First record of the Ordovician fauna in Mila-Kuh, eastern Alborz, northern Iran/Ordoviitsumi fauna esmaleid Pohja-Iraanis (Mila-Kuh, idapoolne Alborz).
The position of the Cambrian-Ordovician boundary in the Alborz Region, northern Iran, was a matter of controversy for almost half a century since the publication of the pioneering paper by Stocklin et al. (1964) where the Palaeozoic sequence of northern Iran was described for the first time. The key problem was the age of the uppermost part of the Mila Formation introduced by Stocklin et al. (1964). The unit contains rich mid- to late Cambrian trilobite and conodont faunas, while the Ordovician trilobites were not reported in the original publication. Stocklin et al. (1964) subdivided the Mila Formation into five informal members, which later found a broad application in the Alborz Region, although they have no defined boundaries. The Ordovician age was inferred by some researchers for the Mila Formation Member 5 (Peng et al. 1999; Bruton et al. 2004). Muller (1973) described and illustrated Cambrian (Furongian) conodonts, including Cordylodus proavus Muller, 1959, from the uppermost part of the Mila Formation Member 4 in the Mila-Kuh section. Kushan (1973) pointed out the occurrence of Mictosaukia rotunda (Kushan, 1973) and Pagodia sp., which he considered as proof of the Tremadocian age of the uppermost part of the Mila Formation Member 4, while elsewhere both genera occur below the presently defined position of the Cambrian-Ordovician boundary (Jell & Adrain 2002). There are no previous reports on fossil occurrences in the Mila Formation Member 5 of the Mila-Kuh section except a brief note of Kushan (1973) on the occurrence of Pagodia sp.
The Mila-Kuh section was not a subject of geological and palaeontological studies for almost 40 years. For some time, the Mila Formation Member 5 has been considered as a stratigraphical equivalent of the Lashkarak Formation; though, a recent study by Ghobadi Pour et al. (2011a) gave sufficient evidence that the Lashkarak Formation, as it was originally defined by Gansser & Huber (1962), has Mid-Ordovician (Darriwilian) and younger age.
To resolve the existing uncertainty with the position of the Cambrian-Ordovician boundary and correlation of the upper part of the Mila Formation, our research team revisited the type section of the Mila Formation in Mila-Kuh in 2013.
The Mila-Kuh area is situated in the eastern Alborz Mountains, about 50 km west-southwest of Damghan, about 6.5 km southwest of the Tuyeh village and about 5 km northwest of the road connecting Semnan to Damghan (Fig. 1). It is the type area for the Middle Cambrian to Lower Ordovician Mila Formation introduced by Stocklin et al. (1964). The geographic coordinates of the studied section are 35[degrees]58'54"N, 53[degrees]48'44"E. The measured section starts from the top of the Mila Formation Member 4, exposed along the western side of an unnamed valley. Rocks dip at 50[degrees] southeast.
According to the original report by Stocklin et al. (1964, p. 21), the Mila Formation Member 5 comprises up to 47 m grey compact quartzites in the lower part. These are succeeded by silty shales, more than 70 m thick, with brown sandstone intercalations (shown also as brown limestone on the stratigraphic column, Stocklin et al. 1964, fig. 5) in the middle part. The Mila Formation is overlain by the Devonian Geirud Formation.
The criteria, which define the lower boundary of the Mila Formation Member 5, were not discussed in its original description, except a brief note that the lower boundary of the unit is accentuated by 'a quartzite bed, which suggests break between Member 4 and Member 5' (Stocklin et al. 1964, p. 24). Therefore the lower boundary of the Mila Formation Member 5 can be placed at the top of the last bioclastic limestone bed in the Cambrian part of the Mila Formation. The succession is as follows in the ascending order (Fig. 2):
Unit M5/1-26.5 m of intercalating sandstones and siltstones with a few beds of quartzose sandstones with Cruziana trace fossils and two horizons rich in nodules at 7.4-7.9 m and at 16.0-17.6 m above the base of the unit. A bed of flat-pebbled breccia is present at 17.6-17.8 m above the base of the unit. The uppermost 1.5 m contains rare trilobites Mictosaukia sp. (sample MK-4/8). The upper boundary of the unit is a sharp uneven surface with large ripple marks about 1 m in length.
Unit M5/2-10.0 m of dark to light brown and greenish-brown, fine to medium-grained quartzose sandstones with tabular, low-angle cross-lamination (about 5[degrees]) and with bidirectional cross-bedding at the top of the unit.
Unit M5/3-4.6 m of greenish-grey argillite with a few beds of sandstone.
Unit M5/4-6.1 m of sandstone. The lower interval about 1 m thick is medium- to coarse-grained, calcareous sandstones with three shell beds comprising disarticulated valves of billingsellide brachiopods and with bedding surfaces covered by Cruziana trace fossils in the upper part. The interval from 1.0 to 1.3 m represents a matrix-supported breccia with abundant brachiopod coquinas. The upper 4.8 m comprise fine- to medium-grained quartzose sandstone.
Unit M5/5--more than 60 m of greenish-grey argillite and siltstone with beds of calcareous sandstone at 31.3-31.5 m and 34.7-34.9 m above the base of the unit. The lower boundary of the unit coincides with a sharp change from sandstones to argillites. There are four fossil horizons within the unit. The lowermost horizon (sample MK-5/10) at 5.8 m above the base of the unit contains the trilobites Chashania cf. chashanensis Lu & Sun in Zhou et al., 1977, Geragnostus cf. yangtzeensis Lu, 1975 and Conophrys gaoluoensis (Zhou in Zhou et al., 1977). The next two fossiliferous horizons (sample MK-5/11 at 16.2 m and samples MK-5/12 and MK-5/12A at 31.8-31.95 m above the base of the unit) contain the trilobites Asaphellus inflatus Lu, 1962, Geragnostus cf. yangtzeensis, Conophrys gaoluoensis and Dactylocephalus levificatus sp. nov. and the brachiopod Tritoechia tenuis sp. nov. The uppermost fossiliferous horizon at 39.5-39.6 m above the base of the unit (sample MK-5/14) yields a different trilobite assemblage, including Psilocephalina lubrica Hsu, 1948, Asaphellus sp. and Orometopus sp.
