Abstract
A fragmentary specimen of the diplomoceratid heteromorph Diplomoceras cylindraceum , one of three ammonite markers used for the definition of the base of the Maastrichtian Stage, is recorded from the middle Campanian of the Roztocze Hills, southeast Poland. The new find, well constrained by associated ammonites and inoceramid bivalves, represents the earliest well-dated occurrence of this species worldwide to date. Therefore, the first occurrence (FO) of D. cylindraceum is conspicuously below the traditional base of the Maastrichtian as defined by the FO of the belemnite Belemnella lanceolata . Indeed, this is also far below the boundary designated in the Global Stratotype Section at Tercis les Bains (Landes, France). A comparison of the FOs of D. cylindraceum in sections acrossEurope clearly shows a significant diachroneity, which diminishes its value for definition of the Campanian/Maastrichtian boundary.
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