Decapod crustacean from the Qom Formation (Lower Miocene) of the Isfahan area, Central Iran

Introduction

A number of studies on fossil decapod crustaceans of Iran were published in the last fifteen years. For such a geologically diverse country, this is, however, still insufficient and the knowledge on the decapod fossil record of Iran remains poor. To our knowledge, the first report of fossil decapods was that by Withers 1932, Glaessner 1933. Subsequently, Förster and Seyed-Emami (1982) on an erymid lobster from the Middle Jurassic of northern Iran. Cretaceous decapods were discussed by McCobb and Hairapetian (2009), Yazdi et al. (2009, 2010), Jagt et al. (2014), and Bahrami et al. (2020); the alleged lobster of uncertain affinities described by Feldmann et al. (2007) was later reinterpreted to be an isopod (Hyžný et al. 2020). Cenozoic decapod occurrences from Iran include those from Eocene, Oligocene, and Miocene strata. Eocene decapods were reported by Garassino et al. (2014), Khodaverdi Hassan-Vand et al. (2016), and Ossó et al. (2023), whereas the Oligocene decapods were reported by Bahrami et al. (2023). The first report on Miocene decapod crustaceans of Iran was an abstract by Torabi and Yazdi (2002), presenting a portunid crab from the Isfahan area. Much more attention was dedicated to Miocene decapod assemblages from the Mishan Formation (Vega et al. 2010, 2012; Heidari et al. 2012; Hyžný et al. 2013; Yazdi et al. 2013; Key et al. 2017; Hyžný et al. 2021; Khosravi et al. 2022). The present study focuses on the description of the decapod from the lower Miocene of the Qom Formation exposed in the Isfahan area.

Fig 1- Geographic map of Iran with study area location in central Iran (left) and the Late Oligocene–Early Miocene palaeogeography of the Tethyan Seaway and adjacent regions, with the indication of Qom Basin (after Harzhauser and Piller 2007; Reuter et al. 2009).

Geological setting

In general, the Rupelian–Burdigalian (Early Oligocene–Early Miocene) Qom Formation was deposited at the northeastern margin of the Tethyan Seaway in three northwest–southeast-trending basins, including the Central Iran back-arc basin, the Urumieh–Dokhtar magmatic arc (intra-arc) basin, and the Sanandaj–Sirjan fore-arc basin (Schuster and Wielandt 1999; Daneshian and Ramezani Dana 2007; Reuter et al. 2009; Mohammadi et al. 2013, 2015; Mohammadi 2023). Based on Reuter et al. (2009), at first, the fore-arc basin was inundated by the marine transgression, of the Qom Sea during the late Early Oligocene (Ru 3 cycle), while in the back-arc basin marine environments were not established until the beginning of the Late Oligocene (Ru 4/Ch 1 cycle). Subsequently, normal marine conditions prevailed in both basins throughout the Oligocene. In the Isfahan–Sirjan basin this condition continued during Aquitanian and early Burdigalian, while in the Qom Basin due to the compressive tectonics, the gates to the open ocean gradually became restricted in the Early Miocene and during the the Aquitanian and Burdigalian restricted marine conditions prevailed and episodic precipitation of evaporate deposits occurred.

Berberian (1983) attributed the formation of the Oligo–Miocene Qom Basin to the subduction of Neotethys oceanic crust beneath Central Iran, which formed a back-arc opening, deposition of the Qom Formation, and alkaline volcanic processes, but did not point to the formation process of fore-arc basin. Based on Rahimzadeh (1994), the closure of the Oligo–Miocene, central Iran basin was possibly the result of the eustatic falling of sea level, orogenic movement, or the contemporaneous effects of both. However, Reuter et al. (2009) believed that the closure of the Tethyan Seaway and the disruption of connections of the Qom Basin with the open water of Proto-Indopacific was caused by the collision of the African-Arabian and Eurasia plates. According to Bassi et al. (2009), the closure of the connection between the Mediterranean Sea and the Indian Ocean took place before the Langhian (Figs 1–2).

