The biostratigraphy and palaeogeography of acritarchs, chitinozoans, and cryptospores from the Upper Ordovician Ghelli Formation, Khoshyeilagh region, southern Caspian Sea, Alborz Mountains, northern Iran

In the Khoshyeilagh region, 70 km north of Shahroud city, northern Iran, a siliciclastic succession of Lower Palaeozoic strata, including the Lalun, Mila, Lashkarak, and Ghelli formations occurs alongside igneous rocks. These strata contain various organic-walled microfossils, spanning acritarchs, chitinozoans, cryptospores, scolecodonts, and graptolite remains. This study assesses the biostratigraphical significance of marine microfauna (chitinozoans), microflora (acritarchs), and terrestrial cryptospores in this part of the Alborz Mountains. Eighty-eight surface samples were collected from the above-mentioned formations, with six samples barren due to red sediments or igneous rocks. The remaining samples yielded 118 palynomorph taxa: 55 species of acritarchs (28 genera), 48 species of chitinozoa (18 genera), and 15 cryptospore species (10 genera). Scolecodonts and graptolite remains were observed but not studied in detail. No palynomorphs were found in the Lalun, Mila, and Lashkarak formations, while the Ghelli Formation contained well-preserved, abundant organic-walled microfossils. These findings suggest that the Alborz Mountains were part of the Peri-Gondwana palaeo-province during the Late Ordovician. The co-occurrence of acritarchs, chitinozoans, and cryptospores in the Ghelli Formation indicates their potential for global chronostratigraphical correlation. The palynological assemblages suggest a shallow inner shelf environment during the Late Ordovician in the Khoshyeilagh region. Two hiatuses were identified: between the Lalun and Lashkarak formations and between the Ghelli and Soltan Meidan formations. This study also introduces 15 new morphotypes, including ten new species of acritarchs and five new species of chitinozoa, detailed in the systematic sections.

伊朗北部沙赫鲁德市以北70公里的霍什耶拉格（Khoshyeilagh）地区，发育有下古生界硅质碎屑岩系，包含拉伦（Lalun）、米拉（Mila）、拉什卡拉克（Lashkarak）与盖利（Ghelli）地层，且与火成岩伴生。这套地层产出各类有机质壁微化石，涵盖疑源类（acritarchs）、几丁虫（chitinozoans）、隐孢子（cryptospores）、虫颚（scolecodonts）以及笔石（graptolite）化石残体。本研究针对阿尔博兹（Alborz）山脉该区域的海洋微动物群（几丁虫）、微植物群（疑源类）以及陆生隐孢子的生物地层学意义展开评估。研究人员从上述地层中共采集88件地表样品，其中6件因受红色沉积物或火成岩影响而不含微化石。剩余样品共获得118个孢粉型（palynomorph）分类单元：包括55种疑源类（隶属于28属）、48种几丁虫（隶属于18属）以及15种隐孢子（隶属于10属）。本次研究虽观察到虫颚与笔石残体，但未对其展开详细研究。拉伦、米拉与拉什卡拉克地层均未发现孢粉型化石，而盖利地层则产出保存完好且数量丰富的有机质壁微化石。上述研究结果表明，晚奥陶世时期阿尔博兹山脉隶属于冈瓦纳周缘古生物地理区（Peri-Gondwana palaeo-province）。盖利地层中疑源类、几丁虫与隐孢子的共生组合，表明其具备全球年代地层对比的潜力。孢粉组合特征显示，霍什耶拉格地区在晚奥陶世时期处于浅内陆棚沉积环境。研究还识别出两处沉积间断：分别位于拉伦地层与拉什卡拉克地层之间，以及盖利地层与苏丹迈丹（Soltan Meidan）地层之间。本研究同时建立了15个新形态型，包含10个疑源类新种与5个几丁虫新种，相关细节将在系统古生物学章节中详细阐述。