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WALKER AND BRETT—POST-PALEOZOIC PATTERNS IN MARINE PREDATIONPaleozoic, Triassic, or Cenozoic (Table 1). Hermitcrabs evolved in the Jurassic (Glaessner, 1969), andwhile they may crush mollusc shells (Vermeij,1987), it is difficult to assess their overall importanceas predators on molluscan groups. Hermit crabs canbe scavengers, carnivores, filter feeders, ordetritivores (Schram, 1986).Chondricthyes.—The rapid radiation of sharksand marine reptiles (Figs. 1, 4) in the middleMesozoic may have been triggered by the rise ofvast numbers of squids and actinopterygian fishes,including semionitids and basal teleosts (Theis andReif, 1985). The advances of increased swimmingefficiency and maneuverability, and sensory abilityenabled the neoselachians to pursue fast-swimmingthin-scaled fishes and squids in nearshoreenvironments (Packard, 1972).Hybodont sharks of the Triassic (Fig. 5.4) werelargely supplanted by the expanding neoselachiansharks during the Late Jurassic. The evolution ofhighly flexible, hyostylic jaws clearly marked anew level of sophistication in shark predation(Maisey, 1996). In hyostylic suspension the upperjaw is loosely articulated to the braincase and canby swung downward and forward on thehyomandibular bone. This enables sharks to thrustthe jaws forward and gouge out large chunks offlesh from prey. This adaptive breakthroughfomented an adaptive radiation of sharks, whichcontinued through the present day. Modern sharksshow varied feeding modes, including grasping andswallowing, suction feeding, cutting, gouging, andcrushing (Moss, 1977). One strongly modifiedclade from within the neoselachian shark lineageis the highly successful Batomorpha: rays andskates. These first appeared in the Late Jurassicbut diversified in the Cretaceous. The dental platesof rays and chimaeroids of this type may be usedfor digging up shelled invertebrate prey, and thencrushing them, leaving only fragments.Osteichthyes.—Among the Jurassic bonyfishes there is evidence for common piscivoroushabits; for example, the famed Upper JurassicSolnhofen Limestone provides many instances ofpredator-prey interactions (Voihl, 1990). Most“fossilized interactions” involve fish carcassescontaining partially ingested smaller fish. Jurassicpycnodont reef fish developed deep-bodiedmorphologies. For example, Daepedium (Fig. 5)was a deep-bodied Jurassic marine fish with heavyganoid scales, but with pebble-like teeth forcrushing. Jurassic pycnodonts evolved batteries ofrounded, shell-crushing teeth, plus specializednipping teeth. A few pycnodontids even developedstout pavement teeth possibly for crunching corals;rare specimens have been found with coralfragments in the gut (Viohl, 1990). The generalmorphology of these fishes overlaps with that ofdeep-bodied platysomids of the Paleozoic andmany reef-dwelling Cenozoic teleosts.Fish with durophagous dentition, such asSemionotidae (Lepidotes, Heterostrophus),Pycnodontidae (Mesturus), as well as hybodontsharks (Asteracanthusare) and chimaeroids(Brachymulus, Pachymylus, Ischyodus), arethought to have been predators of ammonoids fromthe Middle Jurassic of the Lower Oxford Clay ofEngland (Martill, 1990). Many well-preservedammonoid fragments are thought to be the resultof fish predation rather than physical factors(Martill, 1990). One ammonite specimen, aKosmoceras, was found to have bite marks thatwere similar to the dental battery of the semionotidfish, Lepidotes macrocheirus (Martill, 1990).Sea turtles.—Turtles are the only livingreptiles that are fully adapted to a marine existence(except for egg laying). Many fossil sea turtlesare only known from their plastron and carapace(Nicholls, 1997). The earliest sea turtles are thePlesiochelyidae, possible predators that lived inshallow, coastal waters.Sauropterygians: Plesiosaurs and pliosaurs.—The plesiosaurs are thought to have diversified intotwo major grades during the Jurassic (Fig. 5;Table2): the short-necked forms as fast-swimmingpursuit predators (pliosaurs), and the long-neckedforms as lurking ambush predators (plesiosauroidsand elasmosaurids). O’Keefe (2002), however, hascalled this an oversimplified view of their actualmorphological diversity.A cladistic analysis revealed that plesiosaurspresent a spectrum of body forms, and do not135