Summary of Conodont Biostratigraphy of the Silurian System of North America

Because of the early paucity of Silurian conodont studies, it is still necessary to establish a comprehensive zonation and provide the sound systematics requisite for zonation of the Silurian System in North America. Current research indicates that the broad outlines of North American zones will closely parallel those defined in the Carnic Alps by Otto Walliser in 1964. In detail, however, modification is required, and the problems of provincialism and migration indicate need for some geographically restricted zones. The oldest Silurian conodont zone recognized in the Midwest, northern Michigan, and Ontario is yet to be named. Above it the limits of the Icriodina irregularisAssemblage Zone have been established in the same area, but provincial differences between the northern and southern areas are recognized. The next overlying zones, the Neospathognathodus celloniand the Pterospathodus amorphog-nathoides-Spathognathodus ranuliformisAssemblage Zones, have been documented in the Midwest and in the Niagara Gorge area. Younger zones have not been established, but a number of guide fossils of Walliser's zones have been recognized. Kockelellahas been found in the upper part of the Louisville Limestone and in the Mississinewa Shale Member (Wabash Formation) in Indiana; Spathognathodus sagittais present in the Lockport Dolomite in the Niagara Gorge and the St. Clair Limestone in Arkansas; Polygnathoides siluricusand Kockelella variabilisare included in the diversified fauna of the Bain-bridge Formation in Missouri and in the Lafferty Limestone in Arkansas; P. siluricusis also present in the Brownsport Formation and Decatur Limestone in Tennessee; Spathognathodus snajdriis found in the Kokomo Limestone Member (Salina Formation) in Indiana and the top of the Lockport Dolomite in New York; Spathog-nathodus eosteinhornensisis present in the Stonehouse Formation in Nova Scotia; and Spathognathodus remscheidensisis reported from the Syracuse Formation and younger Silurian strata in New York and from the upper part of the Decatur Limestone in Tennessee.