The Devonian-Carboniferous boundary in the Graz Paleozoic (Eastern Alps, Austria) and its global significance

 

El límite Devónico-Carbonífero en el Paleozoico de Graz (Alpes orientales, Austria) y su importancia global

 

Sandra I. Kaiser1,*, Bernhard Hubmann2

 

1 State Museum of Natural History Stuttgart, Germany, Rosenstein 1, 70372 Stuttgart, Germany

2 Karl-Franzens University of Graz, Institute of Earth Sciences, Universitaetsplatz 3 8010 Graz, Austria.

 

Corresponding author: (S.I. Kaiser) This email address is being protected from spambots. You need JavaScript enabled to view it.   

 

How to cite this article:

Kaiser, S.I., Hubmann, B., 2024, The Devonian-Carboniferous boundary in the Graz Paleozoic (Eastern Alps, Austria) and its global significance: Boletín de la Sociedad Geológica Mexicana, 76 (3), A210524. http://dx.doi.org/10.18268/BSGM2024v76n3a210524  

 

Manuscript received: April 7, 2024; corrected manuscript received: April 28, 2024; manuscript accepted: May 12, 2024

 

ABSTRACT

Pelagic, outer-shelf sedimentary successions across the Devonian-Carboniferous boundary (DCB) in the western Graz Paleozoic at Trolp (Eastern Alps, Austria) display exceptional, continuous limestone facies of the Hangenberg Crisis interval. These enable the study of conodont successions, lithofacies, and geochemistry without noteworthy interruptions by siliciclastic intervals, which are equivalents of the Hangenberg Black Shale and Hangenberg Sandstone. The praesulcata, kockeli, and sulcata/ kuehni conodont Zones are recognized by stratigraphically significant markers in the early siphonodellids and early protognathodids within thin-bedded limestones. The major conodont biofacies change and initial mass extinction phase (costatus-kockeli Interregnum) of the Hangenberg Crisis is succeeded by radiations among conodonts in the kockeli and sulcata/kuehni Zones. Carbon isotopes (δ13Ccarb) record positive excursions and enhanced Corg burial episodes; the characteristic signals can be used as chemo-indicators of the early, middle and late phases of the Hangenberg Crisis. The global common Siphonodella sulcata Morphotype 5 and Protognathodus kuehni have their first occurrence at the base of the Tournaisian at Trolp and elsewhere in Europe, Asia and North Africa, and thus can be used to define the base of the sulcata/kuehni Zone and the current DCB level. `Protognathodus´ faunas with a high morphological variability are recorded in the praesulcata and kockeli Zones, but spread mainly in the sulcata/kuehni Zone, or even later in the lower Tournaisian. Therefore, the extended kockeli Zone suggested recently, and even leaves the current DCB level undivided, as well as the use of Protognathodus kockeli as index fossil for the DCB, are not applicable as suggested herein.

A precise biozonation concept as well as multidisciplinary approaches used at Trolp are needed for understanding ultimate causes of the 1st order Hangenberg mass extinctions and major environmental changes at the DCB. Multidisciplinary approaches comprise taxonomically significant index conodonts, extinctions and radiations, as well as lithofacies and isotopic analyses. These markers at Trolp can be well correlated with those from many other settings worldwide, reflecting the global significance of the Graz Paleozoic.

Keywords: Hangenberg Crisis, conodont mass extinctions, stable isotopes, Trolp quarry, Protognathodus fauna, kockeli Zone, sulcata/kuehni Zone.

 

RESUMEN

Las sucesiones sedimentarias pelágicas de la plataforma exterior a lo largo del límite Devónico-Carbonífero (DCB) en el Paleozoico occidental de Graz en Trolp (Alpes orientales, Austria) muestran facies de piedra caliza continuas y excepcionales en el intervalo de la crisis de Hangenberg. Estos permiten el estudio de sucesiones de conodontes, litofacies y geoquímica sin interrupciones notables por intervalos siliciclásticos, que son equivalentes de Hangenberg Black Shale y Hangenberg Sandstone. Las zonas de conodontes praesulcata, kockeli y sulcata/kuehni se reconocen mediante marcadores estratigráficamente significativos en los primeros sifonodélidos y los primeros protognatódidos dentro de calizas de estrato delgado. El cambio de biofacies de conodontes principales y la fase inicial de extinción masiva (interregno costatus-kockeli) de la crisis de Hangenberg es seguida por radiaciones entre conodontes en las zonas de kockeli y sulcata/kuehni. Los isótopos de carbono (δ13CCarb) registran excursiones positivas y episodios de entierro de Corg mejorados; las señales características pueden utilizarse como quimioindicadores de las fases temprana, media y tardía de la crisis de Hangenberg. El morfotipo 5 de Siphonodella sulcata, común a nivel mundial y Protognathodus kuehni tienen su primera aparición en la base del Tournaisiano en Trolp y en otras partes de Europa, Asia y el norte de África y por lo tanto, pueden usarse para definir la base de la zona sulcata/kuehni y el nivel DCB actual. En las zonas praesulcata y kockeli se registran faunas de ‘Protognathodus’ con una alta variabilidad morfológica, pero se extendieron principalmente en la zona sulcata/ kuehni, o incluso más tarde en el Tournaisiano inferior. Por lo tanto, la zona de kockeli extendida sugerida recientemente, e incluso deja indiviso el nivel actual de DCB, así como el uso de Protognathodus kockeli como fósil índice para el DCB, no son aplicables como se sugiere en este documento.

Se necesita un concepto de biozonificación preciso, así como enfoques multidisciplinarios utilizados en Trolp, para comprender las causas últimas de las extinciones masivas de primer orden del Hangenberg y los principales cambios ambientales en el DCB. Los enfoques multidisciplinarios comprenden conodontos índice taxonómicamente significativos, extinciones y radiaciones, así como litofacies y análisis isotópicos. Estos marcadores pueden correlacionarse bien con los de muchos otros entornos en todo el mundo, lo que refleja la importancia global del Paleozoico de Graz.

Palabras clave: Crisis de Hangenberg, extinciones masivas de conodontes, isótopos estables, cantera Trolp, fauna  de Protognathodus, zona kockeli, zona sulcata/kuehni

 

1. Introduction

The Graz Paleozoic belongs to the Upper Austroalpine Nappe System (Schmid et al., 2004) of the Eastern Alps and consists of Silurian to Carboniferous sedimentary rocks which crop out in the surroundings of Graz (Styria, southeastern Austria, Figure 1). In general terms, the internal tectonic architecture of the Graz Paleozoic has a structure consisting of a lower, tectonically, and metamorphically more stressed nappe system and an upper one comprising less metamorphic sequences (Gasser et al., 2010; Figure 2). The slightly metamorphic and in places very fossiliferous sequence of the upper nappe system has been the subject of numerous paleontological and sedimentological studies (e.g., Histon et al., 2010; Ebner and Hubmann, 2012). In particular, biostratigraphic and geochemical studies of the stratigraphic younger parts (e.g., Nössing, 1974a, 1974 b; Ebner, 1976b, 1980a; Kaiser, 2005; Bojar et al., 2013; Kaiser et al., 2020) have provided important new data on environmental dynamics during the 1st order mass extinctions of the multiphase Hangenberg Crisis at the Devonian-Carboniferous boundary (DCB; Kaiser et al., 2016; Becker et al., 2016a).

Worldwide the DCB is characterized by either transgressive Hangenberg black shales and regressive Hangenberg sandstone deposits, or by unconformities with stratigraphic gaps (Kaiser et al., 2016). The latter phenomenon is also known from the Graz Paleozoic east of the Mur River (Figure 3). In contrast, stratigraphically comparable sections west of the Mur River show exceptionally continuous limestone sequences. The best outcrop is located in the abandoned Trolp quarry (Figures 2 and 3) and was examined with regard to high resolution biostratigraphy, lithostratigraphy and chemostratigraphy. 

The Trolp section focused on herein, consists of pelagic cephalopod limestones of the upper Famennian Steinberg Formation and Tournaisian Sanzenkogel Formation. The DCB is located between both units (Figures 4-8). A positive carbon isotope excursion (δ13Ccarb) measured previously in thin-bedded limestones coincides with the major change in conodont biofacies and the initial mass extinction episode of the Hangenberg Crisis (Figure 9-11). After Kaiser et al. (2020), recent high-resolution geochemical and biostratigraphic studies at Trolp, and the correlation with the Grüne Schneid section in the Carnic Alps (see reviews by Schönlaub, 2018; Spalletta et al., 2021), reveal the utility of the joint sulcata/kuehni Zone, as well as uncertainties in the taxonomy of the Protognathodus fauna (Figures 12-14).

During the 1980s, the DCB working group at the time initially considered the base of the kockeli (upper praesulcata) Zone as a DCB position. However, this idea with the first occurrence (FO) of Protognathodus kockeli was rejected after intensive studies that included visits to relevant sections worldwide (pers. comm. C. Sandberg, 2019). The current DCB, based on the FO of Siphonodella sulcata, was established by the Heerlen Congress in 1935, confirmed by the DCB Working Group in 1988, and since has been applied worldwide. However, the current GSSP at La Serre E´ (Montagne Noire, France) was reconsidered by Kaiser (2009) due to former uncertainties in the identification of early siphonodellids (Kaiser and Corradini, 2011; Spalletta et al., 2017; see summary in Becker et al., 2016a; Kaiser et al., 2020; Hartenfels et al., 2022). Also, according to Ziegler and Sandberg (1996), the current GSSP position at La Serre É between Bed 88 and 89 was chosen hastily and despite the objections of conodont specialists in the DCB working group. Kaiser´s studies in 2005 and 2009 reveal an older FO of the first morphotype (M5) of the index fossil Siphonodella sulcata at La Serre É in Bed 84 (not in Bed 86 as shown in figure 1, Aretz and Corradini, 2021). However, from today’s perspective (Aretz et al., 2021; Hartenfels et al, 2022), the profile at La Serre É would hardly be an option for a suitable GSSP.

Based on taxonomic studies during the last decades, 1. uncertainties of the early siphonodellids (Siphonodella praesulcata, Siphonodella sulcata) resulted in morphotype groups established by Kaiser and Corradini (2011) and are used worldwide (see Kaiser et al., 2020; Hartenfels et al., 2022), 2. the Morphotyp 5 of Siphonodella sulcata is used as index fossil for the current DCB (e.g., Kaiser, 2009; Kaiser et al., 2020; Becker et al., 2021; Hartenfels et al., 2022), 3. the `siphonodelloids´ were discriminated from the early siphonodellids (Becker et al., 2013), and 4. it is recently discussed to what extent Protognathodus kockeli, which was proposed as a supplementary index fossil for the DCB, would be applicable (Becker et al., 2016a; Corradini et al., 2011, 2016; Kaiser et al., 2020; Aretz et al., 2021; Hartenfels et al., 2022).

