Early Ordovician ( Tremadocian ) brachiopods from the Eastern Alborz Mountains , Iran

Six linguliform and two rhynchonelliform brachiopods, including three new species Eurytreta ahmadii, Wahwahlingula kharbashi and Nanorthis bastamensis are described from Tremadocian strata (Paltodus deltifer deltifer conodont Biozone) in the Deh-Molla area southwest of Shahrud, Northern Iran. The fauna is dominated by micromorphic lingulides and acrotretides and shows distinct similarity to the contemporaneous micromorphic brachiopod association from Tremadocian chalcedonites of the Holy Cross Mountains, Poland. New data on the early ontogeny of the enigmatic lingulide Diencobolus show a very distinct pattern, including the presence of a metamorphic protegulum ornamented with flat-based pits and a single pair of larval setal bundles, which links this taxon to Paterula and suggests close phylogenetic relationships of both taxa to the Discinoidea.


INTRODUCTION
The Ordovician brachiopod faunas of Iran are still very poorly documented.In particular, Early Ordovician brachiopods are presently known only from a single section in the Eastern Alborz Mountains, where they were reported from the uppermost Tremadocian Paroistodus proteus Biozone and overlying Floian deposits (Popov et al. 2008(Popov et al. , 2009a)).The newly discovered brachiopod fauna from the Deh-Molla section, which co-occurs with conodonts of the Paltodus deltifer deltifer Biozone, bridges the gap between earlier described faunas and the Cambrian (Furongian) brachiopod associations of the Mila Formation (Popov et al. 2009b).The linguliform brachiopods are represented by six taxa, including Acrotreta dissimilis (Biernat, 1973), Akmolina minor (Biernat, 1973), Diencobolus sp., Elliptoglossa sp., Eurytreta ahmadii sp.nov.and Wahwahlingula kharbashi sp.nov.The assemblage shows distinct similarity to the association of micromorphic lingulates described earlier by Holmer & Biernat (2002) from Tremadocian chalcedonites of the Holy Cross Mountains, Poland, where they also occur in association with conodonts of the Paltodus deltifer Biozone.Only two rhynchonelliform brachiopods have been found through the Tremadocian interval of the sequence, including Nanorthis bastamensis sp.nov.and Tritoechia sp.

GEOLOGICAL SETTING
The brachiopod fauna described in the present work was sampled in the Deh-Molla area about 15 km southwest of the city of Shahrud in the Eastern Alborz Mountains (Fig. 1).The only published source on the Ordovician sequence in the vicinity of Shahrud is a paper by Ghavidel Syooki (2006), where he reported for the first time on the presence of Tremadocian deposits north of the Kalat-e Molla village and subdivided the Cambrian (Furongian)-Ordovician sequence into six acritarch assemblage zones.He referred the Lower Ordovician part of the sequence to the Lashkarak Formation, which is discontinuously overlain by the Upper Ordovician deposits referred to the Ghelli Formation.However, the lithostratigraphical subdivision of the Ordovician in the Alborz Mountains is currently under revision and therefore it is not used in our paper.
The studied section is exposed on the southern side of the unnamed valley about 1 km northeast of the Qalyankesh Mountain, close to the Shahrud University mine at about 8.5 km north of the Kalat-e Molla village.The Cambrian-Ordovician boundary in the section is placed provisionally at the base of the unit of green and grey argillite (up to 58 m thick) with several thin beds of bioclastic limestone (Fig. 2).The lowermost sample (DM-A/5, geographical coordinates 36°21′21.12″N,54°44′44.1″E)was taken from the argillite at 15.5 m above the base of the unit, containing abundant trilobites, including Asaphellus inflatus Lu, 1962; Chungkingaspis sinensis (Sheng, 1958); Conophrys simehensis Ghobadi Pour, 2006;Dactylocephalus mehriae Ghobadi Pour, 2006; Geragnostus sp. and a few brachiopods, including Eurytreta sp. and Nanorthis bastamensis sp.nov.
The main part of the brachiopod collection was sampled from four beds of bioclastic limestone in the middle of the unit.Two uppermost limestone horizons yielded an abundant conodont fauna indicative of the Paltodus deltifer deltifer Biozone.
Sample DM-A/7 was taken at 21.4 m above the base of the unit.It contains brachiopods Eurytreta ahmadii sp.nov.and Wahwahlingula kharbashi sp.nov.
