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1. Introduction
The Oligo-Miocene Qom Formation of Iran, because of its highly fossiliferous deposits, has been the subject of investigations since very long (Schuster and Wielandt, 1999; Khaksar and Maghfori-Moghaddam, 2007; Reuter et al., 2009; Behforouzi and Safari, 2011; Mohammadi et al., 2011; Yazdi et al., 2012; Mohammadi and Ameri, 2015; Zágoršek et al., 2017; Hyžný et al., 2021). However, brachiopods are still poorly known from the Qom Formation. Although their presence was mentioned from different sections (Nouradini et al., 2014, 2015, 2019; Pedramara et al., 2019), the only taxonomical description of brachiopods from the Qom Formation is that from the Varton section where six micromorphic species, i.e. Lacazella mediterranea (Risso, 1826), Megathiris detruncata (Gmelin, 1791), Argyrotheca cuneata (Risso, 1826), A. bitneraeDulai, 2011, Joania cordata (Risso, 1826), and Platidia anomioides (Scacchi and Philippi, 1844), were recognized (Pedramara et al., 2019).
In this study, we present the new records of brachiopods from the Lower Miocene deposits of the Qom Formation, collected at two outcrops situated at Bagh and Ramsheh, Central Iran (Figure 1). The brachiopods are of low diversity, represented by three micromorphic species.
2. Geological setting
The Qom Formation was deposited in the interval from late Early Oligocene until the end of the Early Miocene in northern and central Iran (Abaie et al., 1964; Stöcklin and Setudehina, 1991; Reuter et al., 2009; Yazdi et al., 2012). The Qom Formation crops out along the Zagros fold belt and is mainly composed of marine marls, limestones, gypsum and siliciclastic strata (Reuter et al., 2009). In general, the Lower Red Formation underlies the Qom Formation that is conformably overlain by the Upper Red Formation (Stöcklin and Setudehina, 1991). The Qom Formation was divided by Furrer and Soder (1955) into six members: a) basal limestone, b) sandy marl, c) alternating marl and limestone, d) evaporate, e) green marl, and f) limestone. Reuter et al. (2009) recognized two basins for the Qom Formation: the Qom back-arc basin and Isfahan-Sirjan fore-arc basin. Subsequently, Oligocene-Miocene sedimentation in the Qom basin took place along a carbonate ramp including intertidal, shelf lagoon, platform margin and open marine environments (Seyrafian and Toraby, 2005; Daneshian and Ramezani Dana, 2007; Behforouzi and Safari, 2011). During the Early-Middle Miocene periodic connections prevailed between the Mediterranean Sea, Paratethys province, Indo-Pacific region and the Atlantic Ocean (Rögl, 1998; Popov et al., 2004; Harzhauser et al., 2007). Intermittent seaway connections and regional closings were mainly driven by regional and global geotectonics and sea-level fluctuations (Rögl, 1998).
The Lower Miocene carbonate deposits of the Qom Formation are well exposed in the Bagh outcrop, situated at approximately 55 km northeast of Isfahan, Central Iran (Figure 1). The GPS coordinates are 32˚57’61”N and 52˚0’95”E. The thickness of the Qom deposits in the Bagh section reaches 85 m. Lithologically they are mainly represented by greenish-grey, fissile, fossiliferous calcareous shale interbedded with thin beds of argillaceous limestone. For details see Nouradini et al. (2015) and Zágoršek et al. (2017).
The Ramsheh section is located approximately 135 km southeast of Isfahan (Figure 1). The WGS coordinates of the base of the section are 31°30’2.85”N and 52°48’48.91”E and the top 31°31’10.41”N and 52°48’52.22”E. The section consists mainly of marly and sandy limestones interbedded by sandstones and black shales (Figure 2).
3. Material and methods
The brachiopods investigated herein were collected in two outcrops situated at Bagh and Ramsheh, Central Iran (Figure 1). All specimens come from the sediment bulk samples collected during the fieldworks carried out in 2014, 2018 and 2019. The samples were washed and/or soaked with solution of hydrogen peroxide if necessary, then wet-sieved. The residuum was checked for brachiopods using a binocular microscope. The total number of specimens collected in the Bagh section is 485, while in the Ramsheh section it is 31 specimens.
Specimens selected for scanning electron microscopy were mounted on stubs, coated with platinum and photographed using a Philips XL-20 microscope at the Institute of Paleobiology, Warszawa. The material studied is housed at the Geology Museum of the University of Isfahan, Iran under UIGM acronym.
4. Results
The Early Miocene brachiopod fauna collected at the Bagh and Ramsheh sections is of low diversity, containing three micromorphic species belonging to two families (Megathyrididae and Platidiidae) and one order (Terebratulida).
Family Megathyrididae Dall, 1870
Argyrotheca bitneraeDulai in Dulai and Stachacz, 2011
Figure 3 Brachiopods, Early Miocene, Iran. A-G. Argyrotheca bitneraeDulai, 2011, Bagh section; A-D, Dorsal views of articulated specimens (UIGM2026-2028), and enlargement (D) of the umbonal part to show details of foramen and deltidial plates; E-G, inner views of dorsal valves (UIGM2029-2030) and oblique view (G) to show a septum without serrations. H-M. Joania cordata (Risso, 1826); H, ventral view of articulated specimen (UIGM2031), Ramsheh section; I-K, dorsal views of articulated specimens (UIGM2032-2033) and enlargement (J) of umbonal part to show details of beak, Bagh section; L, inner view of partly broken ventral valve (UIGM2034), visible hooked teeth and marginal tubercles; M, inner view of dorsal valve (UIGM2035), visible high septum and distinct marginal tubercles. All SEM.