In this review article we provide a brief overview of the stratigraphic architecture of the Graz Paleozoic Rannach Nappe and present the faunistic, geochemical and sedimentologically important markers at the DCB in the western Graz Paleozoic. We also highlight the potential for correlation with regions outside the study area. We discuss criteria currently proposed – 1. the extended kockeli Zone suggested by Spalletta et al. (2017), and 2. the FAD of Protognathodus kockeli suggested by Hartenfels et al. (2022) for a revised DCB position and their application to the Graz Paleozoic as well as to other European, North African, and Asian areas. This discussion requires an evaluation of the applicability of the kockeli and the sulcata/kuehni Zones (= current DCB level). To this end, we provide a discussion of aspects of conodont biostratigraphy, biofacies, and taxonomy; stable isotope geochemistry (δ13Ccarb) and lithoand microfacies aspects are also included, as are earlier and more recent biozonation concepts (see Figure 12).

Abbreviations: DCB = Devonian-Carboniferous boundary, GSSP = Global Stratotype Section and Point, HBS = Hangenberg Black Shale, HBSE = Hangenberg Black Shale Event, FAD

= first appearance datum, FO = first occurrence, ckI = costatus-kockeli Interregnum, HSS = Hangenberg Sandstone, HSSE = Hangenberg Sandstone Event.

 

 

2. Overview of geology and facies development in the Graz Paleozoic

 

2.1. OVERVIEW OF GEOLOGY

Within Austria, weakly metamorphosed Paleozoic units are irregularly distributed throughout the Alpine orogen (Figure 1). Their original, pre-Alpine geographical location is uncertain, but it is generally accepted that the units were part of the northern Gondwana margin (e.g., Neubauer et al., 2022). According to tectonic concepts of the Eastern Alps (e.g., Schmid et al., 2004) – except for the Paleozoic remnants south of the Periadriatic fault – they are part of the uppermost tectonic nappes of the Eastern Alps, the Upper Austroalpine Nappe System. The Upper Austroalpine Nappe System consists of highly varied crystalline units with a dominant pre-Alpine metamorphic overprint, and is tectonically overlain by the Greywacke Zone, which itself is largely unconformably overlain by thick Mesozoic sediments of the Calcareous Alps. The Graz Paleozoic and some isolated outcrops in southern Burgenland which both lack Permo-Mesozoic cover sequences, as well as the Gurktal nappe, are part of the Drauzug-Gurktal Nappe System, which is itself part of the Upper Austrian Basement Nappes.

The structure of the Upper Austroalpine Nappe System was triggered by the closure of the Meliata Ocean (a marginal basin of the Tethys) during the Early Cretaceous. Therefore, the Upper Austroalpine Nappe System is sealed by Upper Cretaceous to Eocene sediments. Later, the Upper Austroalpine Nappe System was dismembered by lateral extrusion, i.e., eastward displacement of the eastern parts of the Eastern Alps (Ratschbacher et al., 1991; Neubauer et al., 2000) and thus brought into its current position.

The internal architecture of the Graz Paleozoic (Silurian to Carboniferous) is composed of a high-grade metamorphosed lower nappe system and a low-grade metamorphosed upper nappe system (Gasser et al., 2010; Figure 2). Each of them is differentiated into individual nappes. In short, the volcanosedimentary sequence contains basal pre-Devonian volcaniclastics, and a pronounced platformto basinal-facies development during the Devonian. In the structurally higher units, especially in the Rannach Nappe, the depositional environment deepens steadily from the Upper Devonian to the Mississippian, and in the Pennsylvanian the depositional environment becomes abruptly shallow marine (Figure 3).

Resting on metamorphic basement, the nappes of the Graz Paleozoic are unconformably overlain by an Upper Cretaceous sequence, the “Kainach Gosau” (Ebner and Rantitsch, 2000) in the west and by Neogene sedimentary rocks of the Styrian Molasse Basin in the south (Gross et al., 2007).

In contrast to the Lower Nappe System which experienced stronger deformation as well as higher metamorphosis – up to lower amphibolite facies – sequences of the upper nappe system, among them the sequence of the Rannach Nappe, are quite fossiliferous.

 

 

2.2. RANNACH NAPPE OF THE GRAZ PALEOZOIC

The sequence of the Rannach Nappe (Figures 3-5) starts with predominantly alkaline metavolcanites (tuffs, lavas) which pass into slightly calcareous sediments and contain fossils from the late Silurian to early Devonian (Ludfordian Lochkovian). This is followed by various Lower Devonian (Lochkovian) to Upper Devonian (Frasnian) shallow-water platform carbonates. Formations of this depositional episode are comprised to the Rannach Group (Flügel, 2000). Approximately during the Frasnian to Famennian transition, the depositional environment changed to an open, pelagic marine area. This persisted until the late Mississippian (Serpukhovian). The formations of the pelagic facies are combined into the Forstkogel Group (Flügel, 2000). The end of the Rannach Nappe stratigraphic series is formed by lower Pennsylvanian (Bashkirian) shallow-marine birdseye limestones and phytoclast-rich shales of the Dult Group (Flügel, 2000). Detailed descriptions of formations of the Rannach, Forstkogel and Dult Groups, their lithologies, stratigraphic extents and the extensive literature can be found in the explanations for the “Austrian Stratigraphic Chart 2004” (Hubmann et al., 2013).

The pelagic facies of the Forstkogel Group, which consists of micritic deep-marine cephalopod limestones of the Steinberg and Sanzenkogel Formations (Figures 4 and 5), reaches a maximum thickness of 100 m. In the Rannach Nappe east of the Mur (= Eastern Graz Paleozoic) it reduces down to about 30 m. The reasons for the reduced thickness are intraformational stratigraphic gaps in the area of the DCB, which were created by karstification (Ebner, 1978, 1980a, 1980b).

Elementary geochemistry used for interpretions of bathymetry shows clear geographical differences: While in the Graz Paleozoic west of the Mur (= western Graz Paleozoic) the depositional depth in the Mississippian period was around 200 m, in the eastern Graz Paleozoic the water depth was less than 50 m (Nössing, 1974b; Ebner and Prochaska, 1989; Buchroithner et al., 1979). Continuous sequences covering the DCB interval only occur in the western Graz Paleozoic. In the eastern Graz Paleozoic, however, erosion gaps occur in this time interval, with stratigraphic gaps increasing in temporal extent towards the east (Figure 3).

 

 

3. The Devonian-Carboniferous boundary (DCB) in the western Graz Paleozoic

The Forstkogel Group is, according to conodonts, divided into the Upper Devonian Steinberg Formation and the Lower Carboniferous (Mississippian) Sanzenkogel Fm. (Figure 4). The strongly condensed and only 220 cm thick Lower Sanzenkogel Fm., comprising the Early Tournaisian Siphonodella sulcata Zone to the Late Tournaisian Scaliognathus anchoralis Zone, occurs only in the western Graz Paleozoic. Marker beds within the Upper Sanzenkogel Fm. are the Trolp Phosphorite Bed (shale and lydite layer with phosphorite nodules) in the western parts of the Rannach Facies (Ebner, 1978; Bosic, 1998, 1999; Flügel, 2000).

Numerous conodont studies from the Graz Paleozoic have been published since the 1950s because the pelagic limestones of the Forstkogel Group/Rannach Facies yielded rich conodont faunas, which enable a fine-stratigraphic subdivision of the Paleozoic succession. Comprehensive and detailed conodont studies were conducted by Fritz Ebner (e.g., Ebner, 1976a, 1977a, 1977b, 1978, 1980a, 1980b) in the western and eastern Graz Paleozoic. Also, conodont studies from the western Graz Paleozoic were published, for example, by Flügel and Ziegler (1957); Kodsi (1967); Khosrovi-Said (1962); Flajs (1966); Nössing et al. (1977); Surenian (1978), Buchroithner et al. (1979); Buchroithner and Ebner (1981), and succeeding conodont biostratigraphic and geochemical, high-resolution studies at the DCB at Trolp provide important comprehensive data towards a better understanding of the Hangenberg Crisis at the DCB (Kaiser, 2005; Kaiser et al., 2008, 2009, 2020; Bojar et al., 2013).

An overview of localities with DCB successions in the western Graz Paleozoic (Section 3.1), where successions of the Famennian Steinberg Fm. and Tournaisian Sanzenkogel Fm. crop out, is followed by a detailed description of the currently easily accessible Trolp Quarry (Section 3.2).

3.1. OVERVIEW OF LOCALITIES

Faunal and lithologic markers from different lithologic successions from the Graz Paleozoic, such as phosphorite nodules, lydites, fissure fillings, conodont index fossils and mixed conodont faunas, can be easily correlated (Figure 4) and resulted in the reconstruction of the depositional environments (see Section 2.2). The stratigraphic gap between the Steinberg Fm. and the Lower Sanzenkogel Fm. increases from west to east (Figure 3), and complete Famennian and Tournaisian sediments, including the DCB, are recorded at Forstkogel in the Steinberg area (Surenian, 1977, 1978; Nössing, 1974b, Buchroithner et al., 1979; Ebner, 1980a), and at Eichkogel in the Rein area (Figures 4 and 5; Nössing, 1974a; Nössing et al., 1977). Successions at Eichkogel are situated in a forest area and currently covered by forest soil. At Forstkogel, Buchroithner et al. (1979) described several quarries where DCB successions crop out, among them the famous Trolp quarry (Figure 6).

Complete successions of the Steinberg Fm. in the western Graz Paleozoic, reaching a thickness of 70 m, are reported at Forstkogel, Eichkogel and Weihermühle in the Gratberg-Au area (Figures 4 and 5; Ebner, 1980a, 1980b). Complete successions of the Sanzenkogel Fm., reaching a thickness of up to 35 m from the Siphonodella sulcata to the Gnathodus bilineatus bollandensis conodont Zones, are reported at Forstkogel, while a hiatus is already reported from Weihermühle from the lower Tournaisian (Figure 4). The Tournaisian successions at Forstkogel were selected as type locality for the Lower Sanzenkogel Fm., established by Nössing (1974b, 1975). Sedimentologic investigations of the Lower Carboniferous (Mississippian) at Forstkogel were summarized by Ebner and Prochaska (1989).

In the western Graz Paleozoic, DCB successions at Trolp (see Section 3.2) and at Kanzelkogel (north of Graz, Kodsi, 1967) consist characteristically of condensed thin-bedded micritic limestones. At Kanzelkogel, the Upper Devonian and Lower Carboniferous (Mississippian) are separated by a several-decimetre-thick sequence of thin-bedded limestones with mixed Devonian and Carboniferous conodont faunas (Kodsi, 1967, Figure 12). However, the Protognathodus or Siphonodella index conodont faunas were not recorded at Kanzelkogel probably due to a stratigraphic gap, or condensation and an incomplete sampling. Unfortunately, these successions do no longer exist due to mining activity.

