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BRETT AND WALKER—PREDATION IN PALEOZOIC MARINE ENVIRONMENTSburrowing bivalves occupied an upper endofaunaltier, 0 to -6 cm (i.e., at and up to 6 cm below thesediment–water interface), from Cambrian timesonward. Invasion of an intermediate (-6 to -12 cm)tier by venerid and pholadomyid bivalves occurredlater during the Devonian; pholadomyids pusheddownward into the deep, -12 to -100 cm, tierslightly later in the Lower Carboniferous. Bottjer(1985) attributes this increased tiering to the mid-Paleozoic escalation of predators. Deependobenthic modes of life were limited during thePaleozoic by the absence of mantle fusion and lackof true siphons in most bivalve clades (Stanley,1970, 1977) (Fig. 13).Cementation of shells to hard substrates alsomakes them harder to dislodge by durophagouspredators (Harper, 1991). Alexander (2001) notesthat two major groups of brachiopods (productidesand orthotetaceans) show an increased frequencyof cemented forms during the later Paleozoic (Fig.13). During the Carboniferous, pseudomonotidbivalves also adopted a cemented mode of life andevolved shell spines, presumably in response toincreased predation pressure.SUMMARYThere is a growing body of evidence thatpredation on hard-shelled marine organismsintensified during the middle Paleozoic (Fig. 13).The direct fossil record of potential predators showsa substantial increase in durophagous shellcrushingpredators, as well as pelagic pursuit andambush piscivorous predators. Trace fossilevidence provides a strong case for the existenceof predatory attack on shelled organisms as earlyas the Cambrian.Predation in marine environments evolvedthrough several phases of intensification withminor setbacks following mass extinctions(Fig. 13). The first phase might be termed theCambrian Revolution. In this phase, largepredators, such as anomalocaridids, otherenigmatic arthropods, and perhaps trilobites, firsthad an impact on marine communities.A middle Paleozoic phase of predationintensification, emphasized by Signor and Brett(1984), involved the rise of nautiloid andammonoid cephalopods, phyllocarids, asteroids,and several lineages of gnathostome fishes. Thelatter group in particular radiated rapidly duringthe Devonian to produce diverse durophagous andpiscivorous placoderms and sharks. Major LateDevonian extinctions terminated the placoderms,but their guilds were rapidly replaced by evolvingsharks, holocephalans, and bony fishes.Brachiopod and molluscan shells and trilobiteexoskeletons show evidence of healed bite marksand peeling from the Cambrian onward, but with amarked increase in frequency in the later Paleozoic.Predatory drill holes with stereotypical position andprey-species preference are found in brachiopods(Cambrian onward) and mollusks (Ordovicianonward), but boreholes also show increasedfrequency in the middle Paleozoic.The Permo-Triassic extinction crisisconstituted a major setback for all marinecommunities. This certainly included manypredatory taxa (e.g., many ammonoids, nautiloids,phyllocarids, predatory archeogastropods).However, it is likely that certain active predatoryorganisms (e.g., fishes) were not as stronglyaffected as others. Studies by Knoll et al. (1996)note that many sedentary benthic organisms (e.g.,brachiopods, echinoderms) have a lower capacityfor controlling CO 2concentrations than do some“high energy” organisms, including activepredatory arthropods and vertebrates. Theseauthors postulate preferential extinction of manyfilter-feeding invertebrates during an interval ofhypercapnial stress.Drilling predation appears to have beencommon in the Paleozoic (Kowalewski et al.,1998). The evolution of Paleozoic drilling actuallyoccurs in two phases: a Precambrian to Silurianstage, and a Silurian to Carboniferous phase(Kowalewski et al., 1998, their figure 3). Drillingwas globally widespread in the Permian, but thefrequency of drilling bivalve or brachiopod preywas relatively low (Kowalewski et al., 2000). Asmore data are added to our knowledge of drillingpredation, it appears that there is not an everincreasingescalatory trajectory through the111