Redefinition of the Sandbian–lower Katian conodont biozonation of Baltoscandia

Recent development in the Upper Ordovician conodont biostratigraphy of Baltoscandia highlights the mismatch between the traditionally used conodont zonation and ranges of the eponymous species. Practical application of the zonation is further complicated by the fact that the morphology of the long-ranging species Amorphognathus tvaerensis, the key taxon of the eponymous conodont zone, changes through its distribution interval, and its older and younger representatives are quite different. The latter one was recently described as a new species, A. viirae. Also, it appeared that the specimens assigned earlier to A. ineaqualis in the northern Baltoscandian region are conspecific with A. tvaerensis and A. ineaqualis is missing here. As a result, the A. ineaqualis Conodont Zone has to be abandoned from the regional zonal scheme. Restudy of conodont collections from the Bliudziai-150 (Lithuania) and Kovel-1 (Ukraine) core sections demonstrated the absence of A. ineaqualis and the presence of A. viirae also in the southern Baltoscandian area and Ukraine. This paper contains a formal description of the new, emended conodont zonation for Sandbian and the lowermost Katian of the Baltoscandian Palaeobasin and its correlation to the regional chemostratigraphic standard.