Ammonites of the Pachydiscus neubergicus group from the mid-Maastrichtian chalk of Denmark: alternative taxonomic interpretations

Despite their rarity, ammonites of the Pachydiscus neubergicus group rank amongst the stratigraphically most important macrofossils in the Maastrichtian of Europe. Here, their taxonomy is studied on the basis of material from mid-Maastrichtian chalk exposed at Hillerslev in Denmark. The studied sample consists of ten specimens in mouldic preservation, collected from a c. 10 m thick chalk interval. Their taxonomic identification is based on comparison with ‘type populations’ of known members of the P. neubergicus group, primarily with the roughly coeval material of P. neubergicus (v. Hauer, 1858) from Neuberg in Austria. To date, only P. neubergicus has been identified from this group in the Danish chalk. Based largely on differences in ribbing density, the Hillerslev material has conventionally been subdivided into several taxa, namely P. neubergicus, P. sp. cf. neubergicus, P. jacquoti (Seunes, 1890), P. sp. cf. jacquoti, P. sp. aff. armenicus Atabekian and Akopian, 1969, and Pachydiscus sp. Results of the Principal Component Analysis (PCA) and one-sample t test for the studied material are compatible with such identifications of the Hillerslev specimens. However, an alternative taxonomic interpretation is possible that the identified morphospecies, with the potential exception of P. sp. aff. armenicus, are members of a single, highly-variable biospecies, dominated by a sparsicostate morphotype closer to P. jacquoti than P. neubergicus. Additional material from the mid-Maastrichtian levels of Danish chalk is needed to test the latter hypothesis.