View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002(Massare, 1987). Importantly, the range in tooth formand function in Jurassic (and Cretaceous) marinereptiles was at least as great as that of modern marinemammals (Massare, 1987).Gastropods.—Although naticid mesogastropods(Fig. 6.4) existed in the Jurassic, boreholes are quiterare. However, recent discovery of drilled shellsproves that the capacity for drilling predation didexist (Kowalewski et al., 1998).Nautiloids.—Nautiloids were very diverse inthe Paleozoic, but there were few nautiloids in theMesozoic (House and Senior, 1981). Nautiloids arethought to have continued with their Paleozoicpredatory mode of life, perhaps scavenging orpreying on crustaceans (Fig. 1). A nautiloid with acomplete jaw apparatus (rhyncolites) is knownfrom lithographic limestone, Upper Jurassic ofsouthwestern Germany (Dietl and Schweigert,1999). Modern nautiloids can repair their shells(Meenakshi et al., 1974), although little is knownabout shell repair in Mesozoic nautiloids.Ammonoids.—Shell shape in ammonoids issometimes used to infer directly whether or notthey were predatory. For example, largemacroconchs of oxyconic forms are interpreted tobe mobile predators (Westermann, 1996). Shellshape, sculpture (Fig. 6.1), and size (especially formacro- and microconchs) can also be explainedby sexual dimorphism (Westermann, 1996). Interms of direct evidence, there is only one EarlyJurassic example of an ammonoid (Hildoceras)with aptychi of juvenile ammonids within its bodychambers (Westermann, 1996).Middle Jurassic ammonoids appeared tooccupy a number of trophic functional groups, fromplanktonic to demersal forms that presumably fedon ostracodes and microgastropods in algal mats(Westermann, 1996), although there is no data ongastric contents to confirm this. The lower ToarcianPosidonia shale (northwestern Europe) is knownto have clusters of fragmented harpoceratineammonoids, presumably from cephalopodpredation (Lehmann, 1975). In turn, the stomachcontents from a harpoceratine indicate that it preyedon small or juvenile ammonoids (Lehmann, 1975).Finally, Late Jurassic ammonoids hadtrophically complex functional groups similar tothose in the Middle Jurassic. Some ammonites mayhave fed on both the plankton and the benthos,depending on food availability and benthic anoxia.Ammonoid forms at this time had costae or nodosemacroconchs, and microconchs with horns on somespecies; smooth shelled ammonoids were alsocommon. Numerous records of ammonoid aptychiare reported from the body chambers of haploceratidammonites, indicating predation; and specimens ofthe Late Jurassic ammonoid, Neochetoceras, haveaptychi of conspecific juveniles within their bodychambers, indicating cannibalism (Westermann,1996, p. 676). A rare find of a Saccocoma crinoidamong the stomach contents of Physodoceras isknown from the Solnhofen Limestone (Milson,1994). Saccocoma is variously interpreted as eitherplanktic or benthic in habit (Milson, 1994), anddepending on the interpretation of the life mode forSaccocoma, the ammonoid is interpreted as either aplanktic or a benthic feeder (the latter interpretationis favored by Westermann, 1996).Echinoderm Predators.—Living families ofasteroids (e.g., Forcipulatida and Notomyotida)have their roots in the Early Jurassic (Hettangian)of Germany and Switzerland (Blake, 1993).Complete asteroids are exquisitely preserved inpelletoidal calcarenite from this time period.Modern forcipulatids are known to prey on otherechinoderms, molluscs, barnacles, and many othertypes of invertebrates. The presence of many armsin asteroids (e.g., solasteroids) suggests that theywere predators of active prey, such as otherasteroids. Predation on active prey by solasteroidsmost likely evolved in the Jurassic (Blake, 1993).Asteriids, in contrast, continued to feed onmolluscs and other benthic prey as their Paleozoicancestors did. During the Jurassic, asteriids hadprominent adambulacral spines that their moderndescendants no longer have; it is thought that thesespines functioned to trap prey (Blake, 1993).Decapods.—Despite the common assumptionthat shell-crushing crabs evolved during the Jurassic,in reality, only one group of lobsters (theNephropidae) is known to have evolved during thistime. All other groups evolved during either the134