Middle Ordovician ostracods from the Lanna and Holen Limestones, south-central Sweden

The macrolithologically monotonous section of the Middle Ordovician Lanna Limestone and Holen Limestone in Hällekis, Västergötland, south-central Sweden, yields a rich and diverse ostracod fauna. The taxonomical composition in its lower part is comparable to that in North Estonian sections, whilst other species are known from the Kunda Stage of Estonia, Latvia, and Lithuania or the Lower Red and the Upper Red Orthoceras Limestone of Dalarna, Sweden. The ostracod assemblage structure and diversity index shows considerable variation throughout the section. A low-diversity assemblage occurs in the lowermost Lanna Limestone and the uppermost Holen Limestone. A high-diversity assemblage occurs at the base of the Holen Limestone (Täljsten marker horizon). Judging from the faunal composition, the boundary between the Volkhov and Kunda stages in Hällekis should be placed lower than previously thought.