3.2. TROLP SECTION (FORSTKOGEL, STEINBERG AREA)

The Trolp section is an abandoned quarry (ÖK Sheet 163 Voitsberg) located at Forstkogel about 8 km from the city Graz, beside the street from Steinberg to Rohrbach (Figures 5 and 6). Trolp is the type locality of the Late Devonian conodont Polygnathus styriacus first described by Ziegler (1957). The Famennian-Tournaisian pelagic cephalopod limestones at Trolp have been intensively investigated due to rich conodont faunas, and sampling by Sandberg and Ebner in 1980 was followed by conodont studies of, for example, Ebner (1980a), Nössing (1975), Kaiser 2005, Kaiser et al. (2008, 2020). It was formally proposed as a candidate for the GSSP at the DCB, but the scarcity of macrofossils did not fulfill the criteria required (Ebner, 1980a; Ziegler and Sandberg, 1984a; Ji et al., 1989).

The tectonically inverted (upside-down) successions at Trolp are dated as Late Famennian expansa-praesulcata Zone to Late Tournaisian Gnathodus typicus Zone (Ebner, 1980a; Kaiser, 2005). A detailed, bed-by-bed or cm-by-cm sampling of DCB beds is required at Trolp due to the condensation of successions (Figures 7 and 8) which may produce a bias of the conodont record; re-numbering of DCB beds was necessary due to weathering and missing old numbers, as well as due to the high-resolution sampling of the successions. As shown in Figure 9, sample and bed numbers of Kaiser (2005) are correlated with bed numbers established by C. Sandberg and F. Ebner in 1980, and Ebner (1980a). The praesulcata and kockeli Zones, and the sulcata/kuehni Zone (DCB level) are readily recognized by their zonal index fossils (Ebner, 1980a, Kaiser, 2005; Kaiser et al., 2009, 2020; Figures 9 and 10a). The Middle praesulcata Zone of the standard conodont zonation (Ziegler and Sandberg, 1984b), recognized at Trolp by Sandberg and Ebner in 1980, Ebner (1980a), Bojar et al. (2013), Kaiser (2005), and by Kaiser et al. (2008) was not considered recently due to the asynchronous last occurrence of Palmatolepis gracilis gonioclymeniae defining the base of the Middle praesulcata Zone (see discussions in Kaiser, 2005; Kaiser et al., 2009; Corradini et al., 2016; Spalletta et al., 2017). The asynchronous last occurrence of this taxon resulted in different levels of the base of the Middle praesulcata Zone as previously recorded at Trolp by the above mentioned authors.

The eastern part of the quarry which is well accessible consists of well-exposed Lower Carboniferous beds, representing the lower member of the pelagic Sanzenkogel Formation (“Untere Sanzenkogel-Schichten”), and of bedded to massive upper Famennian beds, representing the Steinberg Formation. The succession consists of light-grey to brownish pelagic nodular cephalopod limestones and micritic Flaser-limestones of the Famennian Steinberg Fm. and Tournaisian Sanzenkogel Fm.; continuous limestone successions (Figures 7-10a and 10b) deposited during the Hangenberg Crisis interval are recorded at Trolp, which is globally characterized by black shales, sandstones or hiatuses. The stratigraphically lower part of the section consists of bedded to massive limestones of the praesulcata Zone, and a lithologic change is marked by a thin, ~1 cm thick shaly layer (basal Bed 10) at the base of thin-bedded limestones that disappears laterally (N 47º04’08’’, E 15º19’09’’). Bojar et al. (2013, Figure 3d) recorded a continuous limestone succession lacking this layer in a lateral, different position next to previous mentioned sampling points (N 47º04’29’’, E 15º19’15’’). This shaly or marly level was regarded by Flügel and Ziegler (1957) as a tectonic disturbance, which locally separated the Devonian and Carboniferous at Steinberg. Due to its stratigraphic position, Kaiser (2005) suggested an equivalent level of the Hangenberg Black Shale (HBS) since the lithologic change is accompanied by the major conodont biofacies change (Figure 11) indicating the main extinction phase of the Hangenberg Crisis at the costatus-kockeli Interregnum (ckI, after Kaiser et al., 2009 = mass extinction-based level within the Middle praesulcata Zone, correlative with the HBSE and HSSE). This interpretation is supported by the onset of conodont radiations and the entry of Protognathodus meischneri, Protognathodus collinsoni, Protognathodus semikockeli and Protognathodus kockeli immediately above in thin-bedded limestone successions conformably overlying the shaly layer (Kaiser et al., 2020; Figures 9 and 10a). The occurrence of stratigraphically leaked younger Carboniferous faunas mainly siphonodellids and Gnathodus in Devonian samples were recognized about at the level of this shaly layer, or in younger levels, respectively.

The base of the Carboniferous has been fixed at Trolp in Bed 16 by the first occurrence of index conodonts at the same stratigraphic levels (Figures 8, 9 and 12): Polygnathus purus subplanus, an index conodont for the base of the Tournaisian (e.g., Kaiser et al., 2009; Spalletta et al., 2017), as well as Siphonodella sulcata (M5) and Protognathodus kuehni, which were used to define the joint sulcata/kuehni Zone and the DCB (Kaiser et al., 2020; Becker et al., 2021). Conodont index fossils previously recorded by Ebner (1980a) and Nössing (1975) at Trolp, and their correlation with index fossils recorded by Kaiser (2005) and Kaiser et al. (2020) are shown in Figure 9; the biostratigraphic corrrelation of the Hangenberg Event and the DCB from the Carnic Alps and Rhenish Massif is shown in Figure 8. At stratigraphically younger levels, Siphonodella bransoni (Siphonodella duplicata M1), as well as Pseudopolygnathus multistriatus were recorded (Ebner 1980a). The condensed Tournaisian sequence is intercalated by the 20 cm thick Trolp Phosphorite Bed (Figures 4 and 14), marking the beginning of the Upper Sanzenkogel Fm. (base of the upper Tournaisian) after Nössing (1975), or the base of the middle Tournaisian as suggested by Kaiser (2005), respectively. It is correlative with the end of the erosional gap within parts of the Rannach Nappe, and interpreted as deepening and upwelling at the shelf margin at the base of the Scaliognathus anchoralis Zone (base of upper Tournaisian; Ebner, 1998; Ebner et al., 2000).

Conodont biofacies at Trolp (Figure 11) indicates a dominance of Bispathodus, Branmehla and Palmatolepis in pre-Hangenberg Crisis (pre-extinction) levels in the praesulcata Zone (Kaiser 2005). The conodont mass extinction in the ckI at Trolp (base of Bed 10), and major biofacies change to an impoverished Polygnathus-Protognathodus biofacies with stunted faunas, is connected to the extinction of Famennian taxa which predominated in the Famennian, such as Branmehla suprema, Bispathodus costatus, Palmatolepis gracilis expansa, and Pseudopolygnathus marburgensis trigonicus. The beginning of radiations in the basal kockeli Zone, recorded in the basal part of the thin-bedded limestones (Bed 10/11), is succeeded by faunal recovery and the 2nd radiation phase (Figure 10a) in the basal Lower Carboniferous sulcata/kuehni Zone. It is connected to the return of normal-sized conodont faunas, and an increasing abundance of pseudopolygnathids and siphonodellids, but the Polygnathus-Protognathodus biofacies remains in the Lower Carboniferous.

Faunal evolution during the Hangenberg Crisis is precisely documented at Trolp (Kaiser et al., 2020). A faunal assemblage of the Lower Protognathodus fauna, with Protognathodus meischneri and Protognathodus collinsoni, enters at a level with the initial lithoand biofacies change (Bed 10, Figures 9, 10a and 11), and the absence of Bispathodus costatus and Protognathodus kockeli at the same level in Bed 10 is interpreted as the event-based ckI. The succeeding entry of Protognathodus semikockeli recently established by Hartenfels et al. (2022) occurs immediately above, and the entry of Protognathodus kockeli s.str. slightly above marks the start of the kockeli Zone (Figures 9 and 10a). The Protognathodus fauna at the DCB at Trolp even occurs in association with Siphonodella praesulcata and Siphonodella sulcata which was also recorded by Sandberg et al. (1983). At Trolp, a phyletic lineage of the early protognathodids (Protognathodus meischneri, Protognathodus collinsoni, Protognathodus semikockeli, Protognathodus kockeli s.str., advanced Protognathodus kockeli, Protognathodus kuehni (see figure 6 in Kaiser et al. 2020) as proposed by Ziegler (1973), Ziegler and Leuteritz (1970) and Hartenfels et al. (2022) is recognized on the basis of the shape and the ornamentation of the cup of typical morphotypes (Figures 12 and 13). Many specimens, however, can be designated as atypical morphotypes (Figure 13; see Kaiser et al., 2019 unpublished) recorded by Nössing (1975), Ebner (1980a) and Kaiser et al. (2020).

Stable isotopes (δ13C) at Trolp (Kaiser et al. 2008; Figure 10b), used for the reconstruction of changes in the marine dissolved inorganic carbon reservoir, revealed a positive excursion in δ13Ccarb in the ckI and kockeli Zone to values of up to 3‰ (V-PDB). Two minor positive δ13Ccarb peaks were recorded in the praesulcata Zone, with values reaching 2.5‰ and 2.7‰, and one positive peak was measured in the Upper expansa Zone (Bispathodus ultimus Zone after Becker et al., 2021). The onset of continuously decreasing carbon isotope values is at the base of the sulcata/kuehni Zone (DCB). A δ13Corg excursion was measured from the sedimentary organic matter at Trolp, and oxygen isotope

values of conodont apatite at Trolp are remarkable high indicating low seawater temperatures (unpublished data). Element geochemistry conducted by Bojar et al. (2013) at Trolp indicates a decrease of detrital influence in the lower Tournaisian and was interpreted as transgression.

4. Discussion

4.1. LITHO,BIO, -AND CHEMOSTRATIGRAPHY AT TROLP AND CORRELATIONS OUTSIDE THE REGION

Characteristic markers and abundant conodonts at Trolp allow a high time-resolution study of major environmental changes around the DCB. These can be well correlated with coeval markers from continuous limestone successions as well as siliciclastic-dominated successions in North-America, Asia, Europe, North-Africa from pelagic, hemipelagic and neritic realms as discussed as follows, indicating environmental changes on a global scale.

4.1.1. LITHOSTRATIGRAPHY AND LITHOFACIES

At Trolp, a characteristic lithofacies change (Figure 7), from bedded or massive limestones to thin-bedded limestones (mudto wackestones, Kaiser 2005), is correlative with the global change to siliciclastics (HBS, HSS) and the anoxic episode of the Hangenberg Crisis in the extinction-based ckI and kockeli Zone (Figure 8-10). In continuous, pelagic limestone successions in the Carnic Alps (Grüne Schneid, Figure 8), and in hemipelagic to pelagic, more proximal successions in the Montagne Noire (La Serre É, e.g., Flajs and Feist, 1988; Feist et al., 2021) and in the Moravian Karst (Kumpan et al., 2021), a lithofacies change from bedded to thin-bedded, condensed limestones can be correlated with equivalent changes at Trolp. This lithofacies change is globally recorded, and correlative for example with black shales and/or sandstones in pelagic but more deeper settings in the Carnic Alps (Kronhofgraben), Pyrenees (Milles), as well as in hemipelagic settings in the Rhenish Massif (Figure 8), for example at Drewer, or in neritic successions in the Meseta and Anti-Atlas, Morocco (Ain Jemaa, El Atrous, M´Karig, Lambidia), and in the Alborz Mts., Iran (Mighan, Chelcheli), as reported by several authors (e.g., Cygan and Perret, 2002; Kaiser, 2005; Kaiser et al., 2006, 2007, 2011, 2013, 2018; Becker et al.,

2016b; Bahrami et al., 2021; Parvizi et al., 2021; Königshof et al., 2021).