Sample DM-A/8 was taken at 29.8 m above the base of the unit.The brachiopod assemblage from that sample includes Acrotreta dissimilis, Diencobolus sp., Elliptoglossa sp., Eurytreta ahmadii and Wahwahlingula kharbashi, which occur in association with spicules of hexactinellide sponges and ostracods.
Sample DM-A/9 was taken from the base of a grey bioclastic limestone bed at 38.9 m above the base of the unit.It contains the brachiopods Acrotreta dissimilis, Diencobolus sp. and Wahwahlingula kharbashi.
Sample DM-A/10 was taken from the top of the same limestone bed at about 0.3 m above the sample.It contains brachiopods Acrotreta dissimilis, Akmolina minor, Eoconulus sp., Eurytreta ahmadii and Wahwahlingula kharbashi.
Sample DM-A/9B was taken from the base of the uppermost limestone bed at 41.75 m above the base of the unit and sample DM-A/9A was collected from the same bed just 5 cm above the previous sample (geographical coordinates 36°21′22.5″N;54°44′44.88″E,altitude 1645 m).They contain rhynchonelliform brachiopods including abundant Nanorthis bastamensis and a few Tritoechia sp., linguliform brachiopods Acrotreta dissimilis, Akmolina minor, Diencobolus sp., Eurytreta ahmadii and Wahwahlingula kharbashi.In comparison to the Simeh-Kuh section, which is situated in the vicinity of Damghan about 50 km west of Deh-Molla (Ghobadi Pour 2006;Popov et al. 2008), the Tremadocian part of the Ordovician sequence in the studied section is condensed, while a significant interval from the uppermost Tremadocian Paroistodus proteus Biozone to the Darriwilian Stage is missing.In the absence of diagnostic conodonts, the Lower Ordovician boundary in both sections is defined by the appearance of the characteristic Asaphellus-Dactylocephalus Association (for further discussion see Ghobadi Pour 2006), which includes Asaphellus inflatus, Chungkingaspis sinensis and Dactylocephalus.According to Peng (1990aPeng ( , 1990b))  is placed significantly higher than it was previously defined by Ghavidel Syooki (2006), approximately 35 m above his local Acritarch Assemblage Zone III.

SYSTEMATIC PALAEONTOLOGY
Abbreviations for parameters measured on specimens are (in millimetres): W, L, T = maximum width, length, thickness of the shell; Lv, Ld = maximum length of ventral and dorsal valve; Il, Iw = maximum length and width of pseudointerarea/interatarea; Pl = median length of pseudointerarea; Cw, Cl = width, length of cardinal muscle field; Sa = length of dorsal median septum; Sm = position of maximum height of dorsal median septum; Vl = length of visceral area; BBl = length of brachiophore basis; Ml = length of ventral muscle field; Mw = width of ventral muscle field.The morphological terminology used here for the linguliform brachiopods follows that of Holmer & Popov (in Kaesler 2000).
The illustrated and described material is housed in the National Museum of Wales, Cardiff (NMW).The cited specimens from Poland are deposited in the Institute of Palaeobiology Warsaw (ZPAL Bp).

Remarks.
A single dorsal valve is characterized by the marginal umbo, and well-defined limbus, which are characteristic of Elliptoglossa.There are only three other described Tremadocian species, all restricted in their geographical distribution to Gondwana and Baltica (Popov & Holmer 1994;Mergl 2002), including Elliptoglossa polonica Holmer & Biernat, 2002 and Elliptoglossa linguae (Westergård, 1909) and Elliptoglossa celdai Mergl, 2002.All these species differ from each other mainly in the shape of the vestigial ventral pseudointerarea.There is no doubt about generic attribution of the specimen, but in the absence of data on the dorsal valve morphology its precise taxonomic discrimination down to the species level cannot be made.
