Bedding Plane Co-Occurrence of Biostratigraphically Useful Conodonts and Graptolites in Ordovician Shale Sequences
Bedding plane co-occurrence of biostratigraphically useful conodonts and graptolites in Ordovician shale sequences enhances the overall correlation precision between platform and deep water successions. Darriwilian shale successions in Tarim, western China, and Alabama and Idaho in North America contain the key conodont zonal indicator species Pygodus anitae, P. serra, and P. anserinus (as well as more long-ranging taxa) on bedding planes with Pterograptus elegans to Nemagraptus gracilis Zone graptolites. Three of the Pygodus bedding plane associations appear to be partial conodont apparatuses. The occurrence of bedding plane conodonts with graptolites across the Sandbian-Katian boundary at Black Knob Ridge (Atoka County, Oklahoma, U.S.A.) was a key factor in the selection of Black Knob Ridge as the GSSP for the base of the Katian, the middle stage of the Upper Ordovician Series. The Amorphognathus tvaerensis Zone - A. superbus Zone boundary is tentatively identified at 5.7 meters above the base of the Bigfork Chert in the lower Diplacanthograptus caudatus graptolite Zone.
New collections across the Sandbian-Katian succession at the Hartfell Score section near Moffat, Scotland also contain biostratigraphically important conodonts. Amorphognathus tvaerensis is present 1.6 meters below the FAD of D. caudatus and A. superbus is present 9.4 meters above it. Thus, at Hartfell Score the A. tvaerensis Zone - A. superbus Zone boundary occurs within an interval of 11 meters in the D. caudatus graptolite Zone. These bedding plane co-occurrences provide more precise ties between graptolite and conodont biozonations and support the potential for additional resolution with further collecting at these and other localities. Not all Sandbian-Katian successions have yielded biostratigraphically important conodonts. For example, the succession on Bornholm yields is dominated by long-ranging, coniform conodont taxa (e.g., Scabbardella altipes). This suggests that graptolite-bearing dark shale successions may contain at least two distinct conodont biofacies.