View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002thus escape fossilization, although it is not likelythat this will remove the record of the preyed-onspecies altogether. On the other hand, in somecircumstances predation may enhance thepossibilities of fossilization, thereby boosting thediversity record of the prey. Fecal-pellet transportvia planktic predators is today the dominant modeof transfer of plankton to the sea floor, andindigestible tests of the prey are thereby protectedfrom dissolution by the seawater (Honjo andRoman, 1978; Kitchell, 1983).Sampling artifacts may have considerablystronger effects on fossil diversity curves than isgenerally recognized, and it is not unlikely thatmost of the short-term diversity changes reportedfrom the fossil record are in fact a function ofdifferential preservation in the rock record (Raup,1976a, 1976b; Peters and Foote, 2001, 2002).In conclusion, various effects of predation ondiversity may be postulated, but general diversitydata cannot be used to argue levels of predation. Wemay have to be content with “asking what isplausible in theory and what is interesting in themeasurable fossil record of diversity” (Sepkoski,1996). The “interesting” aspects of diversity may inthis case be related not to clades or grades but toconvergent aptations to predator–prey interactions(spinosity, burrowing habit, tube-dwelling,spiculation, sclerotization, etc.) in comparableenvironments. Such data are generally not directlyavailable from the literature, and will have to bespecially compiled to be useful. Later in this chapterI will discuss, however, how some of the availablediversity curves, in particular for stromatolites andprotists, may be of use as proxies for predatorialactivities in the early evolution of life.SIZE INCREASE AS ARESPONSE TO PREDATIONPredators either penetrate their prey or swallowit. In our imaginary Shangri-La, there would belittle need to get big. On the contrary, efficiency inthe exchange of gases and nutrients is a functionof an organism’s surface-to-volume ratio, and sosmaller organisms have the advantage. Anyincrease in size would have to be accompanied byan exponential increase in surface complexity tokeep the surface-to-volume ratio stable.Enter a predator. Now a large surface area maybecome a vulnerability—the more exposed surface,the more there is for the predator to attack. Increasein size may then be a better option, not only becauseit reduces the surface-to-volume ratio thusprotecting against penetration, but also because itmakes the potential prey more difficult to swallow(cf. Guillard and Kilham, 1977). Conversely, it isadvantageous for a swallowing predator to be largerthan its prey (see Hansen et al., 1994), so a positivefeedback loop is created.Also (with the exception of the large landanimals, for which gravitation becomes the majorobstacle), larger organisms can move faster thansmall ones (Bonner, 1965, 1993). Althoughmovement may be an advantage also for anorganism seeking out suitable energy gradients,there is no compelling reason to move quicklyunless someone else does too. Thus, increasingmotility, a corollary of size, may also be selectedfor in predator–prey interactions.Other effects of larger size are division of laborand hence differentiation of tissues and thedevelopment of organs that would have no functionin smaller organisms: respiratory, digestive,circulatory, and muscular structures, for example.These effects may be seen as secondary to theprimary phenomenon of size increase (Bonner,1965, 1993, 1998). They also have a much widersignificance than merely being involved inpredator–prey interactions, so at these higher levelsof organization the connection between predationand size increase becomes weak.Although increase in cell/body size, at least forsimpler organisms, may thus be a more useful proxyfor predation pressure than taxonomic diversity, acaveat is needed also here. The tendency towardlarger body size and complexity in evolution is soprevalent that it has been regarded as a general law(Cope’s Rule). Whereas individual instances of sizeincrease may be due to specific selection pressures,the general phenomenon does not have to beexplained as anything more than an increase in292