Silurian vertebrate remains from the Oslo Region, Norway, and their implications for regional biostratigraphy

Several vertebrate assemblages are described from the Silurian of the Oslo Region, Norway, based on the review and revision of previous reports of microremains (e.g. by Turner & Turner 1974), as well as unpublished material from museum collections. Articulated thelodont specimens from the Rudstangen Fauna, Ringerike Group, are also described here for the first time, revealing a seemingly monogeneric loganelliid assemblage. The oldest assemblage (mid-Llandovery) only contains the thelodont Loganellia cf. aldridgei, while a single sample from upper Llandovery strata produced four Thelodus sp. scales. These scales share features with those from younger Thelodus taxa and give additional support to an early appearance of this genus. The mid-Wenlock faunas consist of thelodonts Loganellia grossi, Loganellia einari and Thelodus laevis. These are joined by the thelodont Paralogania martinssoni, anaspids Rhyncholepis parvula and cf. Pterygolepis nitida, as well as the osteostracans cf. Tyriaspis whitei and Osteostraci gen. et sp. indet. in late Wenlock and earliest Ludlow faunas. These complement the previously described anaspids and osteostracans of the Rudstangen Fauna based on articulated specimens. The faunas of a number of calcarenite samples collectively contain the thelodonts L. grossi, L. einari, P. martinssoni and T. laevis, and are most likely of early Ludlow age. When the vertebrate-producing samples are analysed and put in the proposed stratigraphical framework for the Oslo Region that has been refined in recent decades, the faunas fit well into the vertebrate biozonation established for the Silurian (Maerss et al. 1995), contrary to previous claims. The earliest faunas in the area show similarities to the Llandovery of Britain, whereas thelodonts and anaspids, but not osteostracans, from late Wenlock and early Ludlow are more closely related to Scottish and Baltic forms. Additional sampling of the area may assist in refining the biostratigraphy and provide insights into Silurian vertebrate distributions in Norway and related regions.