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WALKER AND BRETT—POST-PALEOZOIC PATTERNS IN MARINE PREDATIONdiscretely fall into two basic shapes: from the longnecked,small-headed elasmosaurs to the shortnecked,large-headed pliosauromorphs (O’Keefe,2002). By the Late Cretaceous, these pelagicmarine reptiles were globally distributed (Rieppel,1997). But the taxonomy of this group is still poorlyknown because of the inadequacy of type material,and preservational problems such as skull-lessskeletons (Carpenter, 1997).The plesiosaurs (clade Plesiosauria) wereamong the most diverse, geologically long-lived,and widespread of the Jurassic to Cretaceousmarine reptiles, with a fossil record extending fromthe Triassic-Jurassic boundary to the LateCretaceous (Figs. 5.5, 5.7) (Carroll and Gaskill,1985; Rieppel, 1997). These large reptiles (up to15 m long) had long paddle-shaped limbs(considered hydrofoils), short tails, long necks,needle-shaped conical teeth, and may have swamlike modern sea lions (Godfrey, 1984; Carroll andGaskill, 1985). Plesiosaurs are the only marineanimals in which both forelimbs and hindlimbsperformed as lift-based appendicular locomotion(i.e., as hydrofoils; Storrs, 1993). Pliosaurs had largeskulls (up to 3 m) and jaws with large fang-like teeth(Taylor, 1992), and were capable of dismemberingtheir prey (Taylor and Cruickshank, 1993).The small relative skull size and neck lengthof plesiosaurs, in addition to their dentition,suggests that many of them may have fed on smallfish and soft-bodied cephalopods; some may havealso strained the water for prey (Massare, 1987;Rieppel, 1997). Their evolution may have beenstimulated by the new abundance of largeractinopterygian fishes and sharks in offshoremarine environments. Plesiosaurs from the MiddleJurassic of the Oxford Clay also are thought to havebeen specialists on soft-bodied cephalopods andfish (Martill, 1990). The gastric contents of one LateJurassic plesiosaur, Pliosaurus brachyspondylus,included cephalopod hooklets (Tarlo, 1959). Wetzel(1960) has reported small ammonites in coprolitesattributed to plesiosaurs.Case studies from the Middle Jurassic OxfordClay, United Kingdom, provide a window into themarine trophic relationships of this time period. Thecarnivorous plesiosaurs (Liopleurodon, Pliosaurus)were considered to be at the top of the MiddleJurassic food chain, presumably feeding on fish and“naked” (without a shell) cephalopods (Martill,1990). The ichthyosaur Ophthamosaurus wasthought to be a specialist on naked cephalopods,while marine crocodilians (Metriorhynchus,Steneosaurus) presumably fed on fish and nakedcephalopods (Martill, 1986a, 1986b, 1990). Massare(1987) examined the conical pointed teeth form ofsome Jurassic ichthyosaurs and plesiosaurs, andconcluded that the teeth functioned to pierce softprey. Fish from these deposits were either planktonfeeders or fed on smaller fish, indicating that theMiddle Jurassic had a highly complex marine foodweb (Martill, 1990).Ichthyosaurs.—Lower Jurassic localities fromEurope (e.g., Lyme Regis, England; Holzmaden,Germany) indicate that marine reptile guilds at thistime were dominated by a diverse array ofichthyosaurs (Figs. 5.5, 5.9) (Massare, 1987). Gastriccontents from ichthyosaurs are known from LowerJurassic localities in Europe (Pollard, 1968; Keller,1976; Massare, 1987). The majority of preservedfood remains were cephalopod hooklets (Massare,1987, her table 1, p. 128). For example, preservedcephalopod hooklets (interpreted to be frombelemnites), fish remains, and wood were presentin the gastric contents from the small (< 3 m) LowerJurassic ichthyosaur Stenopterygius (Keller, 1976).Putative phragmoteuthid cephalopods also werepreserved in the stomach contents of the smallLower Jurassic icthyosaur, Ichthyosaurus (Pollard,1968). No belemnite hardparts (besides hooklets)have been reliably found in ichthyosaur gutcontents (Massare, 1987; but see Pollard, 1968).In contrast to the Lower Jurassic, Middle to LateJurassic assemblages indicate a number of changesin the vertebrate predatory ensemble (Massare,1987). Although the same functional feeding types(based on tooth form and wear) were present, thereptile groups were different, with pliosauroids andcrocodiles dominating the assemblages, and withreduced ichthyosaur diversity (Massare, 1987). TheMiddle Jurassic cephalopod-eating ichthyosaur,Ophthalamosaurus (Fig. 5.9) is inferred to have137