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. 2004, 2006; Kaiser et al. 2006, 2011; Buggisch 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. 1999, 2008; Streel et al. 2000, 2001; Caputo et al. 2008; Brezinski et al. 2008, 2010; Lakin 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 2005, 2007; Kammer 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. 2009, 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 1968, 1971; Stepanov 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. 2002, 2005; Leven 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 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 meta-diabases 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 section described here (Figs. 1 and 2) is located close to the northern end of the N–S striking mountain range Kuh-e-Bande-Abdol-Hossein which is located southwest of the Kuh-e-Lakh metamorphosed complex, about 34 km southwest of Anarak (sheet 6756 Anarak, 1:100,000; WGS coordinates: base of the section 33° 10′ 44.79″ N, 53° 52′ 23.83″ E; top of the section 33° 10′ 33.78″ N, 53° 52′ 22.90″ E; see Figs. 1 and 2).

Fig. 1
figure 1

a Structural units of Iran and location (1) of the Anarak section. b Road map showing the position of the Anarak section northeast of Isfahan (modified after Bakhtiari 2005)

Fig. 2
figure 2

Geological map and location of the investigated section A`—A (modified after Sharkovski et al. 1984)

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 cliff-forming, coarse and poorly sorted limestone breccia. The topmost part is composed of Upper Mississippian (Viséan–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 carbonate–siliciclastic Absheni Formation (formerly “Sardar 2”, early Moscovian; see Table 1).

Table 1 Stratigraphy and formations applied to the Upper Devonian–Permian strata of Central-Eastern Iran, modified after Leven et al. (2006)

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 shallow-water 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.

Table 2 Conodont element distribution and number of elements of the Anarak section (Devonian part, samples A2–A29)
Table 3 Conodont element distribution and number of elements of the Anarak section (Carboniferous part, samples A30–A53)

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.

Fig. 3
figure 3

a, b, e Panoramic views of the studied interval (Bahram, Shishtu and Ghaleh formations). c Cliff-forming brecciated grey to whitish limestone (package 4). d Grey, fossiliferous thick-bedded limestone (package 5). f Grey skeletal limestone (package 3). g Goniatite bearing red marly nodular limestone at the top of package 3. h Thin-bedded grey fossiliferous limestone including, corals and brachiopod shells in package 3

Fig. 4
figure 4

Stratigraphic log, samples position, conodont occurrences and biozonation of Anarak section

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 shallow-water sections, for instance, by Bahrami et al. (2018, 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) are absent, the upper boundary of this biozone corresponds to the last occurrence of Polygnathus cf. politus Ovanatanova 1969; Polygnathus webbi Stauffer 1938; and Polygnathus cf. alatus Huddle 1934 in level A6 in our section. All mentioned species become extinct in the linguiformis Zone (Ziegler and Sandberg 1996, 2000; Ovnatanova and Kononova 2001, 2008; Bultynck 2003). Therefore, this assemblage belongs to an Upper Frasnian, Upper rhenana-linguiformis interval or more updated global biozones Palmatolepis winchelli to Ancyrognathus ubiquitus. Polygnathus aequalis and Polygnathus cf. xylus are the other important associated species. 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).

Palmatolepis triangularis Zone (samples A7–A9)

Spalletta et al. (2017) proposed the Palmatolepis subperlobata Zone which correlates with the lowest part of the former Lower triangularis Zone. The new base is defined by the first appearance datum (FAD) of Palmatolepis subperlobata. Within the studied section (samples A7–A9), the index conodont Palmatolepis subperlobata was not observed. Thus, the lowermost part of the Famennian according to the new standard zonation (Spalletta et al. 2017) was not proven by conodonts. The base of this following interval was recognized by the entry of Palmatolepis triangularis Sannemann 1955a in level A7. This zone corresponds to the upper part of the former Lower triangularis Zone. The top is defined by the entry of Pelekysgnathus inclinatus Thomas 1949 and Ancyrognathus sinelaminus (Branson and Mehl 1934a) at the base of the next zone. Polygnathus brevilaminus and Polygnathus asplundi asplundi were also recovered in this zone (Fig. 5).

