Stegocornu and associated brachiopods from the Silurian ( Llandovery ) of Central Iran

A Llandovery (mid-upper Aeronian) brachiopod fauna is described for the first time from the Niur Formation of Central Iran. It is dominated by two succeeding rhynchonellide species Stegocornu procerum Dürkoop, 1970 and Stegocornu denisae sp. nov. In addition, there are three common and four rarer brachiopod species, including Dalejina? rashidii sp. nov., Isorthis (Ovalella) inflata sp. nov. and Striispirifer? ocissimus sp. nov. The Stegocornu Association gives a distinct biogeographic signature to the mid to late Llandovery rhynchonellide-dominated shallow-water brachiopod faunas of Central Iran, Kope-Dagh and Afghanistan. Its proliferation in temperate latitude peri-Gondwana was one of the earliest signs of biogeographical differentiation of the brachiopod faunas in the early Silurian. The affinities of Stegocornu and Xerxespirifer are discussed.


INTRODUCTION
The only published report on the occurrence of Silurian brachiopods in Central Iran is the paper by Brice (1999).Her study was based on a small collection sampled by the late A. F. de Lapparent in 1973, from a small outcrop of Silurian sediments, now assigned to the Shabdjereh Formation, in the vicinity of the Khanuk village.A more substantial record of the Silurian brachiopods from Kope-Dagh in northeastern Iran can be found in the paper by Cocks (1979) where he described 23 taxa.His study was based on a collection sampled at 2.5 km northeast of Robat-e-Qarabil, from a single bed about 33 m above the base of the Qarabil Limestone Formation designated in that publication.This is the type locality of Xerxespirifer iranicus Cocks, 1979, which for some time became a taxonomic puzzle.In particular, Carter et al. (1994) suggested that Xerxespirifer is not a spiriferidine, but more likely has rhynchonellide or leptocoellid affinity.Alvarez & Rong (2002) questionably assigned this genus to the athyridide family Retziellidae, whereas Brice (1999) and Savage et al. (2002) considered Xerxespirifer as possible junior objective synonym of Stegocornu, which has found support in the present study.
The rhynchonellide Stegocornu is a core taxon in the shallow-water fauna of the Benthic Assemblage Zone 2 (BA 2) in Central Iran, which can be termed as the Stegocornu Association.It occurs also in Robat-e-Qarabil and the Saluk Mountains, both located in the Kope-Dagh region of northeastern Iran (Fig. 1; Cocks 1979 and new unpublished observations by M. Ghobadi Pour), and in Afghanistan (Dürkoop 1970).The age of the Stegocornu fauna is defined as mid-to late Aeronian by co-occurrence with characteristic conodonts.

Kashmar region
The major source of the studied brachiopods in the area is Silurian deposits of the Niur Formation, exposed in the Boghu Mountains about 25 km southwest of the town of Kashmar (Fig. 1).The geographical coordinates of the base of the measured section are 34°04′6.2″N,58°15′48.8″E,altitude 1222 m.The geographical coordinates for the top of the section are 35°04′43.3″N,58°15′14.7″E,altitude 1266 m.Here a succession of the lower to middle Silurian sediments, assigned to the Niur Formation, is well exposed on the southern flank of the Boghu Mountains.It is underlain by an unnamed formation of Upper Ordovician siliciclastic rocks.The sampled section consists of highly fossiliferous limestones, shales and sandstone/siltstone units.The lowermost part of the measured section comprises green siltstone/shale beds intercalating with thin tentaculitid limestones and sandstones, which were assigned to the Niur Formation and dated by conodonts as lowermost Llandovery on the geological map of Kashmar (GSI 2001a, sheet 1 : 100 000).However, the occurrence of Icriodella in association with Amorphognathus within this interval (Fig. 2, unit 1, samples K5-K10) clearly indicates an Upper Ordovician (Sandbian-Katian) age (P.Männik and C. G. Miller, pers. comm. 2011).
