VARIATION IN SEPTAL SPACING OF EXTERNALLY SHELLED CEPHALOPODS FROM THE LATE CRETACEOUS OF NORTH AMERICA

Key features in the morphology of externally shelled cephalopods are the internal partitions (septa) between chambers. Variation in the spacing of septa may reflect changes in metabolism, growth rate, and the environment during the lifetime of the organism. Previous studies examined the ontogenetic pattern of septal spacing in multiple cephalopod taxa. For example, an ontogenetic pattern is shared within an ammonoid family of Desmoceratoidae. The factors that determined the intraspecific variation of septal spacing have not been sufficiently explored. In our study, we explore variation in septal spacing in Late Cretaceous Hoploscaphites, co-occurring Baculites and Eutrephoceras. The specimens are from the Pierre Shale in Western Interior Seaway, present day Montana and South Dakota, representing a relatively shallow marine environment probably less than 100 m in depth. We sectioned the specimens along the median plane and photographed the polished median plane using a digital microscope. In Baculites, we measured the linear distance between septa (mm) and in Hoploscaphites and Eutrephoceras, we measured the angular distance between septa. All specimens were collected at approximately the same locality and stratigraphic horizon, reducing variation in environmental conditions.