First report of Devonian corals from the Bitlis-Pötürge Massif (SE Turkey): a rare occurrence of corals on the northern margin of Gondwana

DOI: 10.1344/GeologicaActa2021.19.1  J. Denayer and I. Hoşgör, 2021 CC BY-SA J . D e n a y e r a n d I . H o şg ö r G e o l o g i c a A c t a , 1 9 . 1 , 1 1 1 ( 2 0 2 1 ) D O I : 1 0 . 1 3 4 4 / G e o l o g i c a A c t a 2 0 2 1 . 1 9 . 1 First Fransinian corals from SE Turkey 2 and Flügel (1961) described some rugose and tabulate corals, along with stromatoporoids and chaetetid sponges, from Yahyalı (Aladağ Mts.), probably collected in Middle Devonian formations. Hubmann (1991, 1992) described the fauna collected by Kiratlioğlu (1959) near Feke. This fauna is younger than the one described by Flügel and Flügel (1961) and attributed to the upper Givetian and Frasnian (Middle-Upper Devonian) by Kiratlioğlu (1959). Oekentorp and Schröder (2001) reported a single occurrence of rugose coral in the Frasnian (Upper Devonian) in the Sultan Dağ in the Geyik Dağ Unit. Coen-Aubert et al. (2013) reported a Frasnian coral fauna composed of cosmopolitan genera also from the Geyik Dağ, in the Kozan area near Adana. Here, we document for the first time the Devonian coral fauna from the Meydan Formation in the Bitlis-Pötürge Massif (SE Turkey, Fig. 1) and discuss its age and affinities.


INTRODUCTION
Devonian corals have rarely been reported from Turkey, and only few of them have received a proper description. Heritsch and Gaertner (1929) and Charles (1933) reported some tabulate and rugose corals from the Yılanlı Formation, NW Turkey, mostly of Lower-Middle Devonian age, later Kullmann (1973) reported small solitary undissepimented rugose corals from Lower-Middle Devonian deep-water sediments in the Istanbul area. The first and yet the only comprehensive study of Devonian corals from NW Turkey was provided by Birenheide and Kaya (1987) who described a diverse coral fauna of Eifelian age (Middle Devonian) from Adapazarı. Tchiatcheff (1897) cited some corals from southern Turkey in the faunal list of his monography, and Penecke (1903) was the first to describe corals from southern Turkey based on material collected by Schaffer (1901) in Hadschin near Feke. Heritsch (1928) and Ünsalaner (1951) reported some colonial rugose and tabulate corals from the Taurides (in the vicinity of Yedi Oluk and of Saimbeyli respectively) but most of those species are nomen obliterum. Flügel (1955) and Flügel and Flügel (1961) described some rugose and tabulate corals, along with stromatoporoids and chaetetid sponges, from Yahyalı (Aladağ Mts.), probably collected in Middle Devonian formations. Hubmann (1991Hubmann ( , 1992 described the fauna collected by Kiratlioğlu (1959) near Feke. This fauna is younger than the one described by Flügel and Flügel (1961) and attributed to the upper Givetian and Frasnian (Middle-Upper Devonian) by Kiratlioğlu (1959). Oekentorp and Schröder (2001)  Here, we document for the first time the Devonian coral fauna from the Meydan Formation in the Bitlis-Pötürge Massif (SE Turkey, Fig. 1) and discuss its age and affinities.

GEOLOGICAL SETTINGS
The Bitlis-Pötürge Massif forms an arcuate metamorphic belt, about 30km wide and 500km long, rimming the Arabian Platform in SE Turkey. It is separated from the Arabian Platform by a narrow belt of Upper Cretaceous to Eocene flysch and ophiolitic mélange, and Miocene sediments (Oberhanslı et al., 2010) (Fig. 1). The massif includes a Precambrian basement and an overlying Phanerozoic sequence (Cavazza et al., 2018;Okay et al., 1985). In an early description, Tolun (1953) interpreted the metamorphic rocks of the Bitlis-Pötürge complex as forming the basement of the region. According to Kellogg (1960) and Göncüoğlu and Turhan (1984) the Bitlis-Pötürge metamorphics are the equivalents of the Arabian autochthonous succession and, accordingly, a Devonian to Upper Cretaceous age was assigned for the metasediments.