In the Boghu section Stegocornu procerum Dürkoop, 1970 was sampled from two horizons (Fig. 2).In the lowermost sample K25 it occurs in association with a few specimens of Clorinda sp. and Mesoleptostrophia (Mesoleptostrophia) sp., whereas in sample K28 it forms a monotaxic association.The first occurrence of Stegocornu denisae sp.nov. is documented from sample K29, about 25 m above sample K28.The second horizon with Stegocornu denisae in association with a few Striispirifer? ocissimus sp.nov. is about 15 m higher (Fig. 2, sample K30).

Derenjal Mountains
The area is located about 65 km northwest of Tabas (Fig. 1; see also Ruttner et al. 1968;Hairapetian et al. 2008).Here Silurian sediments assigned to the Niur Formation are exposed on the eastern side of the Dahaneh-e-Kolut gorge and have a faulted contact against the lower Early-Middle Ordovician Shirgesht Formation (Bruton et al. 2004;Ghobadi Pour et al. 2006).The upper boundary of the Niur Formation represents a conformable contact with the presumably Lower Devonian Padeha Formation (Ruttner et al. 1968).
A measured section of the Niur Formation, in total 551 m thick, is exposed in three major hills named A, B and C (Hairapetian et al. 2008).The sedimentary succession through Hill A (units 1-4) consists of mediumto thin-bedded limestones, siltstones and argillites with abundant corals, brachiopods, ostracods, crinoids and trilobites (Fig. 2).Unit 2 is sandwiched between two horizons of volcanic rocks representing submarine basaltic lava flows (Fig. 2, units 1 and 3).The sedimentary unit was dated as mid-Aeronian, based on ostracods and brachiopods (Hairapetian et al. 2011).
Silurian deposits exposed on Hill B (base of the section at 34°5′2.9″N,56°48′14″E; altitude 1040 m) comprise sandstone and bioclastic sandy limestone.The limestone beds yield an abundant fauna of brachiopods, corals, cephalopods, tentaculitids, conodonts, ostracods and trilobites.The Stegocornu Brachiopod Association was sampled from a single sample S14 in the middle part of Unit 7 (Fig. 2 A conodont assemblage from the underlying Unit 6 (Fig. 2, sample S12) contains Wurmiella excavata cf.puskuensis (Männik, 1994), Distomodus staurognathoides (Walliser), Ozarkodina sp. and a few elements of Panderodus and Oulodus?suggesting a mid-to late Aeronian age (unpublished data by P. Männik, C. G. Miller and V. Hairapetian).Similar conodonts were recovered also from the overlying Unit 7 (Fig. 2; S15, S16 and S17).The occurrence of Pterospathodus amorphognathoides lennarti Männik, the index species of the conodont biozone, in sample S19 in the uppermost part of Unit 11 (Fig. 2) indicates a mid-Telychian age (unpublished data by P. Männik, C. G. Miller and V. Hairapetian).Thus from the available conodont data the age of the brachiopod fauna from sample S14 is probably mid-to late Aeronian.

Kerman region
Geographical coordinates of the locality, in the vicinity of the Khanuk village published by Brice (1999) are undoubtedly erroneous and the precise location and sedimentary logs of the sampled section remain unknown.Probably it was located somewhere southeast of the village, where the Shabdjereh Formation is exposed (Fig. 1).The specimens of Stegocornu aff.procerum Dürkoop, 1970 (here re-assigned to Stegocornu denisae sp.nov.) have been identified from two individual samples.According to Brice (1999), they occur in association with Hedeinopsis spp.and Nikiforovaena? sp.The latter shells are probably assignable to Striispirifer? ocissimus.
The precise taxonomical affiliation of two other taxa referred to Howellella and Rhynchotrema is questionable.
Another small collection from the Kerman region was handed to T. L. Modzalevskaya by the late Prof. Stepanov (St Petersburg University) and has recently been donated to the Central Geological Research and Exploration Tschernyshev Museum (St Petersburg).These brachiopods were sampled by an anonymous Iranian geologist from an outcrop in the vicinity of the Shabdjereh village (Fig. 1).Geological and geographical information for the collection is not available.The specimens were most likely derived from a loose sample, as they include both Stegocornu procerum and Stegocornu denisae, which do not co-occur in the same horizon.

