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BRETT AND WALKER—PREDATION IN PALEOZOIC MARINE ENVIRONMENTS(Vermeij, 1987). In addition, specimens ofpriapulid worms from the Burgess Shale have beenfound with hyolithids in their gut trace (ConwayMorris, 1977; Briggs et al., 1994, their fig. 73).The earliest report of cannibalism is also knownfrom the Burgess Shale. One specimen of Ottoia,a priapulid, had a proboscis of another Ottoiapreserved in its gut (Briggs et al., 1994). Modernpriapulids are also known to be cannibals. Thus,priapulid feeding behavior has remainedremarkably similar for 530 million years (Fig. 1).Nautiloid cephalopods appear late in theCambrian, but they only attain abundance and largesize during the ensuing Ordovician Period. Allknown living cephalopods are carnivorous, butearly forms were small, poor swimmers that mayhave been bottom-feeding scavengers (Bandel,1985; Lehmann, 1988), and only later didnautiloids develop as major predators.Trace Fossils.—Circular borings made byputative predators that are 0.1 mm to 4 mm indiameter occur on shells from the Early Cambrianonward (Bengtson and Zhao, 1992; Conway-Morris and Bengtson, 1984; Miller and Sundberg,1984). Minute pits are reported in the enigmaticphosphatic shell of the Early Cambrian Moburgella(Bengtson and Zhao, 1992; Conway Morris andBengtson, 1994). These borings were evidentlyproduced by an organism capable of drillingphosphatic shells. This borer may have persistedinto the middle Paleozoic. Chatterton andWhitehead (1987) reported similar cylindrical drillholes on about 10% of the valves of a lingulatebrachiopod from the Silurian of Oklahoma.Putative predator borings are also known tooccur in the exoskeletons of agnostoid trilobites(Babcock, 1993). Some of these tiny pits have pearllikeplugs, evidently secreted by the trilobites inresponse to the predatory action of the predator (orparasite). These ancient pits resemble borings madeby modern nematodes (Sliter, 1971) and providecircumstantial evidence for the existence of boringnematodes as far back as the Early Cambrian.Häntschel et al.’s (1968) compendium oncoprolites cites only 25 reports of pre-Devoniancoprolites; most of these are small and phosphatic.Subsequently, there have been several reports ofCambrian coprolites containing trilobite sclerites,echinoderm ossicles, and fragments of inarticulatebrachiopods (Sprinkle, 1973; Conway Morris andRobison, 1986, 1989; Babcock, 1993; Nedin, 1999).Pelagic Predators?—Seemingly, there was littleto no development of a pelagic predator-prey systemduring the Cambrian, as there are relatively fewdefinite pelagic forms. Cephalopods remained smallbenthic forms, and open swimming vertebrates,other than possible conodont animals, had yet toappear. It is possible that large nektonic arthropods,such as Sidneyia, may have preyed upon each otheror on conodont animals.CAMBRIAN RESPONSE:EARLY PALEOZOIC MARINEPREDATOR REVOLUTIONIn many ways, the Cambrian revolution ofpredators was the first major episode of escalationin marine ecosystems, although the effect of newlyevolved groups of biting and drilling predators isso pervasive that it might be overlooked. With theexception of tiny boreholes in some of the earliestsmall calcareous shelly organisms, Cloudina(Bengtson and Zhao, 1992), there is, as yet, noevidence of predators in the latest Proterozoic(Vendian). This observation led to the scenario ofa nearly predator-free, early “Garden of Ediacara”phase in Earth’s history (McMenamin, 1986;McMenamin and McMenamin, 1990).The first wave of predation may have instigatedthe acquisition of hard skeletons by numerous taxaduring the Cambrian explosion (Bengtson, 1994;Conway Morris, 2001). The apparently “explosive”development of phosphatic and calcitic sclerites,valves, and armor in the Early Cambrian may wellhave been driven by biting organisms. The early andevolutionarily critical rise in Cambrian predators isreviewed by Babcock (in press), who proposes tocall this the “early Paleozoic marine revolution.”The appearance of skeletons was geologicallyrapid, probably encompassing no more than tenmillion years, and was one of the most dramaticepisodes of convergent evolution in the history of97