Diversity and biostratigraphic utility of Ordovician brachiopods in the East Baltic.

The stratigraphy of the Ordovician carbonates of Baltoscandia was initially based, during the 19th century, on the stratigraphical ranges of macrofossils, mainly trilobites, but other fossils (brachiopods, echinoderms and cephalopods) were also used. During the 20th century, their importance in biostratigraphy gradually decreased due to a greater reliance on microfossils, especially conodonts and chitinozoans, which enable accurate correlation of carbonate successions where graptolites are absent or very rare. New methods have further reduced the attraction of macrofossils for biostratigraphy, although they are useful tools in different fields of geology such as palaeobiogeography and palaeoecology. The revised data on species diversity and the stratigraphical distribution of articulated brachiopods with carbonate shells (rhynchonelliformeans) in the East Baltic are used here for the evaluation of their role and potential in the modern stratigraphy of the Ordovician System. The 106 stratigraphical units (mainly formations and members) belonging to 17 Ordovician and the lowermost Silurian regional stages are analysed based on the taxonomic composition of their brachiopod faunas comprising in total more than 400 species. The cluster analysis used in this stratigraphical experiment captures the major dissimilarities between and continuity of the regional subseries, stages and subregional units by the similarity of their brachiopod faunas. Bryozoans, another important group of benthic organisms in shallow-water facies, are analysed for comparison, providing a test for the correlations based on brachiopods