Middle Ordovician acritarchs from the island of Odinsholm, Estonia

Fifteen acanthomorphic acritarch species belonging to the genera Baltisphaeridium, Goniosphaeridium and Peteinosphaeridium from the Middle Ordovician Uhakuan Stage of the island of Odinsholm (Osmussaare) in Estonia are described by means of stereo-scanning technique. Seven species have been found to be stratigraphically confined to the Viruan Series (Middle Ordovician). The acritarch assemblages are biostratigraphically compared with assemblages from other Baltoscandian regions, for example, Gotland, Öland and Östergötland.