BIOGEOGRAPHIC PATTERNS OF LATE CRETACEOUS NORTH AMERICAN CHONDRICHTHYAN: IS THE ATLANTIC COAST FAUNA DISTINCT?

Attempts to explore biogeographic patterns among North American marine vertebrates during the Cretaceous have largely involved qualitative comparisons of generic or species lists from various well-documented rocks units. These analyses have typically involved comparisons of chondrichthyan, osteichthyan, and reptilian taxa. More recent qualitative analyses have focused on chondrichthyan and osteichthyan taxa from the Late Cretaceous of the Western Interior Seaway (WIS). Both qualitative and quantitative analyses of vertebrates of the WIS and western Gulf Coast (GC) have subdivided the regions into distinct northern and southern subprovinces. To date, no quantitative analysis has considered WIS and GC faunas with those of the Atlantic Coast (AC). To test whether AC chondrichthyan faunas represent a distinct biogeographic subprovince we compiled data on the presence or absence of chondrichthyan species from 26 Late Cretaceous (Santonian-Maastrichtian) North American localities from the WIS, GC and AC. Similarity scores were calculated from the presence/absence data using the Kulczynski coefficient. Similarities scores were used to conduct a group-averaging cluster analysis and a non-metric multidimensional scaling analysis (MDS). Three primary clusters are identified: one comprised of all WIS localities, one comprised primarily of AC localities with three Gulf localities, and one comprised of a mixture of WIS, AC, and GC localities. A similar pattern is recovered from the MDS. Localities also group temporally in the MDS forming distinct groups of Santonian, Campanian, and Maastrichtian localities. The regional and temporal influences appear more important than the influence of depositional environment as both the cluster analysis and MDS do not appear to discriminate localities based upon environment. This result may reflect differing methodology between researchers for determining the depositional setting, or may reflect the limited portion of marginal marine faunas examined in this study. Inclusion of osteichthyan and reptilian components of these faunas, particularly the terrestrial portion of the faunas, may allow discrimination based upon depositional environment.