View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002PREDATION ON AND BYPROTEROZOIC PROTISTSModern planktic predators are efficient grazerson phytoplankton (Steele, 1974; Stanley, 1976b),and most of the morphology of planktic protists isprobably a response to predation (Smetacek, 2001).The development of spines or other externalprocesses is widespread and is considered to bemainly a way for the potential prey to expand itsexposed surface beyond the size that a predator ofthe same size order is able to handle, or to reduceits nutrient-to-volume ratio (Burzin, 1997;Butterfield, 1997).Planktic ecosystems are often not veryaccessible to paleontological investigations, butfossil data are potentially of great value to testthe several hypotheses that place important phasesof early metazoan evolution in the plankton(Nielsen, 1985, 1995, 1998; Runnegar, 2000). Onepossible source of such data would be thedemonstration of antipredatory devices in earlyphytoplankton, represented by acritarchs (looselydefined as organic microfossils of unknown andprobably varied affinity; cf. Mendelson andSchopf, 1992a), as indicative of the presence ofgrazers in the water column.There are a number of problems in theinterpretation of such data. Acritarchs are a verydisparate group of fossils, and their ecology is inmany cases unknown. Not all are planktic (seediscussion in Butterfield and Chandler, 1992;Butterfield, 1997), and all may not be protists.Processes may be of different kinds and of differentfunctional significance (for example, they may alsobe selected for as a means to increase water frictionor adhesiveness). The presence of process-bearingacritarchs is therefore not a definite indication ofthe presence of predators/grazers. Conversely,however, a biota of simple spheromorphicacritarchs of consistently low diversity would bestrongly suggestive of the absence of selectivepressure from plankton-eaters.Acritarchs undergo a dramatic diversificationnear the Precambrian–Cambrian boundary (e.g.,Moczydlowska, 1991), with a wealth of complexand process-bearing forms introduced. Diversebiotas of Neoproterozoic large process-bearingacritarchs have been discovered during the lastdecades (Chen and Liu 1986; Zang and Walter,1989; Mendelson and Schopf, 1992a; Zang andWalter, 1992; Knoll, 1994b; Vidal andMoczydlowska-Vidal, 1997; Zhang et al., 1998).Occurrences of process-bearing forms before 1 Gaare exceedingly scarce, though weakly spinyacritarchs are known already from about 2 Ga(Hofmann, 1971; Mendelson and Schopf, 1992b).A recently reported 1.5 Ga biota with processbearingacritarchs (Javaux et al., 2001) is a notableexception to an otherwise rather consistent seriesof Paleoproterozoic and Mesoproterozoic simplespheromorphic assemblages. The total curve ofacritarch species (Fig. 1) suggests that diversitieswere low between 2 and 1 Ga and then rose to apeak before a decline during the greatNeoproterozoic ice ages (the “Snowball Earth”episodes of Kirschvink, 1992 and Hoffman et al.,1998). Another peak after the last of these ice ageswas followed by an extinction event and asubsequent Cambrian bloom.Though this evidence is tentative, it may benoted that the rise in acritarch diversity during theNeoproterozoic is an approximate reciprocal of thedecline seen in stromatolite “diversity” (Fig. 1), andthat both trends may reflect an increase of predatorialactivity. An alternative explanation is that this dualpattern reflects a general diversification of protists,which ecologically displace the mat-formingprokaryotes. However, modern microbial matsusually incorporate protists (red, brown, and greenalgae, diatoms, etc.), which help to stabilize thesediment (Bathurst, 1967; Ward et al., 1992; Riding,2000), so there is no evidence that mat-formingprokaryotes and protists are mutually exclusive.Non-acritarch eukaryotes in the Neoproterozoicalso show probable antipredatory morphologies. The“vase-shaped microfossils”, or melanocyrillids(Bloeser, 1985), have flask-shaped tests andresemble modern testate amoebae (Porter andKnoll, 2000); plate-shaped microfossils ofprobably siliceous composition resemble scales ofvarious Phanerozoic groups of biomineralizing298