Palaeontology and biostratigraphy of the Middle– Upper Coniacian and Santonian inoceramids of the US Western Interior

Abstract

The taxonomy of the Middle-Late Coniacian and Santonian inoceramids of the US Western Interior, including some specimens from the Canadian Western Interior, is revised, based mainly on the extensive collections of the US Geological Survey. The classic Meek and Hayden material is discussed. Forty-four species are described of which 5 are new: Inoceramus americanus, Inoceramus sokolovi, Inoceramus robertsoni, Inoceramus glacierensis, and Sphenoceramus gilli. The Middle Coniacian to Santonian inoceramids of the Western Interior represent a uniform Euramerican fauna. This allows the application of a uniform biostratigraphical zonation throughout the whole biogeographical region. Starting in the Late Coniacian, inoceramid faunas are characterised by relatively strong north–south biogeographic differentiation.The inoceramid zonation applied is discussed, diagnosed, and compared to previously used schemes, and to the ammonite zonation commonly used in the US Western Interior.