Fig. 5
figure 5

Conodonts from the Anarak section, with a given scale bar of 100 μm. 1 Polygnathus webbi Stauffer, 1938; upper (a) and lower (b) views of IUMC 200, sample A4. 2, 3 Polygnathus aequalis Klapper & Lane, 1985; 2 upper view of IUMC 201, sample A4; 3 upper view of IUMC 202, sample A4. 4 Polygnathus alatus Huddle, 1934; upper view of IUMC 203, sample A4. 5 Polygnathus cf. xylus Stauffer, 1938; upper (a) and lower (b) views of IUMC 204, sample A4. 6, 7 Polygnathus aequalis Klapper & Lane, 1985; 6 upper-oblique view of IUMC 205, sample A2; 7 upper (a) and lower (b) views of IUMC 206, sample A3. 8 Polygnathus brevilaminus Branson & Mehl, 1934a; upper view of IUMC 207, sample A13. 9, 10 Polygnathus aspelundi aspelundi Savage & Funai, 1980; 9 upper view of IUMC 208, sample A9; 10 upper view of IUMC 209, sample A8. 1112 Polygnathus cf. politus Ovnatanova, 1969; 11 upper view of IUMC 210, sample A2. 12 Upper (a) and lower (b) views of IUMC 211, sample A6. 13 Polygnathus cf. alatus Huddle 1934; upper (a) and lower (b) views of IUMC 212, sample A4. 14 Ancyrognathus sinelaminus (Branson and Mehl, 1934); upper view of IUMC 213, sample A10. 15 Icriodus alternatus helmsi, Sandberg et Dreesen, 1984; upper view of IUMC 214, sample A15. 1621 Icriodus alternatus alternatus Branson & Mehl, 1934; 16 upper view of IUMC 215, sample A11; 17 upper view of IUMC 216, sample A12; 18 upper view of IUMC 217, sample A13; 19 upper view of IUMC 218, sample A14; 20 upper-oblique view of IUMC 219, sample A15; 21 upper view of IUMC 220, sample A16. 22, 23 Icriodus alternatus helmsi, Sandberg and Dreesen 1984; 22 upper view of IUMC 221, sample A12; 23 upper view of IUMC 222, sample A13. 24, 25 Icriodus costatus darbyensis Klapper 1958; 24 upper view of IUMC 223, sample A28; 25 upper view of IUMC 224, sample A29. 26 Icriodus cf. cornutus Sannemann, 1955; upper lateral view of IUMC 225, sample A19. 27 Icriodus cornutus Sannemann, 1955; upper view of IUMC 226, sample A22. 28, 29 Polygnathus padovanii, Perri and Spalletta 1990; 28 upper view of IUMC 227, sample A18; 29 upper view of IUMC 228, sample A18. 30, 31 Polygnathus communis communis Branson & Mehl, 1934; 30 upper (a) and lower (b) views of IUMC 229, sample A15; 31 upper-oblique view of IUMC 230, sample A15. 32 Polygnathus triphylatus Helms, 1961; upper view of IUMC 231, sample A19. 3336 Polygnathus semicostatus Branson and Mehl, 1934; 33 upper view of IUMC 232, sample A21; 34 upper lateral view of IUMC 233, sample A21; 35 upper view of IUMC 234, sample A16; 36 upper view of IUMC 235, sample A17. 37 Polygnathus delicatulus Ulrich and Bassler 1926; upper view of IUMC 236, sample A26. 38 Pelekysgnathus inclinatus Thomas 1949; upper-oblique (a) and lateral (b) views of IUMC 237, sample A10. 39, 40 Polygnathus communis collinsoni Druce 1969; 39 upper (a) and lower (b) views of IUMC 238, sample A26; 40 upper view of IUMC 239, sample A29. 41 Polygnathus sp. Upper view of IUMC 236, sample A22. 42 Alternognathus regularis regularis Ziegler and Sandberg 1984; upper (a) and lower (b) views of IUMC 237, sample A21. 43 Polygnathus tichonovitchi Kuzmin & Melinkova, 1991; upper view of IUMC 238, sample A13. 44 Polygnathus aff. subnormalis Vorontsova and Kuzmin 1984; upper (a) and lower (b) views of IUMC 239, sample A18. 45 Scaphignathus velifer velifer Helms 1959; upper view of IUMC 240, sample A21. 46 Gen. et sp. indet. Upper view of IUMC 241, sample A25. 47, 48 Bispathodus stabilis vulgaris (Dzik 2006) Branson & Mehl, 1934; 47 upper view of IUMC 242, sample A21; 48 upper lateral view of IUMC 243, sample A22

