Late Devonian (Famennian) to Carboniferous (Mississippian-Pennsylvanian) conodonts from the Anarak section, Central Iran

A relatively complete conodont record from Famennian to the Mississippian/Pennsylvanian boundary was investigated in the Anarak section, Central Iran. The studied interval belongs to the Bahram, Shishtu, Ghaleh and Absheni formations. The Famennian part of the section (Bahram Formation) ranges from the Palmatolepis triangularis Zone into the Bispathodus ultimus Zone. Not all conodont zones could be defined due to the lack of indicative species. Furthermore, it seems likely that a hiatus occurs around the Devonian/Carboniferous (D/C) boundary (most probably from the Siphonodella praesulcata to the ?Siphonodella sulcata–early Siphonodella crenulata conodont zones) based on the lack of stratigraphically important conodonts as well as on sedimentological criteria. The lack of representative siphonodellids and protognathodids at the base of the Mississippian prevents detailed stratigraphic position of the D/C boundary. Lower Carboniferous (Mississippian) rocks are characterized by red nodular limestone which is unique in comparison with other studied sections of the same age in Central Iran. Within the studied section, we could define the Mississippian/Pennsylvanian boundary. The mid-Carboniferous boundary was defined by the occurrence of Declinognathus noduliferus s.l. Conodont biofacies changes (Mississippian genera Gnathodus and Lochriea have been replaced by Pennsylvanian genera Declinognathus and Idiognathodus) are recognized in this section as well.


Introduction
The mid-Palaeozoic, particularly the Devonian/Carboniferous (D/C), transition was a critical interval in Earth's history which was characterized by dramatic climate and faunal changes, severe anoxic intervals, frequent sea level changes and intensified volcanism which finally led from global greenhouse conditions to icehouse conditions (Caplan et al. 1996;Caplan and Bustin 1999;Streel et al. 2000;Joachimski and Buggisch 2002;Joachimski et al. 2004Joachimski et al. , 2006Kaiser et al. 2006Kaiser et al. , 2011Buggisch et al. 2008;Caputo et al. 2008;Isaacson et al. 2008;Marynowski et al. 2012;Kumpan et al. 2014a, b;Paschall et al. 2019). The first-order mass extinction at the end of the Devonian (Hangenberg Crisis) caused not only a loss of major fossil groups such as conodonts (the main extinction among conodonts occurred during the global deposition of the Hangenberg Black Shale, see review by Kaiser et al. 2016) but also entire ecosystems such as metazoan reefs. In the aftermath of the Kellwasser Crisis around the Frasnian/Famennian boundary, the reefs were significantly reduced but no reef complex survived the Hangenberg Crisis (e.g. Webb 2002). The D/C transition is characterized by several transgressive/regressive cycles, and widespread ocean anoxia have been recognized along continental margins or epicontinental basins known as the Hangenberg Black Shale (HBS) Event. Close to the D/C boundary, a major sea level fall (Hangenberg Sandstone Event, HSS) can be recognized in many sections around the world. This eustatic sea level fall is most probably associated with a glaciation on Gondwana (e.g. Caputo 1985;Isaacson et al. 1999Isaacson et al. , 2008Streel et al. 2000Streel et al. , 2001Caputo et al. 2008;Brezinski et al. 2008Brezinski et al. , 2010Lakin et al. 2016). The Mississippian shows a transition between early Palaeozoic stable warm and greenhouse conditions to more late Palaeozoic oscillating climates, including several major glacial episodes (e.g. Powell 2005Powell , 2007Kammer and Ausich 2006;Montanez et al. 2007;Mullins and Servais 2008;Heim 2009;Lowry et al. 2014;Sardar Abadi et al. 2017).
Glacial deposits have been reported from the Viséan and Serpukhovian with a maximum extent during the Bashkirian and the Moskovian (Garzanti and Sciunnach 1997;López-Gamundi 1997;López-Gamundi and Martínez 2000). It is believed that the Late Palaeozoic Ice Age (LPIA) is one of the most important ice ages that cover much of the Late Devonian to Permian when the ice waxed and waned across southern Gondwana (Veevers and Powell 1987). The LPIA consisted of several discrete glacial climates with warmer periods of glacial minima. Ice minimum and maximum intervals were reported from low latitudes (e.g. Soreghan and Giles 1999;Bischoff et al. 2009Bischoff et al. , 2010 as well as from high latitudes (e.g. Caputo et al. 2008;Isbell et al. 2008a, b).
During the Palaeozoic, Iran was part of the northern margin of Gondwana. Marine conditions occurred in Northern and Central Iran from the Middle Devonian to Early Frasnian and persisted into the Early Pennsylvanian (Berberian and King 1981;Husseini 1991;Sharland et al. 2001). A widespread uplift during the latest Carboniferous led to continental environments before the onset of a new marine cycle during the Early Permian. The entire sequence from the Late Devonian, Carboniferous into the Permian deposits of Central Iran is divided into five lithostratigraphic units: the Bahram Formation (Givetian-Famennian), the Shishtu Formation (Tournaisian-Serpukhovian), the Sardar Group (Bashkirian-Moscovian), the Zaladu Formation (Gzhelian-Asselian) and the Jamal Formation (Permian).
The first detailed data on the Upper Devonian to Lower Permian successions in Central and Eastern Iran were obtained during the geological mapping of the Tabas and Kerman areas (Huckriede et al. 1962;Stöcklin et al. 1965;Ruttner and Stöcklin 1966;Ruttner et al. 1968;Stöcklin 1968Stöcklin , 1971Stepanov 1971;Walliser 1984;Weddige 1984). More recently, numerous publications provided comprehensive stratigraphic and palaeontologic data about this region (Korn et al. 1999;Yazdi 1999;Wendt et al. 2002Wendt et al. , 2005Leven and Gorgij 2009;Leven et al. 2006;Hairapetian et al. 2006;Hashemie et al. 2015). Bahrami et al. (2014) described the first conodont data from the Mississippian/Pennsylvanian boundary interval in Central Iran. The aim of this study is to describe new conodont assemblages from the Late Devonian (Famennian) to the Mississippian/Pennsylvanian boundary of the Anarak section.

