THE MIDDLE LUDFORDIAN EVENT IN THE SILURIAN EVOLUTION OF BRACHIOPODS OF THE EUROPEAN PROVINCE

The Ludfordian sections of shallow-water carbonates, which are exposed in the Urals, Arctic islands, and northeastern part of the East Europ ean platform, yielded diverse brachiopod assemblages (BA-2) with domin ant atrypids and athyrids. The rather monotonous Lithology of these se ctions (Gerdyuss Superhorizon) opens an opportunity to trace the compl ete stratigraphic ranges of genera and to detect their phylogeny. Repr esentatives of the Greenfieldia-Didymothyris lineage associate with ot her athyrids, such as Squamathyris and Homeathyris forms and have a co mplicated inner structure resembling that of Didymothyris species. All the indicated genera gradually died out during the middle Ludfordian time and were replaced by new brachiopod assemblages, which included r hynchospirinids, spiriferids, and rhynchonellids in addition to surviv ing athyrids. The distinct diversification of Ludfordian atrypid speci es was also confined to this time. These taxonomic changes in brachiop od assemblages are used for defining the boundary between the Gerdyuss and Greben horizons, Along the western and southwestern margins of th e platform (Baltic region, Podolia), the middle Ludfordian extinction and subsequent radiation of brachiopod fauna are implied only by fragm entary data. Distribution ranges of brachiopods are insufficiently com plete here because of frequent facial changes inside the sequence of s hallow-water deposits. The inferred variations in diversity of brachio pod assemblages can be applied for correlation of the upper Ludlow dep osits throughout the European province. The paper includes characteris tics of the new genus Homeathyris and its type species Homeathyris ins ularis from the Ludfordian deposits of Dolgii Island.