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PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002TABLE 6—Evolutionary findings concerning whether conchiolin serves an anti-predatory function againstdrilling predators for corbulid bivalves. Alternative hypotheses are discussed in text.EvolutionaryconclusionAdaptationExaptation*EvidenceConchiolin arises at the same time asdrilling predators in the CretaceousConchiolin arises in the Middle Jurassic,well before origin of drilling naticids;*however, no temporal trend in drilling predationReferenceHarper (1994)Kardon (1998)Not Anti-predatory No temporal trend in drilling predation Anderson et al. (1991);Anderson (1992)PhylogeneticConstructional ArtifactConclusions drawn from synthesis ofthe literatureThis paperexample, suggested that Recent corbulid specieswere less likely to be completely drilled than fossilspecies. Kardon (1998, p. 73) also suggests that intemporally and spatially separated fossil samplesof corbulid bivalves, conchiolin layers are effectivedeterrents of naticid predation. The second is thatdrilling predation actually increased in corbulidsover their evolutionary history. For example,although using a limited data set, Taylor et al.(1983) suggested that corbulids showed enhancedpredation from the Late Cretaceous to Eocene. Andlastly, others suggest that there is no spatial ortemporal trend in drilling predation in corbulids.For example, in Late Cretaceous to Pliocenecorbulid fossils from Europe and North America,De Cauwer (1985) found no trend toward increasedcomplete drilling was found. Similarly, forMiocene to Pleistocene fossil corbulids from theDominican Republic and Florida, there appears tobe no spatial or temporal pattern in complete orincomplete drilling, strongly indicating thatconchiolin layers are not effective deterrents tonaticid predation (Anderson, 1992). Likewise,Harper (1994) reported that there was nosignificiant difference in drilling frequency amongalmost all geological samples examined from theCretaceous to Plio-Pleisotocene. Further, Harper(1994) found that there was no significantdifference in possession of conchiolin sheetsbetween temperate and tropical localities. Giventhese contrasting findings, it is important toexamine some of the salient evolutionaryhypotheses regarding conchiolin as anantipredatory deterrent, such as cost-benefitanalyses and whether conchiolin is an adaptationor exaptation (or neither) against predation.Corbulids have small size and effective valvearmor (i.e., relatively thick valves with conchiolinsheaths), and thus, according to Kitchell et al.’s(1981) cost-benefit model, would represent a highdrilling investment with low benefits (De Cauwer,1985). For example, Kelly (1988) found thatpredation on corbulids was lower than would bepredicted by the cost-benefit model (but seeAnderson, 1992). Yet corbulids are heavily drilledin many localities, and this may be a result of theirtendency to cluster, their shallow burrowing depths,and their sluggishness (De Cauwer, 1985). Perhapsdrilling predators may mistakenly drill empty shellsin the presence of chemical attractants in theexhalant water of the corbulid associations(Carriker, 1981; De Cauwer, 1985); or there maybe hydrodynamic and taphonomic reasons for thepreponderence of drilled corbulids in some174