risk in present-day terrestrial vertebrates, Purvis and colleagues (Purvis et al., 2000b, 2005a; Davies et al., Chapter 14, this volume) found mixed, but significant, effects for body size, a consistent inverse relation between extinction risk and both abundance and geographic range, and either a positive relation or no effect for habitat specialization. Similar patterns are seen in the fossil record. For example, the geographic range is a significant determinant of Cretaceous and Cenozoic molluscan species duration or survivorship [Hunt et al. (2005) and Jablonski and Hunt (2006) and references therein], and Paleozoic crinoids show a significant positive relation between habitat breadth and species duration (Kammer et al., 1998). Predictable interactions among factors can also be seen, although this aspect needs much more work. Molluscan genera containing many widespread species tend to be more extinction-resistant, with a median duration of 130 million years (Myr), than genera having just a few, localized species, which show a median duration of 32 Myr, and the genera with the other combinations give intermediate values (Jablonski, 2005). These are not theoretically surprising results, but it is encouraging that the paleontological outcomes so clearly match expectations.
Extinction selectivity appears to change significantly at the most severe mass extinctions, however. The rules of survivorship changed during the K-T extinction, such that species-richness and species-level range failed to predict genus survivorship, singly or in concert [Jablonski (1986a, 2005) and see Kiessling and Baron-Szabo (2004) for comparable results for K-T corals]. In fact, survivorship of marine invertebrates in the K-T mass extinction is unrelated to a number of factors that have been shown or hypothesized to be important during more normal times. Besides the two already mentioned, these factors include local abundance, mode of larval development (which is in turn related to fecundity and species-level dispersal capability), estimated generation time, living position relative to the sediment–water interface, and trophic strategy (Jablonski, 2005).
Despite this loss in effectiveness of a variety of organismic, species-and even clade-level traits, survivorship at mass extinction boundaries is not random. Every event seems to show some degree of selectivity, but one factor that seems to have promoted survival for most major groups and most mass extinctions is broad geographic distribution at the clade level (i.e., genera), regardless of species-level geographic ranges. This effect, which has been recorded for many groups and all of the major mass extinctions [see Jablonski (2005) for a tabulation], is again further corroborated in an extensively revised version of Jablonski and Raup’s (1995) data on K-T bivalves (Fig. 10.1A and B). This is more than a simple binary effect: bivalve genus extinction is inversely related to geographic range, with strong concordance between the new and old data (Fig. 10.1C). The 70% extinction suffered by the genera found in just one or two biogeographic