Geological setting
The Anarak area belongs to the NW part of the Central-East Iranian Microcontinent, which is juxtaposed with the Great  Sharkovski et al. 1984) Kavir Block and the Sanandaj-Sirjan Zone, is characterized by a complex tectonic history (Davoudzadeh et al. 1981;Soffel et al. 1996;Korn et al. 1999;Bagheri and Stampfli 2008). The section is composed of a 5000-8000-m-thick series of sedimentary, volcanoclastic and metamorphic rocks (Soffel et al. 1996;Almasian 1997;Korn et al. 1999;Leven et al. 2006;Aghanabati 2010).
The latter ones contain marbles, schists, gneisses and metadiabases which are unconformably overlain by a series of approximately 1200-m-thick Palaeozoic sediments, ranging stratigraphically from the Ordovician Shirgesht Formation to the Permian Jamal Formation (Lensch and Davoudzadeh 1982;Leven et al. 2006;Hairapetian et al. 2015). Major crustal movements associated with basaltic and ultrabasic volcanism and deposition of preferably shallow-marine deposits led to a complex geology exposed in this area. Several thrust, horst and graben structures and lateral facies changes occur. The entire succession contains some hiatuses due to erosion and/or tectonic uplift.
The base of the section is composed of 700 m of volcanic and sedimentary deposits of ?Late Cambrian-Late Ordovician age (Hairapetian et al. 2015). These series rests upon the Doshakh metamorphic complex (Sharkovski et al. 1984;Schallreuter et al. 2006;Muttoni et al. 2009). The overlaying sedimentary sequence belongs to the carbonate-siliciclastic deposits of the Silurian Niur Formation. Intercalated in this succession are volcanic rocks, such as basalts. Reddish sediments of the Lower to Middle Devonian Padeha Formation with intercalated volcanic rocks at the base cover this succession. The Padeha Formation is commonly overlain by dolostones of the Sibzar Formation which gradually passes into limestones of the Bahram Formation (Bahrami et al. 2014). The Late Devonian (Famennian) Bahram Formation is conformably overlain by the Early Mississippian Shishtu Formation which is composed of red, marly limestones.
Some limestones are rich in fossils and provided an age range from Viséan to late Namurian (Korn et al. 1999). The red nodular limestones are conformably overlain by a cliffforming, coarse and poorly sorted limestone breccia. The topmost part is composed of Upper Mississippian (Viséan -Table 1 Stratigraphy and formations applied to the Upper Devonian-Permian strata of Central-Eastern Iran, modified after Leven et al. (2006)     Namurian) grey fossiliferous thick-bedded calcareous mudstone (Sharkovski et al. 1984). Leven et al. (2006) studied the Pennsylvanian succession (Sardar Group) of the Anarak section. The Sardar Group (previously Sardar Formation) is divided into two formations: the Ghaleh Formation (formerly "Sardar 1", early Bashkirian age), which is mainly composed of carbonate, and the siliciclastic or mixed carbonatesiliciclastic Absheni Formation (formerly "Sardar 2", early Moscovian; see Table 1).

