TRUE IS IT THAT WE HAVE SEEN BETTER DAYS? BIOGEOGRAPHY AND SURVIVORSHIP IN THE CRETACEOUS WESTERN INTERIOR SEAWAY

In As You Like It Shakespeare quipped that we have seen better days; with the fate of our planet’s biota uncertain due to the twin threats of invasive species and climate change, this remark may provide an unfortunate parable to biology. The Cretaceous Western Interior Seaway of North America (WIS) serves as an excellent paleontological window onto some of those better days. Given its habitus of extreme global warmth in a biogeographically mixed setting, species responses in the face of changing conditions during this time could serve as a harbinger of the future fate of the planet’s biota. Here, we focus on the controls of species’ biogeographic patterns through time to understand the mechanisms governing macroevolution. We use Geographic Information Systems (GIS) to quantitatively analyze changes in the distribution and range size of species in 16 molluscan genera during the last 35 million years of the Cretaceous Period.

Species’ distributions were compared to patterns of changing sea level and previously identified areas of endemism to determine how distribution and range size are reflected in species survivorship and faunal stability, and how these may vary across biogeographic boundaries during periods of environmental and ecological change. GIS spatial analysis was also used to test for differences in survivorship between invasive species and endemics. Preliminary results suggest that large range size alone may not have increased the likelihood of species’ survival across multiple stages. Further, while the number of species occupying Kauffman’s (1984) WIS endemic center increased through time, presence of all or part of a species’ range within the endemic center may not significantly affect survivorship. Instead, species seem to respond to environmental perturbations individualistically, and consequently it is important to consider the relationship between biogeographic patterns and processes at different hierarchical levels (species and regional biotas) in order to understand the effects of pronounced global warmth and species invasion in the distant past. Such a focus may elucidate the extrinsic pacemakers of evolution and extinction in deep time and may even outline what was better about the good old days.