Peri-Gondwanan acritarchs and chitinozoans from the Lower-Middle Ordovician Lashkarak Formation in the Alborz Mountain Ranges, northern Iran: Regional stratigraphical significance and palaeogeographical implications
Creators: name: Ghavidel-Syooki, by Mohammad
name: Piri-Kangarshahi, Mohammad Hossein
Publisher: name: Taylor & Francis
Year: 2023
Subjects: subject: Microbiology
subject: FOS: Biological sciences
subject: Evolutionary Biology
classificationCode: 59999
subject: Environmental Sciences not elsewhere classified
subject: Ecology
classificationCode: 20199
subject: Astronomical and Space Sciences not elsewhere classified
subject: Sociology
subject: FOS: Sociology

description: The Lashkarak Formation (Lower-Middle Ordovician) in the Gerdkuh locality, 10 km west of Damghan city, northern Iran has been found to contain acritarchs and chitinozoans. This study aimed to understand these chitinozoan and acritarch successions as well brachiopods in this part of the Alborz Mountains, in a novel manner. Ninety-Seven surface samples from this succession were collected and analysed. Thirty samples yielded well-preserved palynomorph taxa such as acritarchs, chitinozoans, and scolecodonts, as well as graptolite remains. In total, 53 taxa were identified from acritarchs (38 species from 21 genera) and chitinozoans (15 species from 10 genera). Although scolecodonts and graptolite remains were also observed, they were not studied in detail. The palynological analyses revealed the presence of several diagnostic acritarchs in the Lashkarak Formation, including <i>Vulcanisphaera simplex</i>, <i>Arbusculidium filamentosum</i>, <i>Coryphidium bohemicum</i>, <i>Dactylofusa velifera</i>, <i>Striatotheca mutua</i>, <i>Arkonia virgata</i>, and <i>Orthosphaeridium ternatum</i>. These acritarchs allowed recognition of five acritarch assemblage zones. Similarly, seven chitinozoan biozones were recognised, including <i>Eremochitina brevis</i>, <i>Desmochitina ornensis</i>, <i>Belonechitina henryi</i>, <i>Cyathochitina protocalix</i>, <i>Cyathochitina calix</i>, <i>Siphonochitina formosa</i>, and <i>Laufeldochitina clavata</i>. These findings confirm the inclusion of the Alborz Mountains in the peri-Gondwana palaeoprovince during the Early-Middle Ordovician. The co-occurrence of the acritarch and chitinozoan taxa with previously identified brachiopods allowed for the establishment of a more detailed Early-Middle Ordovician biozonation, demonstrating their potential usefulness for global chronostratigraphy. Based on palynological and brachiopod assemblages, a shallow marine environment of inner shelf setting is suggested for the Early-Middle Ordovician at the Gerdkuh locality. Moreover, in this study, four new morphotype species were erected, consisting of one new chitinozoan (<i>Cyathochitina gerdkuhensis</i> sp. nov.) and three new acritarchs (<i>Baltisphaeridium razii</i> sp. nov., <i>Navifusa alborzensis</i> sp. nov., and <i>Orthosphaeridium iranense</i> sp. nov). However, <i>Othosphaeridium</i> cf <i>ternatum</i>. was left with open nomenclature. Morphotypes from the Lower-Middle Ordovician (Lashkarak Formation) in the Alborz Mountains. 15 chitinozoan species (10 genera) recorded from the Lashkarak Formation, S. Caspian Sea. 38 acritarch taxa (21 genera) were identified, attributing to the N. Gondwanan Domain. The diagnostic acritarch taxa consist of <i>Arbusculidium</i>, <i>Coryphidium</i>, <i>Striatotheca,</i> and <i>Arkonia</i>. Chitinozoan biozones are: <i>E. brevis, D. ornensis, B. henryi, C. protocalix, C. calix, S. formosa</i>, <i>L. clavata</i>. <i>Baltisphaeridium razii</i> sp. nov., <i>Navifusa alborzensis</i> sp. nov., <i>Cyathochitina gerdkuhensis</i> sp. nov., in the first time. The hiatus corresponds to the Late Ordovician, Silurian, and Early-Middle Devonian