Material and methods
In order to improve and update the biostratigraphy of the Anarak section, fifty-six conodont samples of approximately 4 to 5 kg each were taken from carbonate rock and processed by standard methods (see Jeppsson and Anehus 1995). The process was repeated until samples were dissolved. The washed residues were dried in an oven (~40°C) and later sieved and separated into three different fractions. Conodonts were handpicked utilizing a binocular microscope. Depending on the depositional facies setting, the number of conodonts per sample was highly variable; e.g. in dolostones, no conodonts were found whereas in shallowwater limestones, a good number of species occurred in distinct horizons. Herein, the conodont zonation scheme follows Ziegler and Sandberg (1990), Lane et al. (1999), Kaiser et al. (2009), Hartenfels (2011 and Spalletta et al. (2017). A total number of 373 conodonts were obtained from the residues which led to the identification of 71 species and subspecies within eighteen genera: Alternognathus, Ancyrognathus, Bispathodus, Branmehla, Clydagnathus, Declinognathus, Idiognathus, Gnathodus, Locheria, Icriodus, Mehlina, Palmatolepis, Pelekysgnathus, Polygnathus, Protognathodus, Pseudopolygnathus, Rhachistognathus and Scaphignathus (Tables 2 and 3).
Overall, the preservation of the conodont elements was good, and several specimens are broken or incomplete as a result of sediment transport. The conodont collection is stored at the Department of Geology (sample numbers: EUIC), University of Isfahan, Islamic Republic of Iran. Repository numbers of the figured specimens are given in the explanations of plates. The colour alteration of conodonts (CAI, Epstein et al. 1977) in the Givetian and Frasnian limestones is CAI 4-4.5 (Shakeri 2017), whereas in the Famennian and Mississippian, the colour gradually changes to lower CAI of 2-2.5.

Lithology
Based on field observations and sedimentological characteristics, the Anarak section was subdivided in five units, which are summarized from base to top (Figs. 3 and 4). It is not the aim of this paper to provide a detailed sedimentological analysis which will be published elsewhere. The base of the section (package 1, samples A1-A6, thickness 15 m) starts with grey, medium to thick-bedded fossiliferous limestone. This succession is conformably overlain mainly by greyish to white, thin-bedded nodular limestone (package 2, samples A7-A29, thickness 30 m). The portion contains some reddish shale horizons with grade upwards into skeletal limestone. These sediments are overlain by red, nodular limestone which yielded a number of macrofossils, such as gastropods, brachiopods and solitary corals (package 3, samples A30-A48, thickness 68 m). Three meters below the top, a 20-cm-thick marker horizon with goniatites occurs. The next succession (package 4, samples A49-A50, thickness 27 m) is mainly composed of whitish, cliff-forming brecciated limestone and dolostone with rare macro fossils which is overlain by grey, fossiliferous thick-bedded mudstone and limestone (package 5, samples A51-A56, thickness 38 m). Some sedimentological characteristics are shown in Fig. 3.

