THE RELATIONSHIP BETWEEN GENUS RICHNESS AND GEOGRAPHIC AREA IN THE LATE CRETACEOUS: EPICONTINENTAL SEAS VERSUS OPEN-OCEAN SETTINGS

Despite their utility for comparing biodiversity among regions worldwide, species-area relationships (SARs) among ancient marine organisms have not been investigated extensively. Most studies of present-day SARs are conducted in terrestrial settings, given difficulties in sampling modern marine settings over broad geographic regions. In the fossil record, however, marine organisms are commonly preserved extensively at a range of geographic scales, and paleontological databases that catalogue the occurrences of fossil organisms in a geographic context provide opportunities to study ancient marine SARs over broad regions.

Different marine settings may be characterized by different degrees of geographic variation in biotic composition, and comparative analyses of SARs should provide a more definitive window into these signatures. In particular, because of differences in the areal characteristics of their respective environmental gradients, open-ocean shallow-water regions on continental margins may exhibit different SARs than epicontinental seas. Against this backdrop, we are working at the genus level to analyze genus-area relationships (GARs) for Late Cretaceous (Turonian through Campanian) marine settings using the Paleobiology Database (PaleoDB; http://www.paleodb.org). This is an ideal interval because: a) extensive Late-Cretaceous data are now available worldwide; b) the interval contains well-preserved biotas from epicontinental seas and open-ocean shallow-water regions; c) field locations are available for investigations at the fine spatial/geographic scales of individual horizons at single localities and among closely-spaced localities as companions to the broader-scale analyses based on the PaleoDB.

To date, we have developed protocols using the R programming language for GARs analyses with data from the PaleoDB. Results demonstrate clear relationships between genus richness and area for regions worldwide, and indicate that as area increases, genus richness increases at different rates in epicontinental seas versus open-ocean settings. To analyze finer-scale GARs, we are currently collecting field data from a paleoenvironmental gradient preserved in the late Turonian through early Campanian Ladd Formation in the Santa Ana Mountains, California.