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WALKER AND BRETT—POST-PALEOZOIC PATTERNS IN MARINE PREDATIONthe biomechanical loading was oversimplified, andneeds to be reanalyzed using various ammonitemodels in addition to testing various loadingfunctions attributed to the jaws of mosasaurs. Thelack of exact matching between jaw shape andputative bite marks is explained by the fact thatlike most marine reptiles, mosasaur jaws were notperfectly occluded; the lower jaw was loose enoughto pivot laterally. Further, while well-preservedlimpet fossils are found in the Pierre Shale, theyhave yet to be found in the Bear Paw Formation ofAlberta. Thus, Tsujita and Westermann concludethat putative predatory holes on ammonites maybe only rarely associated with limpet home scars,and the vast majority are from mosasaur predation.No one has done a quantitative comparision of thesize of the bite marks, the size of the limpet homescars, the diameter of the preserved limpets, andrelated it to the range of tooth sizes found incontemporaneous mosasaurs.Gastric contents from mosasaurs includecephalopod hooklets, fish, belemnites, turtle bones,and birds (Massare, 1987, her table 1, p. 128). Forexample, gastric contents from a single specimenof the mosasaur Clidastes included a marine sharkand a diving marine bird, Hesperornis (Martin andBjork, 1987). At least one squid gladius from thePierre Shale exhibits bite marks attributable to amosasaur (Stewart and Carpenter, 1990). Dollo(1913) reported a broken test of the echinoid,Hemipneustes, between the teeth of the mosasaurCarinodens; and numerous ammonites have toothmarks, presumably from mosasaur predation(Kauffman and Kesling, 1960; Kauffman, 1990).Some of these tooth marks, however, may also belimpet homing scars on some specimens (Kase etal., 1998). To date, no ammonites are known frommosasaur gastric contents (Martin and Bjork, 1987;Massare, 1987), and this may be due to taphonomicbias against the preservation of aragonititc ammonitesin gastric contents (Tsujita and Westermann, 2001).Sea and Shore Birds.—Finally, in the LateCretaceous, two orders of marine diving birdsevolved: the flightless, foot-propelledHesperornithiformes and the swimming-wingedIchthyornithiformes. Both taxa had elongate beakswith rows of sharply pointed teeth, presumably forfish capture. Fish remains have been found incoprolites associated with Hesperonis (Benton,1997, p. 273). Presumably, these taxa filled theguild presently occupied by diving sea birds,although the Cretaceous orders were evolutionarydead ends. One mosasaur specimen also containsingested Hesperonis skeletal elementsJURASSIC–CRETACEOUSBENTHIC PREY AND THEIRPOSSIBLE ANTIPREDATORYRESPONSESPossible Behavioral Responses ofInvertebrates.—During the Jurassic and Cretaceous,a host of organisms from sponges to worms,barnacles, and bivalves independently evolved anability to bore into stiff mud, rock, carbonate,hardgrounds, shell substrates, and wood (Palmer,1982; Seilacher, 1985; Wilson and Palmer, 1990,1992). Submarine crypts and caverns seeminglyprovided a refuge for certain primitive groups, suchas sclerosponges, many bryozoans, sedentary tubedwellingpolychaetes, and pediculate brachiopods(Palmer, 1982; Wilson and Palmer, 1990). Anumber of sedentary invertebrate groups persistedon exposed hard substrata during the Mesozoic.But these, in particular, show allegedly strongantipredatory skeletal adaptations (Fig. 3): they arestrongly cemented (oysters, corals, barnacles), havethick, heavy shells (e.g., rudists, oysters),camouflage, and spines/spicules or toxins.A major decline in free-resting benthicinvertebrates occurred in the Mesozoic (relative tothe Paleozoic). Quasi-infaunal forms, such asgrypheid and exogyrid oysters, remained commonon Mesozoic soft substrates, but these organismswere partially hidden and evolved thick, robustshells (Fig. 3). Exposed epifaunal brachiopods,corals, and crinoids were greatly reduced or absentfrom shallow marine soft-substrate settings duringthe Mesozoic (Thayer, 1983; Vermeij, 1987).Thayer (1983) argued that this decline in epifaunalsuspension feeding may have been fostered by therise of deeply burrowing infaunal “bulldozers,”147