HABITAT OF THE UPPER CRETACEOUS HETEROMORPHIC AMMONOID DIDYMOCERAS IN THE WESTERN INTERIOR SEAWAY

The preferred habitat of torticonic (i.e., irregulary coiled) ammonoids from the Western Interior Seaway has long been controversial. The contradicting interpretations include three habitats: pelagic, demersal, and vertical-migrants. Traditional research on these ammonoids has used morphology and taphonomy to elucidate the habitat and ecology of this group of cephalopods, but new work using geochemical proxies for past water temperatures, salinities, and productivity may provide insight into the paleoecology of this group. The Western Interior Seaway is believed to have been isotopically stratified, and because mollusks secrete their shells in isotopic equilibrium with seawater, the ammonoids should produce isotopic values indicative of their habitat. This study will examine δ¹⁸O and δ¹³C isotope signatures of well-preserved aragonitic shells of the nostoceratid ammonoid Didymoceras to determine if this genus lived within the water column or was bottom-dwelling.

Fossils used for this study were collected from the Middle and Upper Campanian Pierre Shale of South Dakota, Montana, and Wyoming. These specimens are reposited within the Black Hills Museum of Natural History Collection, the United States Geological Survey Denver Collection, and the University of Wyoming Collection of Fossil Invertebrates. To ensure that the original shell chemistry was preserved, shells were screened for presevational quality using five visual characters, Scanning Electron Microscopy (SEM), and X-ray Diffraction (XRD) analysis. The chosen mollusk shells were then sampled along a 0.5 cm ontogenetic transect from the nacreous layer using a 1.0 mm diameter drill. One milligram of carbonate was taken from each shell. The oxygen and carbon isotope composition of each sample was determined by analysis at the University of Wyoming Stable Isotope Facility. Differences in mean δ¹⁸O and δ¹³C values for Didymoceras and associated mollusks were evaluated using a standard One-Way ANOVA.

Results from this study will provide new information on the position of this enigmatic ammonoid within Cretaceous marine ecosystems as well as more information about the paleoenviromental conditions within the Western Interior Seaway.