The biostratigraphy and palaeobiogeography of Cambrian and Ordovician acritarchs and chitinozoa from the Simeh-Kuh, NW Damghan City, the Alborz Mountains, northern Iran.

A study in Simeh-Kuh, northern Iran, focuses on Ordovician strata, documenting acritarchs and chitinozoa, and has helped to understand ancient environments. The aim is chronostratigraphical calibration, using palaeontological data including brachiopods, trilobites, and graptolites. Ninety-three surface samples were collected and analysed, yielding a rich collection of 117 morphotypes, including 59 acritarch species and 58 chitinozoan species. Eight acritarch assemblage biozones (A–H) and 14 chitinozoan biozones (1–14) were established. Biozones A–F indicate Late Cambrian to Middle Ordovician ages in the Lashkarak Formation, while biozones G–H indicate the Late Ordovician in the Ghelli Formation. The Eremochitina brevis-Linochitina pissotensis biozones suggest late Floian to late Darriwilian ages, and the Belonechitina robusta-Armoricochitina nigerica biozones support a Late Ordovician age. These findings place the Alborz Mountains in the North Gondwana Domain during the Ordovician. Palynomorphs with a thermal alteration index of 4.0–4.5 indicate overmature organic matter in Lower Palaeozoic deposits, potentially linked to the Ordovician metamorphic event in the southern Caspian Sea. Additionally, four new species were identified: Cyathochitina sp. A, Tanuchitina hosseini-nezhadi sp. nov., Dactylofusa kazzazii sp. nov., and Inflatarium alborzensis sp. nov. Highlights: A palynological study at Simeh-Kuh in the Alborz Mountains, northern Iran has uncovered a richly preserved assortment of Cambrian–Ordovician acritarchs and chitinozoa. Fifty-eight chitinozoan species, spanning 22 genera, were identified from the Lashkarak and Ghelli formations at Simeh-Kuh. This facilitated the delineation of 14 distinct chitinozoan biozones. Fifty-nine acritarch species across 33 genera were identified from the Late Cambrian to Late Ordovician strata, contributing to recognising eight distinct assemblage biozones. The Lashkarak Formation has yielded four distinctive acritarch genera: Arbusculidium, Coryphidium, Striatotheca, and Arkonia. These genera hold notable significance as they are characteristic elements of the Peri-Gondwanan palaeoprovince. The identification of chitinozoans in the Lashkarak and Ghelli formations has facilitated the establishment of 14 chitinozoan biozones. Notably, these biozones are considered classical elements of the Northern Gondwana Domain/Peri-Gondwana palaeoprovince. The Belonechitina robusta, Tanuchitina fistulosa, Acanthochitina barbata, and Armoricochitina nigerica biozones were recognised in the Ordovician deposits at Simeh-Kuh. This discovery marks the region's first confirmation of Sandbian–Katian strata. A significant hiatus exists between the Ghelli and Geirud formations, spanning the Late Ordovician (latest Katian–Hirnantian), Silurian, and Early–Middle Devonian periods. Four new species have been established: Cyathochitina sp. A, Tanuchitina hosseini-nezhadi sp. nov., Dactylofusa kazzazii sp. nov., and Inflatarium alborzensis sp. nov. [ABSTRACT FROM AUTHOR]