The appearance of Rosenella (stromatoporoid sponge) in Baltica: new species from the Pirgu Stage (Katian, Upper Ordovician) of Estonia

The Katian Age, part of the Late Ordovician Epoch, was a period of significant stromatoporoid diversification. However, their fossil record is often less continuous, hindering our understanding of their diversity and diversification. Estonia, which was part of Baltica during the Ordovician, is one of the most intensively studied areas for stromatoporoids. In this study, we describe a new species of the stromatoporoid genus Rosenella, named R. hosholmia sp. nov., from the Upper Ordovician Pirgu Stage Adila Formation (late Katian in age) of Estonia. The specimens were collected from the Hosholm locality on Vormsi Island. R. hosholmia is characterised by high variability in cyst size and an alternation of sporadic layers of denser, either smaller or larger, cyst plates. The cyst plates display short, thin, sharp denticles, appearing as tightly clustered points in tangential sections. Ichnofossils Planolites and Coprulus occur with this new Rosenella species. This discovery marks the first record of Rosenella in the Ordovician of Baltica, extending the palaeogeographic range of this genus beyond its known occurrences in Gondwana, peri-Gondwanan terranes, and the Altai–Sayan Fold Belt of southwestern Siberia during the Middle to Late Ordovician interval. The finding highlights the potential for further discoveries in well-studied areas, and suggests that stromatoporoid diversity in Baltica may be higher than previously known. The lack of stromatoporoid faunal similarity between Baltica and any other continents, including Laurentia, during the Ordovician contrasts with other organisms such as cephalopods and brachiopods, indicating asynchronous dispersal patterns across different groups.