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Biostratigraphy of the Ordovician Nemagraptus gracilis Zone

Summary The middle Ordovician Nemagraptus gracilis Zone has long been used as a prominent, reliable correlation datum, having been recognized on every continent except Africa and Antarctica. However, there is some disagreement today on its definition and its boundaries. A re-examination of the original definition of the zone and evaluation of pertinent graptolite and conodont biostratigraphical data from key sections in Scotland, Wales, Scania, New York, Texas, Alabama and Australia indicate that the first appearance of N. gracilis is synchronous worldwide and can thus be used to define the base of the zone. Additional studies, however, are necessary before an internationally recognized upper boundary of the zone can be defined. The Berrybush Burn section in Scotland is proposed as a general reference section of the N. gracilis Zone graptolite assemblage, with the Fågelsång and Albany sections in Scania and New York, respectively, as secondary reference sections. The Fågelsång, Sweden and Calera, Alabama sections are proposed as local reference sections for the lower boundary of the zone in, respectively, Europe and North America.