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This study presents a new analysis of the Late Ordovician to earliest Silurian melanosclerites from the Kaugatuma core on Saaremaa Island, 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 Alfred 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 map to determine their palaeogeographical potential. Although the spatial distribution of melanosclerites shows patchy occurrences resulting from sparse research efforts, a tendency to occur in low-latitude, warm-water regions can be detected. All the reported occurrences are confined within the tropics and subtropics, with most of them restricted to 30° north and south of the equator during the Early Palaeozoic. This distribution pattern is compared with that of Palaeozoic hydroids, showing an overlap in palaeogeographical distribution, with both occurring in Baltica, Laurentia, Siberia, South China, and the northeastern part of Gondwana, but absent from high-latitude regions. If future work can confirm their postulated close biological affinity, the palaeogeography of the two types of fossils can be combined to complement each other.

Melanosclerites from the Late Ordovician to earliest Silurian of Estonia and their palaeogeographical implications