SELECTIVITY IN SPECIES TURNOVER DURING THE LATE ORDOVICIAN MASS EXTINCTION: TESTING THE INFLUENCE OF BIOTOPE AND GEOGRAPHIC RANGE ON GRAPTOLITE EXTINCTION AND ORIGINATION RISKS

The Late Ordovician Mass Extinction (LOME) brought about a change in regime from diplograptine to neograptine graptolite species. This turnover was the result of a combination of environmental changes. Previously we investigated the changing biotope affinities of graptolites through the LOME. By utilizing a Bayesian analysis of the graptolite occurrence patterns within the biozones, we determined that the previously dominant diplograptine species shifted from an epipelagic to a mesopelagic depth designation prior to going extinct. The neograptine species radiated in a similar pattern, diversifying from strictly epipelagic species to occupy both biotopes in the Silurian. For this study, we investigate the role that biotope plays on the rate of turnover of the species within the LOME compared to other, known, extinction risk factors. Utilizing the Horizon Annealing (HA) procedure in R, we have generated a composite of the 90 sections containing graptolite observations that will be binned based on four different binning schemes: equal time, equal duration, bounded by sequence boundaries, and bounded by maximum flooding surfaces. We will estimate biotope affinities for the graptolites within each of these sets of bins. In addition, graptolite turnover rates will be determined by two methods: Capture-Mark-Recapture (CMR) and boundary-crosser (Foote) metrics. Finally, the geographic ranges of each of the species will be estimated using summation of minimum spanning trees, the number of paleogeographic regions occupied, and counts within paleogyres. The explanatory power of extinction risk of the species biogeography and species biotope will then be compared. We hypothesize that a species biotope will have a greater impact on extinction risk than biogeography during the LOME.