The Early Ordovician trilobite distribution and zonation of the East Baltic

Biozones of the East Baltic Early Ordovician trilobites were studied based on the trilobite distribution in northwestern Russia, Estonia, and Sweden. New data on trilobite distribution reveal the diachroneity of the lowest calcareous sediments of the Mäeküla Member, which forms the upper part of the Leetse Formation in the East Baltic. Trilobites of the Tremadoc Apatokephalus serratus and Megistaspis (Ekeraspis) armata biozones are missing in the East Baltic, in contradiction to the suggestion of Balashova (Vopr. Paleontol., 1966, 5, 3–22). Some poorly preserved specimens probably belonging to Megistaspis (Paramegistaspis) planilimbata (Angelin) and also records of Pliomerops? linnarssoni (Wiman) indicate that the Arenig Megistaspis (P.) planilimbata Biozone of the Hunneberg Stage may be represented in some localities. The final delimitation of biozones in the Mäeküla and Päite members should improve when the ontogenetic and phylogenetic variation of the M. (Paramegistaspis) lineage has been fully documented. Until then the Evropeites lamanskii Biozone is used in the East Baltic following Balashova, as indicated by the appearance of that taxon in most studied localities, marking the beginning of the Billingen Age. The succeeding Megalaspides (Megalaspides) dalecarlicus Biozone is represented by the occurrence of at least its upper part in the Vassilkovo Beds in the St. Petersburg region, where Megalaspides paliformis Tjernvik occurs together with the index taxon (these taxa co-occur in the upper part of this zone in Sweden). The Päite Member in the East Baltic is correlated with the Megistaspis (P.) estonica Biozone of the upper part of the Billingen Stage in Sweden.