Conodont succession
Although carbonate samples of 4 to 5 kg per sample were dissolved for biostratigraphic analysis, the overall number of conodont elements is relatively low as it was shown in other shallowwater sections, for instance, by Bahrami et al. (2018Bahrami et al. ( , 2019 and Ariuntogos et al. (in press). As a result, we are not able to recognize all of the conodont zones used in the revised Conodont Standard Zonation (see Hartenfels 2011;Spalletta et al. 2017). As it is common practice in high-resolution stratigraphic conodont studies, only Pa elements were identified, as many multielement reconstructions are still doubtful and incomplete. However, studied conodont elements lead to the discrimination of 22 biostratigraphic intervals (Fig. 4).
Palmatolepis winchelli to Ancyrognathus ubiquitus zones (samples A2-A6) Although the indicative species Palmatolepis winchelli, Palmatolepis bogartensis and Ancyrognathus ubiquitus (Girard et al. 2005)  The Frasnian-Famennian boundary of the Anarak section seems to be continues with no interruptions, no evidence of any disconformity observed in the field which means lithologically the F/F boundary does not show characteristic sediments, such as black limestones or black shales as it is known from many places around the world (see Carmichael et al. 2019), which is a result of overall shallow-water palaeoenvironment. For more detailed conodont biostratigraphic framework and biofacies analysis of the Givetian and Frasnian part of the Anarak section (Kuh-e-Bande-Abdol-Hossein), see Bahrami et al. (2019).  at the base of the next zone. Polygnathus brevilaminus and Polygnathus asplundi asplundi were also recovered in this zone (Fig. 5).

Palmatolepis triangularis
Palmatolepis delicatula platys to Palmatolepis minuta minuta zones (samples A10-A13) The Palmatolepis delicatula platys Zone corresponds exactly to the former Middle triangularis Zone of Ziegler and Sandberg 1990. We did not find the zonal name-given conodont species, but the base of this interval (sample A10) was discriminated by the first occurrence of Pelekysgnathus inclinatus Thomas 1949 which ranges from Middle triangularis Zone to Upper praesulcata Zone (Sandberg and Dreesen 1984;Huang and Gong 2016) and Ancyrognathus sinelaminus Branson and Mehl 1934a which ranges from the Middle triangularis Zone into the Uppermost crepida Zone (Ziegler and Sandberg 1990). Palmatolepis perlobata perlobata Ulrich and Bassler 1926 enters in level A12 which is the other important indicator to define the lower limit of this interval. Icriodus alternatus alternatus and Icriodus alternatus helmsi were the other species recovered in this interval.

Palmatolepis crepida Zone (samples A14-A15)
The Palmatolepis crepida Zone corresponds to the former Lower crepida Zone (Ziegler and Sandberg 1990), and the base can be discriminated by the first appearance of Palmatolepis minuta loba Helms 1963 which ranges from the base of the P. crepida Zone to P. rhomboidea Zone (Spalletta et al. 2017

Palmatolepis gracilis gracilis Zone (sample A19)
This interval corresponds to the former Upper rhomboidea Zone (Ziegler and Sandberg 1990). The lower limit can be identified by the first entry of Palmatolepis gracilis gracilis Branson &Mehl, 1934a andPolygnathus triphylatus Helms, 1961, and Bispathodus stabilis vulgaris in level A19, the first occurrence of all mentioned species, corresponds to the Palmatolepis gracilis gracilis Zone (see Metzger 1994;Klapper and Ziegler 1979;Spalletta et al. 2017).

Bispathodus aculeatus aculeatus Zone (sample A25)
This zone corresponds to the former Middle expansa Zone and can be recognized by the first entry of Bispathodus aculeatus aculeatus Branson & Mehl, 1934a in level A25 which ranges from Middle expansa Zone (Ziegler & Sandberg, 1984)-texanus Zone (Lane et al., 1980). Clydagnathus ormistoni Beinert et al., 1971 becomes extinct in this conodont zone.