The lithofacies change was caused by a global carbonate crisis at the end of the Devonian (Li et al., 2022) due to ceased carbonate sedimentation, enhanced Corg burial and marine anoxia during the Hangenberg Black Shale Event (HBSE), and the succeeding “Hangenberg glaciation” during the Hangenberg Sandstone Event (HSSE; e.g., Isaacson et al., 2008; Kaiser et al., 2006; Lakin et al., 2016). The main regressive phase which led to

the global deposition of Hangenberg Sandstone equivalents cannot be recognized by a clear sedimentological marker at Trolp, while the onset of the anoxic episode may be represented by the thin shaly layer (base of Bed 10), which is, however, only locally developed.

Tournaisian cherty sediments at Trolp (Trolp Phosphorite Bed, Figures 4 and 14), can be correlated with the basal middle Tournaisian as suggested by Kaiser (2005), and with the faunal turnover during the Lower Alum Shale Event which is globally characterized by a lithologic change from limestones to cherty rocks, the equivalents of the Rhenish Lower Alum Shale of the Middle crenulata Zone (Becker, 1993a, 1993b; Kaiser et al., 2018). It is coeval with transgressive shales, partly associated with phosphorite nodules, for example at Kronhofgraben, Carnic Alps (Schönlaub, 1969; Kaiser, 2005), Puech de la Suque and La Serre É, Montagne Noire (Lethiers and Feist, 1990; Kaiser, 2005; Feist et al., 2021), and at El Atrous, Jebel Ouaoufilal, M´Fis, M´Karig, Anti-Atlas, Morocco (Kaiser et al., 2011, 2013, 2018). The base of the upper Tournaisian anchoralis Zone is globally characterized by an end of an erosional gap, deepening and upwelling. It is correlative with cherty sediments all over the Graz Paleozoic and with the end of an erosional gap within the Rannach Nappe, interpreted as deepening and upwelling event by Ebner (1998) and Ebner et al. (2000).

4.1.2. BIOSTRATIGRAPHY AND CONODONT BIOFACIES

A continuous record of biofacies change at the DCB can be well recognized at Trolp, as well as in continuous limestones in the Carnic Alps, Grüne Schneid (Figure 11), and there is no hiatus in each phases of the Hangenberg Crisis. The dominance of the Protognathus fauna and the Polygnathus purus group in the kockeli and sulcata/kuehni Zones at Trolp, Grüne Schneid and elsewhere characterizes the 1st (FO of Protognathodus kockeli, = base of kockeli Zone) and 2nd (FO of Protognathodus kuehni, FO of Siphonodella sulcata M5, = basal Tournaisian transgression) faunal recovery phases after the Hangenberg extinction (Figure 14). The 3rd radiation phase is in the middle sulcata/kuehni Zone (FO of Siphonodella sulcata M4, Gattendorfia level = Tournaisian transgression).

At Trolp, the Hangenberg extinction-based ckI is recognized by the absence of Bispathodus costatus and Protognathodus kockeli at same level, and the extinction of almost all upper Famennian conodont taxa among the pseudopolygnathids, polygnathids, bispathodids, branmehlids and palmatolepids (see Kaiser et al., 2009, 2020). It is recognized by a major biofacies change from a Famennian Bispathodus-Branmehla-Palmatolepis biofacies to a characteristic, impoverished latest Famennian and earliest Tournaisian Polygnathus-Protognathodus biofacies. The ckI and the major conodont biofacies change are recorded at the base of the thin-bedded limestones at Trolp, as well as in continuous pelagic and hemipelagic limestone successions at the base of thin-bedded limestones in the Carnic Alps at Grüne Schneid (Kaiser, 2007; Figure 11), in the Moravian Karst at Krtny (Kumpan et al., 2021), and in the Montagne Noire at La Serre É (Flajs and Feist, 1988; Feist et al., 2021, Kaiser unpublished data) at correlative levels, and is caused by major environmental changes during the HBSE and HSSE.

Conodont faunal distributions and lithofacies at Trolp display distal, pelagic depositional environments, with sediment deposition in ca. 200 m water depth (e.g., Nössing, 1975). For comparison, lithoand biofacies, and the depositional environments in the Carnic Alps at Grüne Schneid (Schönlaub et al., 1988, 1991) are similar to that of Trolp, and a rich Protognathodus fauna occurs in both regions (Kaiser et al., 2020). Differences only related to single occurrences of Protognathodus specimens in pre-Hangenberg extinction levels (praesulcata Zone) at Grüne Schneid, with minor differences in biofacies, only. Accordingly, a slightly higher abundance of Bispathodus than of Palmatolepis occurs in the Famennian in pre-crisis levels at Trolp (Figure 11) which is an indicator for more shallow depositional environments according to previous biofacies models (see Kaiser et al. 2017). For example, Bispathodus abundantly occurs in neritic settings in Iran (Alborz Mts., e.g., Mighan, Chelcheli), Morocco (Tafilalt, e.g., El Atrous), and in hemipelagic settings in the Rhenish Massif (e.g., Hasselbachtal), Montagne Noire (e.g., La Serre, Puech de la Suque), Moravian Karst (e.g., Lesni Lom), as reported by several authors (e.g., Kaiser, 2005; Hartenfels and Becker, 2016; Feist et al., 2021; Kaiser et al., 2011, 2018; Bahrami et al., 2011, 2021; Königshof et al., 2021; Kumpan et al., 2021; Parvizi et al., 2021).

In contrast, siphonodellids, as indicator for more deeper environments (see Kaiser et al., 2017) are slightly more abundant at Trolp in pre-event levels when compared to Grüne Schneid, and Siphonodella is not recorded in the ckI and kockeli Zone at Grüne Schneid and elsewhere in the Carnic Alps. Protognathodids, previously suggested as an indicator for more shallow environments, occur at Grüne Schneid in pre-event levels, while this faunal group is not recorded in pre-event levels at Trolp and elsewhere in the Graz Paleozoic. In summary, faunal distribution of Protognathodus and Siphonodella indicates more shallow depositional environments at Grüne Schneid than at Trolp in pre-event levels, which is, however, not in accordance with the Bispathodus biofacies at Trolp as mentioned above.

In this respect, previous biofacies concepts for example of Sandberg and Ziegler (1979), Dreesen et al. (1986) and Ziegler and Weddige (1999), have to be re-considered (see Kaiser et al., 2017, 2020). Paleogeographic provincialism and migration pattern, competition and feeding (Mossoni et al., 2015), but also decreasing ecological niches around the DCB due to global shelf anoxia (Algeo et al., 1995; Li et al., 2022) and major eustatic sea-level fall with longterm and short-termed regressions (e.g., Johnson et al., 1985; Kaiser et al., 2011; Babek et al., 2016), as well as climate cooling (Issacson et al., 2008; Brezinski et al., 2009; Lakin et al., 2016), caused more likely characteristic faunal assemblages and biofacies changes. The occurrence of either Protognathodus or Siphonodella around the DCB is thus more likely related to major environmental changes during anoxic, transgressive and and regressive phases of the Hangenberg Event, and is regarded as biotic opportunism by several authors (e.g., Kaiser, 2005, Kaiser et al., 2008, 2009; Corradini et al., 2011; Spalletta et al., 2017). Thus, disregarding taxonomic uncertainties as discussed below, the occurrence of Protognathodus in pre-Hangenberg Event levels in the Carnic Alps and other regions in Europe, Asia and North-Africa could thus be linked with an opportunistic lifestyle during environmental changes probably related to the Drewer Event (early Hangenberg phase) in the praesulcata Zone (see Section 4.1.3).

4.1.3. STABLE ISOTOPES AND CHEMOSTRATIGRAPHY

Chemostratigraphy as a formal stratigraphic method is a useful tool for global correlations of chronological boundaries (see Sial et al., 2019), correlations of event-related successions (e.g., Algeo and Shen, 2024), and Hangenberg event-related successions at the DCB (Kaiser et al., 2016). In this respect, positive carbon isotope excursions recorded at Trolp from different levels in the Upper expansa (ultimus Zone) and praesulcata Zones, as well as in the ckI and kockeli Zone (Figure 10b) can be well correlated outside the region (Figure 10c), and thus indicate enhanced marine Corg burial and global perturbations of the cabon cycle. Accordingly, a minor positive peak in δ13Ccarb in the Upper expansa Zone at Trolp, with increasing values of more than 2‰, is widely known from the Middle (costatus Zone) and Upper expansa Zone as distinctive positive carbon isotope excursions of up to 3‰ in the Carnic Alps (Kaiser et al., 2008), Franconia, southern Germany (unpublished data, Kaiser et al., 2022), and the Moravian Karst (unpublished data, Kumpan et al., 2018), and were correlated previously with the small-scaled “Epinette” (Middle expansa) and “Etreoungt” (Upper expansa) bioevents (Kaiser et al., 2008). Increasing δ13Ccarb values and positive carbon isotope excursions in shallow-water successions were also recorded from the Middle and Upper expansa Zone in South China (costatus and ultimus Zone, Zhang et al., 2019), in northern Iran (Parvizi et al., 2021), and in Belgium (Kumpan et al., 2014b). In deeper pelagic settings (Kronhofgraben, Carnic Alps), a positive δ13Corg spike of 2‰ is recorded as well in the middle part of the Middle expansa Zone (Kaiser et al., 2008). Worldwide, transgressive shale and black shale deposits (Algeo et al., 1995; Li et al., 2022; Sahoo et al., 2023), and enhanced Corg burial episodes, as also indicated by an increasing δ13Ccarb baseline on a global scale (Zhang et al., 2019), are recorded in the expansa Zone probably connected with the Dasberg Crisis in the Lower and basal Middle expansa Zones (see Hartenfels, 2011), and with the Epinette and Etreoungt Events in the middle Middle and Upper expansa Zone (e.g., Kaiser et al., 2008; see also Di Pasquo et al., 2019).

Two minor positive δ13Ccarb peaks in the praesulcata Zone at Trolp, reaching values of more than 2.5‰, can be correlated with coeval positive peaks and/or increasing carbon isotope values of up to 3‰ recorded from the Carnic Alps at Grüne Schneid, Rio Boreado, Casera Malpasso, Großer Pal (Figure 10cA; Kaiser et al., 2006, 2008), and in Franconia (Kaiser et al., 2022, unpublished data), as well as by positive Corg peaks in the Rhenish Massif (Figure 10c B; Kaiser et al. 2006). These geochemical anomalies were interpreted as anoxic phases of the Drewer Event during the early phase of the Hangenberg Crisis in the praesulcata Zone (Kaiser et al., 2020). The Drewer Event was previously correlated with the regressive Drewer Sandstone and equivalents elsewhere (Streel, 1999; see also Kaiser et al., 2011, 2016), and was previously regarded as a short-term glaciation episode during a humid event at the LL-LE miospore transition (e.g., Streel et al., 2000). The Drewer level in the Rhenish Massif is characterized by at least two distinctive shale horizons (Becker et al., 2021). Wet climate and enhanced detritus influx ( = regressive Drewer Sandstone Event) probably caused enhanced marine bioproductivity and suceeding global shelf anoxia already in the praesulcata Zone, well before the HBSE, and may expressed by enhanced carbon burial as indicated by the positive δ13Ccarb peaks.

