Southeast Anatolia represents the northern margin of the Arabian Plate, located in the southern hemisphere as a part of the Gondwanaland during the Palaeozoic (Göncüoğlu and Kozlu, 2000). However, well-dated Lower Silurian rocks (Tanf Formation), represented by an alternation of organic rich shale and sandstone, have been described only from one location (Nusaybin area) (Bozdoğan et al., 1987 and Bozdoğan and Ertuğ, 1997). A regional depositional break during the Early Silurian is followed by Late Silurian deposits (Dadaş Formation), which are restricted to the central and northern parts of Southeast Anatolia (Diyarbakir Basin). Deposition of the Dadaş Formation, which consists of predominantly organic rich shale, sandstone, limestone and dolomite of restricted marine environment, is completed by a regressive cycle during the Early Devonian (Bozdoğan et al., 1987 and Yılmaz and Duran, 1997).

Well-preserved and diverse palynomorphs, including acritarchs and chitinozoans belonging to the Tanf and Dadaş formations have been recovered from some wells (Erkmen and Bozdoğan, 1979 and Steemans et al., 1996). Sixty acritarch species belonging to thirty-three genera and eight chitinozoan species belonging to seven genera have been identified. Late Llandovery-Early Wenlock samples are characterized by the presence of the acritarch species Domasia bispinosa, Dateriocradus monterosae and Carminella maplowoodensis and Ludlow-Přídoli samples are characterized by the presence of Hapsidopalla spongiosa, Cymbosphaeridium pilar and Leonella carminae acritarch species and Calpichitina corinnae chitinozoan species (Bozdoğan et al., 1987 and Ertuğ et al., 1998).

Within the Dadaş Formation (Fig. 2), three members are distinguished based on different lithological compositions, which are reflected in log characteristics (Bozdoğan et al., 1987). The Dadaş 1 Member consists of dark, organic-rich shales with some limestone interbeds; the Dadaş 2 Member is composed of similar shales alternating with some sandstones and the Dadaş 3 Member consists of an alternation of sandstones, marls and calcareous siltstones. The Dadaş Formation lies unconformably on the Middle-Upper Ordovician Bedinan Formation and is overlain conformably by the Devonian Hazro Formation (Fig. 2) (Bozdoğan et al., 1987, Kellog, 1960 and Perinçek et al., 1991).

The individual members of the Early-Late Silurian (Wenlock- Pridoli) to Early Devonian (Lochkovian) age Dadaş Formation of the Diyarbakır area in SE Turkey were evaluated with respect to their potential for petroleum formation based on some organic-geochemical, petrographic and biostratigraphical analyses (Bozdoğan et al., 1994). The sedimentary sequence of the Dadaş–1 and –2 formations was evaluated on data from several deep boreholes, because due to the geologic situation, rock samples of this formation are not available as outcrops near the Diyarbakır-Bismil area (Bozdoğan et al., 1994, Erkmen and Bozdoğan, 1979 and Steemans et al., 1996).

The Dadaş Formation was deposited on a restricted inner shelf, which was developed on the irregular paleotopography of the eroded Bedinan Formation. The respective shelf became shallower and was gradually converted to a tidal flat towards the top of the sequence (Bozdoğan et al., 1994 and Yılmaz and Duran, 1997). The Dadaş Formation is equivalent to the Qalibah Formation in Saudi Arabia, Akkaş Formation in Iraq, Tanf Formation in Syria, and Mudawwara Formation in Jordan (Lüning et al., 2005).

Silurian rocks (Dadaş-1 and lower Dadaş-2 members) are not exposed in Diyarbakır area; however, they have been penetrated in several boreholes in southeastern Turkey (Fig. 1). Bahar-1 well (Longitude: 40° 30′ 30 986″E, Latitude: 37° 59′ 06 7114″N) (Fig. 2) is drilled to test the production potential of Silurian-Dadaş (primary target) and Devonian-Hazro (secondary target) sequences. The well field is located in Petroleum District X-Siirt, the southeastern part of Anatolia.

