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An Early Ordovician Trilobite assemblage from the Lashkarak Formation, Damghan area, northern Iran

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A low diversity Lower Ordovician trilobite assemblage is described from the middle part of the Lashkarak Formation at Simeh-Kuh, northwest of Damghan, northern Iran. Co-occurrence with conodonts of the lowermost Paroistodus proteus Biozone suggests
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  Original article An Early Ordovician Trilobite assemblage from the Lashkarak Formation,Damghan area, northern IranAssemblage a`trilobites de la Formation de Lashkarak, Ordovicieninfe´rieur de la re´gion de Damghan, Nord de l’Iran Mansoureh Ghobadi Pour a , Muriel Vidal b, *, Mahmud Hosseini-Nezhad c a  Department of Geology, Faculty of Sciences, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan 49138-157, Iran b UMR 6538, domaines oce´ aniques, universite´ de Bretagne occidentale, de´  partement des sciences de la Terre, 6, avenue le Gorgeu,CS 93837, 29238 Brest cedex 3, France c  Department of Geology, Damghan University of Sciences, Damghan, Iran Received 11 January 2005; accepted 22 April 2005Available online 24 May 2007 Abstract A low diversity Lower Ordovician trilobite assemblage is described from the middle part of the Lashkarak Formation at Simeh-Kuh, northwestofDamghan,northernIran.Co-occurrencewithconodontsofthelowermost Paroistodusproteus BiozonesuggestsanuppermostTremadocianage.This assemblage includes four taxa, including one new genus and two new species, Damghanampyx ginteri nov. gen. nov. sp. and Asaphellus fecundus nov. sp. The third diagnosable species from the formation is Taihungshania miqueli (Bergeron,1894) which has previously been reportedfrom southern France and the Turkish Taurides, suggesting north Gondwanan faunal affinities. # 2007 Elsevier Masson SAS. All rights reserved. Re´sume´ La partie moyenne de la Formation de Lashkarak dans la coupe de Simeh-kuh au Nord de l’Iran (NW de Damghan), a livre´un assemblage a`trilobitespeudiversifie´(4taxons)detypebiofacie`sa`raphiophoride´s.D’apre`slesconodontesassocie´s,ilestattribue´auTremadocienterminal,Biozonea` Paroistodus proteus . Cet assemblage comprend des formes ende´miques de´crites pour la premie`re fois, avec un nouveau genre de raphiophoride´s :  Damghanampyx ginteri nov. gen. nov. sp. et une nouvelle espe`ce du genre Asaphellus , A . fecundus nov. sp., mais aussi une forme typiquementgondwanienne, Taihungshania miqueli (Bergeron, 1894), signalant des affinite´s fauniques avec la Turquie et le Sud de la France (Montagne Noire). # 2007 Elsevier Masson SAS. All rights reserved. Keywords: Trilobites; Lower Ordovician; Tremadocian; Iran  Mots cle´ s : Trilobites ; Ordovicien infe´rieur ; Tremadocien ; Iran 1. Geological and stratigraphical setting, and faunalaffinities There are a considerable number of publications on theCambrian trilobites of Iran (Kushan, 1978; Peng Shanchi et al.,1999and references here) but for a long time only a fewOrdovician trilobites have been described in a short paper byPillet (1973)and recently a more comprehensive study with adescription of 20 Ordovician trilobite taxa have been publishedbyBruton et al. (2004). All other information came fromreports of the Geological Survey of Iran and from a few brief reviews of the Iranian Ordovician faunas (seeHamedi et al.,1997and references cited). This information is clearlyincomplete and needs revision.This present work is part of a wider systematic study of theOrdovician