The last outcrops of the argillaceous unit are separated from the lowermost exposures of the Devonian Geirud Formation by 23 m of alluvial sediments. Thus the contact between the Devonian and the underlying strata was not observed.
The validity of the Mila Formation, as the litho-stratigraphical unit introduced by Stocklin et al. (1964), is questionable. Geyer et al. (2014) pointed out that formally it represents a junior homonym of the 'Mila Formation' introduced a year earlier by Assereto (1963). The latter by original designation includes also Devonian sediments. Therefore, pending a general revision of the early Palaeozoic lithostratigraphy of the Alborz Region, here we avoid the assignment of the Lower Ordovician sediments exposed in Mila-Kuh to any existing formal lithostratigraphical unit.
The Alborz Region is usually shown as situated within the Gondwana domain in the palaeogeographical reconstructions for the Ordovician (e.g. Torsvik & Cocks 2009). Nevertheless, faunal evidence indicates that Alborz was most probably a separate microplate located in temperate latitudes somewhere between the Gondwana margin and South China (Ghobadi Pour 2006, fig. 17; Ghobadi Pour et al. 2011a) through the early Palaeozoic.
BIOSTRATIGRAPHY AND AGE OF THE FAUNA
The end of the Cambrian Period in eastern Alborz was the time of a lowstand, when a unit of cross-bedded quartzose sandstones with Cruziana trace fossils and billingsellide brachiopod shell beds was deposited within shoal systems nearshore. The 47.2 m thick stratigraphic interval from Unit M5/1 to Unit M5/4 with confidence can be compared with the lower sandstone unit of the Mila Formation Member 5 as it was characterized by Stocklin et al. (1964). However, it contains a characteristic Cambrian trilobite Mictosaukia sp. (sample MK-4/8) and brachiopod shell beds not recorded in the original description. These 'Cruziana' sandstones are widely recognized in eastern Alborz, in particular in Deh-Molla (Ghobadi Pour et al. 2011a).
The overlying argillites (Unit M5/5) have a sharp contact with sandstones. They contain three succeeding trilobite assemblages. The lowermost assemblage (samples MK-5/10) is characterized by the occurrence of the trilobites Chashania cf. chashanensis, Geragnostus cf. yangtzeensis and Conophrys gaoluoensis. At 10.4 m upsequence it is succeeded by a more diverse faunal assemblage, including the trilobites Asaphellus inflatus, Geragnostus cf. yangtzeensis, Conophrys gaoluoensis and Dactylocephalus levificatus sp. nov. and the brachiopod Tritoechia tenuis sp. nov., which occur in samples MK-5/11, MK-5/12 and MK-5/12A in the lower part of Unit M5/5. At 39.5 m above the base of Unit M5/5 it is replaced by another trilobite assemblage with Psilocephalina lubrica as the dominant taxon, which occurs in association with Asaphellus sp. and Orometopus sp. The observed succession of trilobite assemblages closely resembles the succeeding Asaphellus inflatus-Dactylocephalus and Psilocephalina lubrica associations described by Ghobadi Pour (2006) from the Tremadocian of the Simeh-Kuh section northwest of Damghan.
The Asaphellus inflatus-Dactylocephalus Association is indicative of the base of the Ordovician succession in northern Iran. In eastern Alborz Asaphellus inflatus invariably appears in the lower part of the shale unit, which rests with a sharp contact on cross-bedded 'Cruziana' sandstones (Fig. 3); however, associated trilobite fauna is varying in diversity and taxonomic composition. In particular, in Simeh-Kuh (Ghobadi Pour 2006) it co-occurs with Dactylocephalus mehriae Ghobadi Pour, 2006 and Conophrys simehensis Ghobadi Pour, 2006; both genera are represented in Mila-Kuh by different species. In South China the Asaphellus inflatus-Dactylocephalus Biozone is recognized as the second biostratigraphical unit for the Tremadocian (Peng 1990a; Zhou & Zhen 2009). In the Nantsinkwan Formation of western Hunan Asaphellus inflatus appears above the local Monocostodus servierensis-Cordylodus intermedius conodont Biozone of the uppermost Cambrian age (Peng 1990a, 1990b). Remarkably, in western Hunan it co-occurs with Geragnostus cf. yangtzeensis and Conophrys gaoluoensis. Both taxa are also present in Mila-Kuh. Chashania chashanensis was previously considered as endemic to South China, where it has been documented from the lower Tremadocian (Cordylodus angulatus Zone equivalents) according to Zhou & Zhen (2009).
