View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002dived to depths of 600 m, based on an analysis ofits eyes and bone condition (Motani et al., 1999).Ichthyosaurs are also thought to have regurgitatedhardparts of indigestible food. In the Peterboroughquarry in England, Peter Doyle (unpublished,2002) discovered “ichthyosaur regurgitates” of 160million-year-old acid-etched juvenile belemnites.The acid-etched fossils indicate that they were oncewithin the ichthyosaur stomach.Marine Crocodiles.—Little is known about thefossil history of marine crocodiles (SuborderMesosuchia) compared to other marine reptiles (Huaand Buffetaut, 1997). The earliest crocodiles(Teleosauridae) are known from the Early Jurassic.This group shows little adaptation for marine life,and it is only because they are found in shallowmarine deposits that they are inferred to have beenmarine crocodilians (Hua and Buffetaut, 1997).Later in the Jurassic, these forms showed anatomicalfeatures that were more characteristic of life inmarine conditions (e.g., streamlined skull, reductionin bony armor, and reduction of the forelimb). Someforms (Steneosaurus) had long, slender teeth andmay have been piscivorous. Other teleosaurids hadblunt teeth, and more robust jaws, and are thoughtto have been durophagous predators on ammonoidsor sea turtles (Hua and Buffetaut, 1997).The Early Jurassic to Early CretaceousMetriorhynchidae include crocodilians with bothlong (longirostrine) and short (brevirostrine) snoutsthat may reflect dietary differences (Hua andBuffetaut, 1997). This group, because of its morestreamlined body form and a skull similar tomosasaurs, is thought to have been pelagic. Thestomach contents of a brevirostrine form(Metriorhynchus) contained ammonites, belemnites,pterosaurs (Rhamphorhynchus), and the large fishLeedsichthys (Martill, 1986b). Metriorhynchus andtheir ilk were probably lunging ambush predatorsthat captured their prey by sudden bursts ofswimming (Massare, 1987).Two other groups of marine crocodiles, thePholidosauridae (Lower-Upper Cretaceousboundary) and Dryosauridae (Upper Creatceous tolate Eocene) had fresh- and salt-water members (Huaand Buffetaut, 1997). The marine species ofPholidosauridae (Teleorhinus) are thought to havebeen piscivorous. Two groups of marine dryosaurids(Phosphatosaurinae and Hyposaurinae) are known:the phosphatosaurins had blunt teeth and robustjaws, and are thought to have preyed upon turtlesand nautiloids; the hyposaurins, most common inthe Paleogene, had long slender jaws and pointedteeth and were probably piscivorous (Hua andBuffetaut, 1997). Crocodilians are known to undergorapid changes in dental morphology in response toenvironmental change related to dietarymodification. It is thought that the piscivorous modeof life became more common after the Cretaceousmass extinction, when ammonoids and hardshelledmarine reptiles were not as common(Denton et al., 1997). However, the extinction ofthe dryosaurids in the Eocene is thought to haveresulted from the expansion of whales, which mayhave competed with them for food (Hua andBuffetaut, 1997). Crocodiles also regurgitate theirprey and such remains have been reported fromthe Paleocene of Wyoming (Fisher, 1981a, 1981b)but not from the Cretaceous.CRETACEOUS PREDATORSThe Early Cretaceous marked the beginningsof a major reorganization of marine predators,including the rise of neogastropods, numerouscephalopod predators, and several new vertebratepredatory guilds (Figs. 1, 4–6). The Early Cretaceoussaw the radiation of large teleost fish (> 3 m inlength) and sharks, and the non-dominance of marinereptiles (Massare, 1987). Massive shell-crushingmosasaurs (e.g., Globidens) did not evolve untilthe Late Cretaceous. This major specializedfunctional feeding type had been essentially absentthroughout most of the Mesozoic, since theextinction of Triassic placodonts (Massare, 1997).Late Cretaceous marine reptiles were dominatedby ambush predators, such as mosasaurs; marinefish (including sharks) were much more commonat this time and became more dominantcomponents of the predator functional feedingguild than ever before in the Mesozoic. Marinereptiles such as plesiosaurs and ichthyosaurs wereminor components of the Cretaceous predatory138