macrofauna of Iran. Here we describe a trilobiteassemblage from the middle part of the Lashkarak Formation,previously reported byYazdi and Hosseini-Nezhad (2002). A http://france.elsevier.com/direct/GEOBIOGeobios 40 (2007) 489–500* Corresponding author. E-mail address: Muriel.vidal@univ-brest.fr(M. Vidal).0016-6995/$ – see front matter # 2007 Elsevier Masson SAS. All rights reserved.doi:10.1016/j.geobios.2005.04.007  brief outline of the Upper Cambrian and the Lower Ordoviciangeology and stratigraphy of this area was published recently byLefebvre et al. (2005).Bruton et al. (2004)reported on the occurrence of five trilobite taxa from the Lashkarak Formationof the western Alborz Mountains, none of them occurs in thenewly described faunal association.The material described was sampled from the Lashkarak Formation at the Simeh-Kuh section, located in the EasternAlborz Range about 13 km NW of Damghan, northern Iran(Fig. 1). The Lashkarak Formation at this locality is about283 m thick and rests conformably on the Upper Cambriancarbonate deposits of the Mila Formation (Fig. 2). Trilobiteswere collected from the locality ‘Simeh-Kuh-T’ (geographicalcoordinates N 36 8 12 0 44.8 00 , E 54 8 13 0 30.1 00 , altitude 1382 m)and are confined to the middle part of the unit of olive to grey,argillaceous siltstone, about 35.5 m thick. The associatedfaunal assemblage includes brachiopods, a few echinodermfragments, rare conodonts and the graptolites Didymograptus sp. and Tetragraptus ex gr. quadribrachiatus. Conodonts of thelower Paroistodus proteus Biozone have been collectedimmediately below (sample C-2) and above (sample Tc)trilobitebearingbeds(Fig.2).Theconodontassemblageamongother species includes Drepanodus arcuatus Pander, Drepa-noistodus aff. amoenus (Lindstro¨m), Paltodus cf. subaequalis (Pander) and Paroistodus proteus (Lindstro¨m). Co-occurrenceof  Drepanoistodus aff. amoenus , Paroistodus proteus and Paltodus cf. subaequalis suggests the lowermost Drepanois-todus aff. amoenus Subzone ofthe Paroistodus proteus Biozoneof the Baltoscandian Lower Ordovician conodont sequence(Lo¨fgren, 1994; Lehnert, personal communication 2004) whichmainlycorrespondstothe Megistaspisarmata trilobite Biozoneand the lowermost Hunnebergian Regional Stage.The trilobite assemblage includes four taxa: Damghanam- pyx ginteri nov. gen. nov. sp., Asaphellus fecundus nov. sp., Taihungshania miqueli (Bergeron, 1894) and Asaphopsis ? sp.This low diversity assemblage is comprised of 53% Asaphellus fecundus (with about half of the specimens in the collectionpreserved as complete exoskeletons), 40% Damghanampyxginteri nov. gen. nov. sp. (only a small proportion preserved ascomplete exoskeletons), and about 7% of disarticulated Taihungshania miqueli . The assemblage may represent theRaphiophorid Biofacies, srcinally described byFortey andOwens (1978)from the Lower Ordovician of South Wales,which is characterized by asaphids (  Merlinia ), raphiophorids(  Ampyx ) and rare trinucleids. Trinucleids are very scarce orabsent in the Lower Ordovician of the North Gondwananmargin but the Iranian assemblage described here is otherwisevery similar. These biofacies are characterized by trilobites of low convexity, with a high ventral surface/volume ratio, and/orpresence of genal or pygidial spines (Fortey and Owens, 1978),and Taihungshania shares these morphological features.According toFortey and Owens (1978, 1987), this biofaciesis characteristic of an outer shelf environment between thedistal part of the upper offshore and the proximal part of thelower offshore (see also,Vidal, 1998b).So far, Taihungshania has been recorded from South China,TurkeyandSouthernFrance.Thespecies Taihungshaniamiqueli was srcinallydescribedfromthe lowerArenigofthe MontagneNoire (Bergeron, 1894;Courtessole et al., 1981).Dean and Monod(1990)alsoreportedontheoccurrenceof  Taihungshania cf. miqueli in the Arenig Seydis¸ehir Formation of the easternTaurus Mountains in central Turkey. The new occurrence of thisspecies in the eastern Alborz Mountains expands the geogra-phical area occupied by Taihungshania miqueli further east andindicates North Gondwanan faunal affinities for the Lashkarak trilobite assemblage. By contrast, Asaphellus is widespread inmany areas of Gondwana and also occurs in the Uralian marginof Baltica (Ancygin, 2001). The new raphiophorid genus  Damghanampyx is endemic to the Alborz terrane . 2. Systematic palaeontology The specimens studied and figured herein are deposited inthe Department of Geology, University of Esfahan (EUITmuseum numbers); a reference collection is also deposited inthe National Museum of Wales, Cardiff. Fig. 1. Geographical map of northern Iran, showing the location of the Simeh-Kuh section.Fig. 1. Carte ge´ographique du Nord de l’Iran avec la localisation de la coupe de Simeh-Kuh.  M. Ghobadi Pour et al./Geobios 40 (2007) 489–500 490  Fig. 2. Stratigraphical column of the lower part of the Lashkarak Formation in the Simeh-Kuh section, showing position of the trilobite locality and stratigraphicaldistribution of selected conodont species (modified fromGhobadi Pour, 2006).Fig. 2. Colonne lithologique de la partie infe´rieure de la Formation de Lashkarak, coupe de Simeh-Kuh, avec la localisation de la faune a`trilobites et de quelquesespe`ces se´lectionne´es de conodontes (modifie´d’apre`sGhobadi Pour, 2006).  M. Ghobadi Pour et al./Geobios 40 (2007) 489–500 491  Fig. 3. Asaphellus fecundus nov. sp.; Lower Ordovician, Tremadocian, lower Paroistodus proteus Biozone, Lashkarak Formation, sample ‘T’, Simeh-Kuh section,eastern Alborz, Iran; 1 , EUIT5042, internal mould of incomplete exoskeleton of large holaspis, Â 4; 2 , EUIT5047, holotype, internal mould of cephalon and anteriorpart of thorax of large holaspis, Â 1.5; 3 , EUIT5070, latex cast of right librigena, Â 2; 4 , EUIT5022, internal mould of incomplete exoskeleton of largeholaspis, Â 0.7; 5 , EUIT5050, latex cast of cephalon and anterior part of thorax of small holaspis, Â 2.5; 6 , EUIT5060, internal mould of pygidium, Â 1.1; 7 ,EUIT5065, latex cast of exoskeleton, Â 3.8; 8 , 13 , EUIT5023, internal mould of cephalon, Â 1.2, details of right palpebral lobes, eye and librigena, Â 3.4; 9 ,EUIT5049,internalmouldofcephalonandanteriorpartofthoraxofsmallholaspis, Â 2.3; 10 ,EUIT5055,internalmouldofexoskeletonofsmallholaspis, Â 3.7; 11 ,EUIT5066, latex cast of exoskeleton of small holaspis, Â 2.6; 12 , EUIT5046/2, internal mould of pygidium, Â 2.6; 14 , EUIT5041, latex cast of exoskeleton of smallholaspis, Â 3.8; 15 , EUIT5035a-c, cluster of three exoskeletons, internal moulds, Â 1.7; 16 , EUIT5048, pygidium, Â 1; 17 , EUIT5066, latex cast of exoskeleton of small holaspis, Â 2.8.Fig. 3. Asaphellus fecundus nov. sp. ; Ordovicien Infe´rieur, Tremadocien, partie infe´rieure de la Biozone a` Paroistodus proteus , Formation de Lashkarak, niveau ’T’,coupedeSimeh-Kuh,Alborzoriental,Iran ; 1, EUIT5042,mouleinterned’unecarapaceincomple`te,stadeholaspis, Â 4 ; 2 ,EUIT5047,holotype,mouleinterned’unce´phalon et de la partie ante´rieure du thorax d’un stade holaspis, Â 1.5 ; 3 , EUIT5070, moulage en latex d’une librige`ne droite, Â 