CEPHALOPOD DIVERSITY AND PALEOBIOGEOGRAPHY DURING THE CENOMANIAN-TURONIAN INTERVAL OF THE LATE CRETACEOUS

The Cenomanian-Turonian (C-T) was a critical interval in the Cretaceous, marked by global sea level rise, greenhouse warming, ocean anoxia, and the formation of extensive epeiric seas. Mobile marine animals might have responded to these changes in one of two ways. As sea levels rose and the latitudinal temperature gradient decreased, clades might have become less diverse with expanded geographic ranges as marine corridors for dispersal opened up. Alternatively, clades might have become more diverse with a decrease in geographic ranges, as groups diversified locally within newly-formed epeiric basins. As diverse, mobile marine animals with a rich C-T fossil record, cephalopods provide a model group with which to evaluate these two predictions. A database of North American cephalopod occurrences was combined with global data downloaded from the Paleobiology Database. After editing for duplications and taxonomic consistency, species and genus richness were tabulated for the six C-T substages. Geographic ranges and latitudinal distributions of genera were determined using ArcGIS spatial analysis tools on rotated paleocoordinates.

The most diverse C-T cephalopod group, Superfamily Acanthoceratoidea, increased its species richness from the Middle Cenomanian through the Early Turonian before declining sharply. At the genus level, however, the decline started with a modest drop-off across the C-T boundary. Most other groups showed no significant change in diversity or a drop between the Late Cenomanian and Early Turonian at both species and genus levels. While geographic ranges did fluctuate, most superfamilies show no statistically significant change in generic geographic range or median, maximum, or minimum latitude through the C-T interval. These results imply that most cephalopod groups did not expand their geographic range or shift northward during the C-T interval. Acanthoceratoids, however, show a southward shift between the Late Cenomanian and Early Turonian, as seen in decreased median, minimum, and maximum latitudes. It may be that the primarily nektobenthic acanthoceratoids underwent endemic radiations during the Late Cenomanian, boosting their species richness, while cephalopods with deep-water affinities were relatively unaffected by global changes during the C-T interval.