The TransAtlantic Petroleum Corp-Bahar-1 well penetrated about 376 m of very dark gray to black shale, white, light gray limestone and sandstone, of which only the middle 12 m was cored (Fig. 2). The middle part 12 m (core 1, 2865–2877 m) contained the bivalve Cardiolinka bohemica (Fig. 3) and very poorly preserved orthoconic cephalopods.

The systematic arrangement of higher taxa largely follows the scheme proposed by Carter et al. (2011). The studied samples are from BHR1 GDA 01 and deposited in the Palaeontological Collection of Middle East Technical University by archive Nr: METU BHR1 GDA 01.

Class Bivalvia Linné, 1758

Subclass Autobranchia Grobben, 1894

Infraclass Pteriomorphia Beurlen, 1944

Order Cyrtodontida Scarlato and Starobogatov in Nevesskaja et al., 1971

Suborder Nepiomorphia Kříž, 2007 (=Praecardiidina Newell, 1965)

Hyporder Praecardioidei Newell, 1965

Superfamily Cardioloidea Hoernes, 1884

Family Cardiolidae Hoernes, 1884

Generally, the Bohemian type of fauna is characterized especially by epibyssate representatives of the families Cardiolidae (Kříž, 1999a). The Silurian family Cardiolidae Hoernes, 1884 consists of 13 genera: Copenychia Kříž, 2005 (Late Telychian); Cardavia Kříž, 2005 (Late Telychian); Cardiobeleba Kříž, 1974a (Early Sheinwoodian–Early Homerian); Carnalpia Kříž, 1974a (Late Sheinwoodian); Cardiolopsis Stache, in Heritsch, 1929 (Late Sheinwoodian); Cominicula Kříž, 1974a (Late Sheinwoodian); Cardicarnia Kříž, 1974a (Late Sheinwoodian); Nutricula Kříž, 1974b (Early Homerian); Cardiola Broderip, in Murchison, 1839 (Late Sheinwoodian–Early Pridoli); Isiola Kříž, 1976 (Late Sheinwoodian–Larly Přídolí); Cardiolinka Kříž, 1981 (Late Ludfordian–Late Pridoli); Pygolfia Kříž, 1974b (Early Ludfordian–Late Pridoli); Snoopyia Kříž, 1976 (Late Ludfordian–Late Pridoli) (Kříž, 2007) (Fig. 4).

Genus Cardiolinka Kříž, 1981

Type species. Cardiolita bohemica (Barrande, 1881), Bohemia, Prague Basin, Pridoli, Požáry Formation.

Discussion

The genus Cardiolinka represents most probably infaunal Cardiolidae and occurs from the Ludlow to Uppermost Pridoli mainly within the Gondwana, Perunica, Baltica and Arctic Canada regions (Kříž, 1999a and Kříž, 1999b).

Cardiolinka bohemica (Barrande, 1881)

Fig. 3A–C.

1881 Cardiola bohemica Barrande, pl. 164, figs. IV/19–22; PL. 168, figs. 1–5, 6, II/3–4, III/1–6, IV/1, 2, V/3, 4, VI/3, 4, VIII/1, 2, X/1, 2, XI/1–4, XII/1, 2; PL. 169, figs. 1–5, 14–23, 26–28, 31/38; pl. 170, figs. 8–20.

1979 Cardiolita bohemica (Barrande, 1881), Kříž, p. 106–110, pl. 32, figs. 1–10; pl. 39, figs. 1–3, 5–11; pl. 40, fig. 7 (for complete previous synonymy see this paper).

1996 Cardiolinka bohemica (Barrande, 1881), Kříž, p. 48, pl. 4, figs. 7, 12, 16.

1999c Cardiolinka bohemica (Barrande, 1881), Kříž, p. 287, pl. 6, figs. 28–29; pl. 7, figs. 1–3.

Lectotype (designated by Kříž, 1974b). Internal mould of a left valve figured by Barrande in 1881 on pl. 169 as figs. 16–19, re-figured by Kříž (1979) on pl. 32 as figs. 4,7,10; deposited in the National Museum, Prague under no. NM L 6874.