Remarks on the ontogeny.The finely pitted microornament of the larval and adult shell in Diencobolus was reported previously by Holmer et al. (2000Holmer et al. ( , 2001) ) and by Holmer & Biernat (2002), but no further details were provided.The Iranian specimens of Diencobolus sp.exhibit a larval shell, about 180 µm wide, surrounded by a distinct halo.The second halo delineates a circular swollen area about 60 µm in diameter, which most probably represents a metamorphic protegulum.The surface of the protegulum is differentiated into a single pair of lateral lobs terminated anteriorly near the halo and divided medially by a shallow cleft (Fig. 3K).The protegulum and the larval shell are ornamented with shallow, densely placed flat-based pits varying from 4.5 to 5.1 µm in diameter (Fig. 3H).The mature shell surface is also bearing flat-based imprints, but the diameter of pits is only 2.8 to 3.2 µm and they are more deeply impressed than the juvenile pits (Fig. 3I).
The dorsal metamorphic protegulum of Diencobolus shows distinct similarity to the 'first-formed shell' of ________________________________________________________________________________________________________ Fig. 3. A-E, G  Paterula described in detail by Williams (2003, p. 70, text-fig. 2).In both genera the dorsal metamorphic shell is only about 60 µm; it bears a pair of lateral lobes and is ornamented with flat-based imprints.The only difference is that the diameter of pits in Paterula (on average 2.3 µm) is significantly smaller than in Diencobolus sp.The 'brephic zone' on the Paterula dorsal forms a narrow band just 7 µm wide, ornamented by elliptical pits.It is succeeded outwards by the mature shell ornamented with characteristic rhombic imprints.In Diencobolus the larval shell outside the protegulum, which in the ontogenetic sequence is comparable to the area of the 'brephic zone' after Williams (2003, textfig. 2C), is significantly larger and shows no change in characters of flat-based imprints unlike Paterula.The most significant change in the shell ornamentation occurs outside the second halo, which forms the boundary with the mature shell.
It is unclear if the metamorphic shells of Diencobolus and Paterula represent the embryonic protegulum formed shortly before hatching, or if these shells were secreted by the whole surface of the mantle at the beginning of a free swimming stage, as in discinids (Chuang 1977).By analogy with the paterinides (Williams et al. 1998), the paired lobes in both genera can be interpreted as showing the position of two larval setal sacks.The slight wrinkling of the shell surface suggests that the metamorphic protegulum in both genera was flexible and most probably not mineralized.
The closest analogy to the ontogenetic pattern observed in Diencobolus and Paterula can be found in the extant discinids.According to Lüter (2001), Discinisca sp.cf.tenuis develops a pair of larval setal bundles at the end of the embryonic stage shortly before hatching.However, larval setae are becoming lost at the beginning of the free swimming stage and are absent at the stage when the protegulum is secreted (Chuang 1977).It is possible that larval setae in Diencobolus and Paterula were lost later in metamorphosis, shortly after the metamorphic protegulum was formed.The second halo in Diencobolus and the outer boundary of the 'brephic zone' in Paterula most probably mark a transition from the pelagic to the benthic stage of the life cycle.It may suggest that the pelagic stage in Paterula was significantly shorter than that of Diencobolus.
Data on the ontogeny suggest that Diencobolus and Paterula are closely related and they show more similarity to discinoids than to other lingulide families in the development pattern.It supports an earlier suggestion by Holmer & Popov (in Kaesler 2000) and Williams (2003, text-fig. 6) that the family Paterulidae represents a sister group of Discinoidea.Remarks.The Iranian shells show a finely pitted surface of the larval and adult shell, a moderately convex sagittal profile more strongly curved in the umbonal area in the dorsal valve, whereas a single ventral valve shows a submarginal umbo.All listed features are characteristic also of Diencobolus subovalis Holmer & Biernat 2002 from the Lower Ordovician, Tremadocian chalcedonites, Wysoczki, Holy Cross Mountains, Poland; however, the Iranian shells lack a ventral visceral platform that is characteristic of the Polish species.The only other known Ordovician species of the genus is Diencobolus naukatensis (Holmer, Popov & Bassett, 2000) from the Lower Ordovician (presumably Floian) olistolith in the Silurian Pulgon Formation of the Alai Range in southern Kyrgyzstan.Both species are closely similar in the dorsal valve morphology and can be distinguished mainly in the position of the ventral umbo, which is submarginal in Diencobolus subovalis and excentric in Diencobolus naukatensis.In addition, the latter species also lacks a ventral visceral platform.

Wahwahlingula kharbashi sp. nov.