SIGNIFICANCE OF THE FAUNA
In Central Iran Stegocornu is represented by two species, namely Stegocornu procerum and Stegocornu denisae, which succeed each other in the sequence.Both species either dominated in low-diversity associations, usually including Striispirifer? (Kashmar and Kerman regions), or were an important part of more diverse associations together with various orthides, spiriferides and strophomenides (e.g. the Derenjal Mountains and Kope-Dagh).
In the Derenjal Mountains abundant Stegocornu denisae, and Striispirifer? ocissimus and rare Hercotrema sp. and Rhytidorhachis?sp.occur as conjoined valves, whereas Isorthis (Ovalella) inflata and Dalejina?rashidii are preserved as numerous disarticulated valves in shell accumulations with a few disarticulated valves of Stegocornu.It is probable that the brachiopod association from sample S14 represents a mixture of mainly autochthonous rhynchonellide and spiriferide shells, and allochthonous shells of Isorthis (Ovalella) and Dalejina?, which were transported into the area during seasonal storms.
In the type locality in Dascht-e-Nawar, eastern Afghanistan, Stegocornu procerum occurs together with conodonts identified as Lonchodina cf.fluegeli (Walliser, 1964) (Dürkoop 1970, p. 159), which may be an element of Aspelundia? fluegeli (Walliser, 1964) known from the late Aeronian-Telychian.In the Kashmar section, eastern Central Iran, Stegocornu procerum co-occurs with a conodont assemblage, suggesting an age not older than mid-Aeronian.The succeeding species Stegocornu denisae is found in the Shirgesht section well below the first documented appearance of the index species of the mid-Telychian Pterospathodus amorphognathoides lennarti Biozone and within the range of Wurmiella excavata cf.puskuensis (Männik), which is considered as the early member of the W. excavata lineage.These finds suggest that the proliferation of Stegocornu in Afghanistan and Central Iran took place within a narrow time span from the mid-to late Aeronian.
Dispersion of the Stegocornu Association in Central Iran, Kope-Dagh and Afghanistan in the late Aeronian represents a significant regional event.Stegocornu has a short stratigraphic range and its occurrence can be considered as a good indication of Aeronian age, in particular, for the faunas described earlier by Cocks (1979) from Kope-Dagh and by Brice (1999) from the Kerman Region.Boucot & Blodgett (2001) consider the presence of Xerxespirifer (= Stegocornu) in Iran as indication of biogeographical differentiation within their North Silurian Realm in Wenlock time.New data suggest that proliferation of the Stegocornu Association had occurred already in the mid-to late Llandovery.At that time peri-Gondwanan terranes of Central Iran, Kope-Dagh and Afghanistan supported shallow-water faunas bearing a common biogeographical signature, suggesting their position in relative proximity to each other, and probably in temperate southern palaeolatitudes.

SYSTEMATIC PALAEONTOLOGY
Abbreviations for parameters measured on specimens are: W, L, T = maximum width, length, thickness of the shell; Lv, Ld = maximum length of the ventral and the dorsal valve; Iw = maximum width of the interararea; Sw, St = width and depth of the sulcus at the anterior margin; Mw, Ml = width, length of the muscle field; SVl = length of the ventral median septum; BBw = distance between outer margins of brachiophores; BBl = brachiophore length; Sl = distance from the umbo to anterior termination of the median ridge; X = mean; S = standard deviation from the mean; N = number of specimens; max = maximum observed size; min = minimum observed size.All measurements are in millimetres.
The illustrated and described material is housed in the National Museum of Wales, Cardiff (NMW), in the Central Geological Research and Exploration Tschernyshev Museum (St Petersburg) and Azad University, Esfahan (AEU).The authorship of the new taxa is as follows: Popov, Modzalevskaya & Ghobadi Pour.