The upper association (Mutki Group) is a greenschist facies sedimentary succession represented by quartzites, limestones and dolomite lenses, metaclastics and metavolcanics (Okay et al., 1985;Ustaömer et al., 2012) (Figs. 1;2). Fossils from the metasediments indicate a Middle Devonian age for the lower part of the succession (Göncüoğlu and Turhan, 1984). The base of this rock group is composed of the Devonian Meydan Formation comprising meta-conglomerates, quartzites and greenschists with limestone interlayers, "reef" limestones and albite-chlorite actinolite-chloritoid schists of probably volcanogenic origin (Oberhanslı et al., 2010)  been recognised in the Meydan Formation in the east of the Bitlis-Pötürge: Actinostroma spp., Thamnopora sp., Favosites sp. and crinoid fragments. According to these fossils the unit is Givetian-Frasnian in age (Göncüoğlu and Turhan, 1984).
During the Devonian, the Eastern Taurides -including areas that are now part of the Bitlis-Pötürge Massif-were situated along the northern margin of Gondwana, south of the later Cimmerian Terranes rifting line. Wehrmann et al. (2010), based on detailed sedimentological and stratigraphic work, suggested that the Upper Devonian strata of the Eastern Taurides were deposited in a shallow-water, mixed carbonate and siliciclastic shelf. Preliminary data on the Meydan Formation are in agreement with previously published interpretations.

Coral association
The coral specimens studied here were found in the Meydan Formation during a geological survey in the western regions of the Bitlis city, in the Bitlis-Pötürge collision zone of Turkey (Fig. 1). The corals are from the middle part of the Meydan Formation ( Fig. 2A, B) in the Danagözü section, located approximately 7km southwest of Sason. All specimens come from a single clayey limestone bed, 20-30cm-thick, which yielded a very poor fossil assemblage but relatively well preserved compared to the fossils occurring in the recrystallized limestone. The fossiliferous limestone is a bioclastic f loatstone with a weathered ferruginous dolomitic matrix. Besides corals, crinoids, brachiopods and bryozoans are abundant (Fig. 2C). The assemblage is dominated by colonial rugose corals, represented by the cerioid Frechastraea schafferi (penecke, 1903), the fasciculate Peneckiella cf. teicherti Hill, 1954 and the dendroid Pseudopexiphyllum supradevonicum (penecke, 1903). The tabulate corals are represented by the ramose Thamnopora reticulata (de blainville, 1830) and Scoliopora sp. and the massive Alveolites ex. gr. suborbicularis (Figs. 3;4). Solitary rugose corals are under-represented in the sample with the occurrence of a single specimen of Macgeea desioi von scHouppé, 1965. This specimen is very similar, however slightly smaller, than those described by Rohart (1999) from the Frasnian of eastern Iran. It is rather weathered and serves as an anchorage for a small Frechastraea colony. Description. Fasciculate colony with lateral offsets. Corallites cylindrical, 6-6.5mm in diameter (tabularium 4-5mm). Twenty to twenty-two septa of each order. Major septa long, commonly reaching the axis where some are joined, but withdrawn in more mature stages, thin with a cuneiform base. Cardinal major septum shorter in some mature corallites. Minor septa short, entering slightly into the tabularium. Dissepimentarium comprising one  Discussion. This colony is similar to Peneckiella minima (roemer, 1855) (="Cyathophyllum minus" in penecke, 1903) that was previously reported from the Turkish Taurides by Penecke (1903), Flügel (1956) and Oekentorp and Schröder (2001) but has larger corallites and longer septa. It shows many similarities with Peneckiella isylica (bulvanker, 1958), including size but the present species has shorter minor septa and very few dissepiments (one or two rows) whereas