Figures 3A-E  Diagnosis.Shell elongate, suboval, with a large ventral pseudointerarea occupying about three fourths of maximum shell width and an undivided dorsal pseudointerarea.Larval shell smooth; adult shell finely and densely pitted.Ventral interior with a large visceral area occupying posterior half of the valve.Dorsal interior with a weakly impressed visceral area extending anteriorly up to 60% of valve length and bisected by the long, faint median ridge.
Description.Shell slightly dorsibiconvex, elongate suboval in outline, about 120-140% as long as wide.Ventral valve with a pointed umbo and with maximum width slightly anterior to midlength.Ventral pseudointerarea wide, subtriangular, occupying up to 75% of maximum valve width.Pedicle groove shallow and narrow with steep slightly divergent margins.Propareas low, concave in transverse profile, with fine flexure lines expressed by a deflection of growth lines.Dorsal valve gently convex with a low, crescent-shaped, undivided pseudointerarea not raised above the valve floor.Larval shell completely smooth, almost subcircular in outline, about 270-360 µm across, delineated by a strong rim.Shell surface of the brephic and adult shell finely and densely pitted with subcircular pits slightly exceeding 1 µm across.Remarks.Unlike Wahwahlingula antiquissima (Jeremeyew, 1856) revised by Popov et al. (1989Popov et al. ( , 2002)), the Iranian shells have a suboval, rather than a subtriangular shell outline, a larger ventral pseudointerarea occupying about three fourths of maximum shell width, a much smaller size of the pits, which only slightly exceed 1 µm across, and they lack a median groove on the dorsal pseudointerarea.The characters of the microornament on the larval shell of Wahwahlingula antiquissima are as yet unknown.
Wahwahlingula kharbashi sp.nov.differs from Wahwahlingula sevierensis Holmer et al., 2005 from the Ibexian, House Limestone and Fillmore Formation of Utah, USA, in having a smooth larval shell, a large ventral pseudointerarea and a distinct median ridge crossing the dorsal visceral area.
Wahwahlingula? emanuelensis Brock & Holmer, 2004, from the Floian, Emanuel Formation in Western Australia, is also characterized by a smooth larval shell and finely pitted microornament of the brephic and adult shell, but it differs from the newly described species in having an elongate suboval, rather than a teardrop- shaped outline, a wider ventral pseudointerarea and a long median ridge bisecting the dorsal visceral area.
Shells of Wahwahlingula? sp. from the Tremadocian Drepanoistodus proteus Biozone of Simeh-Kuh, Eastern Alborz Mountains, Northern Iran, briefly described and illustrated by Popov et al. (2008), are too poorly preserved to make a more detailed comparison.They are characterized by a microornament of hemispherical pits 5-8 mm across, which is significantly larger than that in Wahwahlingula kharbashi, and probably are not conspecific.
Order ACROTRETIDA Kuhn, 1949 Superfamily ACROTRETOIDEA Schuchert, 1893 Family Discussion.In spite of incomplete preservation, the specimens of Acrotreta from the Deh-Molla section show close similarity to the topotypes of Acrotreta dissimilis (Biernat, 1973) in characteristic features of shell morphology and they are considered here as conspecific.In particular, they are characterized by (1) a procline to catacline ventral valve with height not exceeding commissural shell length and an evenly convex sagittal profile of the ventral pseudointerarea, (2) a narrow dorsal pseudointerarea occupying less than half of maximum shell width, (3) a poorly defined median buttress and (4) a low dorsal median septum.There are minor differences of the Iranian shells from topotypes, including a larger (up to 90 µm wide) pedicle foramen and a slightly smaller (less than 225 µm across) larval shell in comparison to the topotype specimens.
