View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002assemblage. Lastly, for a time-averaged assemblage,shell repair frequencies might be higher than whatwould be found in the living population at any onetime because of the patchiness of predation (andassociated physical factors).The consensus is, however, that conspicuousshell repair (i.e., conspicuously peeled shells withsubsequent repair) is most likely the result ofpredation. That is, only deeply peeled injuries thatare subsequently repaired can reliably be used inthe analysis of shell repair, whereas repaired nipsor edge chippings may not be indicative ofpredation (Walker and Voight, 1994; Walker, 2001).Consequently, although shell repair is not a goodindicator of predation intensity, it is instrumentalin providing a record of predators within a habitatwhen body fossils of the predators are missing.Shell Drilling through Time.—Shell drillingfrequency is less ambiguous in interpretation: acompleted drillhole signifies prey mortality. Also,particular borehole morphologies may beassociated with specific gastropod or octopodpredators (Carriker and Yochelson, 1968; Kabat,1990; Kowalewski, 1993; Kowalewski et al.,1998). Nonetheless, certain caveats also apply tothe study of drilling predation.Escalation studies of drilling predators andtheir prey have not generally taken into accountthe particular facies and associated biota ofanalyzed assemblages (with the exception ofHoffmeister and Kowalewski, 2001). Essentially,all assemblages are treated as if they were the samefacies (e.g., onshore and offshore assemblages aregrouped). Environmental differences betweenassemblages, however, can affect the morphologyof the taxa—some species are larger in nearshoreenvironments than they are in offshore environments(or vice versa). This gradient in morphology maynot be related to predation.Sedimentary facies could also havetaphonomic effects. For example, assemblagesdeposited above storm wave base may sort drilledand undrilled shells differently compared tooffshore assemblages. Drilled and undrilled shellscan be differentially transported in nearshoresettings and thus there may be a bias toward anoverabundance of drilled shells in some localities.Additionally, drilled shells are more prone totaphonomic breakage than undrilled shells, andsuch breakage may be more common in somelocalities than others (Roy et al., 1994). Left vs.right valves of bivalves and pedical vs. brachialvalves of brachiopods are also differentiallytransported and/or preserved (Brett and Allison,1998). Thus, it would be important to know thevalve frequencies of an assemblage, and whetherthey are biased. It would also be important to knowif drilling predators were actually found in the sameassemblage as the drillholes (e.g., Hansen andKelley, 1995), but not all papers that examinedrilling predation discuss this issue.It is also important to examine more than onelocality within a time period, as the record ofpredation is strongly controlled by habitat (Vermeijet al., 1981; Geller, 1983; Hansen and Kelley, 1995;Cadée et al., 1997; Hoffmeister and Kowalewski,2001). Location within a sequence may also affectthe density of drilled shells, as transgressive lagdeposits formed after a major sequence boundary(e.g., extinction?) commonly contain more bioticinformation as a result of longer time averaging(Brett, 1995; Holland, 2000). Therefore, one mustbe careful in interpreting the pattern and processof drilling through the Phanerozoic, as it is not assimple as merely counting drilled taxa per temporalstratigraphic sequence. As Boucot said, “Naturedoes not take place within an ecological vacuum”;nor should evolutionary interpretations using thefossil record be decoupled from facies studies.Given these caveats, based on an analysis ofover 150,000 gastropod and bivalve shells fromthe Gulf and Atlantic Coastal Plain (GACP), Kelleyand Hansen (2001) suggested that the interactionbetween naticid drilling predators and their preydoes not necessarily show escalation from theCretaceous to Oligocene. After examination of anumber of localities, they found that there is anepisodic pattern to drilling frequency, with massextinctions resetting the “arms race” for faunas.Drilling within the most of their Cretaceouslocalities was greater than several of their lateEocene localities and similar to early Oligocene170