Shallow marine Rostroconchia (Mollusca) from the latest Devonian (Strunian) and their significance for rostroconch life style and evolution

Abstract

1. Introduction The late Famennian sedimentary succession in central and western Europe is characterized by regressive sequences, followed by a short transgressive phase, the TR cycle IIf of Johnson et al. (1986) Strunian in age. This final Devonian pulse in the global sea-level curve is mirrored by respective litho- and biofacies within the siliciclastic shelf areas bounding the southeastern margin of Laurussia. This shelf is represented by a narrow, continuous band of siliciclastic, mixed siliciclastic-calcareous and calcareous sediments, in its central part known as the Condroz Shelf, recorded from southern Ireland across southwest England (north Devon), the northern Ardennes (Belgium) and the Bergisches Land (western Germany) further east to northern Poland. Although variable in specific lithology, the shallow subtidal environment of this shelf area was termed as the Pilton-Velbert Facies, subdivided into several regional subfacies (see e.g. Van Steenwinkel, 1984, 1990; Thorez & Dreesen, 1986; Matyja, 1988; Amler, 1995, 2001; Thorez et al., 2006 for details). Due to the semi-arid to subtropical climate, the southeastern margin of Laurussia offered optimal conditions to establish a highly diverse fauna and flora during the latest Famennian transgressive phase contrasting with the sparse fauna of the nearshore middle and late Famennian. Somewhat variable substrate conditions, however, combined with other ecologically significant parameters, e.g. salinity, bottom currents, o