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). Icriodus alternatus helmsi Sandberg and Dreesen 1984 become extinct at the end of this zone. Other associated species in level A14 are Icriodus alternatus alternatus and Polygnathus cf. communis communis.

Palmatolepis termini Zone (sample A16)

This interval is equivalent to the former Middle crepida Zone (Ziegler and Sandberg 1990). The base is characterized by the entry of Polygnathus semicostatus Branson and Mehl 1934a in level A16, which has its first occurrence within the Palmatolepis termini Zone of Spalletta et al. (2017) and extends into the Bispathodus ultimus Zone. Palmatolepis minuta loba, Icriodus alternatus alternatus and Polygnathus cf. communis communis are associated as well.

Palmatolepis glabra pectinata to Palmatolepis rhomboidea zones (samples A17–A18)

The base is well marked by the first occurrence of Palmatolepis glabra pectinata Ziegler, 1962b M1 Sandberg and Ziegler 1973 in level A17 and Palmatolepis quadrantinodosalobata Sannemann, 1955a M1 Sandberg and Ziegler 1973, and both species have their first occurrence in the Palmatolepis glabra pectinata Zone (Spalletta et al. 2017). Icriodus alternatus alternatus Branson & Mehl, 1934a, which become extinct at the top of this interval in level A18 (Bultynck 2003; Spalletta et al. 2017), is the other indicator for discriminating the upper boundary of this interval. Polygnathus cf. communis communis, Polygnathus padovanii, Polygnathus cf. subnormalis and Palmatolepis minuta minuta are also present.

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 and Polygnathus 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). Polygnathus semicostatus, Palmatolepis glabra pectinata, Palmatolepis minuta minuta, P. subnormalis, Mehlina strigosa, Icriodus cornutus and Polygnathus inconcinnus are the other associated conodonts.

Palmatolepis marginifera marginifera Zone (sample A20)

This conodont zone in level A20 is confirmed by the first occurrence of Palmatolepis perlobata maxima Müller, 1956, which has its FAD within the lower part of this conodont zone (Spalletta et al. 2017). Bispathodus stabilis vulgaris, Palmatolepis gracilis gracilis, Palmatolepis minuta minuta, Polygnathus semicostatus and Icriodus cornutus are also present, and Polygnathus triphylatus Helms 1961 becomes extinct within the lower part of this conodont zone (Spalletta et al. 2017).

Scaphignathus velifer velifer to Palmatolepis rugosa trachytera zones (sample A21)

The base of this interval is well defined by the first entry of index species Scaphignathus velifer velifer Helms 1959 and Scaphignathus velifer leptus Ziegler & Sandberg, 1984, and both have their first occurrences in the Scaphignathus velifer velifer Zone (= former Uppermost marginifera Zone). The other associated species are Polygnathus perplexus, Polygnathus granulosus, Alternognathus regularis regularis, Polygnathus nodocostatus, Branmehla bohlenana, Bispathodus stabilis vulgaris, Palmatolepis perlobata maxima, Mehlina strigosa and Polygnathus semicostatus.

