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BRETT AND WALKER—PREDATION IN PALEOZOIC MARINE ENVIRONMENTSplates on the axillaries of the arms. Arthroacantha,a very common and widespread Devoniancamerate, possessed articulated spines on the calyx,as well as spines on the arms (Fig. 12.1) (Keslingand Chilman, 1975). Within this genus there is alsoa trend of increasing spine length into the LateDevonian (G.C. McIntosh, pers. comm., 2001).Aronson (1991) argued that if predationpressure were a significant factor in crinoidcommunities a major decline in crinoid thicketswould be expected between pre-Devonian andCarboniferous benthic assemblages. Thisprediction was based in part on evidence thatstalked crinoids migrated offshore in the face ofthe Mesozoic marine revolution of predators(Meyer, 1985). Aronson made corrections fordifferences in rock volume of various ages andpredicted the frequency of dense crinoidassemblages for each age. He found that densecrinoid thickets did not, in fact, show a declineduring this interval. This provides negativeevidence for the escalation hypothesis and mightsuggest that predation pressure was not, in fact, amajor factor in controlling crinoid density.Alternatively, Aronson suggested that the generallack of reefs in the Lower Carboniferous caused aFIGURE 12—Spinosity in Paleozoic crinoids. 1—Reconstruction of Devonian crinoid Arthroacanthawith attached (coprophagous) Platyceras gastropod, ×2; note jointed spines on calyx and spines onaxillaries of arms. 2—Percentages of spinose genera in three subclasses of crinoids through thePaleozoic Era. 1, Modified from Kesling and Chilman (1975); 2, from Signor and Brett (1984).109