View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002disarticulation of those elements, and statistical andtaphonomic assumptions made in the course of astudy (see especially Gilinsky and Bennington,1994 and references therein). These equations arethus directly applicable only to specimens withsingle-element skeletons and may requiremodifications when applied to multi-elementskeletons (see also below).Discussion of Metrics.—LTF provides a metricsystem that offers the best chance for a biologicallymeaningful analysis of predator-prey interactionin the fossil record (see also Leighton, 2002;Vermeij, 2002). This is because lower taxa are morelikely to represent a single behavioral andecological type of prey, which may interact with asimilar suite of predators through time and space.Also, potential taphonomic biases associated withdifferential preservation of taxa are not as severewhen the analysis is restricted to a single genus orfamily (see also Leighton, 2002).The fact that theestimate is restricted to one prey type also decreasesthe chances for variable behavior of the tracemaker, which may change its behavior dependingon prey type (e.g., drilling predation may beobligatory for bivalve prey that are able to shuttheir valves tightly and facultative for prey withvalves that allow a predator to insert its probosciswithout drilling; e.g., Frey et al., 1986).In contrast, the AF metric, by combining preywith a wide range of morphological and ecologicalcharacteristics, is less reliable both in terms ofbiologic interpretations and potential taphonomicbiases. However, AF offers a significant pragmaticadvantage: it can be computed for any fossil sampleand thus provides a metric that is comparableanalytically (if not biologically) throughout thefossil record. In contrast, few lower taxa arecontinuously abundant through long intervals ofgeological time and virtually none can be used tostudy very long secular trends: an LTF studyencompassing the entire Phanerozoic cannot bedone, except perhaps for such extremelyconservative, long-lived lower taxa like Lingulidae.Thus, in the case of comprehensive long-termstudies, AF is the lesser of two evils: it makes moresense to compare overall assemblage frequenciesbetween the Paleozoic and the Cenozoic than tocompare a specific family of Permian bivalves witha different family of Cenozoic bivalves. AF can bea useful indicator as long as we recognize that it isnot likely to provide estimates for specific predatorpreyinteractions but rather represents a proxy forthe overall predation pressure in the ecosystem.Whereas the credibility of AF has been recentlydebated (e.g., Leighton, 2002; Vermeij, 2002), itis worth pointing out here that assemblage-levelmetrics also provide an important baseline for theoverall intensity of a particular behavior (e.g.,drilling) through time. This baseline can providean important reference standard against whichspecific lineages can be compared. Also, as shownbelow, the metrics tend to correlate highly (theyare obviously dependent) so it may actually notmatter that much which one is used. Thus, althoughAF may be a misleading metric when applied totest a specific model such as the Hypothesis ofEscalation (this is yet to be demonstratedempirically), we should not discard it entirely.LTF MAXresembles AF in that it does not focuson specific interactions of a particular lineage ofprey, but rather tries to estimate the overall intensityin the assemblage by targeting the most frequentlydrilled taxon. AF should tend to be more reliablebecause a maximum is a highly volatile parameterboth in a statistical as well as biological sense.Although ATF is akin conceptually to AF, it doesdiffer fundamentally from AF in that it provides anestimate for how widespread predation was acrossprey taxa rather than across prey specimens. ATFmay also provide an indirect proxy for behavioral,ecological, and maybe even taxonomic diversity ofpredators. Thus, in the best-case scenario, andnotwithstanding all caveats listed above and below,AF may tell us how intense was the overall predationpressure ecologically (what proportion of biota wasbeing killed by predators), and ATF can tell us howintense was predation pressure macro-evolutionarily(what proportion of phylogenetic lineages wasaffected by predators).All four estimates are expected to show somecorrelation with one another: as frequency of tracesincreases the metrics all should go up. This is16