Conodont Biostratigraphy of the Middle and Upper Ordovician of Europe and Eastern North America

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 superbusZones are Middle Ordovician, the upper part of the Amorphognathus superbusand the Amorphognathus ordovicicusZones are Upper Ordovician in age. The Pygodus serrusZone is subdivided into five, the Pygodus anserinusZone into two, and the Amorphognathus tvaerensisZone into three subzones. The conodont zones and subzones established are recognizable not only in the Balto-Scandic area but also, to a large extent, in other parts of Europe and in eastern North America. Although the horizontal distribution of Ordovician conodonts in Europe and eastern North America is still incompletely known, it is evident that the lower Middle Ordovician deposits of this area contain elements of at least three major conodont faunal provinces, namely the American midcontinent province, the North Atlantic province, and the Australian province. Llanvirnian faunas of the Balto-Scandic area are unlike most contemporaneous conodont faunas from eastern North America, although a few common species permit some important correlations. Llandeilian and lower Caradocian faunas are similar in the Balto-Scandic area and in the eastern part of the Appalachian Valley, due mainly to an invasion of Balto-Scandic species into the Appalachian Valley during the deposition of rocks corresponding to the Glyptograptus teretiusculusthrough lower Diplograptus multidensgraptolite zones. Younger Ordovician faunas from eastern North America and Europe have, with some exceptions, little in common; the American faunas are of midcontinent type, whereas a Balto- Scandic type of fauna was distributed over much of Europe (Scandinavia, Estonia, northern England, southern Scotland, Ireland, East Germany, northern Italy, Spain), at least during Upper Ordovician time.