Pseudopolygnathus granulosus Zone (sample A22)

The base of this interval is coincident with the first entry of Palmatolepis gracilis sigmoidalis Ziegler, 1962a and Bispathodus stabilis stabilis (Branson & Mehl, 1934a) [M2], and the top limit can be fully identified by the last occurrence of Icriodus cornutus Sannemann, 1955b, Palmatolepis minuta minuta Branson & Mehl, 1934a and Scaphignathus velifer velifer Helms, 1959 in level A22 which all become extinct in the granulosus Zone (former Upper trachytera Zone) (see Bultynck 2003; Ji and Ziegler 1993; Ziegler and Sandberg 1984; Spalletta et al. 2017). Bispathodus stabilis vulgaris and Mehlina strigosa are also presented.

Polygnathus styriacus to Palmatolepis gracilis manca zones (sample A23)

Sample A23 yielded very few conodonts, and this interval is tentatively assigned between the lower and upper discriminated zones. Scaphignathus velifer leptus Ziegler & Sandberg, 1984 become extinct at the end of this interval in level A23. In fact, Scaphignathus velifer leptus ranges from the lower part of the Scaphignathus velifer velifer Zone to the top of the Palmatolepis gracilis manca Zone (Spalletta et al. 2017). Bispathodus stabilis stabilis, Branmehla bohlenana and Palmatolepis gracilis sigmoidalis are associated species.

Palmatolepis gracilis expansa Zone (sample A24)

The zone is equivalent to the former Lower expansa Zone. The entry of Bispathodus jugosus (Branson and Mehl 1934a) and Palmatolepis gracilis expansa (Sandberg and Ziegler 1979) in level A24, both range from the P. gracilis expansa into the Bispathodus ultimus of Spalletta et al. (2017), defines the base of this interval; associated species are Clydagnathus ormistoni, Bispathodus bispathodus and Bispathodus stabilis stabilis (Fig. 6).

