terning of the bivalve body plan, with highly modified conical shells and a unique, pachyodont hinge structure (Skelton, 1985; Seilacher, 1998). This clade and its bizarre growth form, including the pachyodont hinge, disappeared in the K-T mass extinction (Steuber et al., 2002), but this loss was probably related less to any disadvantage inherent in the remarkable hinge apparatus than to the restricted ranges of rudist clades (Fig. 10.1D), and perhaps to their reliance in at least some instances on photosymbionts [Seilacher (1998), but see Steuber (2000)], as seen in modern reef-building corals. If the range-frequency distribution of rudists played a role in their demise, with correlated morphologies carried along, then we would expect a similar pattern for the other bivalve orders. This is in fact the case: the five bivalve orders with median genus ranges of one or two provinces suffered significantly more severe K-T bottlenecks (median = 93% genus extinction) than the four orders with median genus ranges of three or more provinces (median = 32% genus extinction; Spearman’s rank correlation of median genus range and extinction intensity for orders = 0.74, P = 0.02), as predicted by the hitchhiking argument for rudists. More detailed analyses must await a morphometric or discrete-character study combined with a well-resolved phylogeny of bivalve genera, and these results suggest that such studies would be worthwhile.
The hitchhiking of such striking adaptations on the less flamboyant features that actually determine extinction resistance is probably pervasive, both during background times [hence the large literature on comparative methods and phylogenetic autocorrelation; e.g., Freckleton et al. (2002) and Paradis (2005)] and during mass extinctions. For example, marine bryozoan genera with complex colonies suffer more severely during mass extinctions than simple genera, but colony complexity is also inversely related to genus-level geographic range (Anstey, 1978, 1986), which may well be the ultimate basis for differential survival during the end-Ordovician mass extinction. The end-Ordovician extinction also preferentially removed snails with broad selenizones providing access to the mantle cavity, and planktonic graptolites with multiple stipes creating complex pendant colonies; the K-T extinction also preferentially removed bivalves with schizodont hinges (trigonioids), echinoids with elongate rostra (a clade of holasteroids), cephalopods with complex sutures (ammonites), and a major clade of birds with foot bones that fused from the ankles to the toes (Enantiornithes). All of these losses or severe bottlenecks are more likely to represent correlations, not necessarily with geographic range, but with some other organismic or higher-level factor, rather than direct selectivity on the most striking morphology or functional trait. These extinctions nonetheless truncated or rechanneled evolutionary trajectories through morphospace, and additional examples are plentiful.