The Psilocephalina lubrica Association occurs in the uppermost part of the Lower Ordovician succession in Mila-Kuh. Psilocephalina lubrica is otherwise known from South China, in association with species of Asaphopsis in the Acanthograptus-Tungzuella Zone according to Zhou & Zhen (2009) and from Australia (Northern Territories), from the Tremadocian Pacoota Sandstone Formation (Shergold 1991). In the Mila-Kuh section the associated assemblage includes only an unnamed species of Asaphellus, which is probably a member of the local lineage and Orometopus sp. The former probably represents the species transitional between Asaphellus inflatus and Asaphellus fecundus Ghobadi Pour et al., 2007. In Simeh-Kuh, Psilocephalina lubrica appears as a common component of the more diverse trilobite assemblage, which includes Geragnostus sidenbladhi jafari Ghobadi Pour, 2006, Apatokephalus sp., Asaphopsis elhamae Ghobadi Pour, 2006, Conophrys simehensis Ghobadi Pour, 2006, Kayseraspis ghavideli Ghobadi Pour, 2006 and Presbynileus? biroonii Ghobadi Pour, 2006 (Ghobadi Pour 2006). Only Conophrys simehensis is a transitional taxon from the underlying deposits, whereas all other species are newcomers. The species endemic to the Alborz Region constitute a core of the assemblage, while trilobite genera Apatokephalus, Asaphellus and Asaphopsis are also widespread in the Acanthograptus-Tungzuella Zone of South China. In Simeh-Kuh the assemblage characteristic of the Psilocephalina lubrica Association is replaced upsection by a low-diversity fauna including the single endemic trilobite species Vachikaspis insueta Ghobadi Pour, 2006 and the brachiopod Tarfaya jafariani Popov et al., 2009. The occurrence of conodonts characteristic of the lowermost Tripodus-Drepanodus aff. amoenus Subzone of the Paroistodus proteus Zone in the shell bed with Tarfaya jafariani and Protambonites hooshangi Popov et al., 2009 above the Vachikaspis insueta level (Ghobadi Pour 2006; Ghobadi Pour et al. 2007) suggests that the Psilocephalina lubrica and Vachikaspis insueta levels together may correspond to the upper part of the Cordylodus angulatus Biozone and the Paltodus deltifer Biozone (Fig. 3).
The transition to the Ordovician in the Alborz Region coincided with a significant drowning event (Figs 2, 3). In the absence of conodonts and graptolites precise timing of the initial flooding of the region cannot be defined, while correlation with the Asaphellus inflatus-Dactylocephalus Zone of the Tremadocian succession in South China suggests that it occurred early during the Cordylodus angulatus Zone and was probably synchronous with the transgressive phase of the Black Mountain Eustatic Event of Miller (1984).
Faunal replacement through the Cambrian-Ordovician transition in the Alborz Region shows high turnover rates in trilobite faunas and suggests possible extinction in the regional scale close to the Cambrian-Ordovician boundary. None of the Furongian trilobite genera documented by Kushan (1973) and Peng et al. (1999) from the Mila Formation Member 4 are present in the early Tremadocian trilobite fauna of eastern Alborz. The taxonomic composition of the Asaphellus inflatus-Dactylocephalus trilobite Association shows remarkable similarity to the contemporaneous fauna of South China down to the species level. In addition to the eponymous taxa, such species as Chashania cf. chashanensis, Geragnostus cf. yangtzeensis and Conophrys gaoluoensis are restricted in their geographical distribution to Alborz and South China. The trilobite taxa characteristic of the Psilocephalina lubrica Association retain strong similarity to the South China fauna on the generic level, while most of the species are endemic to the Alborz Region. The Tremadocian trilobite fauna of Central Iran documented by Bruton et al. (2004) is of much lower diversity and has little in common with trilobite assemblages characteristic of the Tremadocian in Alborz.
Brachiopods (by L. E. Popov, M. Kebria-ee Zadeh & C. Baars)
The illustrated and described material is housed in the National Museum of Wales, Cardiff (NMW). Abbreviations in tables of measurements and in the text are: Lv, Ld--sagittal ventral and dorsal valve length; W--maximum width; Iw--width of the interarea; Ml, Mw--length and width of the ventral muscle field; BBw--distance between outer margins of socket ridges; Sl--length of the median ridge.
Order BILLINGSELLIDA Schuchert, 1893 Superfamily POLYTOECHIOIDEA Opik, 1934 Family TRITOECHIIDAE Ulrich & Cooper, 1936 Genus Tritoechia Ulrich & Cooper, 1936
Type species. By the original designation Deltatreta typica Schuchert & Cooper, 1932, Lower Ordovician, Arbuckle Group, Oklahoma, USA.
Tritoechia tenuis sp. nov.
Derivation of name. After Latin tenuis, thin, fine delicate.
Holotype. NMW2012.45G.326 (Fig. 4A-C), dorsal valve (external and internal moulds), Ld = 5.1, W = 6.25, Iw = 5.6, BBw = 2.9, Sl = 1.05; Lower Ordovician, Tremadocian, sample MK-5/12.
Paratypes. Ventral external moulds: NMW2012.45G.335 (Fig. 4K), 360-364; ventral internal moulds: NMW2012.45G.338, 339 (Fig. 4J; Lv = 6.5, W = 8.9, Iw = 6.8, Ml = 2.1, Mw = 2.0), 340, 345-349; dorsal internal moulds: NMW2012.45G.333 (Fig. 4D; Ld = 5.0, W = 6.4, Iw = 5.3, BBw = 2.1, Sl = 0.9), 341, 353-359; dorsal external moulds: NMW2012.45G.334 (Fig. 4L), 336 (Fig. 4H), 342-344, 350-352; all from sample MK-5/11. Ventral valve NMW2012.45G.323; ventral internal moulds: NMW2012.45G.327 (Fig. 4G, Lv = 7.1, W = 8.4, Ml = 2.3, Mw = 2.0), 329 (Fig. 4I, Lv = 7.4, W = 7.9, Ml = 2.6, Mw = 2.5), 330, 365-371, 378; ventral external moulds: NMW2012.45G.324 (Fig. 4E), 325, 332 (Fig. 4F), 337, 372-377; dorsal internal mould NMW2012.45G.331; all from sample MK-5/12. Total 36 ventral and 19 dorsal valves.
Diagnosis. Shell ventribiconvex with a very gently convex to almost planar dorsal valve, a rectimarginate anterior commissure and an apsacline ventral interarea. Radial ornament multicostellate, becoming fascicostellate distally. Ventral interior with widely divergent dental plates and a gently impressed, subtriangular ventral muscle field with the adductor scar slightly longer than diductor scars. No ventral median ridge. Dorsal interior with transverse socket ridges almost parallel to the hinge line, slit-like sockets facing posteriorly and a short dorsal median ridge about one-fifth as long as the valve.