Description.Shell weakly concavoconvex to almost biplanate, transverse subrectangular with maximum width slightly anterior to the hinge line; anterior commissure rectimarginate.
Ventral valve very weakly convex with a low, planar, apsacline interarea; delthyrium covered by a small pseudodeltidium.Dorsal valve planar anteriorly, weakly concave between the umbo and midlength.Dorsal interarea linear, anacline.Radial ornament subequally parvicostellate with one, rarely two parvicostellae between accentuated ribs and with up to 9 ribs per 3 mm in mature specimens.Concentric ornament of regular, very fine, densely spaced fila.
Remarks.In shell outline and characters of ventral and dorsal interior Iranian specimens show close similarity to the shells from the Llandovery (Telychian) of Britain illustrated by Harper & Boucot (1978, pl. 1, figs 12-15, 18).They differ from two other British Llandovery (Telychian) species Mesoleptostrophia (Mesoleptostrophia) tenuis (Williams, 1951) and Mesoleptostrophia (Mesoleptostrophia) ostrina (Cocks, 1967) in having obtuse cardinal extremities, more prominent ventral muscle bounding ridges and subequally parvicostellate radial ornament with only one or two parvicostellae between accentuated ribs.The Iranian shells also lack distinct rugellae in the posterior part of the shell reported in the British species.They may represent a new, not yet recognized species of Mesoleptostrophia (Mesoleptostrophia), but inadequate preservation of the cardinalia makes its formal taxonomic designation impossible.
Dalejina?rashidii sp.nov.Diagnosis.Shell strongly dorsibiconvex, about 80% as long as wide; anterior commissure weakly uniplicate; ventral muscle field large, subcordate, slightly elongated, extending anterior to the midvalve and bisected by long median ridge terminating at about one third valve length from the anterior margin; dorsal interior with undivided cardinal process with a broad shaft on a high notothyrial platform; brachiophores stout, divergent at about right angle; dorsal median ridge terminated at about middle of a weakly impressed dorsal adductor muscle field.Radial ornament equally multicostellate with up to 6 ribs per 3 mm along the anterior margin of mature specimens.Concentric ornament of fine, evenly spaced fila and growth lamellae, in the anterior half of the shell.Ventral valve interior with strong teeth and short, widely divergent dental plates.Ventral muscle field strongly impressed, large, subcordate, about four fifths as long as wide, extending well beyond the midvalve, surrounded by strong muscle bounding ridges.Thickened pedicle callist present in the posterior part of the delthyrial cavity bottom.Ventral median ridge bisecting the entire muscle field, extending anteriorly and terminating at about one third valve length from the anterior margin.Ventral diductor muscle scars large, strongly impressed, enclosing completely weakly defined adductor muscle scars in the posterior half of the muscle field.
Dorsal interior with high, stout brachiophores divergent at about right angles and deep, subtriangular dental sockets supported by socket pads.Fulcral plates present.Cardinal process undivided, with a broad, thickened shaft and crenulated myophore facing posteriorly, situated on a high, subtriangular notothyrial platform.Dorsal adductor muscle field weakly impressed, with only smaller anterior pair of adductor scars clearly discernable.Dorsal median ridge very short, terminated at the midpart of the adductor muscle field.
Remarks.The generic assignment of Dalejina?rashidii sp.nov. is questionable, because, unlike almost all other known species of the genus (e.g.Boucot et al. 1965), it has a simple, undivided cardinal process.It probably represents the oldest documented species of Dalejina.Williams & Harper (2000) considered the stratigraphic range of Dalejina as Wenlock to Emsian, however, there is a species assignable to Dalejina from the Llandovery of the East Baltic, described originally as Rhipidomelloides phaseola Rubel, 1963, from the Adavere Regional Stage (Telychian) of Estonia.Unlike Iranian shells it has a bilobed cardinal process, a strongly dorsibiconvex shell and a larger ventral muscle field extending anterior to midvalve.
