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PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002especially siphonate bivalves, during the Mesozoic.Moreover, predator grazing may be a further causefor the decline of epifaunal suspension feeders(Stanley, 1977).Vermeij (1977) hypothesized that shellbreakingpredation became a more important causeof mortality and was a driving force in evolutionfrom the Mesozoic to Cenozoic. He termed thispostulated major escalation of predator-preyinteractions the “Mesozoic Marine Revolution”(MMR); in fact, most of the trends he described inadaptive morphology continued from the Mesozoicto the Recent, so the present discussion combinesevidence from both eras.Gastropods.—Gastropods show a number oftrends, probably in response to shell crushing/drilling predation pressure (Vermeij, 1977, 1983,1987). These include a further decrease in umbilicateand loosely coiled shells. The few remaining looselycoiled gastropods lived within sponges (sillquarids),or were cemented (vermetids), the latter sometimesforming large aggregates, or “reefs” (Vermeij, 1983).Other trends among Mesozoic gastropods includeincreased proportions of thick-lipped shell apertures,slit-like apertures, varices, and spines or knobs. Thesimultaneous increase in these features suggests acommon evolutionary pressure, presumably theincrease in durophagous predation (Vermeij, 1983,1987). However, as this review shows, most of thesedurophagous predators are generalist feeders,feeding on a variety of hard-shelled prey, and notjust molluscs. More work must be done to determinewhether shell ornamentation really reducessuccessful predation. Shell sculpture and variceshave been shown to decrease predation by sea starsand durophagous crustaceans in laboratoryexperiments (Donovan et al., 1999). Fish predationis also deterred by shell sculpture (e.g., stout spines)on gastropods (Palmer, 1979). However, fieldexperiments demonstrate that shell sculpture maynot always be a deterrant to predation for somegastropods (Ray and Stoner, 1995). For queen conch(Strombus gigas), living in aggregations andattaining large overall size was found to be moreimportant in deterring predation than was shellsculpture (Ray and Stoner, 1995). Longer spines andheavier shells do not necessarily reduce predationmortality in queen conch, especially when predatorsattack through the aperture, as do crustaceans andpredatory molluscs (Ray and Stoner, 1995).Bivalves.—Much of the literature concerningbivalve shell ornamentation in relation to predationhas been based on largely circumstantial evidence(Fig. 3) (Harper and Skelton, 1993). Spondylidbivalves provide an interesting example. These spinyepifaunal bivalves appear in the Middle Jurassic andshow increasingly spinose shells up to the presentday (Harper and Skelton, 1993). However, thesespines apparently do not increase shell strength(Stone, 1998; Carlson, pers. comm., 2000), but doincrease effective size and make shells more difficultto attack. Spines are also commonly worn off, andit is not known how this affects the survival of thesecemented groups (see Logan, 1974).Shell microstructures and the development ofspines in some groups of bivalves may haveoriginated in their Paleozoic ancestors (Table 5).Additionally, changes in thickness and arrangementof shell microstructure may also be primarilycontrolled by water chemistry and temperature,rather than by predation. Some microstructures,however, may secondarily function to reduce crackpropagation, such as cross-lamellar structures, andincrease abrasion resistance (Currey and Kohn,1976). However, there are many ways to build crosslamellarstructures (Schneider and Carter, 2001).Shell microstructure such as spines, thickness ofparticular shell layers, and types of shell layers mayreflect a phylogenetic constraint. Thickening of shellmargins through extensive inductural deposits maybe related to photosymbiosis, and not directly topredation (Schneider and Carter, 2001).Cardiid (Jurassic to Recent) bivalve shellmicrostructure exhibits several evolutionary trendsthat may not be related to predation (Table 5). SomeCretaceous cardiids evolved stronger reflection ofthe shell margins, and increased thickness orsecondary loss of the ancestral prismatic outer shelllayers. However, these changes appear to be relatedto water chemistry and temperature. For example,microstructural convergences may be directly orindirectly tied to ocean chemistry and temperature:148