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PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002Bottjer, 1985). Deep burrows and endolithic cryptsmay have been particularly effective protectionfrom grazing predators, such as placodonts. Thenear-synchronous development of this strategy inat least three independent lineages of venerid(Skelton et al., 1990) and myoid bivalves, as wellas the great increase in endobenthic and endolithicanomalodesmatans—all during the Early Triassic—suggests intensification of some selective pressure(Fig. 3). Bottjer (1985) and Skelton et al. (1990)drew attention to the coincidence of this downwardpush with the Mesozoic Marine Revolution.However, McRoberts (2001) has recently argued thatdurophagous predators may not have beensufficiently abundant or widespread during theTriassic to account for the early radiation ofendobenthic strategies. It is possible that theantipredatory advantages of living cryptically weremerely a side-benefit of adaptation driven by otherpressures, such as competition (McRoberts, 2001).The overall frequency of shell repair, due eitherto predators or to physical factors in theenvironment (see Cadée et al., 1997; Cadée, 1999;Ramsay et al., 2001) is also low during this timeinterval, with repair frequencies evidently evenlower than those of the late Paleozoic, as recordedby Vermeij et al. (1982) (Table 3).Ammonoids.—Vermeij (1987) drew attentionto the fact that surveys of ammonoid shellarchitecture and traces of predation on cephalopodsare critically needed for the whole Phanerozoic. Ageneral view of ammonoids suggests that theirmorphology is related to their pelagic, demersal,or planktic lifestyle (Westermann, 1996, p. 689),rather than to antipredatory features.Early Triassic ceratitic ammonoids fromplatform environments are considered to have beenchiefly nektonic in habit, although some planktonicand demersal forms occurred (Westermann, 1996).Offshore bituminous limestones of the Middle andUpper Triassic in Europe, North America, and Chinaalso contained coarsely costate to smooth ammonoidmorphotypes, all of which were interpreted to bepelagic (including some with planktonic lifestyles).In the Late Triassic (Norian), however, most highlysculpted evolute ammonoid morphotypesdisappeared, whereas smooth involute formssurvived, and the first heteromorphs appeared.Deep outer-shelf and upper-slope environmentsfrom the Early Triassic of China contained bothsmooth and costate ammonoids; deep basinammonoids were smooth-shelled, some with finesculpture, many of which are interpreted to havebeen pelagic (Westermann, 1996). Coarse sculpture,however, is thought to be commonly associated withbasin-slope habitat.Large pelagic predators, such as ichthyosaurs,plesiosaurs, placodonts, and turtles had evolved bythe Late Triassic, and many are thought to haveeaten ammonoids; ceratitic ammonoids do notseem to show classic antipredatory defenses.However, the temporal trends, if any, of ceratiteshell injuries remain to be studied.Echinoderms.—Echinoderms went through anevolutionary bottleneck after the Permianextinction, with at least five classes surviving intothe Early Triassic (Simms, 1990). From lowdiversity in the Triassic, echinoids and crinoidsdiversified in the Middle Triassic, but some cladeswent extinct during the mid-Carnian. A seconddiversification occurred in the Norian and the EarlyJurassic for both groups. Triassic crinoid forms reevolved“passive” filtration systems like theirPaleozoic forebearers. Most post-Paleozoic crinoidsare thought to be anatomically similar to theirPaleozoic ancestors; however, Donovan (1993) andOji (2001) provide evidence that the Mesozoiccrinoids (especially the Jurassic forms) were agileand could actively relocate—this may have provideda selective advantage as predation pressure increased(Meyer, 1985). Pseudoplanktic pentacrinitids,paracomatulids, and the true comatulids evolved inthe Late Triassic and occupied niches altogetherdifferent from their Paleozoic counterparts (Simms,1990). These new modes of life probably do notreflect escalation; overall, predation on theseechinoderms is deemed to have been relatively lowduring the Triassic (Schneider, 1988).The two main clades of echinoids, theDiadematacea and Echinacea, diversified in theLate Triassic and Early Jurassic, but still retainedtheir mid-Paleozoic diversity levels (Simms, 1990).130