COMPARING LATE CRETACEOUS AMMONITE EVOLUTION IN THE WESTERN INTERIOR AS COMPARED TO OTHER BIOGEOGRAPHIC PROVINCES

The modern inventory of life is the result of the many factors, physical and biological, that combine to drive evolution. Unfortunately, many fundamental evolutionary questions, such as those dealing with mode and tempo of speciation, can only rarely be adequately addressed using the fossil record, due to insufficiency of temporal resolution in the stratigraphic record. In this talk we address a fundamental evolutionary question: how do size and setting of a biogeographic unit (such as a province, the specific unit to be examined here) effect characteristic modes of evolution – for our purposes, measured by speciation and extinction rates, and levels of intra-species morphological variability? While it is now known that taxa with larger (in area) geographic ranges show reduced extinction rates to taxa of more limited distribution (summarized in Jablonski, 2008), only recently has it been proposed that not only size but also setting might have significant effects as well (Miller and Foote, 2009). We approach this issue for the following reasons: during the Late Cretaceous, the Western Interior Biotic Province (WIPB) is widely accepted to have contained a large number of (evolutionarily) short-lived ammonite species; it is also an example of an epeiric sea. As such, this province has among the highest number of biostratigraphic zones per time of any Phanerozoic chronostratigraphy. On the other hand, the marine basins of the North Pacific Biogeographic Province (currently known from the West Coast of North America, Japan, and eastern Russia), those in the Biscay area of Western Europe, and the James Ross Island basin in Antarctica -- now all with new, integrated chronostratigraphy--were all facing open ocean. While each of these other provinces has yielded many of the same genera found in the WIPB, the assemblages are generally composed of fewer and apparently longer ranging species with far more species overlap than in the WIBP. Here we present new morphometric and biostratigraphic range data that allow us to compare the longevity of ammonite species ranges and the degree of species overlap between the WIPB and these other provinces as a series of testable hypotheses.