In the material under study this species has been found only at the Bagh section where it is very numerous, represented by 173 articulated specimens, one ventral and four dorsal valves. It is, however, known from the Qom Formation deposits, being identified in the material from the Varton section (Pedramara et al., 2019). The species A. bitnerae is characterized by a triangular, weakly biconvex shell with smooth surface and high beak. Its dorsal septum is high, triangular in profile, without serrations (Figure 3G).
Argyrotheca bitnerae was originally described from the Middle Miocene of the Central Paratethys (Dulai and Stachacz, 2011; Hladilová et al., 2014; Dulai, 2015; Kopecká et al., 2018). In Europe, it was also recognized in the Oligocene deposists of France (Bitner et al., 2013a) and Germany (Dulai and von der Hocht, 2020; Bitner and Müller, 2022).
Joania cordata (Risso, 1826)
This species was identified in both sections; at Bagh it is represented by 146 articulated specimens, three ventral and three dorsal valves, while at Ramsheh it is much less numerous, being represented by 28 articulated specimens and two dorsal valves. Recently, Joania cordata was recorded in the material collected from the Qom Formation deposits at the Varton section (Pedramara et al., 2019). The species is small, varying in outline from heart-shaped to rounded subrectangular, weakly biconvex. Its surface is smooth or weakly ribbed; ribs are usually more distinct on the ventral valve (Figure 3H). Internally, it is characterized by a high median septum with numerous serrations and submarginal tubercles on both valves (Figure 3L, M). In the Middle Miocene of the Central Paratethys J. cordata is one of the most common species (Bitner, 1990, 1993; Bitner and Pisera, 2000; Bitner and Dulai, 2004; Bitner and Kaim, 2004; Dulai, 2007, 2015; Zágoršek et al., 2012; Bitner et al., 2013b, Bitner et al., 2014; Hladilová et al., 2014; Bitner and Motchurova-Dekova, 2016; Kopecká et al., 2018). Its oldest fossil record is from the Upper Oligocene (Bitner et al., 2013a), and today it lives in the NE Atlantic, Mediterranean Sea and Red Sea, occupying the depth from 3 to 600 m (Logan, 2007; Logan et al., 2008).
Family Platidiidae Thomson, 1927
Platidia anomioides (Scacchi and Philippi in Philippi, 1844)
Figure 4 Platidia anomioides (Scacchi and Philippi, 1844), Early Miocene, Iran. A. Dorsal view of young individual (UIGM2036), Bagh section; B, C. Dorsal view of articulated specimen (UIGM2039) and enlargement (C) of umbonal part, Ramsheh section; D-G. Dorsal views of articulated specimens (UIGM2037-2038) and enlargement (E, G) of umbonal part to show details of amphithyrid foramen, narrow interarea and deltidial plates, Bagh section. All SEM.
Although found in both outcrops, Platidia anomioides is very rare in the material from the Ramsheh section, represented by one specimen only. In the Bagh section, however, it is very common, with 152 articulated specimens, two ventral and one dorsal valves. This species was already described from the Qom Formation deposits from the Varton section (Pedramara et al., 2019). The characteristic feature of Platidia is a large, subcircular foramen of amphithyrid type. Platidia anomioides is small, oval to subcircular in outline with smooth surface. Its hinge line is straight and very short.
Known since the Late Eocene (Dulai, 2011), P. anomioides is widely distributed in the Miocene of the Central Paratethys (Bitner, 1990; Bitner and Dulai, 2004; Bitner et al., 2013b; Bitner and Motchurova-Dekova, 2016). In modern waters this species is known from all oceans having a wide depth range from 8 to 2190 m (Logan, 2007).
5. Final remarks
The brachiopod fauna collected in the Lower Miocene Qom Formation deposits at two outcrops, Bagh and Ramsheh, Isfahan province, Central Iran contains three and two species, respectively. The species Joania cordata and Platidia anomioides were found in both sections, while Argyrotheca bitnerae is present only in the material from the Bagh section. All three species were already reported from the Qom Formation deposits at the Varton section (Pedramara et al., 2019), however, in the species composition the Varton assemblage differs from those of Bagh and Ramsheh. At Varton the predominant species is thecideide Lacazella mediterranea (Risso, 1826), absent in the Bagh and Ramsheh material. Also two other species, Megathiris detruncata (Gmelin, 1791) and Argyrotheca cuneata (Risso, 1826), present at Varton were not found at Bagh and Ramsheh. Those differences in the taxonomic composition are difficult to explain, however, we can speculate that the absence of L. mediterranea might be caused by the absence of light-poor, cryptic habitats at both studied localities, preferable environment for this species (Logan, 2008). Additionally, Lacazella mediterranea is a shallow water species with maximum occurrence from 20 to 60 m (Logan, 1979), whereas the presence of Platidia anomioides in significant amount may indicate deeper water environment for the studied deposits. Although having a very wide depth range (Logan, 2007), P. anomioides is considered a deeper water brachiopod, being most common at the depth below 200 m (Logan, 1979; Logan et al., 2008).
All species identified herein are well known from many localities in the Paleogene and Neogene of Europe. Until the closure of the Tethyan Seaway (Harzhauser et al., 2007; Reuter et al., 2009) Central Iran was a part of the Western Tethys Region, including the proto-Mediterranean and the Paratethys seas, and the connection with the Indian Ocean, thus the great faunistic similarities between the Qom Basin and the proto-Mediterranean and the Paratethys are observed among many groups (e.g. Nouradini et al., 2014; Zágoršek et al., 2017; Pedramara et al., 2019; Hyžný et al., 2021).
