Two stratigraphic sections spanning the mid-Carboniferous boundary were examined in the Clare Shale Formation of the Shannon Basin in Western Ireland. Calcareous nodules intermittently occur within these generally non-calcareous organic-rich shales, and these have yielded Serpukhovian and Bashkirian conodont elements. The biostratigraphic range of the Irish material is illustrated here for the first time. Results show that the mid-Carboniferous and Arnsbergian–Chokierian boundaries are coincident at Ballybunion. Gnathodus girtyi is restricted to the lower part of the Serpukhovian Stage (Late Mississippian), while G. bilineatus bollandensis persists into much younger strata, close to the first occurrence of Declinognathodus noduliferus s.l. One element belonging to G. postbilineatus is also present. These findings support the argument that D. noduliferus s.l. developed from G. b. bollandensis, rather than from G. girtyi. The biostratigraphical tool for the identification of the mid-Carboniferous boundary globally, and the suitability of the section at Arrow Canyon, USA as a GSSP, may therefore need to be reassessed.

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