Re-evaluation of the conodont Iapetognathus and implications for the base of the Ordovician System GSSP.

Terfelt, F, Bagnoli, G. & Stouge, S. 2011: Re-evaluation of the conodont Iapetognathus and implications for the base of the Ordovician System GSSP. Lethaia, Vol. 45, pp. 227-237. In 2000, the International Union of Geological Sciences (IUGS) ratified the decision from the International Working Group on the Cambrian-Ordovician Boundary (COBWG) to place the Global boundary Stratotype Section and Point (GSSP) for the base of the Ordovician System in the Green Point section, Newfoundland, Canada, at a point coinciding with the first appearance of the conodont Iapetognathus fluctivagus. However, a restudy of the conodont successions from Green Point shows that this species is not present at the boundary interval, and as a consequence the section does not fulfil the biostratigraphical requirements of a GSSP. The GSSP horizon as now defined is based on a level part-way through the range of I. preaengensis- a species with lower first appearance datum (FAD). The true FAD of I. fluctivagus is above the FAD of planktonic graptolites and well above the FAD of I. preaengensis. As a consequence of these problems, a restudy of the GSSP section and the other sections in the Cow Head Group is necessary. A redefinition of the GSSP horizon is suggested. The following four alternative horizons have potential as new horizons for the GSSP level: the FAD of Cordylodus intermedius; the FAD of Cordylodus andresi; the FAD of Eoconodontus notchpeakensis; and the FAD of the agnostoid Lotagnostus americanus. □ Boundary, Cambrian, conodont, Global boundary Stratotype Section Point, Iapetognathus, Ordovician. [ABSTRACT FROM AUTHOR]