The thickness of the Qom Formation varies from area to area, in the type area, the Qom Formation was laid on the gypsiferous and evaporitic red beds (Lower Red Formation) conformably and overlaid conformably by evaporitic red beds of Middle–Late Miocene age (Upper Red Formation) (Daneshian and Ramezani Dana 2007). Active tectonics led to the formation of complicated local tectonic movements with erosional faces which influenced the lateral thickness in the Qom Formation and produced facies variations (Poroohan et al. 2009; Jalali and Feizi 2010). Because of the major facies changes of the Qom Formation, no stratotype section has been introduced for it, but the Qom area (located approximately 100 km south of Tehran) is proposed as its type area (Rahimzadeh 1994; Aghanabati 2006; Mohammadi et al. 2011, 2013).

The studied locality (Bentonite mine section) is situated in the Isfahan area (Fig 3) in the Isfahan–Sirjan fore-arc Basin with GPS co-ordinates: 32° 56′ 28″ N; 52° 08′ 29″ E), 85 km north of Isfahan, near the Ab-Garm hot spring at the foothill of a bentonite mine. The section includes, grey-yellow to yellow-white highly fossiliferous marls and fossiliferous argillaceous to sandy limestones (Fig. 3a). The fossils, among others, include coralline red algae (Lithophyllum sp., Lithothamnium sp.), molluscs (Spondylus sp., Turritella sp.), and echinoderms (Eucidaris sp., Clypeaster sp., scutellid echinoids), with minor intercalation of marly limestones with the occurrence of foraminifers Neoalveolina melocurdica, Peneroplis evolutus, Dentritina rangi, Meandropsina anahensis, Acervulina sp., and Archaias sp., the foraminifers indicate Aquitanian–Burdigalian age (Pedramara et al. 2019). (Fig 3 c–i).

Accompanying of crustaceans, corals, bryozoans, echinoderms, mollusks and Skolithos burrows in the red algal grainstone-bearing limestone (Fig 3-a) indicates high hydrodynamic (water) energy (Vinn and Wilson 2013; Sedorko et al. 2018), coralline red algae (Lithophylum, Sporolithon, Mesophyllum and Lithothamnium) were the major and most abundant biotic components of photozoan and heterozoan communities of Oligocene–Miocene cool waters (non-tropic) and shallow tropical carbonates (Braga et al. 2010).

Fig 2- a: Map showing the distribution of Urumieh–Dokhtar Magmatic Arc, the Qom Formation outcrops, suture zones of Paleotethys and Neotethys in Iran (after Mohammadi et al. 2013: Mohammadi 2023), b: Geological map of Kuhpayeh area and the location of the study section in Isfahan area (modified from Radfar and Kohansal 2002).

Fig 3- a: stratigraphical column of the studied succession; b: close view of the outcrop with the indication (arrow) of the studied crustacean bearing level; c–i: prominent accompanying fauna, bivalve, echinoid, coral, balanoid and epibiont bryozoans. 

 Material & Methods

From the Burdigalian strata of studied sections, altogether one fossil brachyuran crab was recovered as ex-situ, on the surface (Fig 3a). Photographs were taken with a NIKON D5600 camera under natural light. The specimen is well-preserved and deposited at the Department of Geology, Faculty of Sciences, University of Isfahan, Iran (EUIC). Other abbreviations: NHMUK – Natural History Museum, London, United Kingdom.

Systematic palaeontology

Class Malacostraca Latreille 1802

Order Decapoda Latreille 1802

Infraorder Brachyura Latreille 1802

Section Eubrachyura Saint Laurent 1980

Subsection Heterotremata Guinot 1977

Superfamily Portunoidea Rafinesque 1815

Family Portunidae Rafinesque 1815

Portunus withersi (Glaessner 1933)

Fig 4A–D

1933* Neptunus (Achelous) withersi Glaessner, p. 8, pl. 2, figs. 1–3.

