Trilobite systematics, biostratigraphy and palaeoecology of the Lower Ordovician Komstad Limestone and Huk Formations, southern Scandinavia

The upper Arenigian to lower Llanvirnian Komstad Limestone Formation is a tongue of the Baltoscandian 'Orthoceratite limestone', intercalated in the predominantly shaly Ordovician succession of Scania-Bornholm, southernmost Scandinavia. The stratigraphically equivalent Huk Formation of the Oslo area has been studied only at Slemmestad. Based on collections of about 8000 trilobites, the Komstad Limestone and Huk Formations are assigned to the Zones of Megistaspis polyphemus (former M. lata), M. simon, M. limbata, Asaphus expansus, and A. 'raniceps' of the Volkhov and Kunda Stages. The Komstad Limestone Formation is diachronous. The correlation of trilobite and graptolite zones is briefly discussed. Inferences on the autecology of the most common trilobite groups are summarised. Five recurrent trilobite biofacies are defined: the benthic assemblage succession Nileus→Geragnostus→Symphysurus→Megistaspis represents a depth transect, whereas the Cyclopyge assemblage was pelagic. The correlation of palaeoenvironmental changes as revealed by the alternation of biofacies, ie ecostratigraphy, improves stratigraphical resolution significantly. A relative-sea-level curve is reconstructed on this basis.