A QUANTITATIVE EXPLORATION OF MORPHOLOGICAL VARIATION AND IMPLICATIONS FOR SPECIES DESIGNATIONS IN LATE CRETACEOUS PLACENTICERAS OF ALABAMA

Ammonites of the Placenticeras genus occur worldwide in Late Cretaceous strata, and are known for their high degree of morphological variability. Depending on the location and taxonomist, this morphological variability has been interpreted as either a limited number of species with substantial intraspecific variation, or as an array of numerous intergrading species. The current taxonomy of Placenticeras presents a significant challenge to biogeographic and biostratigraphic studies using this group, yet there have been few attempts to quantify any of this variation or revise the taxonomy of Placenticeras species using morphometric methods. The placenticerids of Alabama in particular have been understudied, but they represent an important geographic link between the placenticerid populations in Europe and in the Western Interior of the United States. A morphometric study of a sufficiently large collection of Placenticeras specimens may be able to refine our understanding about which morphological variables are diagnostic in quantitatively distinguishing between species of Placenticeras.

As a first step to resolving placenticerid taxonomy, we conducted morphometric measurements of the Late Cretaceous (Santonian to Maastrichtian) Placenticeras ammonites of Alabama to quantitatively assess the morphological variation present in this population. We measured every sufficiently-preserved placenticerid specimen in major Alabama collections (including the Alabama Museum of Natural History, the McWane Science Center, and the Geological Survey of Alabama), for a total of nearly 2,000 individual measurements on more than 200 specimens. Collected measurements include standard conch shape parameters (e.g. diameter, whorl height, whorl width, etc.), septal spacing, and other parameters designed to quantify the placement and pattern of ornamentation. Initial observations indicate that within the measured material, diameter seems to have a bimodal distribution (with a break at around 150 mm), which may indicate the presence of dimorphism or multiple species in this population. Data analysis is ongoing and further results will be presented at the conference.