale: A synthesis of U–Pb ID-TIMS ages and conodont stratigraphy. Earth-Science Reviews 76, 175–190], and within uncertainty of the International Commission on Stratigraphy accepted date of 374.5 ± 2.6 Ma. A third date (from sample WVC802, ∼ 8.2 m beneath the F–F boundary) yielded an imprecise age of 357 ± 23 Ma, owing in part to a limited Re/Os range. The initial 187Os/ 188Os (0.45 to 0.47), reflecting contemporaneous seawater Os values, are low but similar to the value of 0.42 reported for the Exshaw Fm (Canada) at the Devonian–Mississippian boundary (ca. 361 Ma) [Selby D., Creaser R.A., 2005. Direct radiometric dating of the Devonian–Mississippian time-scale boundary using the Re–Os black shale geochronometer. Geology 33, 545–548]. This may suggest fairly constant and low global continental weathering rates during the Late Devonian, although in view of the short residence time of Os in seawater (∼ 1–4 × 10 4 yr), further measurements are needed to assess potential short-term variation in seawater Os ratios. Owing to low Os and Re abundances at the F–F boundary, our data are inconsistent with long-term volcanism and bolide impact as potential Late Devonian mass extinction mechanisms. In addition, the Frasnian–Famennian ocean appears to have been depleted with respect to Re, possibly indicating an exhaustion of the Re seawater reservoir owing to high burial rates of redox-sensitive elements under dysoxic/anoxic conditions leading up to the F–F boundary.

Devonian events and correlations—a tribute to the lifetime achievements of Michael Robert House (1930–2002)

The Devonian was a peculiar time in the Phanerozoic evolution of the Earth. Most continents, including the large Gondwana and Laurussia cratons, formed a Pangea-type assembly around the tropical Prototethys and an increasingly hot, global, greenhouse climate prevailed, with a complete lack of major ice sheets, even in polar areas. There was gradual and increasing flooding of the continents, creating huge epicontinental seas that have no modern analogues. Under these conditions the plants finally conquered the land, with the innovation of deep roots in the Emsian, the appearance of seed precursors and trees in the Givetian, and the spread of vegetation into dry uplands in the late Famennian. In the marine realm, the largestknown Phanerozoic tropical reef belts surrounded craton margins and tropical islands. It was the time of the sudden radiation of early ammonoids, of the earliest episodic blooms of calcareousshelled, pelagic zooplankton (tentaculitoids), the rise to dominance of fishes, mostly of armoured forms and with giants reaching 10 m in length, but also including the first sharks, and the appearance of earliest tetrapods in marginal settings. However, the tropical and subtropical areas reaching up to 458 latitude were hardly a paradise. A combination of climatic, plate tectonic/magmatic and still poorly understood palaeoceanographic factors caused the recurrent sudden perturbation of stable ecological conditions by short-term global events of variable magnitude (e.g. House 1985), including two of the biggest mass extinctions that the Earth’s biosphere has experienced—the Upper Kellwasser Event at the Frasnian–Famennian boundary and the Hangenberg Events at the close of the Devonian. The global and regional correlation of Devonian rocks relies strongly on the available fossil record, on the facies distribution of marker fossils, on the recognition of eustatic sea-level change in sequence stratigraphy, and, as a relatively new development, on the application of stable isotope geochemistry and magnetic susceptibility. The Prototethys seaway, fortunately, allowed a free exchange of pelagic faunas through all oceans and seas. In the warm-water areas, there was only moderate endemism in conodonts, ammonoids, pelagic ostracodes and tentaculitoids, which are the main biostratigraphic marker groups. Low diversity benthic assemblages of the outer shelf also include many cosmopolitan groups but they differ strongly from fossil associations of the nearshore, neritic and photic zones. In the latter areas, including reef and biostrome complexes, the evolution and ecologically controlled distribution of brachiopods, stromatoporoids, rugose and tabulate corals, and trilobites provides refined regional time frameworks but the faunas are mostly characterized by endemism that prevents simple correlation across oceans and seaways. Palynomorphs that were washed into the seas enable the correlation of marine, fluvial, limnic and terrestrial deposits. The highly variable palaeoecology and taxonomic composition of the terrestrial, neritic and pelagic faunal assemblages are the fundamental problems of cross-facies correlations within the Devonian. Apart from overlapping facies and palaeobiogeographic ranges of some key taxa, event and physical stratigraphy are the major tools in reconstructing global changes in the Devonian world at high level of time resolution. The International Subcommission on Devonian Stratigraphy (SDS) was the first International Union of Geological Sciences (IUGS) Subcommission to complete the formal designation of all its series and stage subdivisions, but all ratified GSSPs (Global Stratotype Sections and Points), as the norm, were defined in the pelagic facies realm. Their recognition on all continents and in all other facies belts is still a major task to be resolved. As a first step to reach some progress in this wide, international and multidisciplinary scientific field, the Devonian Subcommission and the Institut Scientifique University Mohammed V, Rabat, especially with a major organizational input by its Titular Member and Vice-Chairman, Prof. Ahmed El Hassani, organized in March 2004 an international symposium (El Hassani 2004a) on ‘Devonian neritic–pelagic correlation and events’, followed by an excursion with the same topic to the continually inaccessible Dra Valley of SW Morocco (El Hassani 2004b).

