The trilobite fauna of the upper Tremadoc Bjørkåsholmen Formation in the Oslo Region is revised and redescribed, recognizing 36 species assigned to 28 genera. The regional and vertical distribution of the trilobite fauna are also discussed. The Bjørkåsholmen Formation is found in the Lower Allochthon of the Synfjell Nappe and across the Oslo Region in Norway. In Sweden it crops out in Västergötland, Scania and on Öland. Generally the unit is 60-120 cm thick, comprising micritic to intrasparitic limestone composed of several individual limestone beds, covering the Zone of Apatokephalus serratus. Near the base of the formation a horizon consisting of dark limestone nodules appears, containing a trilobite fauna dominated by Bienvillia angelini. This level is found throughout the studied area and is an important correlatable horizon.
New material of rare species, such as Peltocare modestum Henningsmoen, 1959, Parabolinella lata Henningsmoen, 1957, Falanaspis aliena Tjernvik, 1956, Harpides rugosus (Sars and Boeck, 1838) and Parapilekia speciosa (Dalman, 1827), is described and figured. Orometopus elatifrons (Angelin, 1854) is recognized as distinctly different from British material formerly assigned to this taxon. Three new species are described, Saltaspis stenolimbatus n.sp., Apatokephalus dactylotypos n.sp., and Niobe (Niobella) eudelopleura n.sp. Additionally, the species Apatokephalus cf. sarculum Fortey and Owens 1991 from the upper part of the Alum Shale Formation is described.
Biostratigraphical studies were carried out at six localities distributed across the Oslo Region. A relative-abundance distribution shows that Ceratopyge acicularis dominates the lower limestone beds above the dark limestone nodules and is followed by a small acme of Apatokephalus serratus, then a dominance of Euloma ornatum, and, finally, Symphysurus angustatus in the uppermost fossiliferous beds of the formation. Species of the large asaphid Niobe are present throughout the unit in relatively constant numbers. The remaining species are present in limited numbers. Older views claiming a greater diversity in the Oslo-Asker district compared to the rest of the Oslo Region are erroneous. All data suggest a coherent distribution and diversity across the Oslo Region, with local variations.