Description. Shell ventribiconvex, slightly transverse, subpentagonal in outline, about 80-90% as long as wide with a hinge line about 90% of maximum shell width. Cardinal extremities slightly obtuse; anterior commissure rectimarginate. Ventral valve lateral profile anterior to the umbo gently convex with maximum height at the umbonal region. Ventral interarea high, triangular, planar, apsacline with a narrow, convex pseudodeltidium about twice as high as wide. Dorsal valve very gently convex to almost planar with a low, planar, anacline interarea. Chilidium broad, convex, completely covering the notothyrium. Radial ornament slightly unequally multi-costellate, becoming fascicostellate in the anterior part of the shell in the largest specimens. Ribs tubular, separated by narrow interspaces, varying from 3 to 5 per 1 mm along the anterior margin; 20-22 ribs at the umbonal area and 45-65 ribs along the anterior margin of mature individuals. Concentric ornament of fine densely spaced fila.
Ventral interior with delicate, transverse teeth and long, straight, widely divergent dental plates. Ventral muscle field gently impressed, subtriangular, about one third as long as the valve, bordered anteriorly by the faint muscle bounding ridges. Adductor scar narrow, subtriangular, occupying one-third of the muscle field and slightly longer than gently impressed diductor scars. Dorsal valve interior with a simple, ridge-like cardinal process on a small subtriangular notothyrial platform steeply inclined posteriorly and supported anteriorly by a short median ridge about one-fifth as long as the valve. Socket ridges straight, transverse, bordering anteriorly narrow, slit-like sockets facing posteriorly. Dorsal adductor muscle scars and mantle canals in both valves undiscernible.
Remarks. Tritoechia tenuis sp. nov. is probably the earliest representative of the genus. Unlike most of the species referred to Tritoechia (e.g. Ulrich & Cooper 1938; Laurie 1980; Benedetto 1987, 2001; Popov et al. 2001; Benedetto et al. 2003) it is characterized by a very gently convex to almost planar dorsal valve lacking sulcus, a gently impressed, subtriangular ventral muscle field bounded laterally by long divergent dental plates and with an adductor scar slightly longer than diductor scars, a convex chilidium completely covering the notothyrium, transverse socket ridges almost parallel to the hinge line and a very short dorsal median ridge. Iranian species has no ventral median ridge and aditicules, which are usually considered among diagnostic features of Tritoechia (Rubel & Wright 2000).
Of the Tremadocian species of the genus T. tenuis resembles Tritoechia lewesi Brown, 1948 from the Tremadocian Florentine Valley Formation of Tasmania (Laurie 1980) in having a planar to gently convex dorsal valve, transverse socket ridges parallel to the hinge line and a short dorsal median ridge and in the absence of the ventral median ridge. However, it can be easily distinguished in having multicostellate radial ornament, a convex chilidium completely covering the notothyrium and a ventral adductor scar slightly longer than diductor scars.
The shells of Tritoechia sp. from the upper Tremadocian (Paltodus deltifer conodont Zone) of the Deh-Molla section in the eastern Alborz Mountains, northern Iran (Ghobadi Pour et al. 2011b), unlike T. tenuis, have a strongly elongate ventral muscle field, bounded laterally by long, subparallel dental plates, and a shallow dorsal median sulcus. They represent a separate, not yet formally designated species.
Trilobites (by M. Ghobadi Pour & L. E. Popov)
The illustrated and described material is housed in the National Museum of Wales, Cardiff (NMW). The terminology and systematic classification follows that of the new edition of the Treatise on Invertebrate Palaeontology (Whittington & Kelly in Kaesler 1997).
Class TRILOBITA Walch, 1771 Order AGNOSTIDA Salter, 1864 Superfamily AGNOSTOIDEA M'Coy, 1849 Family METAGNOSTIDAE Jaekel, 1909 Genus Geragnostus Howell, 1935
Type species. Agnostus sidenbladhi Linnarsson, 1869; Lower Ordovician, Tremadocian, Bjorkasholmen Formation (formerly Ceratopyge Limestone), Vastergotland, Sweden.
Geragnostus cf. yangtzeensis Lu, 1975 Figure 5A-F
1990a Geragnostus cf. yangtzeensis Lu, 1975; Peng, p. 12, pl. 1, figs 1-8.
Material. Sample MK-5/10, five cephala NMW 2012.45G.429-433, three pygidia NMW2012.45G.434436; sample MK-5/11, three cephala NMW2012.45G.307 (Fig. 5B), 308 (Fig. 5C), 311.1 (Fig. 5D), six pygidia NMW2012.45G.305.2 (Fig. 5F), 306.1 (Fig. 5E), 153.6, 168.2, 175.4, 321.3; sample MK-5/12, one cephalon NMW2012.45G.182 (Fig. 5A), one incomplete pygidium NMW2012.45G.301.
Remarks. These agonostid trilobites are closely similar to the specimens described and illustrated by Peng (1990a) as Geragnostus cf. yangtzeensis Lu, 1975 from the Tremadocian Nantsinkwan Formation of northwestern Hunan, South China, in all aspects of the cephalic morphology including: (1) shape and proportions of the cephalon, which has a maximum width at about onethird cephalic length from the posterior margin; (2) a gently tapering forward to almost parallel-sided glabella with a faint transverse glabellar furrow situated just anterior to the elongate, suboval glabellar node; (3) broadly triangular basal lobes connected to each other by the low and narrow transverse band (Fig. 5A, B); (4) a cephalic border equally wide anterior to the cephalic mid-length. Peng (1990a) considered the cephalic morphology of his specimens to completely match the holotype of Geragnostus yangtzeensis, but mentioned some pygidial differences such as a tapered axis and a less rounded and somewhat longer posterior segment, so he left his specimens in open nomenclature. This pygidial morphology is also characteristic of most of the specimens from samples MK-5/10 to MK-5/12 of the Mila-Kuh section (Fig. 5F, G). Until a revision of the types, here Peng's (1990a) species assignment is followed, while species attribution of the Iranian specimens is considered as provisional.
