View - Kowalewski, M. - Virginia Tech

VERMEIJ—EVOLUTION IN THE CONSUMER AGEpopulation level, is thus the rate of injury timesthe mean regeneration time:i r= (dn/dt)•t r(4)where i is incidence of regeneration, n is numberof injuries, t is time, and t ris average time ofregeneration. Ideally the rate of injury is measuredon a time scale similar to that of the average timeneeded for regeneration.All comparative studies of predation revealstrikingly high variability in the incidence of bothsuccessful and unsuccessful attack. The incidenceof lethal drilling in the lucinid bivalve Ctena bellavaries from 0.l7 to 0.75 among Recent populationson the small island of Guam (Vermeij, 1980). Similarvariation in drilling has been detected amongmolluscan assemblages from the Eocene of thesoutheastern United States (Allmon et al., 1990;Hansen and Kelley, 1995), as well as amongNeogene turritellid gastropods (Allmon et al., 1990;Hagadorn and Boyajian, 1997) and late Miocenemolluscs from Bulgaria (Kojumdgieva, 1975). Dataon repaired shell breakage also reveal enormousvariations on spatial scales ranging from the localto the regional (Vermeij et al., 1980; Vermeij, 1982b;Schindel et al., 1982; Zipser and Vermeij, 1980;Schmidt, 1989; Cadée et al., 1997). Large-scalespatial and temporal patterns in predation musttherefore be dramatic in order to be discerned abovethe “noise” of variation (Vermeij et al., 1981;Vermeij, 1987; Cadée et al., 1997).Much of the history of predation as inferred fromthe fossil record is necessarily told from the prey’sperspective. This is because many kinds ofpredation, notably drilling and breakage, leavediagnostic traces on fossil skeletons, and becauseadaptations against these forms of predation arereadily identifiable in fossils. Unfortunately, therecord of predators is much spottier. Whenpreserved, the weapons of subjugation—teeth,claws, jaws, seastars’ arms and tube feet, and shellopeningdevices of predatory snails—offer clues tothe performance of predators, as do sensory organsand the structures involved in locomotion; but thesestructures are often not preserved or interpretable.Erickson et al. (1996) and Rayfield et al. (2001), intheir studies of tyrannosaurs and allosaursrespectively, have paved the way to a quantitativeevaluation of forces applied by predators. Theyemploy sophisticated mechanical measurements onmodel prey together with simulations to infer suchforces. These approaches can be applied much morewidely to other fossil predators.Measures of competition among predators canbe made in fossil material in exceptional cases. VanValkenburgh and Hertel (1993) used the frequencyof tooth breakage among carnivoran mammals inthe La Brea tar pits (late Pleistocene of Los Angeles)as an indicator of potential competition among bonebreakingpredators. In a series of carefulcomparisons, they inferred substantially higherintensities of competition among Pleistocenecarnivores than among their living North Americanand African counterparts. The incidence ofregenerating or broken chelipeds (claws) in crabscould probably be used similarly as an index ofcompetition (see Smith, 1992; Juanes and Smith,1995; Brock and Smith, 1998), but fossil material Ihave seen is inadequate. Evaluation of predatorperformance in fossils remains an important goal.THE BEGINNINGS OFTHE CONSUMER AGEThe early history of consumption remainsshrouded in mystery, not only because the reliablefossil record of consumers is very sparse, but alsobecause uncertainties abound concerning whichstyles of consumption are primitive and which arederived in the evolutionary tree. Nonetheless, someconclusions about the early history of consumptionseem justified by the available evidence.The likely earliest form of predation ispenetration and consumption of one bacterium byanother, as described by Guerrero et al. (1986).Given that this interaction occurs betweenprokaryotic organisms, this kind of consumption islikely to have originated some time before 2.5 billionyears ago, that is, during the Archean Eon. The originof the eukaryotic cell, involving the evolutionaryunion of separate lineages of prokaryotic organismsinto a single integrated whole that could function381