1969 Portunus (Achelous) withersi (Glaessner 1933); Glaessner, p. R510, fig. 319.2.

2008 Portunus withersi (Glaessner 1933); Karasawa et al. p. 127.

2012 Portunus withersi (Glaessner 1933); Heidari et al. p. 3, fig. 5.

2013 Portunus withersi (Glaessner 1933); Yazdi et al. p.230, figs. 5.1-4.

Studied material and measurements (in mm): One carapace with cuticle partially preserved, EUIC-87946: length = 38.9; width = 53.5.

Description

Carapace: Carapace subhexagonal, wider than long, widest at the level of last anterolateral spine; regions faintly marked. Front not preserved. Orbits relatively broad; supraorbital margin with two fissures, one medially placed, and another adjacent to the extra-orbital tooth; extra-orbital tooth acute subtriangular, forward directed. Anterolateral margins are widely convex, not completely preserved but retaining the remains of nine short subtriangular teeth alternating in size (extra-orbital tooth included), the last one, the epibranchial tooth, appears to be short. Posterolateral margins are convex, shorter than the anterolateral ones, entire and rimmed. Reentrant of the fifth pereiopod well marked, very broad. The posterior margin is straight, rimmed, with rounded angles. Gastric process barely marked; protogastric lobes slightly swollen; mesogastric region subpentagonal, thin and elongated anteriorly. Urogastric region depressed. Cardiac region diamond-shaped. Both, urogastric and cardiac regions are bounded laterally by marked branchiocardiac grooves. Intestinal region undefined, flattened. The hepatic region flattened. Epibranchial region slightly raised, defined by an arched ridge that ends in the epibranchial tooth; mesobranchial lobes slightly swollen; metabranchial region depressed. Ischium of third maxilliped elongated, inner margin convex, outer margin concave, deep groove paralleling the inner margin; merus subquadrate; exopod narrow, elongate. Thoracic sternum broad, ovate, maximum width at the level of sternite 6. Thoracic sternites 1, 2 not observable; sternites 3 and 4 fused; sternite 3 inverted subtriangular; sternite 4 subtrapezoidal, divided medially by deep sternopleonal cavity, crossed transversely at mid-length by a step; sternites 5, 6, and 7 transversely subrectangular with rounded outer margins, sternite 7 shorter; sternite 8 not preserved. Episternites 4 and 5 are short, curved, downward directed, and separated from sternites by a deep suture. Sternal suture 3/4 visible only laterally; sternal sutures 4/5, 5/6, 6/7, complete laterally. Male pleon broadly subtriangular; pleonal somite 6 broken; somites 3-4-5 fused, narrowing anteriorly; somite 2 extremely narrow transversely, wider than somite 3. Chelipeds and ambulatory legs are not preserved.

Remarks

The specimen described herein showcases strong morphological similarities, both dorsal and ventral, with Portunus withersi, described by Glaessner (1933, p. 8, 9, pl. 2, figs 1, 2; Fig 4C, D), from the Lower Miocene of Sulabdar in SW Iran, near the Persian Gulf. In despite of the poor dorsal preservation of the holotype, preserving only partially front-orbital and anterolateral margins (Fig.4C), the general outline, short epibranchial tooth, small anterolateral subtriangular teeth alternating in size, that is, although all the teeth are small, some are slightly smaller than the others, alternating between them, as Glaessner indicated in his original description, is coincident with the described specimen. Also, the ventral features of both (Fig. 4D) are almost identical, differing only in the degree of preservation. Thus, we can assign our specimen to Portunus withersi.

Subsequent authors, Heidari et al. (2012) and Yazdi et al. (2013), assigned to Portunus withersi several portunid specimens recovered in the Lower and Middle Miocene outcrops of the Persian Gulf Coast of Iran. The present specimen sheds some more light on the dorsal morphology of Portunus withersi, beyond Glaessner's idealized depictions (e.g. Glaessner 1969, fig. 2a), and based on it, perhaps it would be worth reviewing the two aforementioned assignments, to see if they actually correspond to P. withersi. In any case, more and better material would be necessary to know the true nature of the front-orbital and anterolateral margins of Portunus withersi, and thus confirm its belonging to the genus Portunus or to any other portunid genus, such as Achelous De Haan 1833, as Glaessner suggested in his original description.

