Quantitative morphological description of the Late Cretaceous ammonite Baculites inornatus Meek from western North America: implications for species concepts in the biostratigraphically important Baculitidae

摘要

Large collections of well-preserved specimens of the ammonite Baculites inornatus Meek (1862) from two lower to middle Campanian localities on the Pacific coast of North America are analyzed quantitatively to examine both variability and evolutionary change of species-level distinguishing characters. To this end, we present a new method of describing the morphology of the biostratigraphically important Upper Cretaceous zonal index fossil Baculites, using five independent shell characters that can be measured quantitatively. We then use this method to test hypotheses of phyletic evolutionary change in B. inornatus specimens collected from Sucia Island, Washington, USA, and Punta San Jose, Baja California, Mexico.