The Otta brachiopod and trilobite fauna: palaeogeography of Early Palaeozoic terranes and biotas across Baltoscandia

The diverse, Mid-Ordovician shelly fauna from Otta, in the central Scandinavian Caledonides, has prompted much debate regarding the development and palaeogeography of key parts of the Scandinavian Caledonides. During the last 35 years, the Scandinavian Caledonides have yielded a further range of variably preserved fossils from platform, margin and oceanic environments: for example, Early to Mid-Ordovician brachiopod faunas from the Lower Allochthon in Valdres, with Baltic affinities, contrast with the Celtic and Toquima Head biotas from parts of the Upper Allochthon. The Early to Mid-Ordovician faunas thus demonstrate a range of provincial signatures and the relationship between these and the coeval faunas of the parautochthon and the adjacent autochthon provide an alternative palaeogeographic model for the developing orogen. New data from Otta and multivariate statistical analyses of the Iapetus brachiopod assemblages confirm the intraoceanic position of the Otta archipelago. The presence of Platytoechia and Productorthis strengthens links with the Celtic group of faunas, whereas Bockelia was first described from the adjacent Trondheim region. Shelly faunas from the allochthons have little in common with slightly older assemblages from Siberia; the latter are of low diversity and dominated by plectorthoid and syntrophioid brachiopods, within carbonate facies, similar to those of continental Laurentia and other parts of the world. For example, the faunas of the higher Upper Allochthon may be accommodated within the Laurentian marginal Toquima-Table Head province. These together with the Otta fauna demonstrate that the provincial signatures of the Upper Allochthon assemblages are consistent with settings within the Iapetus Ocean associated with one or more microcontinents, seaward of Baltica.