Melanosclerites from the late Ordovician to lowermost Silurian of Estonia and their palaeogeographical implications

This study presents a new analysis of the late Ordovician to earliest Silurian melanosclerites from the Kaugatuma Well on Kuressaare Island in Estonia. By observing a large number of fossil melanosclerites, this study carefully reviews previous records and makes taxonomic emendations. Twelve species (including one in open nomenclature) that can be assigned to nine genera are identified, including the most typical elements, which were first reported by Eisenack eight decades ago. Based on a revised dataset of melanosclerite occurrences from 21 localities around the world, the Palaeozoic palaeogeographical occurrences of these organisms with uncertain affinity are then plotted on an up-to-date palaeogeographical map to determine their palaeogeographical potentials. Although the spatial distribution of melanosclerites shows patchy occurrences resulting from fewer research efforts, a low-latitude, warm-water region gathering distribution tendency can still be detected. All their reported occurrences are confined within the tropics and subtropics, with most of them restricted within 30° north and south of the equator during the Early Palaeozoic. This distribution pattern is compared with that of Palaeozoic hydroids, showing an overlapping in palaeogeographical distribution, with both occurring in Baltica, Laurentia, Siberia, South China, and the northeastern part of the Gondwana, but absent from high-latitude regions. If the future works can confirm their postulated close biological affinity, the palaeogeography of both fossils can be combined to complement each other.