Bispathodus ultimus Zone (samples A27-A29)
This re-defined Bispathodus ultimus Zone is equivalent to the Upper expansa and praesulcata zones and the costatus-kockeli Interregnum of Kaiser et al. (2009). The lower limit of this zone is recognized by the first appearance of Bispathodus ultimus (Bischoff 1957 M1 and M2) which ranges from the Upper expansa Zone into the Middle praesulcata Zone (Ziegler and Sandberg 1984). The assemblage of Bispathodus spinulicostatus, Pseudopolygnathus cf. primus, Bispathodus aculeatus aculateus, Polygnathus communis collinsoni, Bispathodus costatus, Bispathodus bispathodus and Palmatolepis gracilis expansa was found in this interval. According to the conodont zonation scheme proposed by Corradini et al. (2016) and Spalletta et al. (2017), the upper boundary is determined by the lower part of the FAD of Protognathodus kockeli, but due to the lack of Protognathodus and only one species of Siphonodella praesulcata in the Anarak section, there was no evidence for discrimination of the latest Famennian, praesulcata, the costatus-kockeli Interregnum (ckI) and kockeli conodont zones.
?Protognathodus kockeli-L. Siphonodella crenulata zones (samples A30-A32) This interval falls within the first occurrence of red marly nodular limestone at the base of Shishtu Formation. The boundary between the grey limestone of the Bahram Formation and the overlying red nodular limestones of the Shishtu Formation is characterized by a sharp depositional contact. The lack of zonal conodont index species at the base of this interval prevents the identification of the Devonian/ Carboniferous boundary. The rare conodont fauna with Protognathodus collinsoni, Polygnathus inornatus, Polygnathus longiposticus and Polygnathus parapetus (Fig.  7), comprised with Siphonodella sulcata-Lower Siphonodella crenulata zones. The lack of biostratigraphic data might be a result of a depositional hiatus which is related to the Hercynian orogeny. Wendt et al. (2005) also reported a discontinuity from the Anarak section around the same level.
Siphonodella isosticha-Upper Siphonodella crenulata to Upper Gnathodus typicus zones (samples A33-A35) The entry of Gnathodus delicatus, Gnathodus cueniformis, Gnathodus semiglaber and Gnathodus typicus was observed in level 33. Due to the poorness of conodonts in that level, it is difficult to provide a precisely defined conodont zone; thus, only a stratigraphical range can be given.
Scaliognathus anchoralis-Doliognathus latus Zone (samples A36-A37) The lower limit of this interval is recognized by the first appearance of Gnathodus pseudosemiglaber Thomson & Fellows, 1970 in level A35 which ranges from within the anchoralis-latus Zone through the texanus Zone (Lane et al. 1980;Belka and Korn 1994). The conodont assemblage is quite scarce, and only Gnathodus semiglaber and Gnathodus typicus Hass, 1953 have been recovered in this interval.
Upper Gnathodus texanus to Adetognathus unicornis zones (samples A38-A45) The lower boundary of this zone, which is close to the base of the early Viséan, is marked by the FAD of Locheria commutata Branson & Mehl, 1941, and the second conodont found in this level is Gnathodus bilineatus bilineatus Roundy, 1926. Both taxa were recorded as index species of early Viséan (Meischner and Nemyrovska 1999;Nemyrovska et al. 2006;Sudar et al. 2018).
Rachistognathus muricatus Zone (samples A46-A48) The first appearance of Rhachistognathus muricatus (Dunn 1966) in level A46 indicates the Rachistognathus muricatus Zone. This conodont zone is assigned to the late Serpukhovian just below the Mississippian/Pennsylvanian boundary. The upper boundary of the zone coincides with the first appearance of Diclinognathodus noduliferus s.l. (Ellison and Graves 1941), and D. noduliferus conodont Zone conformably lies above the Rachistognathus muricatus Zone. The range of this species is from Upper Mississippian to Lower Pennsylvanian (Krumhardt et al. 1996). Gnathodus girtyi girtyi and Gnathodus girtyi simplex are the other species which occur in this interval (Fig. 7).

