EVOLUTIONARY RESPONSES OF SCAPHITID AND BACULITID AMMONITES TO ENVIRONMENTAL CHANGES IN THE LATE CRETACEOUS WESTERN INTERIOR SEAWAY

The evolution of ammonites in the Western Interior Seaway (WIS) took place against a background of dynamic environmental changes, which likely drove their evolution. However, relatively few studies have quantitatively examined the role that abiotic factors played in influencing ammonite character evolution and speciation. The goal of this project is to examine the controls that various abiotic forces had on the evolution of ammonites in the WIS during the Campanian and Maastrichtian. To examine this question, morphometric and phylogenetic methods were utilized along with environmental proxy data to evaluate evolutionary responses of scaphitid and baculitid ammonites to a range of abiotic variables. This interval was chosen because of its extensive high-resolution, temporally calibrated environmental and paleogeographic records, which document fundamental changes in basin dimensions, facies distributions, temperature, oxygenation, and nutrient input. Scaphitids and baculitids were chosen because of their diverse morphological characters, rapid evolutionary rates, and excellent stratigraphic records. Evolutionarily important environmental data was compared with morphometric and phylogenetic data to determine if there is a relationship between abiotic sequalae and the appearance of certain traits. For example, we documented the degree of whorl compression and incidence of tuberculation in scaphitid species and plotted these parameters against temporal changes in the geographic size of the WIS. We also used phylogenetic analysis and comparative methods to examine the relationship between baculitid character evolution and various environmental factors (e.g., eustatic sea level, geographic size of the WIS, temperature). Preliminary results reveal that eustatic sea level as well as WIS water depth and geographic area drove evolutionary changes among Campanian-Maastrichtian scaphitids and baculitids.