DIPLOMOCERAS MAXIMUM AND D. CYLINDRACEUM – THE GIANT UPPER CRETACEOUS HETEROMORPH AMMONITES AND THEIR STRATIGRAPHIC SIGNIFICANCE
Two phylogenetic interpretations are discussed: 1) maximum evolved and replaced cylindraceum somewhere within the lower part of the upper Maastrichtian; 2) maximum evolved from cylindraceum and these two species coexisted in the upper Maastrichtian. The first hypothesis is advocated. This evolutionary change took place somewhere in the lower part of the upper Maastrichtian, most probably within the equivalent of the lower/middle? part of the European Belemnitella junior Zone.
Additional undoubted records of cylindraceum are reported from the Upper Campanian, as understood traditionally (based on belemnites) and according to the Tercis definition (GSSP for the Campanian/Maastrichtian boundary). Due to diachronity of its FOs in the Middle Vistula (Poland), Kronsmoor (Germany) and Tercis (France) sections, it is recommended to exclude this ammonite species as a potential biomarker for defining the Campanian/Maastrichtian boundary.
The records of D. maximum from the Middle Vistula River section are from the Upper Maastrichtian only, which agrees with its suggested records from the Upper Maastrichtian, both in the Southern and Northern hemispheres. The other records of maximum are reported here from the upper Maastrichtian of Antarctica, Australia, Chile, Alaska, Denmark and less possibly from Belgium located at distances of thousands of kilometers from each other making maximum a truly cosmopolitan citizen of Late Cretaceous oceans.
If the taxonomic and stratigraphic interpretations proposed here are accepted, then maximum is proposed as a new ammonite zonal index fossil. It is distributed worldwide and very easy to identify. The proposed Diplomoceras maximum total range Zone would roughly embrace at least middle and upper parts of the upper Maastrichtian to the very top.