Serpukhovian-Bashkirian (mid-Carboniferous) Boundary
Many conodont specialists indicate that the common early Carboniferous genera Gnathodus, Lochriea and Cavusgnathus become extinct at the end of Serpukhovian, and the first Bashkirian Declinognathodus appeared at the mid-Carboniferous boundary between Mississippian and Pennsylvanian (Brenckle et al. 1997;Lane et al. 1999;Nemirovskaya 1999;Richards and Aretz 2010;Krumhardt et al. 1996). In 1995, International Subcommission on Carboniferous Stratigraphy selected the Arrow Canyon, Nevada (USA), to be the GSSP for the mid-Carboniferous boundary. The first appearances of the index conodont taxon Declinognathodus noduliferus sensu lato, including the subspecies Declinognathodus noduliferus noduliferus, D e c l i n o g n a t h o d u s n o d u l i f e r u s i n a e q u a l i s a n d Declinognathodus noduliferus japonicus, were approved as the biostratigraphic marker for the mid-Carboniferous boundary (Baesemann and Lane 1985;Nemirovskaya and Nigmadganov 1994;Nemyrovskaya 1999;Lane et al. 1999;Gradstein et al. 2004;Özdemir 2012).
Declinognathodus noduliferus (samples A49-A51) The lower limit of this interval is recognized by the first appearance of D. noduliferus s.l. Ellison and Graves, 1941 in level 49, 108 m above the base of studied section. Conodonts from the level between 108 and 135 m above the base of the section are D. noduliferus and Declinognathodus praenoduliferus Nigmadganov and Nemirovskaya 1992. In original definition of Declinognathodus praenoduliferus, it appears earlier than D. noduliferus s.l., but in Iranian sections, both species have the same range starting from the Eumorphoceras-Homoceras boundary and in the D. noduliferus conodont Zone. The last occurrence of D. noduliferus s.l. at 135 m above the base indicates the upper boundary of D. noduliferus Zone which is conformably overlain by the Idiognathoides sinuatus-Rachistognathus minutus Zone. The upper limit also can be identified by the first presence of Rachistognathus minutus minutus (Higgins and Bouckaert 1968) in level A51 at the base of next interval.
Idiognathoides sinuatus-Rachistognathus minutus Zone (samples A51-A53) Conodonts collected from samples A51, A52 and A53 at 135 m from the base of section contained Rachistognathus minutus minutus (Higgins and Bouckaert 1968). The range of this species is lower Morrowan (base of sinuatus-minutus Zone) in North America (Varker et al. 1991). This interval falls within the first occurrence of thick-bedded micritic limestone which is characterized by a sharp boundary to the lower brecciated limestone. The I. sinuatus-R. minutus Zone belongs to the middle Bashkirian and defines the upper boundary of D. noduliferus Zone. Declinognathodus noduliferus and Declinognathodus praenoduliferus are also present in this interval.

Conclusions
Late Palaeozoic Upper Devonian to Lower Carboniferous rocks in Central Iran exhibit a number of different lithologies, which point to a shallow-water shelf setting (Bahrami et al. 2018(Bahrami et al. , 2019Königshof et al. 2017). Several hiatuses exist due to lateral facies changes and/or synsedimentary vertical movements in the late Palaeozoic which is associated with horstand-graben structures in different tectonic blocks. The late Palaeozoic (Late Devonian-Permian) deposits of the Anarak section suggest widespread marine conditions. The gradual transition and lithologic similarities between Devonian and Lower Carboniferous (Mississippian) shows that the depositional regime remained virtually unchanged. On the other hand, many sections in Iran exhibit biostratigraphical hiatuses or facies changes, which particularly concern the conodont record (e.g. Königshof et al. in press). However, some section exhibits a rather complete succession, such as the Anarak section. Sediments of this section range from the Middle Devonian and Frasnian (see Bahrami et al. 2019) to the Late Devonian and Mississippian/Pennsylvanian boundary. University of Isfahan in cooperation with the Senckenberg Research Institute and Natural History Museum, Frankfurt. We thank Jeffrey Over (USA) and an anonymous reviewer for their critical comments which improved an earlier version of the manuscript. This is a contribution to the International Geological Science Programmes IGCP 652 and IGCP 700.
Funding Open Access funding enabled and organized by Projekt DEAL. Funding was provided by German Science Foundation (DFG -KO 1622/16-1).

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Conflict of interest The authors declare that they have no conflict of interest.
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