P. isylica has a larger dissepimentarium. From Pakistan, von Schouppé (1965) described Disphyllum casepitosum tricyclicum n.sp. that most probably is a Peneckiella with larger corallites (8-13mm in diameter and 28 septa of each order) and that developed a third order of septa. From Western Australia, Brownlaw and Jell (2008) reported Peneckiella teicherti Hill, 1954 which has slightly smaller corallites (4-6mm in diameter and 17-22 septa of each order) but having, like the present species, scattered horseshoe dissepiments. A slightly larger species was described by Fan et al. (2003) from Xizang under the name Peneckiella abnorma He in fan et al., 2003 which has dimensions similar to the Turkish colony (6-8mm in diameter), similar number of septa (20-22 thin septa of each order) and a single row of dissepiments, peneckielloid to horseshoe in shape. P. abnorma, together with the present material, probably enters in the range of variability of the Australian species P. techeirti Hill, 1954, but additional material is needed to validate the synonymy. Similarly, the isolated corallites described from the Frasnian strata of the Albroz Mountains (Iran) by Rohart (1999) under the name Peneckiella? cf. cylindrica (yoH, 1937), could possibly be included in that species. Hence, the Turkish colony is provisionally referred with some doubts to P. techeirti Hill, 1954. Occurrence. Small-sized species of Peneckiella occur widely in the Frasnian strata (see MacLean, 2005 for a recent review). P. teicherti Hill, 1954 was described in the Canning Basin (Western Australia) by Hill (1954) then additional material was described by Hill and Jell (1970) and Brownlaw and Jell (2008) from the upper part of the Frasnian (Virgin Hill and Pillara formations). The possible synonyms P. abnorma 0 et al., 2003 and P.? cf. cylindrica (yoH, 1937) Różkowska, p. 29, Pl. 3, fig. 11. 1991Pseudopexiphyllum supradevonicum (penecke, 1903, Hubmann, p. 161, fig. 8, Pl. 2, figs 3-6. Figs. 3C-E Diagnosis. After Penecke (1903), colony dendroid with cylindrical corallites bearing strong rugae, 10-14mm in diameter, having 24-28 septa of each order. Major septa withdrawn from the axis, leaving a free space up to 1/5 of the corallite diameter. Minor septa half as long as the major ones. Dissepimentarium composed of three zones: an external zone of small dissepiments steeply inclined towards the periphery, a single row of horseshoe dissepiments and several rows of smaller and flatter dissepiments becoming more declined towards the tabularium. Tabularium complete, made of sub-horizontal tabulae slightly depressed axially.
Description. Fragment of a small dendroid colony with cylindrical corallites and many offsets, encrusted by bryozoans (Fig. 3E). Corallites 12-18mm in diameter (tabularium 7-8mm), having 28-32 septa of each order. Major septa long, almost reaching the axis but leaving an empty zone of 1-2mm-wide, straight and thick in the dissepimentarium, thin and wavy in the tabularium, some being hooked in the central part of the tabularium. Minor septa long, reaching the inner margin of the dissepimentarium, thinner than the major septa. Both orders of septa carinate in the outer dissepimentarium. Dissepimentarium divided in three zones: an outer zone made of 1-2 rows of large globular dissepiments declined towards the periphery (concavity towards the periphery in transversal section), commonly filled with sediment; a middle zone of thickened horseshoe dissepiments with rounded shape in longitudinal section; an inner zone of 4-8 rows of small densely-packed concentric dissepiments declined towards the tabularium, the innermost row being sub-vertical in longitudinal section. Tabularium made of flat-topped or slightly concave regularly-spaced mesashaped tabulae. Outer wall relatively thin but commonly weathered out.
