Stegocornuand associated brachiopods from the Silurian (Llandovery) of Central Iran

Abstract

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

Silurian stratigraphy of Central Iran – an update

“…It was first described in some detail by Ghavidel-Syooki (2003) who assigned acritarchs from the Silurian part of the succession to the local Acritarch Assemblage Zone 7, which is dated as Llandovery to Wenlock. A detailed stratigraphical column and faunal logs for the Silurian succession in the Boghu Mountains were later published by Hairapetian et al (2012). The Silurian deposits assigned here to the Boghu Formation are underlain by the upper Katkoyeh Formation of Upper Ordovician siliciclastic rocks.…”

“…The Silurian brachiopods and conodonts of Central Iran are currently under extensive study and are partly published (Hairapetian et al 2012;Männik et al 2013Männik et al , 2015Popov and Cocks 2013;Popov et al 2015a). These works have expanded our knowledge of the Silurian geology, stratigraphy and palaeontology of Central Iran and the Kopet-Dagh Region.…”

“…The brachiopod fauna is represented by three succeeding low diversity rhynchonellide associations (Hairapetian et al 2012;Popov et al 2015a), which occur in the middle to upper part of the Boghu Formation (Text- fig. 2; units B9−B13).…”

“…According to Armstrong (1996) this conodont, characteristic of the Rhuddanian and Aeronian, already appears in the uppermost Ordovician, in the Metabolograptus persculptus Graptolite Biozone (GB). The lower boundary of Unit B1 was provisionally considered as the base of the Silurian in the Boghu succession (Hairapetian et al 2012) fig. 3).…”

“…Simplified tectonic maps of Iran showing the location of Silurian outcrops (marked with red colour; small exposures are exaggerated) discussed in the paper (base map modified mainly from Ramezani andTucker 2003 andHairapetian et al 2012); structural data compiled from various sources, e.g., Berberian and King (1981), Lindenberg et al (1984) and Alavi (1991). Abbreviations: AZF -Abiz Fault; DRFDoruneh Fault; KBF -Kuhbanan Fault; KMF -Kalmard Fault; MAF -Mehdiabad Fault; MZT -Main Zagros Thrust; NAF -Nostratabad Fault; NBF -Nayband Fault; NHF -Nehbandan Fault; NNF -Nain Fault; OKF -Ozbak Kuh Fault; RVF -Rivash Fault; SBF -Shahre-Babak Fault; SHF -Syahkuh Fault; TKF -Taknar Fault dence at present, recently documented early Silurian faunas from Central Iran suggest closest links with Afghanistan and the north-east Iranian Kopet-Dagh Range (Hairapetian et al 2012;Popov et al 2014). In the south-west, the Central-East Iranian Platform is bounded by the Sanandaj-Sirjan terrane and the Main Zagros Thrust Zone.…”