View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002FIGURE 9—Cladogram of chondrichthyes (sharks) showing the repeated evolution of durophagy(indicated by “CRUNCH”; after Wilga and Motta, 2000).further defense in the Late Cretaceous–Tertiary,without substantial loss of mobility, in the form oferectile fin spines. These adaptations may indeedhave made the swallowing of whole prey sufficientlydifficult that individuals possessing longer, sharperfin spines were frequently spared and/or avoidedby experienced predators, thus driving adaptivetrends in neoteleosts (Patterson, 1994).Sea turtles.—Although modern turtles are allmorphologically similar, Mesozoic sea turtles werefar more disparate (Hirayama, 1997). There wereup to three separate radiations of sea turtles in theMesozoic (Nicholls, 1997). The Plesiochelyidaeevolved in the Jurassic. The second group(Pelomedusidae) is presently restricted to freshwater, but in the Late Cretaceous and Early Tertiary,members of this group were present in shallowmarine environments (Nicholls, 1997). The thirdgroup (Chelonioidea) first appeared in the late EarlyCretaceous and includes the Dermochelyidae,Cheloniidea, and the Protostegidae (Hirayama,1997). Of these, the jellyfish-eating stock(Dermochelyidae) arose in the Santonian and is stillextant, and the other omnivorous and herbivorousgroups (Cheloniidae) arose in the Aptian and arestill extant, having reached a diversity peak duringthe Late Cretaceous (Hirayama, 1997). TheProtostegidae were restricted to the LateCretaceous. The Chelonidae and Dermochelyidaesurvived the mass extinction at the end of theCretaceous, while most other marine reptiles, withthe exception of the crocodiles, went extinct. Theskull of Late Cretaceous Protostegidae turtles issimilar to that of the modern freshwatermolluscivorous turtle (Malayemys). Based on thissimilarity, it may have fed on pelagic ammonoids(Hirayama, 1997). The Protostegidae were thelargest sea turtles known, characterized by massiveheads, like that of the late Campanian Archelon. Thisgigantism was short-lived, as the Protostegidae wentextinct before the end of the Cretaceous (Hirayama,1997). The skulls of the Dermochelyidae areimperfectly known; however, it appears that thenarrow lower jaw and other skeletal features suggestthat the jellyfish-eating mode was developed duringthe Cenozoic (Hirayama, 1997).144