Discussion. This specimen is very similar to those described by Hubmann (1992) from localities close to the type locality of the species (Penecke, 1903). The major difference resides in the thickening of the septa, more intensely developed in the present colony compared to Hubmann's and Penecke's colonies. However, in juvenile corallites, the thickening and carination are not so pronounced. Pseudopexiphyllum occultum scHröder, J . D e n a y e r a n d I . H o şg ö r G e o l o g i c a A c t a , 1 9 . 1 , 1 -1 1 ( 2 0 2 1  First Fransinian corals from SE Turkey 6 2004 differs from P. supradevonicum (penecke, 1903) by having larger corallites (20-25mm), but has similar number of septa (29-33). The genus Pseudopexiphyllum Hubmann, 1992 can be regarded as a colonial Macgeea webster, 1899 with which it shares many structural elements, notably the multizonal dissepimentarium (see Hubmann, 1992;Schröder, 2004). MacLean (2005) however considered that these characters are not sufficient to discriminate Pseudopexiphyllum Hubmann, 1992 from Thamnophyllum penecke, 1894 as they appear occasionally in large-sized species of Thamnophyllum. Because the specimens of Pseudopexiphyllum spp. are rare, further studies would be required to confirm that Pseudopexiphyllum Hubmann, 1992 is synonym of Thamnophyllum penecke, 1894 or it has evolved from Macgeea webster, 1899 and therefore would be homeomorphic with Thamnophyllum.
Frechastraea schafferi penecke, 1903 *1903 Phillipsastraea schafferi n. sp.penecke, p. 147, Pl. 6, figs 1-2, Pl. 7, fig.1. non 1958 Phillipsastraea schafferi penecke -Bulvanker, p. 121, Pl. 58, fig. 2, Pl. 59, fig. 1a-b. 1959 Phillipsastraea schafferi penecke - Kiratlioğlu, p. 17, 22. 1992 Mixogonia schafferi penecke -Hubmann, p. 159, Pl. 6, figs. 1-2. Fig. 4A-D, G-I Diagnosis. After Penecke, 1903, cerioid colonies with corallites 5-6mm in diameter having 26-28 septa. Major septa reaching the axis where they commonly fused in bundles of 2-3 septa. Minor septa confined in the dissepimentarium, no longer than half of the corallite radius. Septa rather constant in thickness in the dissepimentarium but thin in the tabularium. Axial ends of septa joined by axial part of the tabulae. Thickened inner row of dissepiments forming an inner wall. Dissepiments large and horizontal in periphery, smaller and more declined towards the axis, commonly vertical at the tabulariumdissepimentarium boundary. Tabulae dome-shaped in the axial part of the corallite. Description. Massive low-dome colonies with concave base and marked growth striae. Corallum pseudocerioid limited by a zigzag septotheca locally broken into segments, with polygonal corallites. Corallites 4-5mm wide with a tabularium 1.8-2.4mm in diameter. Thirteen to sixteen septa of both orders. Major septa long reaching the axis where they joined in bundles or connect to the upturned tabulae, locally forming an aulos-like structure open towards the cardinal fossula or completely close. Minor septa short, limited to the dissepimentarium or entering very shortly into the tabularium. Both orders of septa spindle-shaped thickened in the dissepimentarium, reaching a maximum thickness at the tabularium-dissepimentarium boundary. Both orders of septa locally carinate (yardarm carina). Major septa thin and straight in the tabularium. Dissepiments in 3-4 rows, the innermost being thickened in contact with the septa. In longitudinal section, dissepiments globose, larger and horizontal in the outer dissepimentarium, smaller and sub-vertical in the inner dissepimentarium. No horseshoe dissepiments. Tabulae slightly concave, with axial tabellae upturned towards the axis, occasionally forming an auloslike column. Trabecular fan in the thickened part of the inner dissepimentarium.
