MOVING ON UP: LATITUDINAL DIVERSITY PATTERNS OF AMMONOIDS WITHIN THE CRETACEOUS WESTERN INTERIOR SEAWAY OF NORTH AMERICA

Ammonoid cephalopods are remarkable for their rapid rates of evolution. Processes promoting diversification are likely to be closely tied to biogeography. The Late Cretaceous Western Interior Seaway (WIS) of North America offers paleontologists a superb laboratory for studying the biogeography of diversification. The Western Interior Seaway's north-south orientation makes it especially suitable for evaluating changes in latitudinal diversity gradients as rising sea level expands the seaway and brings warm-water faunas into the basin. Such a study requires compiling, manipulating, and analyzing geospatial data. Geographic Information Systems (GIS), while widely used by other geoscientists, have not been fully exploited by paleontologists as a tool for analyzing the spatial distributions of fossil taxa. Happily, this reluctance to utilize GIS methods is changing. MacKenzie has compiled a GIS-compatible database of over 12,000 occurrences of ammonoid cephalopods within the Cretaceous WIS, and we have begun using the database to explore ammonoid evolution.

Genus-level latitudinal diversity gradients at 2° intervals were compiled for the six predominant ammonoid superfamilies found within the WIS. Of these six higher taxa, the Acanthocerataceae show the highest generic diversity, peaking in the Late Cenomanian. The Turrilitaceae are a distant second, also peaking in the Late Cenomanian. Other taxa show low generic diversity (<5 genera), although species-level diversity may be higher (e.g., Baculites spp.). Third-order sea level rises in the Cenomanian-Turonian and Campanian push diversity gradients northward and correlate with peak diversities. During these times, southern latitudinal boundaries remain constant or shift south, rather than shifting north, resulting in increases in total latitudinal ranges. Despite this sea level control, the Early Turonian is notable for a shift south in the diversity gradient – survivors of the Cenomanian-Turonian extinction are concentrated in the southwestern portion of the seaway. Northern boundaries shift back south in the Late Maastrichtian relative to the Early Maastrichtian; this southern retreat may have made ammonoid faunas in North America more vulnerable to the Chicxulub impact.