Material. Sample MK-5/12A, one internal mould of pygidium NMW2012.45G.309 (Fig. 5G), one external mould of pygidium NMW2012.45G.310.
Description. Pygidium strongly convex, subquadrate, about 90% as long as wide. Pygidial axis strongly convex, almost parallel-sided, slightly constricted at the posterior furrow, broadly rounded posteriorly, about three-fourths as long and slightly less than half pygidium width. M1 separated from M2 by a distinct furrow slightly inclined anteriorly, then strongly curved forward adaxially. M2 with a strong median tubercle extended slightly backwards above the transverse furrow bounding its posterior margin. M3 almost half as long as the pygidial axis, delineated anteriorly by the strong transverse furrow. Pleural fields smooth, gradually becoming narrower behind the axis. Border delineated by the distinct border furrow, with maximum width along the posterolateral pygidial margins, at the level of a pair of small, oblique spines.
Remarks. The specimen shows strong similarity in pygidial morphology to Geragnostus crassus Tjernvik, 1956, from the Lower Ordovician, Tremadocian of Baltoscandia, as revised by Ahlberg (1992); however, it lacks a small median tubercle at the posterior part of M3, probably due to imperfect preservation. In the absence of data on cranidial morphology precise taxonomic discrimination of the specimen remains doubtful.
Order PTYCHOPARIIDA Swinnerton, 1915 Suborder PTYCHOPARIINA Swinnerton, 1915 Superfamily PTYCHOPARIOIDEA Matthew, 1887 Family SHUMARDIIDAE Lake, 1907 Genus Conophrys Callaway, 1877
Type species. Conophrys salopiensis Callaway, 1877; Lower Ordovician, Tremadocian, England and Wales.
Conophrys gaoluoensis (Zhou in Zhou et al., 1977) Figure 5H-Q
1977 Shumardia gaoluoensis Zhou; Zhou et al., p. 148, pl. 46. fig. 7.
1990a Shumardia (Conophrys) gaoluoensis Zhou; Peng, p. 14, pl. 2, figs 1-15, text-fig. 6.
Material. Sample MK-5/11, 55 cranidia including NMW2012.45G.153.2-5, 156.2-4, 303.2-6, 304 (Fig. 5I), 305.3, 4, 306.3, 4, 311.3, 4 (Fig. 5K), 311.5-7, 311.11, 12, 312.1, 2 (Fig. 5Q), 314 (Fig. 5H), 315.1, 2, 316, 317.2-9, 321.1 (Fig. 5L), 321.2, 3, 437-454, nine external moulds of cranidia NMW2012.45G.455-463, ten pygidia NMW2012.45G. 175.1 (Fig. 5N), 303.7 (Fig. 5O), 303.8, 306.2, 311.8-10, 317.1 (Fig. 5M), 464, 465; sample MK-5/12, five cranidia NMW2012.45G.203.2, 302, 313 (Fig. 5J), 379, 380.
Description. Cranidium semioval, about three-fifths as long as wide, covered by shallow pits slightly varying in size. Glabella (excluding the occipital ring) about 60-65% as long and about 45-50% as wide as the cranidium, bordered laterally by deep axial furrows. Preglabellar furrow curved forward, deep and narrow, slightly pointed medially. Anterolateral glabellar lobes teardrop-shaped, terminated posteriorly near cranidial mid-length. Longitudinal glabellar furrows slightly inclined anteriorly adaxially, strongly impressed posteriorly, gradually fading anteriorly, terminated at short distance from the preglabellar furrow. L1 and L2 small, button-shaped, usually poorly preserved on the internal moulds. Preoccipital furrow deep, transverse. Occipital ring about 90-95% of maximum glabellar width and slightly more than one-fifth of cranidial length. Occipital node not observed. Preglabellar field narrow (sag.), very gently convex. Fixigenae gently convex, strongly sloping down near the outer margins. Posterior margin of fixigenae slightly transverse, curved backwards distally. Posterior border very narrow (exsag.), separated from fixigenae by a shallow, transverse border furrow terminated at some distance from the posterolateral cranidial margins.
Pygidium subtrapezoidal, slightly more than half as long as wide with the posterior border slightly curved forward. Pygidial axis strongly convex, gradually tapering backwards, rounded posteriorly, with five axial rings, and a very small terminal piece, about 80-85% as long and two-thirds as wide as the pygidium, bordered by deep axial furrows. Pleurae with four, rarely five pleural ribs evenly curved backwards distally. Pleural and interpleural furrows shallow.
Remarks. Conophrys gaoluoensis is a well-defined taxon. Its affinities have been revealed from the phylogenetic analysis of more than forty species of Shumardiidae performed by Waisfeld et al. (2001) and also discussed in detail by Peng (1990a). The specimens from Mila-Kuh show close similarity to the specimens of C. gaoluoensis from the Nantsinkwan Formation of Hunan in South China re-described by Peng (1990a) and considered here as conspecific. The minor differences include more uniformly pitted ornament of the cephalon, a shorter preglabellar field and five axial rings instead of six in the specimens from South China. These differences may be referred to geographical variations within distant populations of the same species.
Two other species of Conophrys have been reported from the Tremadocian of Iran, including Conophrys simehensis (Ghobadi Pour, 2006), which also occurs in Alborz, and Conophrys cf. pentagonalis (Lu, 1975) briefly described and illustrated in Bruton et al. (2004), from the Shirgesht Formation of the Derenjal Mountains in eastern Central Iran.
