Scandinavian Isograptids (Graptolithina, Isograptidae): Biostratigraphy and Taxonomy

The Lower–Middle Ordovician of Scandinavia is largely dominated by endemic Atlantic Province graptolite faunas, which are useful only for local biostratigraphy and paleoecology, but are difficult for correlation on an inter-continental scale. Isograptids, regarded as essential for inter-continental biostratigraphic correlations in the Lower–Middle Ordovician due to their wide palaeobiogeographic distribution, have rarely been recorded from the region. The Scandinavian Tøyen Shale and Bogo Shale formations bear a number of isograptids that range in age from the late Floian to the early Darriwilian, and these provide a good tool for inter-continental correlations of this interval. Isograptus caduceus norvegicus Monsen and Isograptus spinosus Spjeldnaes are interpreted as possible tetragraptid species. The following biostratigraphically important Isograptus species from Scandinavia are described as new: Isograptus spjeldnaesi n. sp., Isograptus rigidus n. sp. Isograptus mobergi n. sp. and Scandinavian zonal names in the Dapingian are revised accordingly.