Graptolites are described from rock units around the Middle-Upper Ordovician boundary in the Oslo Region for the first time. Twelve taxa, including Dicranograptus clingani resicis subsp. nov., are studied systematically. Ten sections have been logged through the Venstøp Shale Formation, the base of which is marked by a phosphorite conglomerate. Below this horizon on Nakholmen (inner Oslofjord) a middle (?) Dicranograptus clingani Zone graptolite fauna is followed directly by Actonian/Onnian Stage trilobites.
Above the phosphorite, graptolites indicating the Pleurograptus linearis Zone occur in association with Pusgillian Stage trilobites. It is concluded that the phosphorite represent a hiatus equivalent to a time interval including the late D. clingani and early P. linearis zones.