Environmental changes at the Frasnian–Famennian boundary in Central Morocco (Northern Gondwana): integrated rock-magnetic and geochemical studies

Abstract Rock magnetic (magnetic susceptibility and hysteresis parameters) and geochemical analyses (major and trace elements) were carried out on whole rock samples of two Frasnian–Famennian boundary sections, Anajdam and Bou-Ounebdou in the Central Morocco (Western Meseta). During the Frasnian, the decreasing trend of the magnetic susceptibility signal, mainly carried by low-coercivity magnetite grains, indicates a gradual reduction of detrital influx. This decrease in detrital input parallels a Frasnian long-term sea-level rise. In the Late Frasnian Kellwasser Horizons, that are classically considered to represent highstand deposits, the magnetic signal exhibits the lowest intensities in connection with maximum diamagnetic contribution of the carbonate fraction. With respect to geochemical data, the two black carbonate-rich Kellwasser Horizons are characterized by noticeable positive anomalies of bottom-water dysoxic proxies and of marine primary productivity markers. Our data thus suggest that in Central Morocco, the Late Frasnian marine environments were marked by a relatively important biogenic productivity favouring the onset of oxygen-depleted conditions during periods of maximum transgression on the continental platforms.

Vertebrate micro-remains from the Upper Devonian of East Greenland with comments on the Frasnian–Famennian boundary

Vertebrate micro-remains of jawless vertebrates (psammosteid heterostracans) and gnathostomes (acanthodians and sarcopterygians) occur in a residue collected from the Late Devonian Kap Graah Group sandstones of East Greenland. Fragmentary and isolated elements of psammosteids are assigned to Psammosteus sp. The acanthodians are represented by scales of acanthodiform type and poorly preserved fin spines. Tooth and scale fragments of sarcopterygians may suggest a porolepiform affinity. This assemblage and the associated macro-vertebrate fauna indicate a Frasnian age for anotherwise assumed Famennian part of the East Greenland succession, and do rather effectively bracket the Frasnian–Famennian boundary.

The fate of the homoctenids (Tentaculitoidea) during the Frasnian–Famennian mass extinction (Late Devonian)

ABSTRACTThe homoctenids (Tentaculitoidea) are small, conical‐shelled marine animals that are among the most abundant and widespread of all Late Devonian fossils. They were a principal casualty of the Frasnian–Famennian (F‐F, Late Devonian) mass extinction, and thus provide an insight into the extinction dynamics. Despite their abundance during the Late Devonian, they have been largely neglected by extinction studies. A number of Frasnian–Famennian boundary sections have been studied, in Poland, Germany, France, and the USA. These sections have yielded homoctenids, which allow precise recognition of the timing of the mass extinction. It is clear that the homoctenids almost disappear from the fossil record during the latest Frasnian ‘Upper Kellwasser Event’. The coincident extinction of this pelagic group, and the widespread development of intense marine anoxia within the water column, provides a causal link between anoxia and the F‐F extinction. Most notable is the sudden demise of a group, which had been present in rock‐forming densities, during this anoxic event. One new species, belonging to Homoctenus is described, but is not formally named here.