Fig. 6
figure 6

Conodonts from the Anarak section, with a given scale bar of 100 μm. 1, 2 Palmatolepis gracilis expansa Sandberg and Ziegler, 1979 M1; 1 upper view of IUMC 244, sample A24; 2 upper view of IUMC 245, sample A26. 38 Palmatolepis minuta minuta Branson & Mehl, 1934a; 3 upper view of IUMC 246, sample A17; 4 upper view of IUMC 247, sample A17; 5 upper view of IUMC 248, sample A22; 6 upper view of IUMC 249, sample A22; 7 upper view of IUMC 250, sample A22; 8 upper lateral view of IUMC 251, sample A22. 911 Palmatolepis gracilis sigmoidalis Ziegler, 1962a; 9 upper lateral view of IUMC 252, sample A22; 10 upper view of IUMC 253, sample A22; 11 upper view of IUMC 254, sample A23. 12 Palmatolepis gracilis gracilis Branson & Mehl, 1934a; upper view of IUMC 255, sample A19. 13, 14 Palmatolepis minuta loba Helms, 1963; 13 upper view of IUMC 256, sample A14; 14 upper view of IUMC 257, sample A15. 15 Palmatolepis quadrantinodosalobata Sannemann, 1955a; upper view of IUMC 258, sample A17. 16 Palmatolepis triangularis Sannemann, 1955; upper view of IUMC 259, sample A7. 17 Palmatolepis perlobata perlobata Ulrich and Basller, 1926; upper view of IUMC 260, sample A12. 18 Palmatolepis gracilis sigmoidalis Ziegler, 1962a; upper lateral view of IUMC 261, sample A22. 19, 20 Polygnathus perplexus, Thomas, 1949; 19 upper lateral view of IUMC 262, sample A21; 20 upper lateral view of IUMC 263, sample A26. 21, 22 Polygnathus granulosus, Branson & Mehl, 1934a; 21 upper view of IUMC 264, sample A21; 22 upper view of IUMC 265, sample A21. 23 Polygnathus nodocostatus Branson & Mehl, 1934; upper view of IUMC 266, sample A21. 24 Polygnathus inconcinnus Kuzmin & Melnikova, 1991; upper view of IUMC 267, sample A19. 25 Bispathodus ultimus Bischoff, 1957; upper view of IUMC 268, sample A27. 26, 27 Bispathodus bispathodus Ziegler, Sandberg and Austin, 1974; 26 upper lateral view of IUMC 269, sample A26; 27 upper lateral view of IUMC 270, sample A29. 28 Bispathodus costatus (Branson, 1934) morphotype 1; upper lateral view of IUMC 271, sample A25. 29 Bispathodus bispathodus Ziegler, Sandberg and Austin, 1974; upper view of IUMC 272, sample A26. 30 Bispathodus cf. ultimus Bischoff, 1957; upper view of IUMC 273, sample A27. 31 Bispathodus aculeatus aculeatus Branson & Mehl, 1934a; upper lateral view of IUMC 274, sample A25. 32 Bispathodus spinulicostatus (Branson, 1934) morphotype 1; upper lateral view of IUMC 275, sample A26. 33 Scaphignathus velifer velifer Helms, 1959; upper view of IUMC 276, sample A22. 34 Bispathodus cf. costatus Branson, 1934; upper lateral view of IUMC 277, sample A26. 35 Bispathodus cf. ultimus Bischoff, 1957; upper view of IUMC 278, sample A27. 36 Scaphignathus velifer leptus Ziegler & Sandberg, 1984. Upper view of IUMC 279, sample A23. 37 Bispathodus jugosus (Branson & Mehl, 1934a); upper view of IUMC 280, sample A24. 38 Bispathodus bispathodus Ziegler, Sandberg and Austin, 1974; 20 upper lateral view of IUMC 281, sample A26. 39 Pseudopolygnathus primus Branson & Mehl, 1934b; upper view of IUMC 282, sample A27. 40 Bispathodus stabilis vulgaris Dzik, 2006; upper view of IUMC 283, sample A21. 41, 42 Palmatolepis perlobata maxima Müller, 1956; 41 upper view of IUMC 284, sample A19; 42 upper view of IUMC 285, sample A19. 43, 44 Palmatolepis glabra pectinata Ziegler, 1962; 43 upper view of IUMC 286, sample A17, X 127; 44 upper view of IUMC 287, sample A17, X 138. 45 Palmatolepis minuta minuta Branson & Mehl, 1934a; upper (a) and upper lateral (b) views of IUMC 288, sample A22. 46 Bispathodus stabilis stabilis (Branson & Mehl, 1934a); upper lateral view of IUMC 289, sample A22. 4750 Branmehla bohlenana (Helms, 1959); 47 upper view of IUMC 290, sample A21; 48 upper view of IUMC 291, sample A21; 49 upper view of IUMC 292, sample A21; 50 upper view of IUMC 293, sample A21

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 costatus Zones (sample A26)

The first appearance of Bispathodus costatus Branson, 1934 M1 (Ziegler and Sandberg 1984) is the indicator of the lower boundary of this interval. Bispathodus bispathodus, Bispathodus spinulicostatus, Pseudopolygnathus cf. primus, Polygnathus communis collinsoni, Bispathodus jugosus, Palmatolepis gracilis expansa, Polygnathus perplexus and Bispathodus cf. costatus are also present.

Bispathodus ultimus Zone (samples A27–A29)

This re-defined Bispathodus ultimus Zone is equivalent to the Upper expansa and praesulcata zones and the costatuskockeli 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 costatuskockeli 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.

