View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002emphasized the role of skeletons in the Cambrianexplosion, the view started to become prevalent thatpredators were absent or at least of little importancein Cambrian ecosystems (Nicol, 1966; Glaessner,1972; Valentine, 1973; Erben, 1975). As theevidence for Cambrian predators and predationmounted, however (Bengtson, 1968; Bergström,1973; Alpert and Moore, 1975; Bengtson, 1977;Birkenmajer, 1977; Szaniawski, 1982; Whittingtonand Briggs, 1982), this view again gave way to thenow-common acceptance of predators as a majorand important part of the Cambrian ecosystems (e.g.,Conway Morris, 1986; Debrenne and Zhuravlev,1997). Let us look at a couple of questions:1. Could the Cambrian explosion have beentriggered by predators?2. What was happening in the plankton?3. How did macrophagous predation enterinto the picture?4. What were the responses to macrophagouspredation?COULD THE CAMBRIANEXPLOSION HAVE BEENTRIGGERED BY PREDATORS?The Cambrian explosion has attracted as manyexplanation attempts as ever did the demise ofdinosaurs, and no smoking gun has yet turned up.There has been a certain tendency to suggest that theproximal cause for the event is whatever object orphenomenon is under study, and predation has notescaped this trigger-happiness. There seems littlereason to doubt that predators played an early andimportant role in the evolving Cambrian ecosystems(Stanley, 1976a, 1976b; Bengtson, 1977, 1994;McMenamin, 1986; Vermeij, 1987, 1990;McMenamin and Schulte McMenamin, 1990;Crimes, 1994; Butterfield, 1997), but more isdemanded of a trigger for the Cambrian explosionthan that things would have been different without it.The search for a trigger may in fact be unfruitful:Any phenomenon relating to an event such as thiscan belong to one of three causal categories:prerequisite, trigger, and effect; or it could have nocausal relationship at all with the event (Bengtson,1994). Prerequisites for the Cambrian explosion aremany (free oxygen, shelf space, regulatory genes,biominerals, etc.), and so are its effects. All theseare parts of cascades, however, whereas a truetrigger should be independent of them, an analogueto (and as elusive as) “free will”. It must eitherarise “spontaneously” or be introduced from the“outside”; i.e., it must have a timing independentof the integrated biological–chemical–physicalsystem that determines the actual course of theevent. Such a trigger might arise from, say, acosmic event, but may not be in any wayspectacular. An actual trigger is not even neededfor the event to take place; the impetus may insteadcome from a critical accumulation of prerequisiteconditions (see also Kauffman, 1989).Predation is probably as old as (cellular) lifeitself, and it is likely to have existed in manydifferent forms and at many different levels duringthe formative phases of the Cambrian explosion.What we can hope for is a better understanding ofhow predation interacted with other ecological/evolutionary forces to produce the specific biotasand food webs of the Cambrian and—in the end—in what way this came to determine the subsequentevolution of the biosphere.WHICH WAY THE PLANKTONREVOLUTION?Planktic ecosystems represent most of themarine biomass in today’s oceans, and predator–prey interactions are probably the single mostimportant factor in their evolution (Kitchell, 1983;Signor and Vermeij, 1994; Verity and Smetacek,1996; Butterfield, 1997; Smetacek, 2001). Theevolution of diverse and complex acritarchs duringthe Neoproterozoic suggests activities by plankticand/or benthic predators, and the possibilities ofopen oceans even during extreme “Snowball Earth”events (Hyde et al., 2000) may have left the plankticrealm as the only part of the biosphere relativelyuntouched by the global freezing (Runnegar, 2000).Thus animal predators on protist photosynthesizersmay have evolved during theNeoproterozoic, survived the “Snowball Earth”300