Re-Os evidence for the origin of Fe-oxide-(Cu-Au) deposits in SW Iberia at the Frasnian-Famennian boundary

Re-Os evidence for the origin of Fe-oxide-(Cu-Au) deposits in SW Iberia at the Frasnian-Famennian boundary

Re–Os depositional ages and seawater Os estimates for the Frasnian–Famennian Boundary

Re–Os depositional ages and seawater Os estimates for the Frasnian–Famennian Boundary

Palynological and bulk geochemical constraints on the paleoceanographic conditions across the Frasnian–Famennian boundary, New Albany Shale, Indiana

Abstract A down-core record of stable isotope and geochemical results is integrated with palynofacies (kerogen) data from the New Albany Shale (Indiana) to reconstruct environmental changes that occurred across the Frasnian–Famennian boundary. Preliminary interpretations are focused on developing several multiproxy linkages that will potentially lead toward a more robust understanding of the occurrence and significance of phytoplankton assemblage variations during the Late Devonian, a time of widespread black shale formation. Development of such linkages will potentially provide a more comprehensive assessment of the various controls on 1) primary production, and 2) carbon sequestration in a large, low-paleolatitude intracratonic basin. An abrupt change in the geochemical and biotic proxies for particulate organic matter across the Frasnian–Famennian boundary coincides with a distinct lithological change, characterized by laminated, brownish-black Famennian mudstones unconformably overlying alternating bioturbated, greenish-gray and non-bioturbated, dark-gray Frasnian mudstones. Elemental and isotopic profiles reflect different patterns of production, degradation, and removal of organic carbon in the two shale facies. A shift from acritarch- to prasinophyte-dominated waters across the boundary indicates the overall importance of bathymetric fluctuations, chemico-physical conditions, and nutrient availability related to eustatic sea-level change. A positive δ 13 C V-PDB shift of 1.1‰ across the boundary is interpreted to be correlative with the global Upper Kellwasser Event. A preliminary model is proposed to explain the sustainable primary production during times of maximum flooding, thereby enhancing organic preservation during black shale formation.

Late Devonian conodonts and the global Frasnian-Famennian extinction event, Thong Pha Phum, western Thailand

The study of Late Devonian Frasnian and Famennian conodonts in western Thailand confirms these species are mostly cosmopolitan. The Thailand section includes the Upper Kellwasser Event recorded in Europe, North America, China, and elsewhere, and probably also includes the upper part of the Lower Kellwasser Event. Stable isotope geochemistry for this interval shows a positive d13C excursion in the Late rhenana conodont Zone followed by a gradual return to normal, followed by a sudden positive excursion near the conodont extinction event. The latter has been chosen elsewhere as the Frasnian-Famennian boundary. The extinction event seems a consequence of unsteady conditions that could include eutrophic, oxygen, carbon dioxide, eustatic, and temperature fluctuations. There was a regional disappearance or extinction of several conodont species at the Frasnian-Famennian boundary followed by recovery and the spread of new species during the early Famennian.

Palynostratigraphy of the Frasnian–Famennian boundary deposits from the Central Devonian Field, western Russia and comparisons with adjacent areas

Detailed palynological research carried out on samples from the three quarries at Khlevnoye and Kamenka (Voronezh region) permitted recognition of the Frasnian–Famennian boundary in the lower Kamenka quarry. Two miospore zones, Cristatisporites deliquescens– Verrucosisporites evlanensis (DE) from the Frasnian and Cyrtospora cristifera– Diaphanospora zadonica (CZ) from the Famennian, have been recognized. Some faunistic data were also obtained from the F–F boundary deposits. Research confirmed the existence of a sedimentological gap between the Livny and Zadonsk horizons, including all of the Volgograd horizon. These new palynological results are compared with data from adjacent areas: Pripyat Depression (Belarus), Timan-Pechora Province and the Volgograd Volga region (Russia), western Pomerania and the Holy Cross Mountains (Poland).