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

Abstract

The Ludfordian sections of shallow-water carbonates, which are exposed in the Urals, Arctic islands, and northeastern part of the East European platform, yielded diverse brachiopod assemblages (BA-2) with dominant atrypids and athyrids. The rather monotonous lithology of these sections (Gerdyuss Superhorizon) opens an opportunity to trace the complete stratigraphic ranges of genera and to detect their phylogeny. Representatives of the Greenfieldia-Didymothyris lineage associate with other athyrids, such as Squamathyris and Homeathyris forms and have a complicated 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 rhynchospirinids, spiriferids, and rhynchonellids in addition to surviving athyrids. The distinct diversification of Ludfordian atrypid species was also confined to this time. These taxonomic changes in brachiopod assemblages are used for defining the boundary between the Gerdyuss and Greben horizons. Along the western and southwestern margins of the platform (Baltic region, Podolia), the middle Ludfordian extinction and subsequent radiation of brachiopod fauna are implied only by fragmentary data. Distribution ranges of brachiopods are insufficiently complete here because of frequent facial changes inside the sequence of shallow-water deposits. The inferred variations in diversity of brachiopod assemblages can be applied for correlation of the upper Ludlow deposits throughout the European province. The paper includes characteristics of the new genus Homeathyris and its type species Homeathyris insularis from the Ludfordian deposits of Dolgii Island.