A total of 15,955 identifiable conodont elements were extracted from the Orthoceras Limestone and its lateral equivalents in the Oslo graben, Norway. The Lower Ordovician conodont zonation devised by M. Lindstrom in 1971 for the eastern Baltic is, for the most part, recognizable in the Norwegian section. In gross aspect, the following equivalencies are noted: Megistaspis Limestone (3c?)/lower Stein Limestone and zone of Paroistodus originalis (BII?); uppermost Asaphus Shale (3c?)/compact limestone of the Endoceras Limestone (3c?)/lower upper Stein Limestone and the lower part of the zone of Amorphognathus variabilis (BIII?). Hence, the Orthoceras Limestone, with the possible exception of the transition beds, Helskjær Shale and Limestone, and uppermost Stein Limestone, are within the upper part of the graptolite zone of Didymograptus hirundo (upper Volkhov/lowermost Kunda). At Rognstrand, the Arenigian-Llanvirnian boundary is placed within the lower half meter of transition beds.
Conodont biostratigraphy of the Lower Ordovician Orthoceras and Stein Limestones (3c), Norway