View - Kowalewski, M. - Virginia Tech

WALKER AND BRETT—POST-PALEOZOIC PATTERNS IN MARINE PREDATIONshow an increase in the Cretaceous and Cenozoic,or it may not, and a closer examination of shellrepair during this time is warranted (Table 3). Littlework has been done for Cenozoic localities toexamine shell repair with respect to habitat, species,and stratigraphic interval.Do lethal shell injuries (or shell repair)increase through late Mesozoic-Cenozoic time?—Traces of non-fatal peeling in molluscs are evidentas scars on the shell (Figs. 8.2, 13; Table 4) thatresult from repair of the outer shell lip by the mantleedge (Robba and Ostinelli, 1975; Raffaelli, 1978;Elner and Raffaelli, 1980; Vermeij et al., 1982;Vermeij, 1982; Allmon et al., 1990; Cadée et al.,1997). Frequency of shell repair is often cited inorder to compare temperate with tropical and deepseawith shallow-sea habitats, as well as to examinewithin- and between-habitat predation, and thetemporal dynamics of shell repair. It appears thatshell repair may increase through the Phanerozoic,with higher incidence of shell repair in theCenozoic—indicating that durophagous predatorsbecome more of a threat to molluscan prey (e.g.,Vermeij et al., 1980, 1982; Vermeij, 1983, 1987;Dietl et al., 2000). But analysis of the data on shellrepair (Table 3) illustrates that there are no realdifferences in shell repair frequency between theMesozoic and Cenozoic, despite the better recordof marine durophagous predators at this time.Shell repair must be interpreted with caution,as researchers use different methods andinterpretations in analyses of shell repair data. Twomethods are used to estimate shell repairfrequencies. First, shell repair frequencies can beestimated by dividing the number of shells withone major repair (jagged scar) by the total numberof shells in the sample (after Robba and Ostinelli,1975; Raffaelli, 1978; Elner and Raffaelli, 1980;Geller, 1983; Vale and Rex, 1988, 1989; Cadée etal., 1997; Walker, 2001). This is the moreconservative estimate for shell repair, as snails cansurvive injury more than once. If the snail is older,it may display more instances of shell repair. Second,shell repair frequency has also been calculated asthe total number of scars in all shells divided by thetotal number of shells in the sample (Table 3)(Vermeij et al., 1980, 1982; Vermeij, 1982). Thismethod does not take into account the fact that oldershells may have more shell repair than youngershells, and thus can result in an overestimate of shellrepair for an assemblage (although Vermeij hasrecognized this problem). Further, more instancesof shell repair than actual sample size are commonlyreported which makes the data difficult to interpret.Therefore, it is important to determine which methodis most useful in examining the fossil record of shellrepair and to be consistent with that method.Comparing papers that use different methods isdifficult and tenuous at best.Interpretations of shell repair must be carefullyevaluated especially in regard to equating frequencyof shell repair with intensity of predation (Cadée etal., 1997; Cadée, 1999). There are several factorsthat complicate the interpretation of shell repair.First, it is difficult to distinguish repair that mayhave been provoked by physical factors, such asburial or crushing between stones (e.g., Raffaelli,1978; Cadée, 1999). Self-inflicted wounds resultingfrom the process of predation that are thensubsequently repaired can also inflate estimates ofshell repair. For example, buccinid gastropods chiptheir outer lips in the process of preying on othermolluscs and then repair their self-inflicted breakage(Nielsen, 1975). Second, shell repair frequencies donot directly correlate with the intensity of predation,as a total absence of scars may mean either thatpredation did not occur or that predators were 100%efficient (Schoener, 1979). Third, the incidence ofrepair on a shell needs to be tied to the age of theorganism, as older snails may exhibit more shellrepair than younger ones. This may be especiallytrue for deep-sea snails that may exhibit slowergrowth rates and increased longevity with depth(Vale and Rex, 1988). Fourth, certain life historytraits (slow growing vs. fast growing, particularbehavior) and feeding mode may affect whether andwhen a shell is exposed to predation. Fifth, somespecies may be more prone to predation than othersin an assemblage (Hoffmeister and Kowalewski,2001; Kelley and Hansen, 2001; Walker, 2001); and,using the metric of only one species’ repairfrequencies can bias the results for an entire169