FIGURE 10.1 Spatial effects in the end-Cretaceous (K-T) mass extinction for marine bivalve genera. (A, B, and D) Victims of the K-T extinction (A) tended to be significantly less widespread than surviving bivalve genera (B), as measured by the number of biogeographic provinces they occurred in during the Maastrichtian stage just before the event [Mann–Whitney U test, P = 0.00001; new analysis based on an extensive, in-progress revision and update of Jablonski and Raup (1995), omitting rudist bivalves (D) as before, note that their inclusion as narrow-ranging victims would strengthen this result]. Adding provinces to fill gaps in observed geographic ranges strengthens the separation between victims and survivors (75% of victims are unchanged in range size and their median range is unchanged at two provinces; 60% of survivors are unchanged and their median increases from four to five provinces). Some caution is needed, because the proportion of survivors is likely to increase with phylogenetic analysis and further taxonomic standardization of early Cenozoic bivalves, but the major pattern is unlikely to change. (C) Significant inverse relation between extinction intensity and the number of biogeographic provinces occupied by bivalve genera during the K-T extinction (Spearman rank test, P < 0.01). Solid line indicates analysis of revised dataset (n = 289 genera). Broken line indicates analysis of previous version of dataset [Jablonski and Raup (1995); n = 297 genera; 28 genera were added and 36 genera were removed in the revision]. (D) Loss of a major adaptation (the pachyodont hinge) by hitchhiking on geographic distribution. The unique pachyodont hinge structure disappeared with the extinction of these genera at the K-T boundary, signaling the termination of the rudist bivalves (Order Hippuritoida).