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

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 degrees 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.

Automated summary

This study presents a new analysis of melanosclerites from the Late Ordovician to earliest Silurian, identifying 12 species and plotting their palaeogeographical occurrences on an up-to-date map. The distribution pattern of these organisms shows a tendency to occur in low-latitude, warm-water regions, mainly confined within the tropics and subtropics. Comparing with Palaeozoic hydroids, both fossils overlap in palaeogeographical distribution, indicating a potential close biological affinity between the two types of organisms.