Dalejina?rashidii sp.nov.differs from Dalejina hybrida (J.de.Sowerby, 1839), which is widespread in the Wenlock to Ludlow of Britain and Baltoscandia, in having a simple, undivided cardinal process, a more strongly dorsibiconvex shell, a weakly defined anterior border of the ventral muscle field, a very short dorsal median ridge and coarser radial ornament.Iranian shells are also almost twice as large as the average size of Dalejina hybrida given by Bassett (1972).
Among the early species of the genus, Dalejina phedodra Bassett, 1972 from the Wollhope Limestone (base of the Wenlock Series) of the Welsh Borderland is probably the closest species to the newly described taxon.It also has a large, weakly defined anteriorly ventral muscle field, an undivided, inflated cardinal process, a short dorsal median ridge and a relatively large shell size.Iranian shells can be distinguished from the British species in complete absence of the dorsal sulcus and in having a ventral median ridge extending beyond the anterior border of the muscle field, a very short dorsal median ridge bisecting only the posterior half of the dorsal muscle field and less strongly transverse adult shells.
Clorinda sp.Remarks.A single specimen from the Niur Formation of the Boghu Mountains is characterized by a low, flattened ventral sulcus and dorsal median fold originating at the umbonal area and smooth flanks.It shows distinct similarity to the ventral valve of Clorinda?sp.illustrated by Cocks (1979, fig. 27).These shells may well be conspecific, but the limited material and absence of data on the interior of both valves makes their precise taxonomic discrimination impossible.
Remarks.Stegocornu was originally described by Dürkoop (1970) from eastern Afghanistan, but was subsequently also identified by Brice (1999)  From the studied collection it is likely that the holotype of Xerxespirifer iranicus is within the margins of significant morphological variability characteristic of Stegocornu procerum, but additional study based on the topotype material is still required to resolve the problem of taxonomic relationships between Stegocornu and Xerxespirifer.
Remarks.The specimens from samples K25 to K28 of the Boghu section are almost identical in morphological characters to the topotypes and considered here as conspecific.The only difference is in the presence of fine crowded concentric lamellae in the anterior half of the Iranian shells, which may be explained by partial exfoliation of the shells studied by Dürkoop (1970), as is evident from the provided illustrations.Earlier Brice (1999) suggested that the majority of the shells illustrated by Cocks (1979)  Paratypes.NMW 2011.11G.271-326, conjoined valves;NMW 2011.11G.270, 327, 328, ventral valves;NMW 2011.11G.329, 330, dorsal valves;sample S14, Derenjal Mountains. NMW 2011.11G.336-354, AEU 1368-1475, articulated shells;NMW 2011.11G.355, ventral valve, NMW 2011.11G.357, 358, dorsal valves, sample K29;Boghu Mountains. NMW 2011.11G.331-333, conjoined valves;NMW 2011.11G.334, ventral  Diagnosis.Subequally biconvex transverse shell with maximum width slightly anterior to the hinge line or at the hinge line, about 50-60% as thick as long; with a strong ventral sulcus and dorsal median fold originating at the umbonal area and occupying about 40% of maximum shell width; radial ornament with a single rib in a sulcus, two ribs on a dorsal fold and 4-7 on valve flanks; ventral median rib of equal size with ribs on valve flanks; ventral beak erect; delthyrial plates merged anterior to the elongate, submesothyrid pedicle foramen.