Conophrys gaoluoensis can be easily distinguished from C. simehensis (Ghobadi Pour, 2006) by its pitted ornament covering all the surface of the cranidium, larger and more prominent anterolateral glabellar lobes and a deep preglabellar furrow not fading medially.
Conophrys gaoluoensis and C. pentagonalis are closely similar in cranidial morphology. The only difference is the less prominent posterior cranidial border distinct only proximally in C. gaoluoensis. Surface ornamentation is unknown either in topotypes of C. pentagonalis or in the specimens of Conophrys cf. pentagonalis from the Derenjam Mountains, while the presence of L1 and position of the occipital node are not evident from the existing descriptions and illustrations of the specimens. The main differences between these two taxa are in pygidial morphology. Unlike C. gaoluoensis, C. pentagonalis has a concave (not transverse) median portion of the posterior pygidial margin and a broad, strongly tapering posteriorly pygidial axis more than two-fifths as wide as maximum pygidial width.
Order PHACOPIDA Salter, 1864 Suborder CHEIRURINA Harrington & Leanza, 1957 Superfamily CHEIRUROIDEA Hawle & Corda, 1847 Family PILEKIIDAE Sdzuy, 1955 Subfamily PILEKIINAE Sdzuy, 1955 Genus Chashania Lu & Sun in Zhou et al., 1977
Type species. By the original designation Chashania chashanensis Lu & Sun in Zhou et al., 1977, Lower Ordovician, Tremadocian, Nantsinkwan Formation, Hubei, China.
Chashania cf. chashanensis Lu & Sun in Zhou et al., 1977
Material. Sample MK-5/10, external mould of cranidium NMW2012.45G.385 (Fig. 6N).
Remarks. This species was recently recovered from the lowermost Tremadocian (Asaphellus inflatus-Dactylocephalus Association) of the Deh-Molla section (Ghobadi Pour, manuscript in preparation). A single incomplete cranidium from Mila-Kuh closely resembles Chashania chashanensis in all aspects of the cranidial morphology; however, it shows a slightly pointed cranidial anterior margin, a short anterior cranidial border and anteriorly situated palpebral lobes which are not preserved in the types.
Order ASAPHIDA Salter, 1864 Superfamily ASAPHOIDEA Burmeister, 1843 Family ASAPHIDAE Burmeister, 1843 Subfamily ISOTELINAE Angelin, 1854 Genus Asaphellus Callaway, 1877
Type species. Asaphus homfrayi Salter, 1866; Lower Ordovician, upper Tremadocian, North Wales.
Asaphellus inflatus Lu, 1962
1962 Asaphellus inflatus Lu; Wang, p. 43, figs 2-4. 2006 Asaphellus inflatus Lu; Ghobadi Pour; p. 102, figs 6.1-7, 9, 12 (for full synonymy).
Material. Sample MK-5/11, five cranidia NMW2012.45G.151 (Fig. 6C), 152 (Fig. 6B), 175.3, 303.1, 466, two librigenae NMW2012.45G.153.1 (Fig. 6F), 156.1, five pygidia NMW2012.45G.154, 155 (Fig. 6H), 201, 202, 468, two external moulds of pygidia NMW2012.45G.322, 467; sample MK-5/12, four cranidia NMW2012.45G.157.1 (Fig. 6A), 158b, 212, 213, 11 pygidia NMW2012.45G.157.2 (Fig. 6J), 159, 203.1, 204-211, one librigena NMW2012.45G.158.1 (Fig. 6E); sample MK-5/12A, one external mould of cranidium NMW2012.45G.318 (Fig. 6D), two incomplete cranidia NMW2012.45G.217, 218, five pygidia NMW2012.45G.160 (Fig. 6I), 161, 214-216, three librigenae NMW2012.45G.162 (Fig. 6G), 219, 220.
Remarks. The occurrence of Asaphellus inflatus in the lower Tremadocian of the eastern Alborz Mountains was first documented by Ghobadi Pour (2006). Unlike the types of A. inflatus described and illustrated by Lu (1962), which were preserved in limestone and retained their original convexity, Iranian specimens of this species occur in shales and are generally flattened and slightly distorted. Nevertheless, in cranidial and pygidial morphology specimens from Mila-Kuh are well within the range of variations of the A. inflatus specimens described and illustrated by Lu (1975) and Peng (1990a), which were also derived from shales.
Figure 6K, L
Material. Sample MK-5/14, one hypostome NMW2012.45G.382 (Fig. 6L), internal mould of pygidium with thoracic segments NMW2012.45G.381 (Fig. 6K), four internal moulds of pygidia NMW2012.45G.383, 392-394, five external moulds of pygidia NMW2012.45G.388-392.
Remarks. A few pygidia of Asaphellus from the Psilocephalina lubrica Association differ from those of Asaphellus inflatus in having a more transverse outline of the pygidium, which is almost half as long as wide, more strongly impressed axial pygidial furrows and a more distinct pygidial border. The associated hypostome has a subtriangular outline, an almost transverse anterior margin very gently curved distally, a rounded posterior margin and an elongate, suboval middle body. The anterior lobe of the middle body is large, subcircular, separated from the crescentic posterior lobe by the discontinuous middle furrow completely fading in the mid-part. Maculae are well defined, situated at the anterolateral parts of the posterior body immediately behind the middle furrow. Anterior wings are large and subtriangular. This hypostome shows close similarity to the hypostome of A. inflatus (Peng 1990a, pl. 6, figs 8, 12), but slightly differs in having a more transverse anterior hypostomal margin and a weaker median furrow. In the absence of data on cephalic morphology, species discrimination of these specimens remains uncertain.
Superfamily CYCLOPYGOIDEA Linnarsson, 1869 Family NILEIDAE Angelin, 1854 Genus Psilocephalina Hsu, 1948
Type species. Psilocephalina lubrica Hsu, 1948; Lower Ordovician, upper Tremadocian, Fenhsiang Formation, South China.
