First record of a receptaculitid algae (Receptaculitaceae) in Ukraine

Most exposures of the Upper Ordovician Subich Formation of western Ukraine are submerged beneath the Dniester Hydroelectric Power Station reservoir, complicating field studies. Nevertheless, several outcrops remain above the water level and provide valuable information on the biota of Late Ordovician ecosystems. Fossils of corals, crinoids, stromatoporoids, bryozoans, brachiopods, cephalopods, bivalves, gastropods, and other faunistic groups have been reported from the Subich Formation; however, no information regarding calcareous algae of the order Dasycladales from these deposits was previously available. We studied a sample of gray clayey-detrital limestone containing remains of an unknown organism, collected by Leonid Ivanovych Konstantynenko from the Subich Formation of the Molodove Horizon on the left bank of Studenytsia Bay of the Dniester Reservoir. Although the fossil was initially interpreted as a coral, detailed analysis revealed a thin (~5 mm) plate composed of rhomboidal cross-section elements (prisms) arranged in a spiral pattern resembling a sunflower inflorescence. These morphological features correspond to the species Receptaculites occidentalis, a member of a problematic group of fossil organisms found in deposits from the Ordovician to the Permian across multiple continents. Historically, these organisms were considered problematic, and their systematic position remains debated. While similar specimens were previously interpreted as corals, a specific type of sponge, or bryozoans, modern studies of their microstructure have clarified their classification as calcareous algae of the order Dasycladales. We report Receptaculites occidentalis from the Ordovician of Ukraine for the first time. This study provides a detailed description of the fossil, includes photographs, and presents a comparison with receptaculitid specimens from the National Museum of Natural History Museum of the NAS of Ukraine, collected from Silurian deposits of the Barrandian region (Czech Republic). These findings enhance our understanding of the palaeontological diversity of the Subich Formation and provide insights into the palaeogeography and environmental characteristics of Late Ordovician marine basins.