Orthacean and strophomenid brachiopods from the Lower Silurian of the central Oslo Region

Brachiopods of the Superfamily Orthacea and the Order Strophomenida from the Llandovery of the Oslo-Asker District of Norway are described taxonomically. The fauna com prises 41 species belonging to 29 genera. Among these, 3 genera, 10 species and l subspecies are new. They include the genera Orthokopis, Crassitestella and Eocymostrophia, the species Orthokopis idunnae, Eridorthis vidari, ? Plectorthis sp., Skenidioides worsleyi, Skenidioides hymiri, Dactylogonia dejecta, Eostropheodonta delicata, Mesopholidostrophia sifae, Eocymostrophia balderi, Fardenia oblectator and Eoplectodonta transversalis (Wahlenberg, 1818) jongensis. Parts of the brachiopod fauna occurring across the Ordovician-Si!urian boundary are unique in representing relict Ordovician taxa. They are related to relatively rare offshore faunas preserved in uppermost Ordovician strata; they survived into the Silurian in the tectonically active, deeper-water environments found in the intracratonic basin of the Oslo Region. The rest of the Llandovery fauna shows very close ties with the faunas and fauna! structure of the Welsh Basin, suggesting minimal geographic separation between Baltica and Avalonia. Similarities at species level between the Gornyj Altaj and the Norwegian-Welsh fauna also suggest that the two areas were situated in the same climatic zone, contrary to some recent paleogeographic reconstructions. A rare fauna of the Southern Midcontinent of Laurentia lived in similar facies as the Norwegian-Welsh fauna and show strong resemblances on a generic leve!. Except for some eurytopic species, however, many of the species are different, indicating that Silurian barriers existed between these locations.