A significant increase of δ13Ccarb values by almost 2‰ at Trolp is at the base of the thin-bedded limestones associated with the conodont mass extinctions and the major conodont biofacies change in Bed 10 as discussed in Section 4.1.2. The positive δ13C Hangenberg-excursion in the ckI and kockeli Zone of values up to 3‰ was globally recorded (Figure 10c, e.g., Kaiser et al., 2006, 2016; Cramer et al., 2008; Kumpan et al., 2014a; Matyja et al., 2021; Qie et al., 2021; Becker et al., 2021). The onset of the positive excursion indicates global changes of the carbon cycle during the Hangenberg Crisis which was caused by enhanced Corg burial during the deposition of the HBS and equivalents (e.g., Kaiser et al., 2006). Enhanced bioproductivity, high rates of continental weathering and terrestrial nutrient influx due to erosion of organic matter during an upper Famennian sea-level lowstand (Babek et al., 2016) contributed to enhanced organic carbon burial. This finally resulted in a succeeding major, glacio-eustatic sea-level fall during wet and cold climate, time-equivalent to the deposition of the regressive Hangenberg Sandstone and global equivalents, and a glaciation pulse (Hangenberg glaciation) at the end of the Famennian (e.g., Kaiser et al., 2006; Isaacson et al., 2008; Brezinski et al., 2009; Lakin et al., 2016).

A distinctive decrease of δ13Ccarb values, starting in the lower Tournaisian sulcata/kuehni Zone at Trolp, reflects an overall long-term trend in the lower Tournaisian at same levels in numerous settings in European, Asian and North-American regions (Figure 10c, Kaiser et al., 2016, and references therein). This trend may be explained by a reduced sedimentation rate of marine organic matter, or by an enhanced delivery of light carbon into the oceans from continental weathering of organic-rich sediments during a short-term warming episode in the basal Tournaisian (Kaiser et al., 2006; Marshall et al., 2020). Decreasing carbon isotope values at Trolp were reported as well by Bojar et al. (2013). However, their data and interpretations are related to uncorrect positions of the kockeli Zone (= Upper praesulcata Zone) and the DCB (see Bojar et al. 2013, Figure 3a). The DCB is fixed by the authors by the late FO of Siphonodella sulcata previously recorded by Ebner (1980a) from his Bed 8 (see Figure 9). The authors ignore that the DCB was correctly fixed by Ebner (1980a) in his Bed 4 with the FO of Polygnathus purus subplanus as an index fossil for the DCB (Kaiser et al., 2009; Spalletta et al., 2017). Bed 4 of Ebner is correlative with Bed 16 of Kaisers studies, according to thickness of the limestones and the FO´s of Polygnathus purus subplanus, Protognathodus kuehni and Siphonodella sulcata (M5) in Bed 16. Thus, main parts of the kockeli Zone and the DCB of Bojar et al. (2013) can be regarded as younger, Tournaisian successions, and these biostratigraphic misinterpretations resulted in misinterpretations of geochemical proxies at Trolp (see Zhang et al., 2019, Figure 3). Further, the transgressive episode as interpreted from element geochemistry, and regarded as DCB-level by Bojar et al. (2013), is more likely the expression of the lower Tournaisian transgression (Gattendorfia level) in the middle sulcata/kuehni Zone (sulcata Event after Kalvoda and Kukal, 1987).

4.2. SCENARIOS FOR A POTENTIAL GSSP POSITION AT THE DCB

4.2.1. AN EVENT-BASED SCENARIO

The event-based, early phase of the Hangenberg Crisis, recognized at Trolp by bio,lithoand chemostratigraphy (Section 4.1), is the initial main Hangenberg mass extinction Event among many marine organisms (e.g., Kaiser et al., 2016, Figure 3). It is correlative with the global deposition of the transgressive Rhenish HBS (= HBSE) and its equivalents elsewhere, and the main anoxic phase of the Hangenberg Crisis, indicated by the first significant positive peak in δ13C  , δ13C (e.g., Kaiseret al., 2006). The global change from cephalopod limestones to black shales, or to high-condensed, thin-bedded limestones, has a worldwide correlation to numerous pelagic, hemipelagic and neritic settings in Asia, America and Europe (e.g., Kaiser et al., 2016; Becker et al., 2016a, 2021; Aretz et al., 2021, Kumpan et al., 2021; Matyja et al., 2021, Königshof et al., 2021; Komatsu et al., 2014; Qie et al., 2021).

The HBSE (former middle part of the Middle praesulcata Zone) can be recognized biostratigraphically for example by miospores, and is dated as the LN miospore Zone (Sandberg et al., 1972; Streel, 2009). The base of the LN miospore Zone provides correlation into the terrestrial realm and into macrofossil-free high-latitude settings. Furthermore, this level can be recognized by sequence stratigraphy and the drowning unconformity at the base of a maximum flooding interval (e.g., Van Steenwinkel, 1993a, 1993b; Kaiser et al., 2011). After summaries in Becker et al. (2016a), this episode of worldwide marine anoxia can be geochemically determined in various regions by organic geochemistry, inorganic carbon isotopes, magnetosusceptibility, element geochemistry, and by gamma ray spectroscopy (e.g., Marynowski and Filipiak, 2007; Kaiser et al., 2006; Cramer et al., 2008; Kumpan et al., 2014a, 2014b, 2015).

In siliciclastic-dominated successions, it is unclear which taxa range through the anoxic and regressive phase (HBSE, HSSE) due to fossil poor sediments, while in continuous pelagic limestone successions at Trolp and Grüne Schneid, the ckI (HBSE, HSSE) marks an important level. This level is characterized by main conodont extinctions, decreasing number of faunas, poor and stunted faunas, and major biofacies change. Alternative zonation concepts (Figure 12) do not consider the main Hangenberg extinction level instead, the level of the ck is the top part of the ultimus Zone (Spalletta et al., 2021), or the base of the extended kockeli Zone (Corradini et al., 2017).

The HBSE was previously proposed as a natural DCB (Walliser, 1984). However, an eventbased scenario is difficult to establish because 1. event-related successions are often accompanied by leakage, reworking, and impoverished faunas, and 2. hiatuses and Corg burial phases well before the HBSE, connected with the Dasberg, Etreoungt and Drewer Events as discussed in Section 4.1.3, resulted in the asynchronous onset of global black shale depositions and stratigraphic gaps (see figure 2a in Bojar et al., 2013).

4.2.2. THE BASE OF THE KOCKELI ZONE

The base of the kockeli Zone (= Upper praesulcata Zone), as well as the extended kockeli Zone (= ckI/ kockeli Zone to base of the bransoni Zone, see Figure 12) are not applicable as a potential DCB position as discussed as follows. The start of the kockeli Zone is probably within the upper LN miospore Zone, but the correlation into the terrestrial realm is difficult (e.g., Becker et al., 2016a). The entry of Protognathodus kockeli in its former definition of Bischoff (1957) related to diagnostic features as curvature (bending) and shape of the cup as well as platform ornamentation is affected by the restricted distribution of Protognathodus kockeli in many regions and widespread facies-controlled, discontinuous conodont sampling across the base of the kockeli Zone due to carbonate-poor deposits and late entry of the index fossil with the first carbonates (see Kaiser et al., 2020). The kockeli Zone is commonly not preserved due to the absence of Protognathodus kockeli at this level, as previously reported and additionally discovered recently in the Alborz Mts., Iran (e.g., Mighan, Chelcheli; Bahrami et al., 2021; Königshof et al., 2021; Parvizi et al., 2021), Carnic Alps (e.g., Kronhofgraben, Plan di Zermula; Kaiser et al. 2009; Spalletta et al. 2021), Anti-Atlas (e.g., M´Karig, El Atrous; Kaiser et al., 2011), Moravian Karst (Krtny; Kumpan et al., 2021), or due to an impoverished Polygnathus-Protognathodus biofacies at the end of the Famennian (partly without Protognathodus kockeli), respectively, for example in the Rhenish Massif (e.g., Oese, Oberrödinghausen; Kaiser et al., 2017; see Becker et al., 2021), Holy Cross Mountains (Kowala; Matyja et al., 2021), Illinois Basin, Indiana, USA (Over, 2021). Stratigraphic gaps are caused by the reduced formation of carbonates due to a global carbonate crisis during this time span, resulting from continuing Famennian sea-level fall (Babek et al., 2016) as the longterm regression continued until the Carboniferous, and from continuing major global cooling (e.g., Isaacson et al., 2008; Brezinski et al., 2009; Kaiser et al., 2022, unpublished data). The absence of the lower Protognathodus conodont assemblages (Protognathodus meischneri, Protognathodus collinsoni, Protognathodus kockeli) does not necessarily suggest stratigraphical gaps because erosional surfaces are missing (e.g., Rhenish Massif, Iran and elsewhere, see Becker et al., 2021; Königshof et al., 2021) but is caused by biofacies changes and the deteriorating environment after the early and middle Hangenberg phase.

The entry of Protognathodus kockeli could be affected by local fossil reworking or leakage, for example in the Carnic Alps (Corradini et al., 2017; see discussions in Kaiser et al., 2020). Reworking and mixing of conodonts at this level is the result of the major eustatic fall associated with the HSSE and the Hangenberg glaciation in the middle Hangenberg phase, while leakage is the result of paleokarstic phenomena in the Graz Paleozoic and Carnic Alps (e.g., Ebner, 1980b; Schönlaub et al., 1991).

Also, the uncertainties of the FAD of the “onerow” (Protognathodus semikockeli after Hartenfels et al., 2022) and the “two-rows” morphotype of Protognathodus kockeli (Protognathodus kockeli s.str. after Hartenfels et al., 2022), and the start of the kockeli Zone either during the middle (regressive episode of the HSSE, based on stable isotopes and the FO of Protognathodus semikockeli and Protognathodus kockeli at Trolp and Grüne Schneid, Kaiser et al., 2022 unpublished data), or late phase of the Hangenberg Crisis (after the facies break related to the HSSE (Hartenfels et al., 2022) have to be considered. The base of the kockeli Zone (former Upper praesulcata Zone) was previously established by the “two-rows” Protognathodus kockeli (Bischoff, 1957), while it was established by the “one-row” Protognathodus kockeli by Corradini et al. (2011). Based on conodont studies at Borkewehr (Rhenish Massif; Hartenfels et al., 2022) from limestone successions overlying the regressive Hangenberg sandstone equivalent, the FO of Protognathodus kockeli s.str. (two-rows) within the evolutionary lineage of Protognathodus semikockeli (one-row) and Protognathodus kockeli s.str. in the post-regressive Hangenberg phase is proposed as future GSSP for the DCB.

