Ammonoid Paleobiogeography In the Cenomanian Western Interior Seaway

The Late Cretaceous Western Interior Seaway (WIS) of North America provides paleontologists with a superb laboratory for studying the evolution and extinction of marine organisms. Decades of detailed field sampling combined with geochemical studies have produced a high-resolution stratigraphic framework into which evolutionary events can be situated. Attention is now turning toward synthesizing available data to provide a high-resolution geographic context for the WIS. The ongoing development of Geographic Information Systems (GIS)-compatible databases for the Western Interior Seaway promises a wealth of opportunities to rigorously explore the critical links between the biogeographic distributions of organisms and their ecology and evolution.

The GIS database we are constructing focuses on Western Interior ammonoid cephalopods. The database currently has over 12,000 occurrences of Late Cretaceous ammonoid cephalopods from the WIS (including Texas and northern Mexico). Biogeographic patterns of acanthoceratacean ammonites during their Middle and Late Cenomanian radiation in the WIS highlight the utility of the database for exploring evolutionary dynamics in a biogeographic context. Genera that are known from locations outside the WIS have significantly larger geographic ranges within the WIS than do genera that are endemic to the WIS. Such differences in dispersal potential can even be seen among species of the same genus. Metoicoceras has the largest generic geographic range among Cenomanian WIS acanthocerataceans. Interestingly, the early species of this genus (which are endemic to the WIS) are widespread only along the western margin of the WIS and in Texas. The last species, M. geslinianum, shows a distinctly different biogeographic distribution within the WIS, hinting at a biological change that enabled it to disperse out of the WIS to become globally distributed in the Late Cenomanian.