Introduction

The existing record of Iranian Silurian brachiopods remains sparse and incomplete apart from the rich Aeronian brachiopod faunas of Central Iran and Kopet-Dagh, which are well documented in publications by Cocks (1979), Brice (1999), and Hairapetian et al. (2012). The major focus of this paper is to describe a small, oligotaxic brachiopod association from the Telychian deposits of the Boghu Mountains in the vicinity of the town of Kashmar, which are situated at the northern margin of the Tabas block in the north-eastern part of the Central Iran plate. The most distinctive component of this association is an unusual rhynchonellide with a strophic shell and a well-developed ventral interarea assigned to a new genus and species Jafarirhynchus alatus.

Institutional abbreviations.—AEU, Azad University, Esfahan, Iran; NMW, Department of Geology, National Museum of Wales, Cardiff, UK; CNIGR, F. N. Chernyshev Central Research and Geological Exploration Museum, St Petersburg, Russia.

Other abbreviations.—Lv, Ld, maximum length of ventral and dorsal valves; max, maximum observed size; min, minimum observed size; N, number of specimens; S, standard deviation from the mean; SLw, SLh, maximum width and height of the sulcus/median folds; W, T; maximum width, thickness of the shell; X, mean.

Geological and geographical Setting

The Boghu Mountains are situated about 25 km south-west of the town of Kashmar (Fig. 1). An outline of the Lower Palaeozoic geology of the area can be found in publications by Ghavidel-Syooki (2003) and Hairapetian et al. (2012). The best exposures are situated on the southern flank of the Boghu Mountains (Fig. 1A). There the Silurian Niur Formation rests unconformably on siliciclastic sediments (green shale member) of the Ordovician Katkoyeh Formation (Fig. 1B). The base of the measured section is situated at 34°4′6.2″N, 58°15′48.8″E, altitude 1222 m. The Niur Formation comprises intercalating beds of argillite, sandstone, siltstone, and bioclastic limestone. The Aeronian part of the section is characterised by a conodont assemblage including Distomodus staurognathoides (Walliser, 1964), Wurmiella ex. gr. excavata (Branson and Mehl, 1933), Ozarkodina spp. and Oulodus? sp., and a low diversity brachiopod fauna dominated by the rhynchonellides Stegocornu procerum Dürkoop, 1970 and Stegocornu denisae Popov, Modzalevskaya, and Ghobadi Pour in Hairapetian et al., 2012 (Fig. 1B).

Fig. 1.

A. Geographic and geologic setting of the studied material showing position of studied section (modified after Taheri 2001). B. Schematic map showing position of studied locality (asterisk). C. Stratigraphical column of the Boghu Section showing the informal lithostratigraphical subdivision of the Niur Formation, position of fossil samples, and stratigraphical distribution of brachiopods and conodonts. Arrows below and above the log indicate that the section continues in both directions (for a complete log see Hairapetian et al. 2012). Arrows at the lowermost of distribution line of some taxa indicates that these also occur in lower strata.

The brachiopods Jafarirhynchus alatus gen. et sp. nov. and Striispirifer? ocissimus Popov, Modzalevskaya, and Ghobadi Pour in Hairapetian et al., 2012 were sampled from the bed of grey bioclastic limestone at 12 m above the base of unit 22 (Fig. 1; sample K33/1). It is separated from the last occurrence of Wurmiella ex. gr. excavata (Fig. 1B; sample K32) by a barren sandstone unit about 61.8 m thick (Fig. 1B; Unit 21). A conodont assemblage collected from sample K36 at 23.3 m above sample K33/1 includes Distomodus staurognathoides and Arianagnathus jafariani Männik et al. (2015).

The uppermost conodont occurrences in the studied succession are in samples K37 and K38 (43.4 m above sample K33/1; Fig. 1B). They contain Kockelella ranuliformis (Walliser, 1964), suggesting an early Sheinwoodian (early Wenlock) age (Hairapetian et al. 2012).

Systematic palaeontology

Table 1.

Basic measurements (in mm) of 13 shells of Jafarirhynchus alatus gen. et sp. nov.; Llandovery, Telychian, Niur Formation, sample K33/1, Boghu Mountains. Abbreviations: Lv, Ld, maximum length of ventral and dorsal valves; max, maximum observed size; min, minimum observed size; n, number of specimens; s, standard deviation from the mean; SLw, SLh, maximum width and height of the sulcus/median folds; W, T; maximum width, thickness of the shell; x, mean.

Fig. 2.

Rhynchonellide brachiopod Jafarirhynchus alatus gen. et sp. nov. from Llandovery, Telychian, Niur Formation, sample K33/1, Boghu Mountains, CNIGR136/12600. A. Transverse serial sections (L = 18.4; W = 23.2; T = 10.5). B. Sagittal section of the shell. C. Dorsal valve interior. Numbers indicate distance in mm from the tip of the ventral umbo.

Fig. 3.

Rhynchonellide brachiopod Jafarirhynchus alatus gen. et sp. nov. (A–C) and spiriferide Striispirifer? ocissimus Popov, Modzalevskaya, and Ghobadi Pour in Hairapetian et al., 2012 (D, E) from Niur Formation, Telychian, Llandovery, Silurian, sample K33/1, Boghu Mountains. A. AEU1500, holotype, a pair of conjoined valves, in dorsal (A₁), ventral (A₂), lateral (A₃), and anterior (A₄) views, enlarged ventral interarea (A₅). B. AEU1501, a pair of conjoined valves, in anterior (B₁), ventral (B₂), dorsal (B₃), and lateral (B₄) views of a strongly asymmetrical specimen. C. AEU1502, dorsal view of a pair of conjoined valves (C₁), enlarged surface ornament (C₂). D. AEU1511, a pair of conjoined valves, in ventral (D₁), lateral (D₂), and dorsal (D₃) views. E. AEU1512, ventral valve exterior. Scale bars 2 mm.

