MORPHOMETRIC SHELL ANALYSIS OF ONE OF THE LAST AMMONITE SPECIES BEFORE THE K/PG MASS EXTINCTION EVENT

We performed a morphometric shell analysis of the ammonite Discoscaphites iris to explore the relationship between shell evolution and environmental variation over time. Discoscaphites iris occurs in upper Maastrichtian strata on the U.S Gulf and Atlantic Coastal Plains representing the last 350 ky of the Maastrichtian (Larina et al., in prep.). The purpose of our study was to quantify the variation in the shell morphology of this species throughout its geographic and stratigraphic range. Specimens were collected at five sites: New Jersey (Tinton Fm.), Missouri (Owl Creek Fm.), Mississippi (Owl Creek Fm.), and Alabama (Prairie Bluff Fm.). Most of the sections are less than 2 m thick with the exception of the Owl Creek type locality in Mississippi, which is 10 m thick. We measured several morphological features of the shell including maximum length (L_max), umbilical diameter (UD), whorl width (W), whorl height (H), and apertural angle, as reported in Landman et al. (2010). Our preliminary results indicate that the morphology of this species is invariant throughout its geographic range. In addition, we did not detect any morphological changes throughout the stratigraphic interval in which this species occurs, suggesting evolutionary stasis. Thus, the record of Discoscaphites iris does not indicate any environmental changes prior to the end-Cretaceous mass extinction that would have affected the morphology of this species.

Landman, N.H., Kennedy, W.J., Cobban, W.A., and Larson, N.L. 2010. Scaphites of the “nodosus group” from the Upper Cretaceous (Campanian) of the Western Interior of North America. Bulletin of the American Museum of Natural History 342: 242 pp.