4.2.2.1. CONSIDERATIONS OF PROTOGNATHODUS

Taxonomic and biostratigraphic uncertainties within typical morphotypes of the early Protognathodus faunas are known for many years and were re-mentioned in Corradini et al. (2011), Kaiser et al. (2019 unpublished, 2020) and Hartenfels et al. (2022). Originally, Bischoff (1957), Ziegler and Leuteritz (1970), and Ziegler (1969) only considered randomly the high spectrum of Protognathodus morphotypes. Generally, the high spectrum of intermediate morphotypes of Protognathodus species attests to the completeness of the stratigraphic record, as at Trolp, Grüne Schneid (Kaiser et al., 2020), Puech de la Suque (France) and Borkewehr (Hartenfels et al., 2022). However, taxonomic uncertainties are related to intraspecific variability concerning juvenile-adult and early-advanced forms, as well as intermediates between all species of the early Protognathodus fauna (Kaiser et al., 2019, 2020; Hartenfels et al., 2022). This was previously recognized in the early siphonodellids, too; the high spectrum of intermediate morphotypes of Siphonodella praesulcata and Siphonodella sulcata at La Serre É and elsewhere have been taxonomically re-evaluated (Kaiser and Corradini, 2011), and morphotype groups of both Siphonodella praesulcata and Siphonodella sulcata were established and widely used (Figure 12, see Section 4.2.3.1).

Diagnostic features of the Protognathodus fauna are related to the cup morphology (curvature and shape of the cup) and platform ornamentation (Bischoff, 1957; Ziegler and Leuteritz, 1970), and a phyletic lineage of the early protognathodids (Ziegler, 1973), based on the typical morphotypes and their transitional forms (intermediates), was suggested at Trolp and Grüne Schneid (Kaiser et al., 2020), comparable to Puech de la Suque from Bed 10 overlying the HBS (Hartenfels et al., 2022). Accordingly, a well-exposed succession of Protognathodus meischneri, Protognathodus collinsoni, Protognathodus semikockeli, Protognathodus kockeli and Protognathodus kuehni could be recorded in these two regions. After Kaiser et al. (2020), the typical morphotype of Protognathodus kuehni at Trolp has at least on one side of the cup distinctive transverse ridges running radial from the platform edge to the carina, and the shape of the cup is wide and slightly asymmetric according to the original diagnosis of Ziegler and Leuteritz (1970).

Three new Protognathodus species are defined based on the arrangement of the cup ornamentation which focused on the developing of row of nodes (Hartenfels et al., 2022), without considering the shape and curvature of the cup, and the platform ornamentation `scattered nodes´ which are discussed by the authors, only (see also Corradini et al., 2011, and Kaiser et al., 2020). Transitional forms, transitional forms between Protognathodus kockeli and Protognathodus kuehni, but also morphotypes previously determined as Protognathodus kuehni are included in Protognathodus kockeli s.str. (Hartenfels et al., 2022), and thus enlarged the spectrum of morphotypes to be determined as Protognathodus kockeli.

Atypical morphotypes of `Protognathodus´ species

with a high morphological variability, related to the shape of cup and the ornamentation of the platform, are recorded in the Graz Paleozoic (Figure 13), Carnic Alps, and other regions (see Kaiser et al., 2020; Corradini et al., 2011, Hartenfels et al., 2022), and as stated in Hartenfels et al. (2022), “... atypical characteristics [within the genus Protognathodus],...result in a significant morphological complexity”. At Trolp, within all species of Protognathodus, typical and also atypical morphotypes at Trolp are observed at the same stratigraphic levels in the uppermost Famennian and lower Tournaisian (Nössing, 1975; Kaiser et al., 2020) which were partly included in the three new Protognathodus species (Protognathodus semikockeli, Protognathodus kockeli s.str., Protognathodus kockeli) recently established (Hartenfels et al., 2022). For example, data at Trolp (Figure 13) indicate that beside specimens with a Protognathodus-like shape of cup, instead extremely narrow, atypical (instead of wide in comparison to holotypes) outlines of the shape of the cup in all species of the protognathodids are recorded from the kockeli to bransoni Zones (Kaiser et al., 2020). Other morphologic characteristics are an extended posterior tip, a strong asymmetry due to an extended outer side of the cup, an enlarged outer margin which is commonly broken, or a strongly bent carina, in contrast to specimens with a straight carina.

Similar to the group of  `siphonodelloids´, which has morphological features of Siphonodella, Polygnathus and Pseudopolygnathus, and were established previously (Becker et al., 2013), the morphological complex group of Protognathodus faunas with atypical diagnostic features could be regarded as Protognathodus–like `protognathoids´, and can have morphological features of the late Protognathodus, of Bispathodus and of Gnathodus regarding platform ornamentation and shape of the cup as discussed below. Also, homeomorphic morphological features could be problematic because the early protognathodids can have overlapping ranges with the late Protognathodus (e.g., Protognathodus praedelicatus, see Habibi et al., 2008; see discussion in Hartenfels et al., 2022), with Bispathodus (e.g., Bispathodus stabilis, Ziegler 1973, 1974) and with Gnathodus (see below). These atypical faunas (`protognathoids´) probably originated during the early phase of the Hangenberg Crisis (Drewer Event) in the praesulcata Zone, with ?Protognathodus meischneri and ?Protognathodus collinsoni, and evolved probably during the anoxic (HBSE) or the regressive phase (HSSE) of the Hangenberg Event, with ?Protognathodus semikockeli and ?Protognathodus kockeli (see Figure 13), and spreaded around the DCB and in the lower Tournaisian. Their occurrence could be related to an opportunistic lifestyle during major environmental changes related to the Hangenberg Event, which can be regarded as “bottleneck” for the evolution of the Idiognathodontidae to which Protognathodus belong. Because protognathoids spread in the kockeli Zone, but mainly in the sulcata/kuehni Zone, or probably even later (Figure 14; see Corradini et al., 2011; Kaiser et al., 2020), the extended kockeli Zone (base of the former kockeli Zone to the base of the bransoni Zone) is not applicable due to the spread of these still unknown faunas regarding their stratigraphic and distributional pattern, and also due to their unknown ancestor and descendent relationship.

At Trolp, a stratigraphically leaked younger fauna is recorded by single specimens of Gnathodus at the same stratigraphic level as the early Protognathodus fauna, and leaked Gnathodus were found in the sulcata/kuehni Zone, or in older (praesulcata Zone) levels. Mixed faunas and homeomorphy (see below) within the Idiognathodontidae family (Gnathodus and Protognathodus) could produce a bias of the faunal record in the lower Tournaisian, especially in 1) condensed successions, 2) in stunted or juvenile-dominated assemblages, 3) in Lower Carboniferous, siliciclastic-dominated successions with fossil-poor limestones and scarce conodont faunas, without associated other diagnostic faunas and/or co-occurrence with `homeomorphic´faunas (e.g., Kaiser et al., 2020; see Königshof et al., 2021 and Parvizi et al., 2021). At Shahmirzad, (Alborz Mts., northern Iran), Protognathodus kockeli even occurs at same level as Gnathodus cuneiformis and Protognathodus praedelicatus in the upper Tournaisian Lower typicus Zone to anchoralis-latus Zone (Habibi et al., 2008, see discussion in Hartenfels et al., 2022). In contrast, leaked faunas among the late (Siphonodella bransoni, Siphonodella duplicata, Siphonodella quadruplicata) siphonodellids are generally easy to distinguish from the early (Siphonodella praesulcata, Siphonodella sulcata) siphonodellids, as at Trolp in mixed faunal assemblages around the thin-bedded limestone successions (Figure 10a).

Lane et al. (1980) provided an extended discussion of the differences between Gnathodus and Protognathodus and the differences in cup shape and cup terminations between early and late Protognathodus, with diagnoses of both genera. However similar diagnostic features between the early Protognathodus fauna, the late Protognathodus fauna ([Protognathodus cordiformis], Protognathodus praedelicatus, see also discussion in Hartenfels et al., 2022), and Gnathodus (Ziegler, 1973; Lane et al., 1980; Dzik, 1997, 2006), and their overlapping ranges in the lower, middle or upper Tournaisian must be considered.

Examples of atypical morphotypes or homeomorphy as discussed as follows are partly described in synonymies for Protognathodus kockeli (Hartenfels et al., 2022). Atypical `Protognathodus´ specimens can have a cup-morphology, which is similar to the late protognathodids regarding the termination of the margin and the wideness of the cup, and also confusion with Gnathodus may be related to the platform ornamentation of Protognathodus kockeli, where, for example, nodes as ridge-forming row of “denticles”, partly with an adcarinal through, are developed (e.g., Corradini et al., 2011, plate 1, figure 18; Kaiser et al., 2020, plate 2, figure 1), or incipient parapet-like morphologies on one side of the platform and a strong asymmetry, e.g., in Sardinia, Corradini et al. (2011, pl. 1, Figure 15),

at Trolp, Kaiser et al. (2020; plate 3, figure 19, 22), in Morocco, Lalla Mimouna, Becker et al. (2013, plate 3, figure 9, with an unusual wide posterior carina, similar to specimen from Oklahoma figured by Over, 1992, see below), or in China, Wang and Yin (1988,; Figure 15).

Protognathodus specimens from the DCB transition were previously determined as Gnathodus in the Rhenish Massif (e.g., Luppold et al., 1984, pl. 6, Figure 2) or as Protognathodus praedelicatus in the Carnic Alps or Montagne Noire (Flajs and Feist, 1988; Schönlaub et al., 1988). Subjective determinations of Protognathodus species from North Africa (Korn et al., 2004), and from China, are evident (Wang and Yin, 1988: pl. 22, Figure 1-19; and compare synonymy list in Corradini et al., 2011, and discussions therein, p. 25). Over (1992) described Protognathodus n. sp. from the kockeli Zone (= Upper praesulcata Zone) from North-America, which can be distinguished after the author from Gnathodus by its relatively symmetrical platform and lack of the offset of the outer and inner anterior platform margins. Specimens of Protognathodus kockeli from this region (Oklahoma, Woodford Shale) have a wide posterior carina, similar to that figured specimen from Morocco (see above, Becker et al., 2013). This species is regarded by Over (1992) as an intermediate between Protognathodus kockeli and Protognathodus sp. B. Over (1992) suggested that Protognathodus sp. B may represent the ancestor of Gnathodus punctatus (with FAD in the late Kinder hookian) which has a broadly expanded posterior carina. Similar specimens have been illustrated from La Serre, Montagne Noire (Flajs and Feist, 1988, pl. 9, Figure 9, 10), and fused nodes on the platform (plate 9, figure 10) could be transitional to Protognathodus praedelicatus but was determined as Protognathodus kockeli after Corradini et al. (2011).