Discussion

There is a general consensus that the earliest “astrophic spirebearers” with calcified brachial supports evolved monophyletically from a rhynchonellide ancestor (Copper and Gourvennec 1996; Alvarez et al. 1998; Popov et al. 1999), while the astrophic shell can be considered as plaesiomorphic feature inherited by the early spire-bearers from rhynchonellides. Rong and Zhan (1996) also published convincing evidence, subsequently supported by a study of the early spire-bearing brachiopods from Kazakhstan (Popov et al. 1999) that spiriferides evolved from an atrypide (probably lissatrypoid) ancestor with a simple, centrally directed or planospiral spiralium. Therefore the strophic shell in spiriferides is an apomorphic character. Remarkably, the strophic shell evolved repeatedly several times in “astrophic spire-bearers”, including the atrypide superfamily Davidsonioidea, the Devonian athyridide genera Anathyrella Khalfin in Gratsianova et al., 1961, and Anathyris Peetz, 1901, the Permian subfamily Comelicaniinae and the Mesozoic superfamily Koninckinoidea (for details see Alvarez and Rong 2002). Wright (1979) argued in favour of the origin of spiriferides from orthides, based on a superficial spiriferide appearance of some genera of the family Platystrophiidae; however, this view was not supported by subsequent studies by Rong and Zhan (1996).

Fig. 4.

Transverse serial sections of a cyrtioid spiriferid brachiopod Striispirifer? ocissimus Popov, Modzalevskaya, and Ghobadi Pour in Hairapetian et al., 2012; NMW 2011.11G.457 from Llandovery, Telychian, Niur Formation, sample K33/1, Boghu Mountains. Numbers indicate distance in mm from the tip of the ventral umbo.

There is no previous record of a similar strophic shell in rhynchonellides through their entire evolutionary history from the Ordovician to Recent. Hence the family Jafarirhynchidae is unique within the order Rhynchonellida in having a strophic shell with a wide, straight hinge line and a well-developed ventral interarea. These features, in combination with cyrtomatodont dentition, are considered diagnostic for the spiriferides (Jaanusson 1971); however, brachiopods assigned to the family Jafarirhynchidae lack calcified brachial supports and show the dorsal cardinalia with a septalium supported by the median septum, a septiform cardinal process and long, raduliform crura, typical for the rhynchonellide superfamily Rhynchotrematoidea. From other side, shells of Striispirifer? ocissimus, which occur at the same locality, have well preserved calcified spiralia (Fig. 4).

Jafarirhynchus itself can be linked to the Aeronian lineage of rhynchotrematid Stegocornu represented by two succeeding species S. procerum Dürkoop, 1970 and S. denisae Popov, Modzalevskaya, and Ghobadi Pour in Hairapetian et al., 2012 in a number of localities across Central Iran (Hairapetian et al. 2012; Popov and Cocks 2013). Indeed, S. denisae shows an erect beak, an almost straight hinge line that is only slightly shorter than maximum shell width and a delthyrium mainly covered by converging deltidial plates (Hairapetian et al. 2012: fig. 6S, T). In comparison with the ancestral taxon S. procerum, S. denisae is also characterized by its wider, dorsoventrally compressed shell and narrower fold and sulcus in relation to the maximum shell width. All these tendencies are progressed further in Jafarirhynchus alatus, which possibly represents the last member of that lineage.

The delthyrium in Jafarirhynchus alatus is completely sealed by the medially merged deltidial plates and there is no functional pedicle foramen in mature individuals. This suggests a liberosessile mode of life for this species, which is unusual in rhynchonellides. Jafarirhynchus alatus is a dominant species in the oligotaxic rhynchonellid association with Striispirifer? ocissimus as a minor component. In the early Telychian of the Kashmar area this association replaced the Stegocornu association characteristic of the Aeronian (Hairapetian et al. 2012). Both associations inhabited the shallow shelf environments on a carbonate ramp and can be assigned to the Benthic Assemblage Zone 2 of Boucot (1975).

Jafarirhynchus shows a remarkable similarity to atrypides of the superfamily Davidsonioidea in the morphology of the ventral interarea and delthyrial covers (Copper 2002: fig. 937.2); however, that similarity is superficial and represents no more than homoplasy, since Jafarirhynchus retains typical rhynchonellide cardinalia with a septalium. Recent rhynchonellides have a spirolophe lophophore with dorsally directed apices of cones. They lack calcified supports, but it is likely that their basic morphology remained unchanged since the early Palaeozoic. The strongly transverse shell renders this type of lophophore dysfunctional in Jafarirhynchus, therefore the eversion model of lophophore cones developed from the laterally directed axis of spiralia, proposed by Rong and Zhan (1996) for spiriferides, was probably applicable also for Jafarirhynchus.

Conclusions

A strophic shell with well-developed ventral interarea convergently similar to those of spiriferides, and a liberosessile mode of life make Jafarirhynchus and the family Jafarirhynchidae unique among the Palaeozoic rhynchonellide brachiopods. In spite of its unusual morphology and life style, previously unknown in the early Palaeozoic representatives of the order, Jafarirhynchus retains characteristic rhynchotrematoid cardinalia and can be considered as a terminal member of a local lineage, which evolved in Central Iran during the Aeronian, where it is represented by two succeeding species of Stegocornu (Hairapetian et al. 2012).