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BRETT AND WALKER—PREDATION IN PALEOZOIC MARINE ENVIRONMENTSrhynchonellides, also appear to have been avoidedby shell boring organisms, at least in the Devonian(Bordeaux and Brett, 1990).Fluted margins may also give shells tighterclosure against the prying action of starfish, as doesinterlocking hinge dentition, and central placementof adductor muscles (Alexander, 2001). LaterPaleozoic brachiopods and bivalves show increasedfrequency of fluting, interlocked hinge teeth, andcentralization of adductor muscle scars.Greater shell thickness could also have beenadvantageous during a time of increaseddurophagous predation, drilling predation, orFIGURE 10—Morphological trends in bellerophontidmollusks. Upper curve shows total number ofgenera; area under lower curve representsproportion of total genera that show indicatedfeature: (a) presence of sculpture; (b) anomphalous(lacking umbilicus); (c) disjunct coiling. Note loss ofgenera with disjunct coiling and increase inproportion of genera with sculpture and lacking anumbilicus. Redrawn from Signor and Brett (1984).parasitism. Leighton (2001b) also notes a tendencyfor brachiopods to develop thickened muscleplatforms in the most drill-prone centrally locatedshell areas. A trend toward increased plate thicknessis evident in late Paleozoic crinoids. CertainPermian taxa are extraordinarily thickly plated(Signor and Brett, 1984).Spinosity.—Spinose skeletons may deter bothdurophagous and drilling predators. An increasein the frequency of taxa with skeletal spines duringthe Paleozoic is documented by Signor and Brett(1984); spines may also increase in length andsharpness. Articulate brachiopods show a strongincrease in the presence of spines on both thepedicle and brachial valves, reflected in the rise todominance of the productides, in the later Paleozoic(Signor and Brett, 1984). Although the spines onthe deeply convex pedicle valve of productides mayhave served as “rooting” spines for these semiendofaunalbrachiopods (Grant, 1966; Rudwick,1970) (Fig. 10), they may also have been functionalin preventing predatory attack, particularly frombelow by infaunal predators. Leighton (2001a)showed that among Late Devonian brachiopods thespinose Devonoproductus had a much lowerfrequency of completed boreholes than eithercontemporaneous atrypids or Douvillina.Among gastropods, relatively few Paleozoicgenera (~5%) show spines; however, here theexceptions may prove the rule. No spinose generaare known from the lower Paleozoic and spinoseforms first appear in the Silurian. Moreover, all ofthe spinose gastropods are inferred to have beenrelatively sedentary. Notably, several species ofspinose platyceratids appear in the Devonian. Apermanently sessile commensalistic/parasiticlifestyle (Bowsher, 1955; Rollins and Brezinski,1988; Baumiller, 1990; Boucot, 1990) may haverendered these gastropods particularly vulnerableto predatory attack, and conferred a selectiveadvantage to species that evolved spines.Trilobites also show an abrupt, but short-lived,burst of spinosity during the Devonian. The wellknown and highly diverse trilobites from theEmsian-Eifelian of Morocco and North Americashow a high frequency of spinose genera in several107