Stig M. Bergström, 1970. "Conodont Biostratigraphy of the Middle and Upper Ordovician of Europe and Eastern North America", Symposium on Conodont Biostratigraphy, Walter C. Stweet, Stig M. Bergstrom
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Detailed investigations of the stratigraphically remarkably complete Middle and Upper Ordovician succession in Sweden have permitted recognition of a sequence of five main conodont zones and ten subzones, principally based on evolutionary changes in rapidly evolving stocks of multielement species. Zonal fossils include species of Amorphognathus, Eoplacognathus, Prioniodus, and Pygodus. The Pygodus serrus, the Pygodus anserinus, the Amorphognathus tvaerensis, and the lower part of the Amorphognathus superbus Zones are Middle Ordovician, the upper part of the Amorphognathus superbus and the Amorphognathus ordovicicus Zones are Upper Ordovician in age. The Pygodus serrus Zone is subdivided into five, the Pygodus anserinus Zone...