Paralectotypes. All other specimens figured by Barrande in 1881, designated by Kříž (1979) as Cardiolita bohemica (=Cardiolinka bohemica) and deposited in the National Museum, Prague.

Type horizon. Lowermost layer of Pristiograptus ultimus zone, Požáry Formation, Pridoli.

Type locality. Dlouhá hora hill near the town of Beroun.

Description and discussion

The specimen is conspecific with Bohemian types. It shows characteristic general shape and size, growth bands divided into unequal parts by a shallow growth furrow with the ventral part larger. The width of radial gutters is smaller than width of the radial ribs; they are deeper than growth furrows. Also the type of radial ribs is characteristic as well as the very reduced swollen band which is practically similar to normal neighbouring growth bands (Kříž, 1979).

Material and dimensions (mm)

One right valve. BHR1 GDA 01, Stage V, L = 2.1, H = 2.2.

During the Paleozoic Era, the interactions between the continents of Laurentia, Baltica and Gondwana were governed by two major oceans: Iapetus and Rheic Ocean (Bozkurt et al., 2008 and Nance et al., 2012). Late Silurian was the time of the major development of the Caledonian orogeny and final closure of the Iapetus. The Rheic Ocean, on the other hand, opened in the Early Ordovician and it is arguably the more important ocean of the two. Stretching from Mexico to the Middle East, it was the Rheic Ocean that separated the great paleocontinents of Gondwana and Laurussia as the principal interior ocean of the Paleozoic (Fig. 4) (Nance and Linnemann, 2008). The northern and southern Rheic domain was a large ocean as indicated by palaeomagnetic and biostratigraphic records from Ordovician and Silurian sediments (Cocks and Torsvik, 2002 and Dojen, 2009a).

The Prague Basin of central Bohemia (Perunica), Montagne Noire, Mouthoumet Massif, Carnic Alps and Sardinia have been long considered as a classic areas for the Silurian bivalve biostratigraphy (Kříž, 1999b) and reference areas for correlation. Many Silurian Bivalvia dominated communities (e.g. Cardiola Community Group, Cheiopteria Community Group and Snoopyia Community Group) and index species were first distinguished and described from these areas (Ferretti and Serpagli, 1996, Kříž, 1999a and Kříž and Serpagli, 1993). During the Late Ludlow and Pridoli, some cardiolid bivalves (Cardiola, Cardiolinka and Snoopyia) became widely distributed outside peri-Gondwana. Cardiolinka is also known from the East European Platform (Poland) and the Moesian Platform (Romania and Serbia) (Korejwo and Teller, 1964, Kříž, 1996, Kříž, 1999a, Kříž, 1999b, Kříž, 1999c, Kříž, 2007, Kříž, 2008, Kříž and Iordan, 1975 and Kříž and Veselinoviç, 1975) (Fig. 4).

The Bohemian-type bivalve Cardiolinka bohemica (Barrande, 1881), which is characteristic for the Perunica, and peri-Gondwana region, occurs in the Latest Ludfordian to Pridoli of the Bohemia, France, and Carnic Alps (Fig. 4). Considering the palaeogeographic reconstructions, the locations of SE Turkey (North Arabian Platform), France, Carnic Alps and Sardinia during the Ludlow-Pridoli times were around 30° southern latitude and mainly concentrated on eastern margins of Rheic Ocean (Cocks and Torsvik, 2002, Tonarova et al., 2012 and Turek and Manda, 2012). However, there are some paleomagnetic data that locate Sardinia and the Carnic Alps at a palaeolatitude of 30–40°S for the Silurian (Schönlaub, 1997). The other occurrences of Cardiolinka are located at latitudes lower and higher than 30°, but on the western margins of the Rheic Ocean in Arctic Canada and on the coasts of Baltica (East European Platform). The position of Bohemia (Perunica) is less well constrained, but according to Kříž et al. (2003), and Cocks and Torsvik (2005) it was situated in more temperate climates northwest of the peri-Gondwanan area at slightly less than 30° S. Cardiolids were also recorded from Tajmyr, Russia (Kříž and Bogolepova, 1995), just north of the Equator, and from Guinea around 60° southern latitude (Kříž, 1979).