Description.Shell subequally biconvex, transverse, semioval in outline, about 80% as long as wide with maximum width slightly anterior to the hinge line or at hinge line and 50-60% as thick as long.Hinge line relatively wide, almost straight; anterior commissure strongly sulciplicate.Ventral valve sagittal profile moderately convex with maximum height at midlength.Beak erect to gently curved.Delthyrial plates thick, conjunct anterior to the elongate, submesothyrid pedicle foramen.Sulcus deep and narrow, with steep lateral slopes, originating at the umbonal area and terminating with a trapezoidal tongue occupying about 40% of maximum shell width.Dorsal valve moderately convex with maximum height at midlength.Dorsal median fold high, originating at the umbonal area.Radial ornament of coarse angular ribs with a single rib in the ventral sulcus, two ribs on the dorsal median fold and 4-7 ribs on flanks of the dorsal valve.Rib in the sulcus about the same size as ribs on the valve flanks.Concentric ornament of fine crowded growth lamellae covering anterior half of the shell surface and with fine, evenly spaced fila in the posterior half of the shell of the mature specimens.Ventral interior with strong, oblique teeth supported by the rudimentary dental plates mainly fused to the valve walls.Ventral muscle field strongly impressed with adductor scars completely enclosed by larger diductor scars.Strongly thickened pedicle callist mainly occupying a bottom of the small delthyrial cavity.Dorsal interior with massive united hinge plates and a small septalium supported by a very short median septum.Cardinal process septiform.Crura faint, radulifer, gently curved dorsolaterally.
Remarks.Stegocornu denisae sp.nov. in the Boghu section succeeds the type species Stegocornu procerum Dürkoop in the stratigraphical sequence.It differs from the former in having a more dorso-laterally compressed shell (only 50-60% as thick as long against 75% in S. procerum), a narrow median fold and dorsal sulcus (on average 40% as wide as the shell against 50% in S. procerum) and an erect ventral beak.The median rib in the ventral sulcus in S. denisae is of about equal size to the ribs on the valve flanks, whereas in S. procerum it is considerably smaller.The number of ribs on the flanks overlaps significantly in both species, however, in S. procerum specimens with four ribs on flanks of the dorsal valve are the most common and the number of ribs does not exceed six, whereas in S. denisae specimens with 5-6 ribs are the most characteristic and shells with seven ribs do occasionally occur.Unlike S. procerum shells of S. denisae are transverse suboval, with maximum width slightly anterior of the hinge line or at the hinge line, and may be slightly alate.
Rhytidorhachis? sp.      but differs in having angular ribs, a subrectangular, not semioval tongue, a more strongly dorsibiconvex shell with a dorsal valve profile having maximum height close to the anterior margin.Without knowing the characters of the cardinalia the generic affiliation of this shell is uncertain, but it is more likely not congeneric with Hercotrema sp. from the same locality.In general shell shape and characters of radial ornament, the specimen resembles Stegerhynchus peneborealis (Twenhofel, 1928), as revised by Jin & Copper (2004) Ventral interior with delicate teeth and thin, short dental plates.Dorsal interior with a short, narrow septalium supported by a short median septum.Cardinal process absent (Fig. 6H, I).
Remarks.The shells from the Niur Formation show strong simple ribs, a ventral sulcus and dorsal median fold originating at some distance from the beak, rudimentary dental plates and a small cruralium lacking a cardinal process, all suggesting affiliation to Hercotrema Jin, 1989.They differ from the type species Hercotrema bulbicostatum in having three ribs in the ventral sulcus and four ribs on the dorsal median fold.
Striispirifer? ocissimus sp.nov.Diagnosis.Shell strongly ventribiconvex, transverse, subtrapezoidal and about three quarters as long as wide, with maximum width slightly anterior to the hinge line; cardinal extremities obtuse; ventral sulcus and dorsal median fold narrow, flattened, slightly more than one quarter as wide as the shell; dorsal median fold bisected by a fine groove; radial ornament of 5-7 plications on flanks and superimposed very fine striae, about 9-11 per 1 mm; dental plates sulcus-bounding, cardinal process smooth.
