TESTING THE CONTROLS ON SPECIES' DISTRIBUTIONS IN THE FOSSIL RECORD: COMPETITION AND BIOGEOGRAPHY IN THE WESTERN INTERIOR SEAWAY

One way the effects of environmental and ecologic change on taxa can be observed in the fossil record is as changes in species range size and distribution. Changes in geographic distribution through time can be caused by environmental change, changing ecological interactions, as well as other factors. In order to tease these processes apart a quantitative and model based predictive approach is necessary. A useful approach for quantifying the spatio-temporal variability of geographic range in the fossil record is through the use of Geographic Information Systems (GIS) and PaleoGIS. GIS/PaleoGIS allow for quantitative analysis of distributional changes in the context of appropriate paleogeography, which may result from habitat tracking (indicating niche conservatism across environmental change).

The Cretaceous Western Interior Seaway (WIS) of North America was used as a model system to study how environmental change and competitive interactions affect species distributions on macroevolutionary time scales. The paleontology and stratigraphy of this region and time interval are particularly well characterized and exceptional collections are available. To examine how ecology and environment interact coherently to affect biogeography and evolution, patterns of distribution change were compared among multiple vertebrate taxa including mosasaurs, sharks, and teleosts. Potential cases of competitive exclusion, taxon replacement, and competitively mediated extinction were examined using PaleoGIS spatial analyses to determine whether species interactions or environmental variables better explained changes in geographic distribution over time. Facies changes within the Cretaceous WIS were then tested for correlation with observed biogeographic patterns to assess degree of habitat tracking. Results suggest that environmental gradients determine distributions of species rather than ecological associations.