Early Silurian chitinozoans from the Qusaiba type area, North Central Saudi Arabia

Sixty-one core samples from three shallow core holes (Qusaiba-1, Baq'a-3, and Baq'a-4) penetrating the Qalibah Formation (Qusaiba Member) in North Central Saudi Arabia were investigated. These core slabs, corresponding to grey-greenish hemipelagic shale or silty-shale, yielded well-preserved and abundant early Silurian chitinozoans. Most recorded chitinozoan assemblages are diverse and include several new species. An informal early Rhuddanian chitinozoan assemblage dominated by Cyathochitina caputoi is documented in the lowermost part of the Qusaiba Member. Six chitinozoan biozones ranging from Aeronian to Telychian are identified: Angochitina qusaibaensis (pro parte), Conochitina alargada, Angochitina hemeri, Angochitina macclurei, Tanuchitina obtusa, and Eunochitina silurica. The presence of these biozones and newly described species (Ancyrochitina alhajrii sp. nov., Ancyrochitina camilleae sp. nov., Armoricochitina crassicarinata sp. nov., Armoricochitina gengi sp. nov., Bursachitina baqaensis sp. nov., Conochitina viviae sp. nov., Cutichitina minivelata sp. nov., Cyathochitina neolatipatagium sp. nov., Fungochitina merrelii sp. nov., Muscochitina oliveri sp. nov., Plectochitina alisawyahensis sp. nov., Plectochitina alnaimi sp. nov., Plectochitina jaquelineae sp. nov., Plectochitina lucasi sp. nov., and Spinachitina geerti sp. nov.) highlight significant diversity and stratigraphic correlation across northern Gondwana.

In addition to chitinozoans, acritarchs, scolecodonts, and eurypterid fragments were noted in the organic residues. The studied intervals suggest varied depositional environments and potential hydrocarbon significance (oil-prone and gas-prone). The Ordovician–Silurian boundary remains undetermined within the borehole, lying likely below the examined depths.