View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002could dive to deeper depths as suggested byavascular necrosis in their bones. Similarly, skeletalelements of modern Belgula whales havepachyostosis, and consequently these whales spendmost of their time in shallow water. However,Belgula whales also are susceptible to the bends ifthey dive too deeply, and this is recorded in theirbones (Sheldon, 1997). Most deep-diving animalsusually do not suffer the bends, although someturtles may show skeletal evidence of having haddecompression syndrome (Motani et al., 1999).Consequently, considering evidence fromdentition, body form, thickness of skeletalelements, and avascular necrosis of the bones,mosasaurs are interpreted to have been top ambushpredators that once foraged in lagoonal to openoceanenvironments. Prey were swallowed whole,crushed, pierced, rammed, and shredded to namebut just a few means of prey demise (Lingham-Soliar, 1998a, 1998b, 1999). The dentition ofmosasaurs was so diverse that dentition patternsfit all five of Massare’s (1987) functional predatorygroups for Mesozoic marine reptiles (i.e., cut,pierce, smash, crunch, and crush guilds), afunctional feat accomplished in just a shortevolutionary time period (Lingham-Soliar, 1999).Because of these varied feeding modes, mosasaursmost likely fed on an array of benthic and pelagicorganisms (Lingham-Soliar, 1999).The West African Pluridens walkeri, forexample, had broad-based and short tooth crownsthat are speculated to be powerful enough to havecrushed thin-shelled invertebrate exoskeletons(Lingham-Soliar, 1998b). Globidens, from theUpper Cretaceous of Belgium, had rounded anddeeply wrinkled mushroom-shaped teeth that arethought to be specialized for crushing thick-shelledmolluscs (Lingham-Soliar, 1999). In fact, Globidens,along with another coeval mosasaur, Carinodens,shows the most remarkable durophagous crushingdentition since the demise of the placodonts(Lingham-Soliar, 1999). While the biomechanicalimportance of such dentition was discussed(Lingham-Soliar, 1999), it still remains to beindependently verified using experiments whetherthe many varieties of mosasaur teeth were capableof crushing ammonite shells or other putative prey,as well as how they crushed the shells.Such biomechanical studies would bebeneficial at least in coming to some conclusionas to how some ammonites received large holes intheir shells; and indeed, Kase et al. (1998)performed such tests using modern Nautilus shellsand a “mosasaur robot” modeled after a putativemosasaur predator of ammonites (Prognathodonovertoni). Seilacher (1998) also performedbiomechanical tests using steel pliers and nautiloidshells. While it can be argued that Nautilus shellsare not analagous to ammonite shells with respectto biomechanical loading (Tsujita and Westermann,2001), it is still important to experimentally testthe predatory hypothesis.Numerous specimens of the ammonite,Placenticeras, from the Late Cretaceous PierreShale and Bearpaw Formation of the westerninterior of North America, show putative mosasaurtooth marks (Fig. 8.1) (Kauffman and Kesling,1960; Kauffman, 1990; Hewitt and Westermann,1990) that have been reinterpreted to be limpethoming scars that were enhanced by diagenesis(Kase et al., 1998). Kase et al.’s biomechanical testsindicated that robot bite marks on live Nautilustypically had jagged edges that did not show theconcentric cracks characteristic of putative bitemarks in Placenticeras. The innermost nacreouslayer was shattered in the experiment, whereasinternal shell layers under the putative mosasaurbite marks on Placenticeras were not (Kase et al.,1998). They also found few examples ofPlacenticeras with holes corresponding tomosasaur jaw shape. Consequently, they concludedthat the holes in ammonites were limpet homescars, and not mosasaur predatory bite marks.Tsujita and Westermann (2001) rejected thefindings of Kase et al. (1998) and provided furtherobservations in support of the mosasaurian originof the holes. In fact, they pointed out that some ofthe experimental robot-induced holes that Kase etal. (1998) figured (e.g., their fig. 3b, p. 948)resembled those of putative mosasaurian bite markson Placenticeras meeki (Tsujita and Westermann,2001, fig. 3a,b, p. 251). Further, they argued that146