HOPLOSCAPHITES FROM THE UPPER CRETACEOUS OF THE WESTERN INTERIOR OF NORTH AMERICA

The Upper Cretaceous Western Interior of North America contains a rich record of Hoploscaphites extending form the middle Campanian Baculites obtusus Zone to the upper Maastrichtian Hoploscaphites nebrascensis Zone, spanning approximately 14 my. The record consists of 45 species, 22 of which are new. Documentation of the biostratigraphic ranges of these species helps complement and refine the existing ammonite and inoceramid zonation of the Western Interior. Most species of Hoploscaphites are limited to one or two biozones, implying a geological duration of approximately 0.5—1.0 my per species. In any one zone, two to four species co-occur, with the exception of the late Campanian and early Maastrichtian, where up to seven species are present at the same time. In the late Campanian, the assemblage includes species otherwise only known from the Vistula River Valley, Poland, testifying to the existence of marine connections to the north. The older species of Hoploscaphites in the Western Interior are characterized by a small coiled portion (= phragmocone), a long shaft, and a sharply recurved hook, whereas the younger species are characterized by a large coiled portion, a short shaft, and a reduced hook. The development of tuberculation on the flanks of the shell first appears in the early Maastrichtian. It probably coincides with the gradual retreat of the Western Interior Seaway at the end of the Cretaceous and the increase in the areal extent of shallow water environments and their associated predators.