Havlíček (1999) argued, based on benthic faunas (mainly brachiopods and trilobites), that Perunica was much closer to Baltica than any other region of Gondwana, specifying its position at the northeastern corner of peri-Gondwana. A similar palaeogeographic scenario is suggested from the faunal distribution of nepiomorphian bivalves (e.g. Kříž, 2007) and particularly nautiloid cephalopods (Manda, 2008 and Turek and Manda, 2012) and jawed polychaetes (Tonarova et al., 2012). It is not surprising that, although separated by an oceanic barrier, similar subtropic palaeolatitudes and shallow marine paleoenvironments are reflected by closely related benthic communities in the Late Silurian of Baltica, Perunica, peri-Gondwana and northern Gondwana.

During the Middle–Late Silurian, the northern parts of the Gondwana margin were quite close to the eastern coasts of peri-Gondwana. Cardiolinka was widely distributed during the Late Silurian in the tropical and subtropical shallow seas of Baltica and Perunica, even reaching the somewhat colder waters of the peri-Gondwanan basins (Fig. 4). Almost no provincialism, but rather the widest cosmopolitanism is characteristic for the Silurian Bivalvia, which were dispersed in most of the regions of the world (Kříž, 1999c and Kříž et al., 2011) due to their relatively long pelagic larval life and the relatively small distances among the basins, islands and continents on the Silurian Globe (Cocks and Torsvik, 2002 and Cocks and Torsvik, 2005). Up to now only a few occurrences of bivalve remains have been reported from the Middle - Late Silurian successions of eastern Taurides. Pojeta et al. (1976) were the first to describe a species Cheiopteria bridgei Pojeta and Kříž, 1976, from the Pridoli deposits at the Halevikdere locality in the Anatolian microplate and from the Cone well in Florida, USA. This palaeogeographically and palaeoecologically important cosmopolitan species was later described as representative of the Cheiopteria Community Group (Kříž, 1999a) from the Pridoli of the Armorican Massif (Kříž and Paris, 1982), Montagne Noire (Kříž, 1996), and Sardinia (Kříž and Serpagli, 1993). Moreover, the new record of the Bohemian-type bivalve Cardiolinka bohemica (Barrande, 1881) from northern Gondwana confirms the idea that the Silurian Bivalvia were mostly cosmopolitan.

Evidence from other fossil groups is also consistent with that of the bivalves. During the Middle-Late Silurian, the Rheic Ocean was not a major barrier against faunal and floral exchanges between Baltica, Perunica and Gondwana. Marine and nonmarine biogeographic patterns based on thelodonts, spores, macroplants, chitinozoans, acritarchs, conodonts, brachiopods, phragmoceratids and trilobites indicate broad similarities and support that the Rheic Ocean separating Baltica and northwestern Gondwana narrowed in the Middle-Late Silurian (Ferretti and Serpagli, 1996, Manda, 2008 and Mergl, 2006). Recent studies of the beyrichioid ostracodes, Hobergiella, Juviella, Hemsiella, and Macrypsilon with these species from the middle and upper Dadaş Formation (Hazro Anticline-NE Diyarbakır) also suggested palaeobiogeographic relationships between North Gondwana and Baltica terranes (Dojen, 2009b).

The bivalve Cardiolinka bohemica (Barrande, 1881), representative of the suborder Nepiomorphia Kříž, 2007, is observed for the first time in the Silurian of Turkey. The species known originally from Bohemia thus documents its distribution in the northern Arabian Plate at northern parts of the Gondawana margin. The occurrence of the Late Ludlow–Pridoli Bohemian type bivalve Cardiolinka bohemica (Barrande, 1881) in the Arabian Plate represents another line of evidence in favour of strong faunistic affinity between Perunica, European peri-Gondwana, and the northern Gondwana margin.