Description.Shell ventribiconvex, strongly transverse, subtrapezoidal in outline, about three quarters as long as wide, with maximum width slightly anterior to the hinge line, and about three quarters as thick as long.Cardinal extremities obtuse; anterior commissure uniplicate.Ventral valve lateral profile strongly convex with maximum height above the hinge line.Ventral interarea high, curved, apsacline with an open, narrow triangular delthyrium.Ventral sulcus narrow, strongly curved, originating at the umbo and terminating in a subtrapezoidal tongue about 25-30% valve width.Dorsal valve moderately and evenly convex with a low, anacline interarea and a narrow, flattened median fold originating at the umbo and often bisected by a fine groove.Radial ornament with 5-6 strong, rounded plications on the flanks of the dorsal valve and 6-7 plications on the flanks of the ventral valve, separated by broad, U-shaped interspaces.Microornament of very fine, striae about 10-12 per 1 mm.Concentric ornament of fine, evenly spaced fila.Ventral interior with rounded, long, slightly divergent, sulcus-bounding dental plates.Dorsal interior with thin, short, slightly divergent crural plates, straight, short rod-like crura (Fig. 11O), and a smooth cardinal process.
Discussion.The Iranian shells are provisionally assigned to the genus, because they have broad, rounded in cross section interspaces between the ribs and a flattened dorsal median fold, unlike other species of Striispirifer (e.g.Boucot 1963;Rong et al. 1974;Rong & Yang 1978).
Rounded in cross section interspaces between the ribs are considered as diagnostic for Macropleura (Boucot 1963).However, Iranian shells differ from other species assigned to this genus in having sulcusbounding, not distinctly extrasinal, dental plates and a relatively narrow dorsal median fold bisected by a faint groove.In addition, they differ from the type species Macropleura macropleura (Conrad, 1840) in having more plications (up to 7) on the lateral sides of the valves and a high, apsacline, not strongly curved ventral interarea.

Fig. 2 .
Fig. 2. Stratigraphical columns of the Derenjal and Boghu sections, showing informal lithostratigraphical subdivision of the Niur Formation, position of sampled horizons and stratigraphical distribution of brachiopods.

Table 2 .
Basic statistics of 21 dorsal valves of Isorthis (Ovalella) inflata sp.nov.; Llandovery, mid-late Aeronian, Niur Formation, Shell strongly dorsibiconvex, slightly transverse suboval, about four fifths as long as wide.Hinge line about two thirds of maximum shell width at midlength, cardinal extremities broadly rounded.Anterior commissure weakly uniplicate.Ventral valve sagittal profile gently convex with maximum height at about one third valve length from the umbo.Ventral interarea low, curved with an open, triangular delthyrium.Ventral sulcus very broad and shallow, almost indistinguishable, originating at 3-4 mm from the umbo.Dorsal valve strongly and evenly convex in sagittal and transverse profiles, with rudimentary, strongly apsacline interarea.
Brice (1999) collection of the Silurian brachiopods sampled by A. F. de Lapparent in 1973 northwest of Kerman.Brice also pointed out the close resemblance of these shells to Xerxespirifer iranicusCocks, 1979from the Silurian Qarabil Formation of Kope-Dagh.There was, however, an issue of taxonomic relationship of the illustrated specimens with the holotype of Xerxespirifer iranicus, which represents a small, strongly biconvex shell with a short hinge line, a single narrow rib in the sulcus and four ribs on flanks of the ventral valve.It looks rather atypical for Stegocornu according toBrice (1999).Subsequently Savage et al.

Table 3
under the name Xerxespirifer iranicus Cocks, with the exception of the holotype, are probably conspecific to Stegocornu procerum.This synonymy is also accepted here.Delthyrial plates in S. procerum are completely merged anterior to the pedicle foramen as shown in Fig.8A.
Derivation of name.After Prof. Denise Brice (University of Lille) who reported for the first time on the occurrence of Stegocornu in Central Iran.

Table 4 .
Basic statistics of 10 shells of Stegocornu denisae sp.nov.; Llandovery, mid-late Aeronian, Fig. 9. Surface of a shell bed showing accumulation of Stegocornu denisae sp.nov.; Llandovery, mid-late Aeronian, Niur Formation, sample S12, Derenjal Mountains; scale bar 10 mm.Remarks.A single specimen from the Niur Formation of the Derenjal Mountains occurs in association with the shell referred to Hercotrema sp.It also has three ribs in the ventral sulcus, four ribs in the dorsal fold,