Early Visean bryozoans from the Shishtu II Member, Shishtu Formation, central Iran

Zoya Tolokonnikova & Mohsen Yazdi-Moghadam Geologos 19, 4 (2013): 291–299 DOI: 10.2478/logos-2013-0016 Early Visean bryozoans from the Shishtu II Member, Shishtu Formation, central Iran Zoya Tolokonnikova1 & Mohsen Yazdi-Moghadam2 Kuban State University, 353235, Aphipskij, post box 30, Russia; e-mail: NIOC Exploration Directorate, Seoul Str., 1994814695, Tehran, Iran; e-mail: 1 2 Abstract Four bryozoan species are described from the upper member (Shishtu II) (Visean, Early Carboniferous=Mississippian) of the Shishtu Formation of central Iran: Nikiforovella ulbensis Nekhoroshev, 1956, Nicklesopora elegantulaformis (Nekhoroshev, 1956), Primorella cf. iranica Gorjunova, 2006, and Nikiforopora intermedia (Nikiforova, 1950). This Visean assemblage shows close palaeogeographical affinities of Iran with Kazakhstan and Russia (eastern Transbaikalia, Kurgan region). Key words: Bryozoa, Visean, Iran 1. Introduction 2. Geological setting Bryozoans are widespread in Carboniferous deposits of most regions worldwide (Ross, 1981; Gorjunova et al., 2004). The systematic composition of bryozoan faunas some areas is unknown or poorly known, however, even though numerous fragments of colonies may be present. Iran is one such region: only three endemic species have been described previously from the Lower Carboniferous: Primorella iranica, Heloclema magnificum, and Worthenopora elbursensis (Gorjunova, 2006). The present contribution aims to characterise the bryozoans from the Visean Shishtu 2 Member of the Shishtu Formation of central Iran. In addition, the regional diversity of this fossil group is evaluated and the palaeobiogeographical affinities in the study area are discussed. The Shishtu Formation was first described by Stöcklin et al. (1965) from the Shotori Mountains. It comprises a succession of several hundreds of metres of dark fossiliferous limestones and shales; these are transitional between the underlying Bahram Formation and the overlying Sardar Formation (Wendt et al., 1997). The Shishtu Formation is divided into two members: the lower Shishtu I and the upper Shishtu II Members. The age of the Shishtu I Member is considered to be Frasnian-Famennian (Weddige, 1984; Yazdi, 1999), whereas the Shishtu II Member is attributed to the Tournaisian to Early Visean (Stöcklin, 1972). The bryozoan fauna described here was recovered from an outcrop at Howz-e-Dorah (Fig. 1). This is located at the southern end of the Shotori Range. The lime- 292 Zoya Tolokonnikova & Mohsen Yazdi-Moghadam 57°30'16" N tori Sho Tabas Teheran 58°17'42" Tabas ge Ran IRAN 0 Sorond 33°48'19" 200 km 0 Howz-e-Dorah outcrop 10 km 33°45'36" Devonian-Carboniferous fault Permian-Triassic Jurassic-Cretaceous study outcrop lithology bryozoans Shishtu II thickness, m road Fig. 1. Geological map and location of the Howz-eDorah outcrop. stones of the Shishtu II Member contain abundant fossils: bryozoans, crinoids, brachiopods, foraminifers, ostracods, and radiolarians (Fig. 2). The age of the bryozoan-bearing beds is estimated as Early Visean. According to Wendt et al. (1997, 2005) and Golonka (2007) the Palaeozoic sediments in northern Iran represent a fragment of Laurussia (Turan Plate). Amalgamation of the Iranian (northern Gondwana) and Turan (southern part of Eurasia) plates occurred during the Mesozoic as a consequence of closure of the Palaeothetys Ocean due to subduction (Golonka, 2002). During the Early Carboniferous the study area represented an isolated carbonate microplate (Wendt et al., 2005). 3. Material and method Cephalopod beds Mush horizon Devonian Famennian Carboniferous Tournaisian-Visean Shisthu Formation system stage formation member post-Cretaceous Fig. 2. Stratigraphical column of the Shishtu 2 Member of the Shishtu Formation. The material for the present study was collected from the Shishtu II Member. The bryozoans were investigated in 20 thin sections using a binocular microscope. The abundant fragments of the bryozoan colonies represent well-known Palaeozoic orders viz the Cystoporata, Trepostomata, Cryptostomata (suborder Rhabdomesina) and Fenestrata. In the available material three rhabdomesines were identifiable at species level: Nikiforovella ulbensis Nekhoroshev, 1956, Nicklesopora elegantulaformis (Nekhoroshev, 1956) and Primorella cf. iranica Gorjunova, 2006. The trepostome bryozoan Nikiforopora imtermedia (Nikiforova, 1950) was 293 Early Visean bryozoans from the Shishtu II Member, Shishtu Formation, central Iran 4.1. Order Trepostomata Ulrich, 1882 also found. Remains of fenestellid bryozoans are abundant, but their identification at species level is difficult because of fragmentation and the absence of good sections. Fenestellids are represented by Fenestella sp., Spinofenestella sp., Minilya sp. and Arborocladia sp. Cystoporates identified only at generic level include Fistulipora sp., Cyclotrypa sp. And Fistulamina sp. Also present are the trepostome Lieoclema sp. and the rhabdomesines Rhombopora sp., Primorella sp. and Nikiforovella sp. The systematic descriptions of the bryozoan fauna are based on measurements of the morphological characteristics, which are specific for each order. Statistical values (mean, standard deviation, variation coefficient, and minimum/maximum values) were calculated according to Köhler et al. (1996). Principal coordinates analysis was carried out applying the Jaccard index to demonstrate similarities between the bryozoan faunas of Iran and some other regions during the Visean (95% concentration ellipses, PAST program; Jaccard, 1901; Hammer et al. 2001). This multivariate ordering technique allowed the data to be projected onto two dimensions, in order to visualise trends and groupings. The studied material is stored in the Palaeontology Department of the National Iranian Oil Company Exploration Directorate. Suborder Amplexoporina Astrova, 1965 4.1.1. Family Stenoporidae Waagen & Wentzel, 1886 4.1.1.1. Genus Nikiforopora Dunaeva, 1964 Type species: Batostomella concentrica Nikiforova, 1927, Lower Carboniferous, Ukraine. Diagnosis: Branched colonies with narrow exozone. Autozooecia with oval apertures. Diaphragms rare, incomplete or centrally perforated; better developed in exozones. Exilazooecia rare. Acanthostyles large, rare. Tubules abundant several locations. Autozooecial walls thin in endozone; strongly and regularly thickened in exozone. Occurrence: Eurasia; Late Devonian to Late Carboniferous. Discussion: Nikiforopora Dunaeva, 1964 differs from Tabulipora Young, 1883 by the presence of rare diaphragms and regularly thickened walls, and from Rhombotrypella Nikiforova, 1933 by the irregularly polygonal sectional shapes of the autozooecia in the endozones. 4.1.1.1.1. Nikiforopora intermedia (Nikiforova, 1950) 1950 Tabulipora? intermedia, Nikiforova, pp. 103–104, pl. 6, fig. 6–6c. (For specimens under study see Figure 3 A-D; Table 1) Material: MZY 3568 (two colonies), MZY 3566 (three colonies). Des (...truncated)