View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002Assemblage-wide studies of drilling in thesoutheastern United States have been interpretedas being inconsistent with the hypothesis ofescalation, because effectiveness against drillingas measured from entire assemblages shows noconsistent trends from the Late Cretaceous to thePleistocene (Kelley and Hansen, 1993, 1996;Kelley et al., 2001). I consider these inferencesinvalid because the biotic composition, especiallywith respect to the relative abundance ofcontrasting adaptive types, varies greatly amongassemblages. An assemblage numericallydominated by slow-burrowing bivalves, whichadapt to drilling mainly through passive resistance,might show a high aggregate effectiveness againstdrilling, whereas an assemblage in which fastburrowingspecies predominate, whose anti-drillingdefense is expressed chiefly in the pursuit phase ofan attack, would show low effectiveness. Whetherthe inferences of an absence of escalation by Kelleyand her coworkers are wrong as well as invalid willhave to await more detailed assessments of thebehavior of individual adaptive types amongpotential prey molluscs. There are no clear trendsin effectiveness, shell thickness, or other potentialdefenses in turritellid gastropods during the lateCenozoic (Hagadorn and Boyajian, 1997), thoughthis result is compromised in that comparisons werenot made within lineages or within physicallycomparable environments. On the Gulf Coastal Plainof the southern United States, Kelley and Hansen(1993) showed that effectiveness among turritellidsincreased from the Paleocene to the Eocene, whereasamong corbulid bivalves it increased between theLate Cretaceous and Paleocene. Lucinid bivalvesshowed no trend. Again, these conclusions mightchange if individual lineages or similar adaptivesyndromes were followed through time in similarenvironments.Increased prey effectiveness through time, ifconfirmed, would indicate a form of escalation inwhich the prey have gained an adaptive advantageover their drilling predators in the long run. Asdiscussed previously, this would be expected if theprey’s adaptive syndrome emphasized passivearmor. Unfortunately, we can surmise little aboutthe adaptations of the predators. Kelley (1992)pointed to an increase in body size (and thuspotentially in the range of available prey) in thetwo lineages of naticid drillers she studied in theMiocene of Maryland, but found no trends in shellshape or aperture shape. Large-footed naticidscapable of edge drilling bivalves and of envelopinglarge prey in the foot are evidently of post-Eoceneorigin, as are edge-drilling muricids.Hagadorn and Boyajian (1997) claimed thatPliocene naticids in the southeastern United Stateswere more stereotyped and more selective in theposition of drilling sites in turritellid prey than wereMiocene naticids, and suggested that there was lessvariation in the size of the predator (as measuredby drill-hole diameter) relative to prey size in theirPliocene samples. If verified, this result could implyadaptations by the predator, but it could also meanthat the prey’s morphology has constrained predatorsto drill at sites where shells are thickest or wherethe shell surface is least easily reached, so increasingprey effectiveness. Still another possibility is thatapparent evolutionary changes in stereotypy bypredators reflect a replacement of one drillingpredator by another. G. Herbert and G. Dietl (pers.comm.) have evidence for this last scenario in thehistory of drilling in the lineage of the veneridbivalve Chione elevata from the late Pliocene toRecent of Florida. Clearly we need clade-level andlineage-level studies of drilling in space and time,in which both predators and their prey are examinedfrom an adaptational point of view.Climate is a factor potentially complicating theinterpretation of temporal patterns in predation.Hansen and Kelley (1995) found evidence of alatitudinal increase in unsuccessful drillingpredation during middle Eocene time (the CookMountain level in the Gulf Coastal Plain). Bycontrast, my analysis of data on effectivenessagainst drilling suggests strongly that assemblagesfrom subtropical and tropical settings show a higheraggregate effectiveness than do those from coolerwaters (represented especially by assemblages inthe middle and late Miocene of Maryland studiedby Kelley et al., 2001). In their detailed study ofthe Recent eastern American lucinid bivalve384