Acrotreta dissimilis is considered as the earliest known species of the genus (Holmer & Biernat 2002).It differs from Acrotreta curvata Klouček, 1919 from the Lower Ordovician, Třenice Formation of Bohemia revised by Mergl (2002) in having a ventral valve as a low cone with not exceeding commissural shell length in height, an almost flat sagittal profile of the dorsal valve, a significantly lower and shorter apical process, weakly impressed ventral mantle canals and dorsal cardinal and central muscle scars.A detailed discussion of the affinity of Acrotreta dissimilis to the derived species of the genus (e.g.Acrotreta subconica Kutorga, 1848 and Acrotreta korinevskii Holmer & Popov, 1994) was given by Holmer & Biernat (2002).Genus Eurytreta Rowell, 1966 Type species.Acrotreta curvata Walcott, 1902 Description.Ventral valve subconical, about 70% as high as long with the maximum height placed at the apex.Sagittal profile of the anterior valve slope almost straight; the lateral slopes are gently and evenly convex.Deltoid pseudointerarea slightly apsacline to almost catacline, straight in the sagittal section, with a weakly defined intertrough.Pedicle foramen small, rounded, about 30 µm in diameter, placed within the larval shell.Dorsal valve transversely oval in outline, 88-90% as long as wide with a weak sulcus slightly more prominent anterior to midlength.Dorsal valve sagittal profile gently convex with maximum height near the larval shell outer boundary.Dorsal interarea slightly anacline, 41-46% as wide as the valve, occupied mainly by a gently concave median groove.Larval shell up to 210 µm wide, ornamented by hemispherical pits of two sizes with larger pits 2.5-3.6 µm across and smaller pits varying from 1.3 to 1.9 µm across.
Ventral interior with a long, narrow, subtriangular apical process, placed anterior to the internal pedicle opening and widening anteriorly.Ventral mantle canals baculate with slightly divergent vascula media.Dorsal interior with slightly thickened elongate suboval cardinal muscle fields occupying 55-68% of maximum valve width and 41-42% of sagittal length.Central muscle scars and dorsal mantle canals indiscernible.Median buttress low, subrectangular, merged with a low subtriangular median septum extending anteriorly for 81-85% of valve length and with the highest point situated almost at midvalve.section in the Eastern Alborz Mountains illustrated in that publication is more likely conspecific with the shells of Eurytreta ahmadii from the Deh-Molla section.
Eurytreta ahmadii shows similarity to Eurytreta fillmorensis Holmer et al., 2005 from the upper Tremadocian Fillmore Formation of Utah, USA, in having a low subtriangular dorsal median septum with the highest point at midvalve and to Eurytreta chabakovi (Lermontova in Lermontova & Razumovskii, 1933) from the upper Tremadocian to lower Floian of the South Urals (Popov & Holmer 1994) in having a less strongly convex and only slightly apsacline ventral valve with the almost straight sagittal profile of the anterior slope, a wider dorsal pseudointerarea exceeding half the maximum valve width.In comparison with Eurytreta fillmorensis, Iranian shells have a larger dorsal larval shell, which is more than 210 µm wide, and a much coarser and strongly differentiated pitted microornament of the larval shell.
Another Tremadocian species Eurytreta sabrinae (Callaway, 1877) was revised and discussed in detail by Sutton (in Sutton et al. 2000).These shells are up to three times larger than those of Eurytreta ahmadii.They also differ in having (1) a less convex ventral valve on average only half as thick as long, (2) an evenly convex sagittal profile of the ventral valve anterior to the apex, (3) a low, broadly triangular apical process and (4) a long, strongly thickened median septum lacking a definite apex.
The main differences of Akmolina minor from Akmolina olentensis include a faint dorsal median ridge and a vestigial apical process occluding the umbonal area, whereas in the latter species it is reduced to the short median ridge supporting the internal pedicle tube.
Family EOCONULIDAE Cooper, 1956 Genus Remarks.The unnamed species is represented in the collection by four dorsal valves, which are characterized by a moderately high conical shape, transverse subrectangular outline about 70% and an excentric position of the umbo.The larval shell is up to 200 µm wide and 165 µm ornamented with circular flat-bottomed pits about 4.5-5.0µm in diameter, surrounded by small hemispherical pits up to 0.7 µm across.
