TESTING FOR CONSISTENCY IN MORPHOLOGICAL SHIFTS ACROSS ENVIRONMENTS FOR AMMONITES OF THE WESTERN INTERIOR SEAWAY

Long-term morphological trends within a clade are frequently linked to shifts in the physical environment. The intensity and direction of this relationship, however, can be difficult to assess at broad spatial and taxonomic scales, as responses are likely individualistic and context-specific. Here, we introduce a comparative framework to explore the role of the environment in driving morphological shifts for fossil ammonites, a diverse, widespread marine group whose shell shapes are often associated with distinct modes of life. Notably, similar morphological forms appear to have arisen repeatedly across the clade's history, due to a number of possible factors including sea level change, environmental stress, and developmental or functional constraints. We focus on ammonites in and around the Cretaceous Western Interior Seaway (WIS) at its earliest stages to test whether taxa undergoing similar environmental shifts - moving into a deepening seaway - also occupy similar positions in morphospace relative to their non-seaway counterparts.

Using the biogeographic zones of Kauffman (1984) and a database of Albian and Cenomanian occurrences compiled from the primary literature and the Paleobiology Database, we identified genera whose constituent species include some found within the WIS and some that appear excluded from it. We digitized aperture shape for each species of interest from photographs of museum specimens and published figures and conducted an elliptical Fourier analysis, the results of which were used to generate a morphospace. The morphospace captures the high variation in shell compression and whorl overlap found in ammonites as well as the morphological distinctiveness of most of the genera. We found no evidence of a common directional shift for species present in the WIS compared to those that were not. However, our results suggest a degree of clustering within some genera by biogeographic region as well as predictable morphological shifts with expansion into the seaway when partitioned by mode of life, highlighting the complex relationship between ecology and environmental change.