EXTINCTIONS IN TIME AND SPACE: SURVIVORSHIP AND GEOGRAPHIC RANGE IN LATE CRETACEOUS AND EARLY CENOZOIC VENEROID BIVALVES

Broad geographic range has been shown to enhance survivorship during mass extinction events. The possible influence of geographic range on survival during small-scale extinctions and across multiple events has received relatively little attention. In this study, I examine the relationship between geographic range and survivorship across the end-Cretaceous (K/T) and end-Eocene (E/O) extinctions in veneroid bivalves. Systematic and stratigraphic data were collected for 140 veneroid subgenera, ranging from the Late Cretaceous through Oligocene of North America and Europe. Latitude and longitude data were compiled for 2182 localities and converted into paleolatitudes and paleolongitudes. Geographic ranges were calculated for each subgenus using an Albers equal area projection. Extinction selectivity and preferential rebound were assessed according to geographic range and regional differences in extinction intensity and patterns of rebound were evaluated. 81% of veneroid subgenera went extinct at the K/T and the extinction appears to be slightly, but not significantly, more severe in North America than in Europe. The E/O event was minor by comparison and involved the extinction of 20% of subgenera. Preliminary results indicate that subgenera that went extinct at the K/T had significantly smaller geographic ranges than survivors. In contrast, geographic range was not correlated with survivorship during the E/O event. In fact, E/O survivors tended to have smaller geographic ranges than victims, although the difference was not statistically significant. One possible explanation for this difference in selectivity lies in the close spacing of veneroid extinctions during the late Eocene. Veneroids undergo another minor extinction at the end of the middle Eocene (mid-E) and it may be that the K/T and mid-E extinctions removed the majority of taxa with small geographic ranges, producing an anomalous pattern of selectivity at the E/O event. These results suggest that the role that geographic range plays in survivorship may vary according to the pressures that a clade has most recently undergone.