Due to the strongly variable shell morphology precise taxonomic discrimination of Eoconulus dorsal valves is difficult and often impossible on the limited material.Together with shells of Eoconulus sp. from Tremadocian chalcedonites of Wysoczki in the Holy Cross Mountains, Poland, illustrated by Holmer & Biernat (2002), Iranian specimens are the only known Tremadocian representatives of the genus.Subphylum RHYNCHONELLIFORMEA Williams et al., 1996Class STROPHOMENATA Williams et al., 1996 Order BILLINGSELLIDA Schuchert, 1893 Superfamily POLYTOECHIOIDEA Öpik, 1934Family TRITOECHIIDAE Ulrich & Cooper, 1936Genus Tritoechia Ulrich & Cooper, 1936 Type species.Deltatreta typica Schuchert & Cooper, 1932  Class RHYNCHONELLATA Williams et al., 1996 Order ORTHIDA Schuchert & Cooper, 1932Suborder ORTHIDINA Schuchert & Cooper, 1932 Superfamily PLECTORTHOIDEA Schuchert in Schuchert & Le Vene, 1929 Family NANORTHIDAE Havlíček, 1977 Remarks.Williams & Harper (in Selden 2007) reassigned Nanorthis to the family Plectorthidae.As a result, the family Nanorthidae was suppressed and the new family Archaeorthidae within the superfamily Orthoidea was introduced to include other genera previously assigned to Nanorthidae.According to Benedetto (2007, p. 275), Nanorthis and Kvania (Havlíček, 1994) are closely related genera.He considered these two genera within the emended family Nanorthidae, which was reassigned to Plectorthoidea.Following the argument given by Williams & Harper (in Selden 2007) and Benedetto (2007), it seems most practical to retain the family Nanorthidae for the earliest known plectorthoids Nanorthis and Kvania.
Remarks.Benedetto (2007) proposed an emended diagnosis of Nanorthis to include shells with multicostellate ornament, a low notothyrial platform and a distinct dorsal median septum (e.g.Nanorthis calderensis Benedetto, 2007 andNanorthis purmamarcaensis Benedetto in Benedetto &Carrasco, 2002), which are rather different from the type species and the Iranian shells described below as Nanorthis bastamensis.The two latter taxa are characterized by a significantly smaller shell size, have a distinctly fascicostellate ornament and lack a dorsal median ridge (Ulrich & Cooper 1938, pp. 88, 89).
Nanorthis bastamensis sp.nov.Diagnosis.Nanorthis with subcarinate ventral valve about as long as wide, fascicostellate radial ornament with up to 42 costellae, closely placed brachiophores with subparallel bases and a weakly defined dorsal median ridge.
Description.Shell ventribiconvex, subcircular in outline, about 90% as long as wide.Hinge line about three quarters maximum shell width at midlength; cardinal extremities broadly rounded.Anterior commissure gently unisulcate.Ventral valve moderately convex, subcarinate.Ventral interarea strongly apsacline with an open, narrow subtriangular delthyrium.Lateral profile of the dorsal valve gently convex with maximum height at about one fourth valve length from the umbo.Dorsal interarea linear, anacline.Dorsal median sulcus shallow and narrow, originating at the umbonal area.Radial ornament fascicostellate with 14-19 primary costae and up to 32-42 angular ribs.Ventral median rib slightly accentuated.
Ventral interior with small teeth, supported by thin, divergent dental plates.Ventral muscle field small, subtriangular, weakly defined anteriorly, occupying less than one fourth sagittal valve length.Adductor scars weakly impressed, broadly subtriangular, about equal length with flanking diductor scars.Dorsal interior with short, bladelike brachiophores flanking a narrow, almost subquadrate notothyrial cavity.No cardinal process and notothyrial platform.Dorsal adductor muscle scars indiscernible.
Remarks.Nanorthis bastamensis differs from Nanorthis hamburgensis (Walcott, 1884) in having subparallel brachiophore bases and complete absence of the notothyrial platform and the cardinal process.Ulrich & Cooper (1936, 1938) mentioned the presence of the rudimentary cardinal process and the notothyrial platform in the original diagnosis of the genus, but the presence or absence of these features in Nanorthis hamburgensis was not specified; however, a rudimentary notothyrial platform occurs in most of the illustrated specimens and a faint, ridge-like cardinal process can be seen on some illustrated shells (e.g.Ulrich & Cooper 1938, pl. 12, fig. 23).
In having a subcarinate ventral valve Nanorthis bastamensis resembles N. carinata Laurie, 1980 from the Lower Ordovician Florentine Valley Formation of Tasmania.However, it differs from the latter by a significantly smaller shell with a coarser, fascicostellate ornament and subparallel brachiophore bases.
Iranian specimens are characterized by a small shell with a fascicostellate radial ornament, a narrow hinge line and a subcarinate ventral valve externally similar to Kvania.However, Nanorthis bastamensis differs from the species assigned to Kvania (Benedetto 2007) in having short, subparallel brachiophore plates not convergent towards the bottom and without tendency to form a septalium-like structure.

