Variation in Adult Size of Scaphitid Ammonites from the Upper Cretaceous Pierre Shale and Fox Hills Formation

The rich fossil record of ammonites provides an opportunity to investigate the size variation of an individual ammonite species over its geographic and temporal range. However, the requirements for such studies are daunting: a sufficiently large sample for statistical analysis, a series of closely spaced geographic localities with detailed stratigraphic data about the distribution of specimens at each locality, a means of precisely correlating from one stratigraphic section to another, a knowledge of the taxonomy of the species that takes into account the possibility of sexual dimorphism, and, finally, an understanding of the ontogeny of the species to facilitate comparisons of specimens at the same ontogenetic stage.

The ammonites in the Upper Cretaceous Western Interior Seaway satisfy many of these requirements. These ammonites are abundant, widely distributed, and occur in a well-documented lithostratigraphic and biostratigraphic sequence (see, for example, the numerous papers by W. A. Cobban and W. J. Kennedy). Among the ammonites from the Western Interior, none surpasses the scaphites in affording excellent material for study. They combine superb preservation, easily identifiable growth stages, and clearly defined dimorphs (Cobban, 1969). As in other ammonites (Calloman, 1981; Davis et al., 1996), the dimorphs are referred to as macroconchs (presumed to be the females) and microconchs (presumed to be the males).