Migration pathways of the late Campanian and Maastrichtian shallow facies ammonite Sphenodiscus in North America

Records of the ammonites Sphenodiscus lobatus and Sphenodiscus pleurisepta from the latest Cretaceous Gulf of Mexico region, the Western Interior Seaway and the Atlantic coast have repeatedly been used as a stratigraphic index for a Maastrichtian age. In recent years, however, high-resolution biostratigraphy on north-eastern Mexican occurrences and its combination with other correlation tools, such as sequence stratigraphy, as well as a refined position of the Campanian–Maastrichtian boundary, led to a different interpretation regarding the age of the earliest Sphenodiscus. The wealth of data allows us to reconstruct their routes of dispersal over the North American continent. Migration appears to have been directed from South to North and is thus opposed to that of Baculites ovatus. Dispersal of all three species is clearly related to changes in the Maastrichtian sea level.