Psilocephalina lubrica Hsu, 1948 Figure 6O, P
1948 Psilocephalina lubrica Hsu, p. 24, pl. 7, figs 8-15, pl. 8, figs 1-14.
1975 Psilocephalina lubrica Hsu; Lu, p. 337, pl. 16, figs 4-24, pl. 17, figs 1, 2.
1977 Psilocephalina lubrica Hsu; Zhou et al., p. 222, pl. 66, figs 7, 8.
1977 Psilocephalina lubrica mut. transversa Hsu; Zhou et al., p. 222, pl. 66, fig. 9.
1991 Psilocephalina sp. cf. P. lubrica Hsu; Shergold, p. 39, pl. 8, figs 1-20.
2006 Psilocephalina lubrica Hsu; Ghobadi Pour, p. 106, figs 5.14, 5.16, 5.18-21, 10, 13.1-3, 13.6-12, 13.15.
Material. Sample MK-5/14, six internal moulds of cranidia NMW2012.45G.386 (Fig. 6O), 396-400, two external moulds of cranidia NMW2012.45G.401, 402, one attached cranidium and librigena NMW2012.45G.403, three librigenae NMW2012.45G.404-406, 15 internal moulds of pygidia NMW2012.45G.387 (Fig. 6P), 408-421, three external moulds of pygidia NMW2012.45G.422-424.
Remarks. Iranian specimens of Psilocephalina lubrica were illustrated and described in detail by Ghobadi Pour (2006). This species is the most common in the uppermost part of the Mila Formation Member 5 of the Mila-Kuh section. Specimens of Psilocephalina sp. cf. P. lubrica described and illustrated by Shergold (1991) from the Pacoota Sandstone Formation of the Amadeus basin in Central Australia are almost identical to the Iranian specimens both in cranidial and pygidial morphology except a slightly more anterior position of the glabellar node and they are considered here as conspecific.
Superfamily TRINUCLEOIDEA Hawle & Corda, 1847 Family OROMETOPIDAE Hupe, 1955 Genus Orometopus Brogger, 1896
Type species. Holometopus? elatifrons Angelin, 1854 from the Bjokasholmen Formation, upper Tremadocian of Sweden.
Material. Sample MK-5/14, two internal moulds of cranidia NMW2012.45G.384 (Fig. 6M), 395.
Description. Cranidium about three-fifths as wide as long with maximum width along the posterior margin. Glabella semiovoid, about four-fifths as long as the cranidium and with maximum width (tr.) about three-fifths of glabellar length, evenly sloping down anteriorly, subcarinate posterior to the glabellar median tubercle and terminating posteriorly with a spine. Occipital ring narrow (sag.). Preglabellar field (sag.) short, evenly convex (tr.). Palpebral lobes small, semicircular about one-fifth of sagittal cranidial length. Posterior border narrow (exsag.), transverse, separated from fixigenae by a broad border furrow. Anterior cranidial margin evenly rounded; posterior margin transverse. Anterior branches of the facial suture slightly divergent, merging with the anterior cranidial margin at the level of the glabellar front. Posterior branches of the facial suture transverse, curved outwards immediately behind the palpebral lobes.
Librigenae, hypostome, pygidium and thorax are unknown.
Remarks. The anterior cranidial border, eye ridges and surface ornament have not been observed, probably because of insufficient preservation of the specimen, while an ovoid shape of the glabella and the presence of the backward-projecting glabellar spine suggest the closest affinity to the type species Orometopus elatifrons as revised by Ebbestad (1999). Iranian specimens differ in having a more transverse anterior cranidial margin, and a wider (exsag.) preglabellar field.
Superfamily ANOMALOCAROIDEA Poulsen, 1927 Family DIKELOKEPHALINIDAE Kobayashi, 1936 Genus Dactylocephalus Hsu, 1948
Type species. Dactylocephalus dactyloides Hsu, 1948; Lower Ordovician, Tremadocian, I-Chang Formation, Hubei, South China.
Dactylocephalus levificatus sp. nov.
Derivation of name. After Latin levificatus, smooth.
Holotype. NMW2012.45G.158.3 (Fig. 7O), internal mould of pygidium, Lower Ordovician, Tremadocian, sample MK-5/12.
Material. Sample MK-5/11, eight cranidia NMW 2012.45G.165.1 (Fig. 7J), 167, 305.1 (Fig. 7D), 311.2 (Fig. 7E), 469-473, seven pygidia NMW2012.45G.164.1 (Fig. 7P), 165.2, 168.1 (Fig. 7T), 305.1, 175.2, 474, 475; sample MK-5/12, one incomplete exoskeleton NMW2012.45G.169 (Fig. 7A), one cephalon and attached part thorax NMW2012.45G.172 (Fig. 7C), one incomplete thorax and attached pygidium NMW2012.45G.170 (Fig. 7B), one incomplete thorax NMW2012.45G.173 (Fig. 7G), six cranidia NMW2012.45G. 192-197, two pygidia NMW2012.45G.171 (Fig. 7S), 174 (Fig. 7R), two external moulds of pygidia NMW2012.45G.199, 200, one librigena NMW2012.45G.163 (Fig. 7K); sample MK-5/12A, seven cranidia NMW2012.45G.176 (Fig. 7H), 177 (Fig. 7I), 178 (Fig. 7F), 183-186, five external moulds of cranidia NMW2012.45G.319 (Fig. 7L), 187-190, one cranidium and librigena NMW2012.45G.179, two internal moulds of pygidia NMW2012.45G.180 (Fig. 7N), 181 (Fig. 7M), two external moulds of pygidia NMW2012.45G.320 (Fig. 7Q), 191.