DISCUSSION AND CONCLUSION
As it was shown in the analysis of biodiversity patterns by Bassett et al. (1999), linguliform brachiopod biodiversity declined significantly by the end of the Cambrian.The early Tremadocian linguliform brachiopod associations are usually oligotaxic, often including species of acrotretides Eurytreta and Ottenbyella, lingulides of the family Elkaniidae and occasionally siphonotretides (Popov & Holmer 1994;Holmer et al. 2001Holmer et al. , 2005;;Popov et al. 2002).The lingulate brachiopod faunas documented from the Paltodus deltifer Biozone of the Eastern Alborz Mountains and Poland (Biernat 1973;Holmer & Biernat 2002) show clear signs of the beginning of recovery.They are still of relatively low taxonomic diversity, yet incorporate the ephippelasmatids, earliest Elliptoglossa, Eoconulus and Acrotreta.All these taxa diversified and became widespread later in the Ordovician.
The Iranian Tremadocian fauna contains two species, Acrotreta dissimilis and Akmolina minor, common with the contemporaneous linguliform microbrachiopod association from Tremadocian chalcedonites of the Holy Cross Mountains, Poland (Holmer & Biernat 2002).However, it does not contain siphonotretides, which are abundant in the Cambrian (Furongian) and Lower Ordovician of the Simeh-Kuh section, just 50 km west of the Deh-Molla section (Popov et al. 2008).
Another similar linguliform microbrachiopod assemblage was reported earlier from the Lower Ordovician (presumably Floian) olistolith in the Silurian Pulgon Formation of the Alai Range in southern Kyrgyzstan (Holmer et al. 2000).The Kyrgyzian fauna also contains Acrotreta, Diencobolus and Eoconulus represented by different species, which occur in association with Ombergia mirabilis Holmer et al., 2000, otherwise known only from the Hunnebergian Regional Stage of Baltoscandia.
The paterulid Diencobolus is transitional from the Cambrian and its relation to Paterula, which appears in the late Floian, is uncertain.However, new data on the early ontogeny of Diencobolus show a very distinct pattern, which links this taxon to Paterula and suggests close phylogenetic relationships of both taxa to early discinoids.

Fig. 1 .
Fig. 1.Geographical map showing the position of the Deh-Molla section southwest of Shahrud.
, in South China, the trilobite association with Asaphellus inflatus and Dactylocephalus spp.appears near the base of the Tremadocian.In particular, in the western Hubei Province, this assemblage appears in the Nantsinkwan Formation above the local Monocostodus servierensis-Cordylodus intermedius conodont Biozone of the uppermost Cambrian age.Therefore, the position of the Cambrian-Ordovician boundary in the Deh-Molla section

Fig. 2 .
Fig. 2. Stratigraphical column of the Lower Ordovician sediments in the Deh-Molla section showing the position of samples and stratigraphical distribution of brachiopods, selected trilobites, echinoderms and conodonts.
Ventral visceral area weakly impressed, with the anterior border situated slightly posterior to midlength.Dorsal interior with a weakly impressed visceral area about 60% as long as the valve.Anterior border of the visceral area with a short median tongue occupied mainly by a pair of weakly impressed outside lateral muscle scars.Central muscle scars large, elongate suboval, situated posterior to midvalve.Visceral area bisected by a long, faint median ridge more prominent anterior to midlength.Posterolateral dorsal muscle fields slightly thickened, but individual muscle scars indiscernible.Dorsal vascula lateralia arcuate, submarginal.
ACROTRETIDAE Schuchert, 1893  Genus Acrotreta Kutorga, 1848 Eoconulus Cooper, 1956 from the Lower Ordovician, Arbuckle Limestone of Oklahoma, USA.Remarks.A few specimens of Tritoechia sp. are characterized by an apsacline ventral interarea with a convex pseudodeltidium, long, slightly divergent dental plates flanking a strongly elongate subtriangular muscle field and saccate ventral mantle canals.They resemble Tritoechia florentinensis Laurie, 1980 from the Lower Ordovician Florentine Valley Formation of Tasmania and Tritoechia tokmakensis Popov et al., 2001 from the Lower Ordovician, Floian, Kurday Formation of southern Kendyktas Range, Kazakhstan, in general shell shape and in characters of dental plates and the muscle field, but poor preservation of Iranian specimens makes further comparison difficult.