Fig 4. A–D: Portunus withersi (Glaessner 1933). A–B: EUIC 87946 from the Lower Miocene of the Vartun section, north of Isfahan, A: dorsal view; B: ventral view. C–D: holotype NHMUK-24479, from the Lower Miocene of Sulabdar, SW of Iran, C: dorsal view; D: ventral view. Abbreviations: a = pleonal somites; cxP1 = coxa of pereiopod 1; ex = exopod of third maxilliped; is = ischium of third maxilliped; me = merus of third maxilliped; st = thoracic sternites. Scale bar = 10 mm. Photographs of C and D by Peter Grugeon.

Discussion

The Oligo–Miocene deposits of the Qom Formation developed in the southeastern margin of the Western Tethys Region (Fig 3). The faunas of this region are important for the interpretation of palaeobiogeography of the circum-Mediterranean region, including the proto-Mediterranean and Paratethys seas, and its connection with the Indian Ocean, as have been discussed by various scholars since the 1990s (Stöcklin and Setudehina 1991; Rögl 1999; Seyrafian and Torabi 2005; Harzhauser et al. 2007; Khaksar and Maghfouri Moghaddam 2007; Daneshian and Ramezani Dana 2007; Reuter et al. 2009; Mohammadi et al. 2011, 2013; Behforouzi and Safari 2011; Yazdi et al. 2013). The timing of the opening and closure of the Tethyan Seaway (especially in the fore-arc and back-arc basins) is still being debated. Bozorgnia (1966) believed that the Qom Sea (Qom back-arc basin) persisted from the Rupelian (early Oligocene) to the Burdigalian (late early Miocene). Schuster and Wielandt (1999) also mentioned that in both foreland basins (Sanandaj–Sirjan and Central Iran), the marine sedimentation began during the early Oligocene and continued until the end of the early Miocene. However, Berning et al. (2009), suggested that during the late Oligocene, the narrowing Tethyan Seaway formed a connection between the eastern and the western Tethys regions.

During the Paleogene, the fossil record of decapod fauna of Iran suggests a potential homogeneity of brachyuran faunas on both sides of the Tethys Realm (e.g. Khodaverdi Hassan-vand et al. 2016; Ossó et al. 2023; Bahrami et al. 2023). This relative homogeneity appears to have persisted until the very early Miocene (Hyžný et al. 2021). However, at the end of the Early Miocene and onward, this trend changes. Indeed, Glaessner (1933, p. 11) noted: “The crabs from the Persian Miocene are rather different from any European fauna”, suggesting a “tropical, Indo-Pacific character” for the Persian Miocene decapod fauna. This is consistent with subsequent works on Miocene decapods of that area, which reported fauna of clear Indo-Pacific affinity (Vega et al. 2010, 2012; Yazdi et al. 2013; Khosravi et al. 2022; Garassino et al. 2024). As for Portunus withersi, it appears to be the dominant portunid species in the siliciclastic sediments of the eastern Tethys area (Reuter et al. 2009), just as P. monspeliensis (Milne-Edwards 1860) was in the siliciclastic sediments of the Proto-Mediterranean and Paratethys areas during the Miocene (see Gašparič and Ossó 2016).

More research is needed to resolve the migratory routes of decapod crustaceans along the Tethyan Seaway, which might change throughout several tens of million years. Fossils from the Qom Basin, being positioned in the middle between eastern and western Tethyan regions, are of great importance in this research.

Conclusions

The new specimen represents an addition to the knowledge of the morphology of Portunus withersi. Further fieldwork to find more and better-preserved specimens will help to understand the diagnostic characteristics of this species and clarify its systematic position within the Portunidae family.

Acknowledgments

The authors would like to thank the University of Isfahan for providing laboratory and fieldwork logistics. We are grateful to Rich Howard (NHMUK) for providing images of the holotype. Thanks to the reviewers for their pertinent remarks.