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PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002skeletons (site selectivity), and (3) among prey withdifferent sizes (size selectivity). This type of analysisis routinely included in detailed quantitative studiesof predation traces and can provide many importantinsights into the nature of predator-prey interactions.Taxon Selectivity.—Predators are often highlyselective in choosing the species (or lower taxon)of their prey. In the most general terms, taxonselectivity simply means that a given taxon isattacked more frequently than is expected by chance.Based on the work of Botton (1984), Alexander andDietl (in press) suggested that this type ofevaluation can be performed using the StraussIndex, which can be defined as follows:Equation 8: L I= R I—P I,where R Iis the percentage of specimens with tracesbelonging to taxon i computed relative to allspecimens with traces found in the assemblage, andP Iis the percentage of taxon i in the assemblage.This index can help us to detect prey taxa withunusually high or low proportion of traces.Another possible strategy is to apply computersimulations to evaluate how likely it is to obtainthe observed distribution of traces across lower taxafor a given sampling design. In an example shownin Fig. 5, all common genera of silicifiedbrachiopods from the Permian of West Texas (4452specimens from 37 genera; data from Hoffmeister,2002) are plotted and the frequency of traces ofpredation (drill holes) is marked by black parts ofthe bars. Notice that holes are generally rare(overall predation rate AF = 1.1%) so many generado not include any specimens with traces (theFIGURE 5—Evaluation of genus selectivity in drilling on Permian brachiopods from West Texas (datafrom Hoffmeister, 2002 and in prep.). See text for details. The SAS/IML code for the Monte Carlo modelshown in the inset plot is provided in Appendix 1.22
KOWALEWSKI—ANALYTICAL METHODSproportion of drilled brachiopod genera ATF =43.6%). It is difficult to assess visually if thedistribution of traces is random or not. A MonteCarlo model (see Appendix 1 for SAS/IML code)was therefore used to draw random samples of4452 specimens assigned to 37 genera ofbrachiopods, mimicking the actual sample sizes forthose genera. The simulated specimens were then“drilled” by the computer with an a priori assignedprobability of 1.1%. The inset plot shows the resultof the simulation. Only one time in 999 iterationswas the simulated ATF value lower than or equalto 43.6%, demonstrating that drill holes aredistributed non-randomly (i.e., if drilling wasrandom, significantly more genera should containholes than is observed in the data). The reportedp=0.002 includes 999 random values and the actualsample (see Manly, 1995).One should be careful in interpreting taxonselectivity, especially in the case of paleontologicaldata. Because of time-averaging, the assemblagemay contain prey taxa that never encountered thepredator even though they are preserved in the sameassemblage; so the “selectivity” may have nothingto do with active selection by predators but maysimply reflect the fact that predators never had achance to encounter some of their “contemporary”prey. Also, and this problem applies toneontological studies as well, the lack of traces maymean that the predators failed in their attacks, andnot that they did not try.Leighton (2002) points to another seriousproblem of selectivity analyses related to thesequential nature of many predator-preyinteractions. Because marine benthic predators tendto encounter one prey at a time, the frequency ofattacks may reflect the frequency of preyencounters, and not a preferential selection by anoptimally foraging predator. If, as Leighton (2002)argues, the “zero-one rule” is in effect (i.e., anyprey type that is ever taken will always be taken),then differences in frequency of drill holes or repairscars observed across prey taxa may reflect therelative abundance and/or accessibility of thoseprey taxa. Although the zero-one rule may bequestioned—many models postulate that thebehavior of individual organisms varies throughtime so any prey type that is ever taken does nothave to be always taken (e.g., Evolutionarily StableStrategy; see Dawkins, 1976 for an excellentreview)—the sequential nature of encountersmakes selectivity analyses based on relativefrequencies of predation traces questionable (seeLeighton, 2002 for more details).Site Selectivity.—The location of trace fossils leftby a predator on skeletons of its prey may provideuseful information about the behavior of that predatorand its interaction with its prey. Many predators,parasites, or amensal organisms are behaviorallystereotyped in showing a preference for a particularlocation for their attack or attachment site.Consequently, biotic traces often display non-randompatterns in terms of their spatial distribution on preyor host skeletons. The evaluation of such siteselectivity is useful for several reasons. First, thepresence of site selectivity provides strong evidencefor the biotic origin of traces (see section above oncriteria for identifying traces of predation). Second,the specific nature of site selectivity may provideclues as to the nature of biotic interactions recordedby traces (e.g., drill holes located around areas thatgive direct access to muscle tissues of victims aremore likely to represent predation than parasitism).Third, changes in site selectivity through time mayoffer a good tool for evaluating various evolutionarymodels (e.g., Hagadorn and Boyajian, 1997; Dietland Alexander, 2000). In exceptional circumstances,multimodal patterns in distribution of trace fossilsalong the growth axis of its prey may be used todetect seasonality of predation and growth curvesof the prey (e.g., Kowalewski and Flessa, 2000).Although the evaluation of site selectivity canbe applied to various types of traces, includingfractures, repair scars, cut marks, and tooth marks(e.g., Babcock and Robinson, 1989; Lyman, 1994;Kowalewski et al., 1997; Kowalewski and Flessa,2000), most of the studies in the marine fossil recordhave focused on drill holes in invertebrate shells(e.g., Reyment, 1971; Negus, 1975; Kelley, 1988;Kowalewski, 1990; Anderson, 1992; Roopnarineand Beussink, 1999; Dietl and Alexander, 2000;Hoffmeister and Kowalewski, 2001; and many23
Page 1: THE PALEONTOLOGICAL SOCIETYPAPERSVo
Page 4 and 5: The Paleontological Society Special
Page 6 and 7: AuthorsRICHARD K. BAMBACHBotanical
Page 9 and 10: THE FOSSIL RECORD OF PREDATIONMicha
Page 13 and 14: INTRODUCTIONTHE FOSSIL RECORD OF PR
Page 15: KOWALEWSKI—ANALYTICAL METHODSTHE
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