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WALKER AND BRETT—POST-PALEOZOIC PATTERNS IN MARINE PREDATIONPlesiosaurs.—Plesiosaurs are thought to havebeen top predators of Mesozoic seas, but there hasbeen little evidence to support this claim (Sato andTanabe, 1998). The oldest firm evidence ofpredator-prey associations between ammonoidsand plesiosaurs is from a Late Cretaceous (UpperCenomanian) outcrop from Hokkaido, Japan. Fromthis locality, Sato and Tanabe (1998) describegastroliths, 30 isolated and disarticulatedammonoid jaws, a shark tooth, and molluscanshells from the putative gastric contents of apolycotylid plesiosaur. While the head of theplesiosaur was missing, comparable teeth in otherpolycotylids suggest that they were poorly adaptedto crush ammonite shells, and may have swallowedtheir prey whole. Plesiosaur gastric contents fromthe Early Cretaceous are known to includecephalopod jaws in association with gastroliths(Sato and Tanabe, 1998). Gastric residue from otherLate Cretaceous plesiosaurs, however, had fishvertebrae, pterodactyl bones, and thin-shelledammonites (Massare, 1987, her table 1, p. 128).Mosasaurs.—Mosasaurs originated anddiversified worldwide in less than 25 million yearsduring the Late Cretaceous (Fig. 10), but met theiruntimely demise during the end-Cretaceousextinction event (Lingham-Soliar, 1999). By the timeof their origin, the ichthyosaurs had gone extinct,and only a few plesiosaur families were still extant(Lingham-Soliar, 1999). Not since the Triassicplacodonts, had a reptile group so dominated thedurophagous functional lifestyle (Massare, 1987).Mosasaurs, with elongated snouts and elongated,fusiform bodies, include the largest marine reptilesever known (e.g., Mosasaurus hoffmanni at over17 m in length; Lingham-Soliar, 1998a). Becausethe Late Cretaceous sea levels were the highestrecorded during the Mesozoic, these giant reptileswere more likely to be preserved than were otherMesozoic marine reptiles, and so we have a betterunderstanding of their habits.Bone microstructure and bone density are usedto infer the ecological distribution of mosasaurs inthe water column (Sheldon, 1997). Reduced bonedensity of two common mosasaurs (Clidastes andTylosaurus) indicates that they lived at great depthFIGURE 10—Temporal distribution of severalgenera of mosasaurs (indicated by letters and theirapparent ammonite prey, based on bite/crushmarks from Cretaceous deposits of the WesternInterior Seaway. From Kauffman (1990).(Sheldon, 1997). Thus, even deep-water LateCretaceous ammonoids that were thought to usedepth as a refuge against predation (Westermann,1996) may not have been immune to their attacks,which may have fragmented the shells completely.Evidence of deep diving in mosasaurs comes fromavascular necrosis of their bones, indicating the“bends”—decompression syndome (Martin andRothschild, 1989; Taylor, 1994). Some mosasaurs,however, had pachyostosis (bone thickening), whichrequired that they increase lung volume to remainneutrally bouyant; in turn, increased lung volumemeans a larger rib cage, and thus more drag on theanimal, making it a slow swimmer (Sheldon, 1997).Mosasaurs with pachyostosis (e.g., Platecarpus)usually lived in shallow waters, but even these forms145