Furthermore, juvenile specimens of Protognathodus meischneri and Bispathodus stabilis are difficult to distinguish (Ziegler, 1973). Both have overlapping ranges in the ?upper/uppermost Famennian and lower Tournaisian, and confusions with stunted or juvenile ?Protognathodus meischneri and Bispathodus stabilis occur at Trolp and elsewhere (Kaiser et al., 2020; see also Corradini et al., 2016). Ziegler et al. (1974) have demonstrated the ancestry of Protognathodus; Bispathodus stabilis Morphotype 2, which has an expanded basal cavity extending to the posterior tip of the blade (end of the platform), was shown to be the ancestor of the Protognathodus lineage by the authors, illustrated on plate 3, Figure 2. Bispathodus stabilis Morphotype 2 lacks only the wider expansion of the cup to distinguish it from Protognathodus meischneri, and Protognathodus meischneri has a distinctly flat basal cavity.

However, as stated in Kaiser et al., (2020), the shape of the cup of adult specimens of ?Protognathodus meischneri at Trolp (Figure 13, narrow shape of cup) can be similar to adult specimens of Bispathodus stabilis stabilis (see Hartenfels, 2011), and thus confusions in the determinations of both species exist, also in regard to the ornamentation of the platform which can be similar between Protognathodus collinsoni and Bispathodus stabilis bituberculatus and Bispathodus aculeatus, or between Bispathodus stabilis stabilis and Protognathodus meischneri (Hartenfels, 2011; Hartenfels and Becker, 2016; see Hartenfels, 2011, plate 31), e.g., Matyja et al., (2021, Figure 5A.d: aff. Protognathodus meischneri); Königshof et al., (2021); Becker et al., (2013, plate 3, Figure 5: Protognathodus meischneri [?with initial nodes on the platform]); Matyja et al., (2021, with a doubtful Protognathodus, Figure 7a-c [similar to her Bispathodus aculeatus]); Kumpan et al., (2021, Protognathodus cf. meischneri, Figure 5.7 [with an initial node on the platform]). The doubtful specimens of Protognathodus meischneri or Protognathodus collinsoni, partly discussed in Hartenfels et al., (2022), occur in pre-extinction levels (praesulcata Zone) or in the Hangenberg Crisis level as single specimens in hemipelagic or neritic settings at La Serre É, Montagne Noire (Flajs and Feist, 1988; Kaiser, 2005, 2009; Feist et al., 2021), Lesní Lom, Moravian Karst (Kumpan et al., 2021), Mighan and Chelcheli, Alborz Mts., Iran (Parvizi et al., 2021; Bahrami et al., 2021; Königshof et al., 2021), Lalla Mimouna, Anti-Atlas, Morocco (Becker et al., 2013), Kowala, Holy Cross Mountains, Poland (Matyja et al.2021), Grüne Schneid, Carnic Alps (Corradini et al., 2017; Kaiser et al., 2020). Also, the occurrence of questionable Protognathodus meischneri and Protognathodus collinsoni in pre-Hangenberg Event levels from different regions (e.g. at La Serre E) must be considered in respect to their FAD and ancestor relationship to the new species Protognathodus semikockeli recently established (see also discussion in Hartenfels et al., 2022).

In summary, beside the diagnostic features of the shape and curvature of the cup of the Protognathodus fauna see Figure 13), morphological complexity of the ornamentation of the platform may resulted in subjective determinations. Therefore, these morphological complexities in shape of the cup and ornamentation of the platform (`protognathoids´) should be focused on in future studies, comparable to the `siphonodelloids´ which are discriminated from the homeomorphic siphonodellids.

4.2.3. THE DEVONIAN-CARBONIFEROUS BOUNDARY AND CURRENT INDICATORS IN THE SULCATA/KUEHNI ZONE

At the base of the sulcata/kuehni Zone, the 2nd faunal recovery among conodonts (Figure 14) is connected with an increasing abundance of pseudopolygnathids and siphonodellids (e.g., Kaiser, 2005; Kaiser et al., 2017), and with a change from poor, rare and stunted conodont faunas in the ckI/kockeli Zone (1st radiation phase) to more abundant and normal-size faunas indicating the post-extinction/ post-crisis level. The 1st and 2nd recovery phases (see Figure 12) each coincide with global post-extinction sea-level rises and deepening episodes, as interpreted, for example, by microfacies at Trolp (Kaiser, 2005) and Grüne Schneid (Kaiser, 2007). At Trolp and Grüne Schneid, it is characterized by more fine-grained micritic limestones – mudand wackestones when compared to underand overlying beds which are packand floatstones. The 2nd faunal recovery phase are about time-equivalent to the return to full carbonate depositional conditions and continuing in the lower Tournaisian (middle sulcata/kuehni Zone, Gattendorfia level, 3rd faunal recovery phase). The onset of a distinctive decrease of δ13Ccarb values started in the sulcata/kuehni Zone at Trolp with the onset of the 2nd radiation phase, and can be correlated to numerous other settings in European, Asian and North-American regions (see Section 4.1.3).

Between the 1st (base of kockeli Zone) and 2nd recovery phases, a short regressive episode (DCB regression) corresponds to significant survivor extinctions in ammonoids, brachiopods, trilobites and foraminifers at the end of the kockeli Zone to a roughly contemporaneous terrestrial extinction, and to abruptly decreasing carbon isotope values globally recorded (Figure 14; e.g., Kaiser et al., 2006, 2016). The level of the DCB regression enables a correlation of pelagic (conodonts, ammonoids) with shallow-water environments (brachiopods, corals, see review in Kaiser et al., 2016), and can also be correlated into the terrestrial realm by the LN/VI miospore Zone boundary (e.g., Clausen et al., 1994). The DCB regression is indicated at Trolp by microfacies change from micritic limestones to bioclastic packto floatstones reported by Kaiser (2005) and Bojar et al. (2013, Figure 3c) although the latter authors assigned this level to the Upper praesulcata Zone based on the uncomplete conodont record as mentioned in Section 4.1.3.

The different morphotypes of Siphonodella sulcata, especially morphotypes 4 and 5, are globally widespread (see Section 4.2.3.1), and Protognathodus kuehni has a widespread geographic distribution which is reported in numerous studies from the sulcata/kuehni Zone (Corradini et al., 2011; Kaiset et al., 2020; see discussion in Hartenfels 2022). However, this taxon can have, as well as Protognathodus kockeli, a late entry, for example at the GSSP at La Serre É (e.g., Flajs and Feist, 1988; Feist et al., 2021). Therefore, the sulcata/kuehni Zone has a high global correlation potential and the FO´s of Siphonodella sulcata and Protognathodus kuehni can be correlated to many other regions globally (see Kaiser and Corradini, 2011; Corradini et al., 2011; and extended references in Kaiser et al., 2020; Königshof et al., 2021; Qie et al., 2021; Corradini et al., 2021; Spalletta et al., 2021; Kumpan et al., 2021; Matyja et al., 2021; Becker et al., 2021; Feist et al., 2021; Komatsu et al., 2014). While the extended kockeli Zone leaves the event-related successions and the DCB undivided, as recently used for example at Grüne Schneid, La Serre E, and Milles (Feist et al., 2021, Spalletta et al., 2021; Aretz et al., 2021; Figure 12), a precise conodont biozonation at the DCB can be applied by the use of Siphonodella sulcata M5 and Protognathodus kuehni. At Trolp, the typical morphotype of Protognathodus kuehni and Siphonodella sulcata M5 first occur at same stratigraphic levels (Figure 10a, 12). Also, Siphonodella sulcata ?M4 was figured in Nössing (1975) in the basal sulcata/kuehni Zone. The sulcata/kuehni Zone can thus be established in the Graz Paleozoic, and at correlative levels in the Carnic Alps and Moravian Karst, based on the co-occurrence of both Siphonodella sulcata M5 and Protognathodus kuehni (Figure 12; Kalvoda et al., 2015; Kaiser et al., 2020; Kumpan et al., 2021). Sampling of continuous limestone successions there supports the coeval FAD of both taxa at the base of the Tournaisian. After Kaiser et al. (2020), this supports the use of a joint biozone in event-related successions, because their co-occurrences are evidenced from these three different regions; the utility of a joint biozone enhances the possibility of using either the biostratigraphic index Siphonodella sulcata or Protognathodus kuehni, if one or the other has a late entry or is as yet unrecorded in a region due to facies-controlled rare or diachronous occurrences (e.g., Kaiser et al., 2009; Corradini et al., 2011; Spalletta et al., 2017).

4.2.3.1. CONSIDERATIONS OF SIPHONODELLA

Within siphonodellids, the abundance of intermediate forms attests to the completeness of the stratigraphic record as at La Serre É (Figure 12; between Siphonodella praesulcata, Siphonodella sulcata M4 and M5, and advanced siphonodellids); the differences between Siphonodella praesulcata and Siphonodella sulcata are stated in Sandberg et al. (1972). However, the occurrence of intermediate forms may have resulted in subjective determination of the first occurrence of Siphonodella sulcata (e.g., Ziegler and Sandberg, 1996; Kaiser, 2009; Becker et al., 2016a), but Siphonodella-morphotype groups can be clearly distinguished and recorded the spectrum of morphotypes and intermediates, which spread in the lower Tournaisian (Kaiser and Corradini, 2011). These groups are used to distinguish between Siphonodella praesulcata (M2, M3) and Siphonodella sulcata (M1, M4-M7), and also between early (M4, M5) and advanced forms (M1, M6, M7) of Siphonodella sulcata, the latter are recorded especially in younger Tournaisian successions at La Serre É (Kaiser and Corradini, 2011). The morphotype groups are based mainly on the curvature of the carina, a feature which had been used for the current GSSP level at La Serre É (Flajs and Feist, 1988, see also Sandberg et al., 1972), as well as shape of the platform, a pseudokeel which contains a pit (basal cavity), and the ornamentation. Siphonodella sulcata Morphotype 4 already has slight constrictions anteriorly but is otherwise similar to Siphonodella sulcata M5. The Siphonodella sulcata M1, M6 and M7 are representatives of advanced forms, and are intermediates between Siphonodella sulcata and Siphonodella bransoni (former Siphonodella duplicata M1) because they are strongly bended and have constrictions anteriorly. Siphonodella sulcata M4 and M5 are easy to recognize, most widespread, and common in North America, Asia, and Europe (Figure 12, Kaiser and Corradini, 2011, figure 7; see Kaiser et al. 2020). The globally most widespread Siphonodella sulcata M5 is the earliest morphotype which occur at the base of the Tournaisian, for example at Trolp, Grüne Schneid (Kaiser et al., 2020), La Serre É (Kaiser, 2009; Kaiser and Corradini, 2011; Feist et al., 2021), Lalla Mimouna (Anti-Atlas, Morocco, Becker et al., 2013), Lesni Lom (Moravian Karst, Czech Republic; Kalvoda et al., 2015), Kowala (together with Protognathodus kuehni and Polygnathus purus subplanus, Holy Cross Mountains, Poland; Malec, 2014; Matyja et al., 2021), Borkewehr (Rhenish Massif, Hartenfels et al., 2022). Siphonodella sulcata M4 was found in lower Tournaisian successions, for example at Bou Tlidat (Maider, Anti-Atlas, Morocco; collection S.I. Kaiser; see Kaiser et al., 2011; Becker and Kaiser, 2023), Mighan (Alborz Mts. northern Iran, Parvizi et al., 2021) and at Shahmirzad (Alborz Mts.; Habibi et al., 2008). Siphonodella sulcata M4 evolved from Siphonodella sulcata M5 based on the record at La Serre É, and represents most probably the marker for the 3rd radiation phase in the sulcata/kuehni Zone at the Gattendorfia level (sulcata Event). Siphonodella bransoni is also recorded from successions at M´Karig, Tafilalt, Anti-Atlas (collection S.I. Kaiser; see Kaiser et al., 2011; Becker and Kaiser, 2023). For the occurrence of Siphonodella praesulcata in neritic settings in Morocco, Iran, China, Vietnam, see Kaiser (2005), Kaiser et al. (2011), Becker et al. (2013), Komatsu et al. (2014), Parvizi et al. (2021), Bahrami et al. (2021), Qie et al. (2021). In shallow-water settings at Gedongguan, Muhua, Nanbiancun and Dapoushang (South China), Siphonodella sulcata occurs, also in association with Protognathodus kuehni, at the base of the Tournaisian (Qie et al., 2021), and Siphonodella sulcata is also reported from shallow-water settings for example from northeastern Vietnam (Komatsu et al., 2014). In summary, siphonodellids also occur in neritic successions, and the sulcata/kuehni Zone can be established in pelagic, hemipelagic and neritic settings based on Siphonodella sulcata and/or Protognathodus kuehni. However, an evaluation of various records from different paleogeographical settings of the earliest Morphotype 5 and Morphotype 4 of Siphonodella sulcata at the base of the Tournaisian is needed (e.g., Becker et al., 1984; Qie et al., 2021; Komatsu et al., 2014; Matyja et al., 2021, Hartenfels et al., 2022; see also Aretz et al., 2021, and references therein).

