Rhopaliophora? asymmetrica, a new acritarch species from the lowermost ‘Arenig’ of Baltica and its biostratigraphic potential

A shallow water analogue of the basal part of the stratotype for the ‘Second stage’ of the Lower Ordovician at Diabasbrottet, Mt. Hunneberg, Sweden, occurs in the Baltic Basin, in the St. Petersburg region, northwestern Russia, where it is represented by the Lakity beds of the lower Leetse Formation, Hunneberg Substage. Palynological investigations of these beds revealed a diverse acritarch association comprising several distinctive species with high biostratigraphic and palaeobiogeographic potential, ranging throughout the upper Tremadoc^lower Arenig interval. Rhopaliophora? asymmetrica sp. nov., which characterises the Leetse Formation, is described from levels independently dated by co-occurring graptolites (Tetragraptus phyllograptoides Zone) and conodonts (Paroistodus proteus^Prioniodus elegans Zones). The possible significance of this species for the recognition in Baltica of the base of the ‘Arenig’ (base of the ‘Second stage’ of the Lower Ordovician Series) is discussed