View - Kowalewski, M. - Virginia Tech

BAMBACH—SUPPORTING PREDATORSmetabolic rates as well as different levels ofactivity). This would imply that the overallenergetics of the predator fauna increased in thePaleozoic and Early Mesozoic. TheNeogastropoda, Malacostraca, Osteichthyes, andtetrapods are all active, high-metabolic-rateorganisms. In the Late Cretaceous and Cenozoicthese groups diversified dramatically, from astanding genus diversity of 164 genera at theAlbian-Cenomanian boundary to 1426 genera atthe Pliocene-Pleistocene boundary, an increase of8.7 times as compared to a 1.44 times increase(from 114 to 164 genera) for other predatory taxaover the same 100 million year interval. It is thisexplosive diversification that is responsible for themajor increase in predators as a proportion of allmetazoan diversity. These increases among taxawith greater energetics and the increase of theproportional representation of predators imply thathigh-quality food (i. e., prey taxa) in greater amountsbecame available to support this diversification.Four lines of evidence from non-predators alsosupport the idea that the base of the food pyramidhas expanded over time, and that this has beenparticularly important in the post-Paleozoic: (1)The diversity of non-predators (prey organisms)has increased (see Fig. 3b). Accompanying that,(2) the biomass per individual of the common preytaxa has apparently increased as diversitydominance (and presumably abundance, as well)has shifted over time (Bambach, 1993). (3) Thedepth and intensity of bioturbation, much of itcaused by the activities of deposit-feeders, hasincreased over time (Thayer, 1983; Sepkoski,Bambach, and Droser, 1991). Bottjer and Ausich(1986) argue that increased predation pressure,something that fits nicely with the data presentedhere, would have been an important selectivepressure to stimulate deeper burrowing by potentialprey. The deeper burrowing by deposit feeders,however, must reflect an increase in the amount ofburied potential food (Bambach, 1993), unless itis because the organic matter that gets buried hasbecome more refractory with time. In the lattercase, it would take longer for bacterial and fungalattack to make the material useable as food; thus,as sedimentation continued, the depth of burial atwhich it would become useful as food wouldincrease. This would require deeper burrowing fordeposit-feeders to gain access to it as a food supply.However, there is no apparent increase in refractoryorganic material in marine phytoplankton or algae,unlike the increase in woody tissues and cuticlesthat evolved as land plants increased in size andinvaded a wider range of habitats; nor was thereany change in refractory components between thegymnosperm floras of the mid-Mesozoic andangiosperm-dominated floras of the Cenozoic.Therefore, even if burial of refractory material fromland plants has been a factor in increased foodsupply to the oceans, much of that change occurredin the Devonian and Carboniferous, not in morerecent geologic time when the depth of bioturbationincreased across the spectrum of marineenvironments (Sepkoski, Bambach, and Droser,1991). Also, deeper burrowing was always presentin high-productivity settings, even in the Cambrian,and the increase in depth of bioturbation hasoccurred over a wider range of facies as time hasgone on, suggesting that it is related to more food←FIGURE 20—Proportion of predator diversity comprising different diversity-dominant groups of taxa.(a) Diversity of the taxa of marine predators that reached their maximum diversity in the Paleozoic andare now extinct, graphed as a proportion of the diversity of all marine predators. (b) Diversity of the taxaof marine predators that achieved high diversity in the Paleozoic and either maintained, but did notincrease it later, or recovered or exceeded that diversity in the Cenozoic, graphed as a proportion ofthe diversity of all marine predators. (c) Diversity of the taxa of marine predators that had low Paleozoicdiversity and only reached their maximum diversity in the Cenozoic, graphed as a proportion of thediversity of all marine predators.349