View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002FIGURE 2—An empirical example of the effect of sieve size on the estimates of drilling intensity. Alarge dataset of Miocene mollusks (> 3500 gastropod and bivalve shells) was obtained by processingbulk samples using fine sieves with mesh below 1mm (Hoffmeister and Kowalewski, 2001; Kowalewskiet al., 2002). An effect of sieve size was then mimicked in a computer simulation by removing from thedatabase all specimens below a given mesh size. For each sieve size, 1000 subsamples of 100specimens were randomly selected and assemblage frequencies (AF; see Equation 1) were computed,including mean drilling frequencies (large solid points) and 95% confidence intervals around them(small solid points). The confidence intervals are based on 2.5 and 97.5 percentile of 1000 AF valuesobtained for each mesh size.only specimens with traces. This is usually notdesirable because such data do not allow us tocompute frequency of traces in bulk samples, andmany other types of analyses (see below) cannotbe conducted. However, this method may beeffective in extracting some quantitative data, whenvery large collections are screened for traces ofpredation. Preferably, a data entry table shouldinclude at least one row per specimen (multiplerows per entry are recommended if multiple tracesare found on the same specimens).Data Reporting.—With increasing use ofonline repository systems, many journals allow forelectronic publication of raw data tables. This isthe most desirable way of reporting data that givesother researchers full access to all informationcollected in a project. Because the posting ofrepository materials may be delayed (authors mayrightly feel it premature to disclose their data), itis also important to report clearly the results of thestudy, so the reader is able to distinguish, forexample, counts of trace fossils from counts offossils with traces and counts of valves of bivalvesfrom counts of shells of bivalves.ANALYSES OF PREDATIONINTENSITY (FREQUENCY)The frequency of traces is arguably the mostimportant and widely used metric in quantitativeanalyses of the fossil record of predation thatestimates the frequency of predator-prey interactionsand may serve as a proxy for predation intensity(but see below). Four different, albeit related,methods have been used for estimating the frequencyof predator-prey interactions in the fossil record.14