Diagnosis. Small size for the genus. Glabella effaced, ornamented with faint terrace ridges; two pairs of indistinct glabellar furrows can be seen only in larger specimens. Preglabellar field short (sag.), divided from the narrow (sag.) anterior border by the broad shallow border furrow. Pygidium transverse, trapezoidal in outline, with five faint axial rings and up to three weakly defined pleural ribs. Pygidial marginal spines short, stout; posterior margin between them gently convex, weakly tipped medially.
Description. Cephalon semioval, about half as long as wide (without genal spines). Cranidium transverse, sagittal length about 60-70% of posterior width. Glabella elongate, subrectangular, about three-quarters of sagittal cranidial length, almost parallel-sided, or very gently tapering anteriorly, with evenly convex anterior margin, bounded by narrow, deep axial and preglabellar furrows. Two pairs of indistinct glabellar furrows can be seen only in larger specimens (Fig. 7I). Occipital ring transverse, parallel-sided, as wide (tr.) as maximum glabella width. Occipital furrow narrow, transverse, slightly deepening abaxially. Preglabellar field short (sag.), gently convex in sagittal profile, with a narrow (sag.) anterior border and a broad shallow border furrow. Palpebral lobes large, semicircular, occupying about two-fifths of sagittal cephalic length. Palpebral furrows shallow, crescentic. Anterior branches of the facial suture slightly divergent proximally, then gradually curved inward at the level of the glabellar anterior margin to merge within the cephalic border. Posterior branches of the facial suture become almost transverse immediately posterior of the palpebral lobe termination, then turn backwards to cross the posterior border at short distance from the posterolateral cephalic margin. Surface ornament of quite widely spaced terrace lines (Fig. 7L), arranged transversely on the anterior part of the glabella and obliquely on the fixigena.
Librigenae wide, with narrow border delineated by the shallow border furrow. Genal spines long, evenly curved backwards. Hypostome unknown.
Thorax probably incomplete, with up to 11 segments preserved. Axis about one-quarter of thoracic width, bordered by narrow axial furrows. Pleural furrows shallow, wide, transverse. Pleural terminations pointed, slightly curved backwards. Facets prominent, moderately inclined forwards.
Pygidium subtrapezoidal, about three-fifths as long as wide with maximum width along the anterior margin. Axis conical, occupying about 30% of cranidial width, terminated at short distance from the posterior pygidial margin. Axial furrows deep and narrow, fading rapidly near the posterior axial termination (Fig. 7Q). Up to five faint axial rings gradually fading posteriorly visible on the internal moulds. Pleural fields gently sloping outwards, with up to three weakly defined pairs of pleural ribs more clearly expressed on the internal moulds. A pair of short, stout marginal spines with triangular bases located at the posterolateral corners of the pygidium. Posterior margin between spines gently convex, slightly tipped medially. Doublure wide (tr., sag.), ornamented by faint concentric terrace ridges.
Remarks. The new species differs from the type species Dactylocephalus dactyloides Hsu, 1948 in having a cranidium with the effaced glabella lacking segmentation, a shorter preglabellar field (sag.), an effaced, transverse pygidium trapezoidal in outline with axial and pleural ribs expressed mainly on the ventral side of the sclerite, and a gently convex posterior margin between marginal pygidial spines.
Dactylocephalus levificatus sp. nov. differs from Dactylocephalus mehriae Ghobadi Pour, 2006, which is the only other species documented from the Tremadocian of Alborz, in having significantly smaller sizes, a cranidium with a narrow (tr.) preglabellar field, a glabella almost lacking segmentation, significantly larger palpebral lobes, a transverse pygidium with weakly defined axial rings and pleural ribs, expressed mainly on its ventral side, an axis terminated in the proximity of the posterior pygidial margin and short, stout pygidial marginal spines with a gently convex posterior margin between them.
Dactylocephalus levificatus sp. nov. can be discriminated from Dactylocephalus latus Peng, 1990a from the upper part of the Nantsinkwan Formation (Tremadocian) of Hunan, South China and Dactylocephalus cf. latus Peng, described by Bruton et al. (2004) from the Shirgesht Formation of the Derenjal Mountains, Central Iran, in having a more transverse anterior margin of the cranidium, a shorter (sag.) preglabellar field, a more transverse pygidium, a narrow axis, more widely spaced and shorter pygidial spines, a relatively straight posterior margin between the spines and up to three weakly defined pleural ribs instead of 4-5 in D. latus.
Received 12 May 2014, accepted 5 September 2014
Acknowledgements. Mohammad-Reza Kebria-ee Zadeh thanks the Payame Noor University for support via a research fellowship. Mansoureh Ghobadi Pour thanks the National Museum of Wales for logistical support to her visits to Cardiff; her work in Iran is supported by the Golestan University. Leonid Popov and Christian Baars acknowledge the support from the National Museum of Wales. We thank David L. Bruton (Natural History Museum, University of Oslo) and Helje Parnaste (University of California, Riverside) for helpful and constructive reviews.
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Mohammad-Reza Kebria-ee Zadeh (a), Mansoureh Ghobadi Pour (b), Leonid E. Popov (c), Christian Baars (c) and Hadi Jahangir (d)
(a) Department of Geology, Payame Noor University, Semnan, Iran; email@example.com
(b) Department of Geology, Faculty of Sciences, Golestan University, Gorgan 49138-15739, Iran; firstname.lastname@example.org
(c) Department of Geology, National Museum of Wales, Cathays Park, Cardiff CF10 3NP, United Kingdom; email@example.com, firstname.lastname@example.org
(d) Department of Geology, Faculty of Sciences, Ferdowsi University, Azadi Square, Mashhad 91775-1436, Iran; email@example.com
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|Author:||Zadeh, Mohammad-Reza Kebria-ee; Pour, Mansoureh Ghobadi; Popov, Leonid E.; Baars, Christian; Jahangi|
|Publication:||Estonian Journal of Earth Sciences|
|Date:||Jun 1, 2015|
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