Discussion. The present specimens, are very similar to those described by Penecke (1903) under the name Phillipsastraea schafferi. Penecke's plate VI, figure 2a is indeed very similar to the present Figure 4A both displaying an aulos-like structure. The present material differs from Frechastraea crassiseptata tsien, 1978, with which it shares similar dimensions and number of septa, by the relatively limited septal thickening and by the higher development of carinae. Hubmann (1992) attributed his material to Penecke's species but consider it to belong to the genus Mixogonaria kong in Kong and Huang (1978). However, this genus is characterized mainly by asymmetrical trabecular fans expanding throughout the dissepimentarium, whereas Hubmann's plate 2, figure 2 shows poorly expressed trabecular fans seemingly situated in the inner dissepimentarium. Consequently, it is suggested to replace Penecke's species in the genus Frechastraea scrutton, 1968, based on the pseudocerioid habitus, the underdevelopment of horseshoe dissepiments and the occurrence of tight trabecular fans near the tabulariumdissepimentarium boundary (see McLean (1989) for discussion of that genus). The aulos-like structure is documented by Hubmann (1992) in his material but also in Penecke (1903) original description. Hubmann (1992) suggested a close relationship between F. schafferi penecke, 1903and Phillipsastraea bethunei tsien, 1978(=Frechastraea crassiseptata Tsien, 1978 but the latter is smaller and has some isolated horseshoe dissepiments but not the aulos-like structure. McLean (1994a) considers Penecke's species to be placed in Hexagonaria güricH, 1896 based on the arrangement of the trabeculae that appear more or less perpendicular to the dissepiments on the original figures (plate VI, figure 1b). However, on Penecke's plate VI, figure 2b, the arrangement of the trabeculae is less clear and this possibly led Hubmann to include the species in Mixogonaria rather than in Frechastraea. In the present material, the trabeculae are arranged in a fan extending in the inner third of the dissepimentarium (see Fig. 4D), allowing to place the species within the genus Frechastraea scrutton, 1968 in the sense of McLean (1994b).

DISCUSSION
Although Early and early Middle Devonian rugose corals show provincialism, they are strongly cosmopolitan in the Frasnian, at least at the genus-level (Oliver and Pedder, 1979) with almost no endemism (Oliver, 1973). However, at the species-level -or at least group-species-leveldifferences exist and could be used in palaeogeographic approaches.
The coral assemblage recovered from the middle part of the Meydan Fm. in the Danagözü section, notably the concomitant occurrence of the genera Frechastraea, Macgeea and Peneckiella, suggests an upper Frasnian age. Similar assemblages are known in the upper Frasnian strata of Western Europe (e.g. Pickett, 1967;Scrutton, 1968), Eastern Europe and Urals Mountains (e.g. Soshkina, 1951;Wrzołek, 1992), Australia (Brownlaw and Jell, 2008;Hill and Jell, 1970), North America (e.g. McLean, 2005;Sorauf, 1998) and South China (Liao and Ma, 2007). Species of the rather rare genus Pseudopexiphyllum have, so far, a poor palaeobiogeographic signature as they have been reported only from two localities, one in Turkey and other in Karakorum, Pakistan, (Schröder, 2004). The occurrences cited by Hubmann (1992) in Iran, Afghanistan and Tibet are not supported by the literature. Further comparisons with time-equivalent coral associations of Northern Gondwana are imprecise as only scattered data have been published so far. In addition to the Turkish occurrences cited above, some upper Frasnian corals were reported from Iran by Ghods (1982) and Rohart (1999Rohart ( , 2000, from Pakistan by Reed (1922), von Schouppé (1965 and Schröder (2004), from Nepal by Flügel and Tintori (1993) and from Afghanistan by Brice (1970). In Western Australia, upper Frasnian coral associations were abundantly described by Hill (1954), Hill and Jell (1970) and Brownlaw and Jell (2008). In all cases, the genera Frechastraea, Macgeea and Peneckiella are represented by distinct and mostly local -if not endemicspecies (Fig. 5). Hence, there is no marker typical of the northern margin of Gondwana. However, Flügel and Tintori (1993) suggested that the depauperate Frasnian coral fauna from these areas could correspond to a distinct province covering the northern margin of Gondwana. This assertion would require an in-detail revision of the coral fauna from the Gondwanian parts of Asia, which is not the aim of the present paper. Based on the close relationship of Upper Devonian conodonts, brachiopods and arthropods from Eastern Taurides with those of Europe, Yalçın et al. (2012) and Gourvennec and Hoşgör (2015) suggested that the Rheic Ocean separating Laurussia and Gondwana was very narrow (Wehrmann et al., 2010). Hence, local species could have evolved on an isolated block of the Cimmerian terranes, off Gondwana and separated by wide seas that acted as faunal barriers (Fig. 5) (Schröder, 2004;Talent et al., 1987).