FRASNIAN STAGE

all those rocks deposited worldwide during the Frasnian Age (374 to 367 million years ago). The Frasnian Stage and the overlying Famennian Stage together constitute the Upper Devonian Series. The stage's name is derived from the town of Frasnes in the Ardennes region of southern Belgium, which has served historically as the type district. Under the authority of the International Commission on Stratigraphy, the name has been retained, but the global stratotype section and point (GSSP) defining the base of this unit was reestablished in 1987 on a hillside exposure at Col du Puech de la Suque, 1.1 km (0.7 mile) southeast of Saint-Nazaire-de-Ladarez in the Noires Mountain region of southern France. The boundary point of the Frasnian is defined on the basis of the first occurrence of the conodont Ancyrodella rotundiloba, which coincided with the base of the conodont Polygnathus asymmetricus' biozone. The former species is part of a well-defined lineage, transitional between A. binodosa and four successor species of Ancyrodella. The goniatite genera Koenenites, Probeloceras, and Manticoceras also have their first occurrences within the lower portion of the P. asymmetricus biozone. The top of the Frasnian Stage records the extinction of many marine invertebrates, especially among the colonial rugose corals; stromatoporoids; orthid, pentamerid, and atrypid brachiopods; and trilobites. Also affected were many key conodonts. This major extinction event defines the top of the stage, as well as the base of the overlying Famennian Stage.