View - Kowalewski, M. - Virginia Tech

WALKER AND BRETT—POST-PALEOZOIC PATTERNS IN MARINE PREDATIONlocalities (De Cauwer, 1985).Anderson et al. (1991) tested the cost-benefitmodel of Kitchell et al. (1981), and showed that acorbulid bivalve (Varicorbula caloosae) was nomore likely to be drilled by a naticid predator thanby a venerid bivalve (Chione cancellata), amongPleistocene fossils from Florida. Anderson (1992)examined many species of corbulids from theMiocene and Pliocene of the Dominican Republicand from the Pliocene and Pleistocene of Floridaand found that the incidence of drilled,incompletely drilled, and multiply drilled valveswas highly variable in space and time. This resultwas similar to other studies on drilled bivalves, andtherefore indicates that conchiolin was generallynot part of the antipredatory arsenal. Rather,alternative evolutionary hypotheses, such asconchiolin as a retardant of shell dissolution or adeterrent to crab-crushing predation (Anderson,1992; Kardon, 1998), for example, need to beadvanced and tested. The main evolutionaryquestion, as Harper (1994) pointed out(paraphrasing Gould and Vrba, 1982), is whetherconchiolin layers are a beneficial trait that isenhanced by natural selection (adaptation), orwhether conchiolin layers are an exaptation, abeneficial trait that is secondarily co-opted foranother function. Experimental testing is requiredto determine if a trait is truly beneficial; and thereshould also be a temporal correspondence betweenthe evolution of the trait and the proposed selectiveagent (Harper, 1994).Accordingly, Kardon (1998) tested threehypotheses concerning the evolutionary importanceof conchiolin: 1) it retards shell dissolution; 2) itincreases shell strength and thus deters crushingpredation; and 3) it inhibites shell drilling by naticidgastropods. She also examined the fossil record ofnaticid drilling predators, and compared it to thatof conchiolin-bearing corbulids (which hail fromthe Middle Jurassic; but see Harper, 1994) toexamine the evolution of the trait in associationwith its putative selective agent (the naticids). Herexperimental results show that conchiolin didretard shell dissolution, although—as she clearlypointed out—the majority of corbulids live incalcium carbonate–saturated regions, and havedone so for most of their geologic history.The most promising line of research concerningconchiolin, however, stems from the finding ofmechanical tests that the conchiolin in corbulids mayfunction to inhibit crack propagation, which in turnmay be a deterrent to shell-crushing predation(Kardon, 1998). It remains to be tested whetherconchiolin layers do inhibit shell-crushing predators.Biomechanical tests using corbulid bivalves, inaddition to feeding experiments with livedurophagous crustaceans, are needed to address thishypothesis. An historical analysis of shell repair incorbulids through time is warranted.Lastly, Kardon (1988) found that naticiddrilling rates were not significantly slowed byconchiolin layers. Further, although Kardon (1998)states that conchiolin has acted as an effectivedeterrent against drilling predation in the corbulidfossil record (p. 73, but see her p. 76), her data donot support this claim (p. 75, her table 2). Her results,in fact, support the findings of Anderson et al. (1991)and Anderson (1992) that there is temporal variationin drilling through time in corbulids, with noapparent trend. It would also be important to knowfrom which facies these corbulids came, and whethertaphonomic (hydrodynamic or biotic) conditionsaffected their preservation.Although Kardon (1998) suggests thatconchiolin is an exaptation, and Harper (1994)suggests that conchiolin is an adaptation, a reviewof the data to date indicates that conchiolin maybe an artifact of construction. Of course, thisstatement needs to be refuted by scientific tests.That is, without further tests with shell-crushingpredators, we cannot know if conchiolin is indeeda beneficial trait, either co-opted or evolved by theorganism against predation. Other hypotheses werediscussed by Harper (1994), such as protectionagainst nonpredatory borers or assistance withhermetic sealing, and these could be rigorouslytested as well (see Table 6). The oldest corbulids(Jurassic Corbulomima) were marine organisms,and had conchiolin before the evolution of naticiddrilling during the Early Cretaceous (Kardon,1998), further suggesting that conchiolin was not175