View - Kowalewski, M. - Virginia Tech

WALKER AND BRETT—POST-PALEOZOIC PATTERNS IN MARINE PREDATIONlocalities (their table 8.1, p. 153). This is asignificant finding given that previously Vermiej(1987) had used naticid drilling as one line ofevidence illustrating escalation in Cretaceous toEocene faunas. In the Cretaceous Vermeij (1987)found limited drilling, but by the Eocene, drillinghad reached modern levels.Kelley and Hansen (1993), in contrast, did notfind an ever-increasing trend in naticid drillingfrequencies from the Cretaceous to Eocene forGACP molluscs. Escalation could also mean that apredator gets better at selecting prey; however,Kelley and Hansen (1993) did not find any temporaltrends toward increased drillhole site stereotypy innaticids. Molluscan prey were found to have moreincomplete drillholes and multiply drilled shells,indicating that prey effectiveness may haveescalated, but Kelley and Hansen’s (2001) data didnot show a trend for most of the periods examined.Kelley and Hansen (2001) also examineddifferences in morphology within molluscan generathat may be related to escalation. Although manygenera were examined, four particularly longrangingMiocene genera from the GACP—twogastropod predators (Euspira heros and Neveritaduplicata) and two frequently drilled naticid prey(Bicorbula idonea and Stewartia anodonta)—wereanalyzed for different morphological charateristics(their table 8.3, p. 159). In this case, however, thegastropod predators are also cannibalistic. Resultsshowed that shell size (height) did not change foreither E. heros, B. idonea, or S. anodonta (no dataare reported for N. duplicata), indicating that theseprey species found no size refuge from predationover time. Shell thickness (which would make a preyitem more difficult to drill) did not change for E.heros, decreased for N. duplicata, slightly increasedfor B. idonea and increased greatly for S. anodonta.Internal volume (an indicator of the amount of fooda predator can take in) did not change within theMiocene. Thus, it appears that most prey charactersdeemed to be directly related to predatory escalationdid not demonstrably change within the Miocene(except for shell thickness in S. anodonta). It wouldbe interesting to know whether drilling frequencyincreased or stayed relatively the same across thevarious assemblages examined.Escalated species are thought to be moresensitive to changes in primary productivity becausemaintaining heavy armor or high speeds to avoidpredators requires high metabolic rates and thus anuninterrupted food source (Vermeij, 1987).Therefore, Hansen et al. (1999) tested whetherpurported escalated species (those withantipredatory adaptations such as heavy armor) weremore vulnerable to extinctions caused by climatechange and associated environmental changes. Tenshell characters deemed important for predatorresistance were evaluated for GAPC molluscs acrossvarious mass extinction events associated withclimatic cooling and/or a decline in primaryproductivity (e.g., Cretaceous-Paleocene; Eocene-Oligocene; middle Miocene; Pliocene-Pleistocene).Importantly, all these assemblages were depositedin relatively shallow shelf environments withroughly similar grain sizes; all but one assemblagewas a bulk collection. Hansen et al. (1999) foundthat escalated species, overall, were not morevulnerable to climate-related mass extinction. Onlyornamented Pliocene gastropod species were moresusceptible to extinction than their weaklyornamented counterparts. In another study, Kelleyet al. (2001) found that recovery faunas after a massextinction event were not more vulnerable toenhanced drilling pressure, contrary to hypothesizedpredictions. Additionally, no overall trend inunsuccessful drilling was seen from the lateCretaceous to Pleistocene.Spatial trends in drilling predation vary byenvironment in fossil studies. Hansen and Kelley(1995) used 27,554 specimens of GACP molluscsfrom the Eocene and found a statistically significantdifference in drilling frequency between the innertomiddle- shelf Moodys Branch Formation andthe outer-shelf Yazoo Formation, the deeper sitehaving a higher frequency of drilling predation.However, for the five other assemblages examinedfrom the Moody’s Branch, there was no significantbathymetric trend. Drilling frequency was alsohighly correlated with the percentage of naticidsand their preferred prey within each assemblage.Hoffmeister and Kowalewski (2001) examined171