2 ; 4 , EUIT5022, moule interned’une carapace incomple`te d’un grand spe´cimen holaspis, Â 0.7 ; 5 , EUIT5050, moulage latex du ce´phalon et de la partie ante´rieure du thorax d’un petit spe´cimenholaspis, Â 2.5 ; 6 , EUIT5060,moule interned’un pygidium, Â 1.1 ; 7 , EUIT5065, moulagelatex d’unecarapace dorsale d’unpetit spe´cimen holaspis, Â 3.8 ; 8 , 13 ,  M. Ghobadi Pour et al./Geobios 40 (2007) 489–500 492  Family ASAPHIDAE Burmeister, 1843Subfamily ISOTELINAE Angelin, 1854Genus Asaphellus Callaway, 1877 Type species : Asaphus homfrayi Salter, 1866, LowerOrdovician, upper Tremadoc Series, North Wales.  Asaphellus fecundus nov. sp.Figs. 3 and 4(1–5) Derivationofname :AfterLatin  fecundus ,fruitful,referringtotherelativelylargesizeofthespeciesanditsabundanceatthetype locality. Holotype : EUIT5047, internal mould of cephalon and fiveanterior segments of thorax (Fig. 3(2)). Lashkarak Formation,sample ‘T’, Simeh-Kuh section, NW Damghan, EasternAlborz, Iran. Lower Ordovician, Upper Tremadocian, lower Paroistodus proteus Biozone. Paratypes : 26 carapaces, 4 cephala with thoracicsegments, 2 cephala, 7 cranidia, 55 pygidia, 3 pygidia withthoracic segments and 30 librigenae from sample ‘T’ and 5cranidia, 9 pygidia, 1 pygidium with thoracic segments, 1thorax and 9 librigenae from sample C-2/1, from the typelocality. EUIT5023, moule interne d’un ce´phalon, Â 1.2, de´tail du lobe palpe´bral droit, Â 3.4 ; 9 , EUIT5049, moule interne du ce´phalon et de la partie ante´rieure du thoraxd’un petit spe´cimen holaspis, Â 2.3 ; 10 , EUIT5055, moule interne d’une carapace dorsale d’un spe´cimen holaspis, Â 3.7 ; 11 , EUIT5066, moulage latex d’unecarapace dorsale d’un petit spe´cimen holaspis, Â 2.6 ; 12 , EUIT5046/2, moule interne d’un pygidium, Â 2.6 ; 14 , EUIT5041, moulage latex d’une carapace dorsaled’un petit spe´cimen holaspis, Â 3.8 ; 15 , EUIT5035a-c, surface avec les moules internes de trois carapaces dorsales, Â 1.7 ; 16 , EUIT5048, moule interne d’unpygidium, Â 1 ; 17 , EUIT5066, moulage latex d’une carapace dorsale d’un petit spe´cimen holaspis, Â 2.8 . Fig. 4. 1–5 . Asaphellus fecundus nov. sp.; Lower Ordovician, Tremadocian, lower Paroistodus proteus Biozone, Lashkarak Formation, sample ‘T’, Simeh-Kuhsection, eastern Alborz, Iran; 1 , EUIT5211, external mould of exoskeleton of large holaspis, Â 0.65; 2 , EUIT5210, internal mould of exoskeleton of holaspis, Â 2.4; 3 ,EUIT5212,internalmouldofexoskeletonoflargeholaspis, Â 1.4; 4 ,EUIT5068,internalmouldofexoskeletonoflargeholaspis, Â 0.9; 5 , EUIT5081,latexcastof exoskeleton of small holaspis, Â 5. 6 , Asaphopsis ? sp., locality and age same as for Fig. 4(1–5); EUIT5026, internal mould of pygidium, Â 1.4.Fig. 4. 1-5, Asaphellus fecundus nov. sp. ; Ordovicien Infe´rieur, Tremadocien, partie infe´rieure de la Biozone a` Paroistodus proteus , Formation de Lashkarak, niveau’T’, coupe de Simeh-Kuh, Alborz oriental, Iran ; 1, EUIT5211, moule externe d’une carapace dorsale d’un grand spe´cimen holaspis, Â 0.65 ; 2, EUIT5210, mouleinterne d’une carapace dorsale holaspis, Â 2.4 ; 3, EUIT5212, moule interne d’une carapace dorsale holaspis, Â 1.4 ; 4, EUIT5068, moule interne d’une carapacedorsale holaspis, Â 0.9 ; 5, EUIT5081, moulage latex de la carapace dorsale d’un petit spe´cimen holaspis, Â 5. 6, Asaphopsis ? sp., Ordovicien Infe´rieur,Tremadocien, Formation de Lashkarak, niveau ’T’, coupe de Simeh-Kuh, Alborz oriental, Iran ; EUIT5026, moule interne d’un pygidium, Â 1.4.  M. Ghobadi Pour et al./Geobios 40 (2007) 489–500 493
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