BUILDING UP THE BIOTA IN NOVEL ENVIRONMENTS: INSIGHTS USING THE FOSSIL RECORD OF EPEIRIC SEAS

Throughout the Phanerozoic, times of rising sea-level were often accompanied by the development of shallow seas on the continents. These epeiric seas formed relatively rapidly in geologic time and differed physically from open marine habitats, with shallower depths and altered salinity, temperature, and circulation. The buildup of diversity within these new habitats must result from one or more of the following processes: uninhibited dispersal of open marine taxa, limited dispersal with ecological filtering of open marine taxa, and one or more rounds of in situ speciation. The paleontological record allows discrimination between these processes and additionally chronicles any accompanying anagenesis. Despite the extensive representation of epeiric seas in the fossil record, little has been done to characterize and determine the source of epeiric biotas.

My focus is on ammonites in the Early and Late Cretaceous, the latter of which is characterized by high sea levels and inland flooding, including creation of a seaway across North America between the present-day Arctic Ocean and Caribbean Sea. Ammonites are an ideal study group because they are fast-evolving, abundant, and well-preserved – features that allow for fine temporal and spatial control. Using geographic and temporal distributions and body size data for over 500 species of ammonite, I find that epeiric species exhibit greater means and wider ranges of body size than do non-epeiric species. I also find that while species tend to occur only in one environment or the other, genera are often composed of a mix of both epeiric and non-epeiric species. Characterizing spatial patterns in the diversity and ecology of Cretaceous ammonites across epeiric and non-epeiric habitats will help to determine the relative importance of the various processes that build diversity in novel environments.