Siphonodella sulcata M4 and M5 can be clearly

distinguished from Siphonodella praesulcata, from the late siphonodellids (see Section 4.2.2.1), and from Siphonodella-like `siphonodelloids´; the latter have overlapping ranges with Siphonodella praesulcata mainly in the praesulcata Zone, as documented e.g., in Becker et al. (2013). The `siphonodelloids´ originated and evolved in pre-Hangenberg event levels in the expansa Zone probably connected with enhanced Corg burial phases and/or shallowing phases during the Epinette and Etreoungt Events (for environmental changes during the events see Kaiser et al., 2008), and spread in the praesulcata Zone; its occurrence is related to biotic opportunism comparable with the spread of `protognathoids´ during the Hangenberg Event (Figure 14).

`Siphonodelloids´ are most widespread in hemipelagic or shallow-water settings, for example in Morocco (e.g., Becker et al., 2013), Iran (e.g., Parvizi et al., 2021), and probably at La Serre É (Figure 14).

5. Summary

Characteristic lithologic, chemoand biostratigraphic markers at the DCB in the western Graz Paleozoic are well displayed at the famous Trolp section in the Steinberg area. Geochemical and lithologic characteristics, conodont extinctions, the end of mass extinctions and radiations, and the coeval FO of index conodonts are recognized at Trolp. These characteristic markers can be easily correlated globally, and afford the possibility for correlations into the pelagic, hemipelagic and neritic realms.

A hiatus is unlikely at Trolp. This is supported by the complete record of conodont zones during the Hangenberg Crisis, the abundance of intermediate forms which attests to the completeness of the conodont stratigraphic record, and by the absence of an erosional (sedimentary) gap. Conodonts are abundant and enable a high-resolution conodont stratigraphy, and radiation and extinction events are well recorded. The main Hangenberg extinction level in the former Middle praesulcata Zone (costatus-kockeli Interregnum = ckI) is easily recognized at the base of thin-bedded limestones, and is accompanied by a succeeding major conodont biofacies change and an impoverished and stunted Polygnathus-Protognathodus fauna. The index conodonts Siphonodella praesulcata and Siphonodella sulcata occur in the DCB beds at Trolp together with the early Protognathodus fauna (Protognathodus meischneri, Protognathodus collinsoni, Protognathodus semikockeli, Protognathodus kockeli, Protognathodus kuehni) in the continuous, thinly bedded limestone succession.

The DCB at Trolp is recognized by the first occurrence of both Protognathodus kuehni and Siphonodella sulcata M5 at same stratigraphic level and document the start of the sulcata/kuehni Zone. The supplementary diagnostic conodont fauna for the Lower Carboniferous, Polygnathus purus subplanus first appears at this level, too. Siphonodella sulcata Morphotype 5 and Protognathodus kuehni have a high correlation potential and a congruent FAD which can be used for defining the start of the joint sulcata/kuehni Zone; their phyletic lineages can be reconstructed in several regions. Thus, the use of the earliest morphotypes of Siphonodella sulcata M5 and probably of Siphonodella sulcata M4 and of Protognathodus kuehni, contribute to global correlations and biozonations. Morphotype 5 and Morphotype 4 are the earliest Siphonodella sulcata-morphotype to occur widely in Europe, North-Africa, Asia and North America at the base of the Tournaisian, without noteworthy overlapping ranges with `siphonodelloids´ (Figure 14).

A well-exposed succession of the early Protognathodus fauna, and the earliest morphotypes of Protognathodus meischneri, Protognathodus collinsoni, Protognathodus semikockeli, Protognathodus kockeli, and Protognathodus kuehni are recorded at Trolp. However, atypical `Protognathodus´ morphotypes – proposed herein as Protognathodus-like `protognathoids´ regarding diagnostic features of the cup and ornamentation of the platform (Figure 13) and partly evaluated by Hartenfels et al. (2022) occur at Trolp and elsewhere around the DCB. Comparable with the Siphonodella-like `siphonodelloids´ which evolve and spread during late Famennian environmental changes in the expansa and praesulcata Zones connecting with the Epinette/Etreoungt and Drewer Events, the atypical morphotypes evolved and spread somewhat later, during late/latest Famennian and earliest Tournaisian environmental changes connecting with the Drewer Event in the praesulcata Zone (early phase of the Hangenberg Event) and in the ckI and kockeli Zone connecting with the middle and late phase of the Hangenberg Event (Figure 14). The record of their precise stratigraphic and distributional pattern is important for future evaluation of phylogenetics because ancestor and descendent relationships of this faunal group are still unclear. Also, homeomorphy between the early and late protognathodids, gnathodids, and bispathodids, and a high morphological spectrum within the genera, could produce a bias of the conodont record in event-related successions. This is especially important due to overlapping ranges between these faunal groups at the DCB as well as in the Tournaisian. Thus, the extended kockeli Zone and the use of Protognathodus kockeli proposed recently as index fossil for the DCB, raises more questions than answers. The extinction-based ckI (HBSE), and the base of the former kockeli Zone (= Upper praesulcata Zone) are, however, needed for a precise biozonation scheme.

Carbon isotopes (δ13Ccarb) at Trolp show characteristic positive excursions and trends in the early, middle and late phases of the Hangenberg Crisis that can be readily correlated to many other regions worldwide. Accordingly, two positive δ13Ccarb peaks in the praesulcata Zone represents Corg burial episodes of the early Hangenberg phase (Drewer Event). The onset of a distinctive positive carbon isotope excursion in the early/ middle Hangenberg phase correlates with the initial conodont mass extinction episode (ckI) of the Hangenberg Crisis during globally widespread deposition of black shales (HBS), which precedes the main regressive episode during globally widespread deposition of regressive sandstones (HSS). This main anoxic episode of the Hangenberg Crisis (HBSE) is recorded in the thinly bedded limestone succession, and its onset may recorded in a locally developed thin shaly layer at the base of this succession. Because the pelagic fossil record of the regressive phase (HSSE) is generally unknown due to globally widespread, conodont-poor siliciclastics, stratigraphic gaps, it is recently unclear whether the main regressive phase of the Hangenberg Crisis is recorded in the thinly bedded limestone succession and needs further studies. The positive carbon isotope excursion in the late Hangenberg phase (kockeli Zone) at Trolp is time-equivalent to the first post-extinction recov ery phase; the onset of a distinctive decrease of carbon isotope values in the sulcata/kuehni Zone is coeval with the second conodont radiation phase; it can be correlated with same trends in many other successions worldwide.

6. Conclusions

The western Graz Paleozoic is of global significance because the phases of the Hangenberg Crisis at the DCB can be well reconstructed in continuous limestone successions at Trolp, and significant markers in bio,lithoand chemostratigraphy can be well correlated outside the region. Conodont extinctions (costatus-kockeli Interregnum), conodont radiations and end of mass extinctions can be recognized at Trolp. The earliest morphotypes of Siphonodella sulcata M5, as well as of Protognathodus meischneri, Protognathodus collinsoni, Protognathodus semikockeli, Protognathodus kockeli and Protognathodus kuehni are recorded at Trolp. The globally common Siphonodella sulcata Morphotype 5 and Morphotype 4, as well as Protognathodus kuehni, have a high correlation potential worldwide and can be used for the joint sulcata/kuehni Zone; their phyletic lineages can be reconstructed from many regions.

Due to the spread of atypical morphotypes of Protognathodus (´protognathoids´) at the DCB and in the Tournaisian related to the Hangenberg Event can be regarded as `bottleneck´ for the evolution of this faunal group. The previously suggested extended kockeli Zone and the recently proposed use of Protognathodus kockeli as index fossil for the DCB are not applicable due to the spread of these still unknown atypical morphotypes, and due to their unknown stratigraphic and distributional pattern and overlapping ranges with other homeomorphic groups. Also, the use of the extended kockeli Zone and the exceptional increasing amounts of morphological complex faunas especially among Protognathodus kockeli biased the conodont record and leaves the DCB and the Hangenberg event-related successions undivided. A conodont biozonation scheme with a fine-stratigraphic subdivision, however, is needed for multidisciplinary, high-resolution studies at the DCB. A strict biozonation and taxonomic concept enables further insights into the first-order Hangenberg mass extinction, also in respect to the age of the enigmatic end-Devonian glaciation. The loss of biozones in Hangenberg Event-related successions would present a potential loss of information and would throwback the scientific affords and progresses for several decades back.

Contributions of authors

Conceptualization: SIK; (2) Analysis or data acquisition: SIK, BH; (3) Methodologic/technical development: SIK; (4) Writing of the original manuscript: SIK, BH; (5) Writing of the corrected and edited manuscript: SIK, BH; (6) Graphic design: SIK, BH; (7) Fieldwork: SIK, BH; (8) Interpretation: SIK, BH; (9) Financing: BH.

Financing

This research did not receive any specific grant from funding agencies in the public, commercial, or non-profit sectors.

Acknowledgements

Thanks go to G.W. Rasser and F. Ebner for their support in field work at Trolp, to C. Gasco (SMNS) for SEM records, and to A. Bahrami and P. Isaacson for their helpful comments and suggestions for the manuscript, and to the late C. Sandberg,

W. Qie and M. Aretz for the review of the former version of the manuscript.

Conflict of interest

There is no conflict of interest

Handling editor

Ali Bahrami.

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