Fig. 7
figure 7

Conodonts from the Anarak section, with a scale bar given for each sample. 1 Protognathodus collinsoni Ziegler, 1969; upper view of IUMC 294, sample A31. 25, 7, 8 Polygnathus inornatus inornatus Branson, 1934; 2 upper view of IUMC 295, sample A30; 3 upper view of IUMC 296, sample A30; 4 upper view of IUMC 297, sample A31; 5 upper view of IUMC 298, sample A31; 7 upper view of IUMC 299, sample A30; 8 upper view of IUMC 300, sample A31. 6 Polygnathus longiposticus Branson & Mehl, 1934; upper view of IUMC 301, sample A30. 9, 10 Polygnathus parapetus Druce, 1969; 9 upper view of IUMC 302, sample A32; 10 upper view of IUMC 303, sample A31. 11 Clydagnathus ormistoni Beinert, Klapper, Sandberg & Ziegler, 1971; upper view of IUMC 304, sample A24. 12 Gnathodus delicatus Branson & Mehl, 1938; upper view of IUMC 305, sample A38. 13 Gnathodus girtyi simplex Dunn, 1966; upper view of IUMC 306, sample A38. 14 Gnathodus typicus Cooper, 1939; upper view of IUMC 307, sample A36. 15, 16 Gnathorius girtyi girtyi Hass, 1953; 15 upper view of IUMC 308, sample A38; 16 upper view of IUMC 309, sample A38. 17 Gnathodus girtyi simplex Dunn, 1966; upper view of IUMC 310, sample A38. 18 Lochriea commutata (Branson & Mehl, 1941); upper view of IUMC 311, sample A39. 1921 Declinognathodus praenoduliferus Nigmadganov and Nemirovskaya, 1992; 19 upper view of IUMC 312, sample A49; 20 upper view of IUMC 313, sample A50; 21 upper view of IUMC 314, sample A50. 2225 Declinognathodus noduliferus s.l. (Ellison and Graves, 1941); 22 upper view of IUMC 315, sample A50; 23 upper view of IUMC 316, sample A51; 24 upper view of IUMC 317, sample A51; 25 upper view of IUMC 318, sample A49. 26 Gnathodus cuneiformis Mehl & Thomas, 1947; upper view of IUMC 319, sample A35. 27 Gnathodus bilineatus bilineatus Roundy, 1926; upper view of IUMC 320, sample A38. 28 Rachistognathodus minutus minutus (Higgins & Bouckaert, 1968); upper lateral view of IUMC 321, sample A51. 29 Rachistognathodus minutas minutas (Higgins & Bouckaert, 1968); upper lateral view of IUMC 322, sample A46. 30 Rachistognathodus muricatus Dunn, 1966; upper view of IUMC 323, sample A48. 31, 32 Gnathodus girtyi girtyi Hass, 1953; 31 upper view of IUMC 324, sample A38; 32 upper lateral view of IUMC 325, sample A38. 33 Gnathodus semiglaber Bischoff, 1957; upper view of IUMC 326, sample A35. 34 Gnathodus pseudosemiglaber Thomson and Fellow, 1970; upper lateral view of IUMC 327, sample A37. 35 Gnathodus pseudosemiglaber Thomson and Fellow, 1970; upper view of IUMC 328, sample A37. 36 Gnathodus girtyi simplex Dunn, 1966; upper view of IUMC 220, sample A39. 37, 38 Idiognathodus sinuosus Ellison & Graves, 1941; 37 upper view of IUMC 329, sample A52; 38 upper view of IUMC 330, sample A52

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 anchoralisDoliognathus 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 anchoralislatus 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, Declinognathodus noduliferus inaequalis and 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 EumorphocerasHomoceras 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 sinuatusRachistognathus 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 sinuatusRachistognathus 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 sinuatusminutus 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. sinuatusR. 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, 2019; Königshof et al. 2017). Several hiatuses exist due to lateral facies changes and/or synsedimentary vertical movements in the late Palaeozoic which is associated with horst-and-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.