Latest Cretaceous occurrence of nodosaurid ankylosaurs (Dinosauria, Ornithischia) in Western North America and the gradual extinction of the dinosaurs

Abstract

ABSTRACT The presence of nodosaurid ankylosaurs in the Lance, Hell Creek, and Laramie formations of western North America is confirmed, thereby extending the geochronological range of this family into the Maastrichtian (Lancian). The material includes a cervical spine and a skull referable to Edmontonia sp., and numerous teeth, plates and a basioccipital, which are questionably assigned to Edmontonia sp. Comparison of the amount of nodosaurid material known from the Judith River Formation (Campanian) with that from the Lancian deposits indicates a substantial decrease in the relative abundance of nodosaurids in the Maastrichtian. Furthermore, the stratigraphic distribution of these nodosaurs is apparently limited to the lower part of the Lancian deposits, suggesting that they became extinct before the end of the Cretaceous. If true, this would support the hypothesis that the dinosaur extinction at the end of the Maastrichtian was gradual, not catastrophic.