Browsing by Author "Rieppel, Olivier."
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Item Osteology of Gobiderma pulchrum (Monstersauria, Lepidosauria, Reptilia). (Bulletin of the American Museum of Natural History, no. 362)(American Museum of Natural History., 2011-12-30) Conrad, Jack L.; Rieppel, Olivier.; Gauthier, Jacques, 1948-; Norell, Mark.; Mongolian-American Museum Paleontological Project.Joint expeditions by the American Museum of Natural History and Mongolian Academy of Sciences have recovered significant new remains of the basal monstersaur Gobiderma pulchrum. We describe these new specimens in detail and also revisit the originally described material in order to more fully understand this pivotal anguimorph taxon. The newly discovered specimens include skull and postcranial materials that add dramatically to the understanding of the osteology of Gobiderma pulchrum. We revise the diagnosis of this species, adding to the previously published diagnosis the following character states: premaxillary nasal process is narrowest mediolaterally; postfrontal and postorbital remain unfused; postorbital extends posteriorly for almost the entire length of the supratemporal fenestra; the Vidian canal is posteriorly enclosed by the parabasisphenoid; an anterior coracoid emargination is present; the pelvis is completely fused; and the lateral plantar tubercle is distally placed. A phylogenetic analysis confirms the placement of Gobiderma pulchrum as a non-helodermatid monstersaur. As such, the fact that it is known from reasonably complete remains makes it pivotal for understanding character evolution within Monstersauria. The Djadokhta Formation includes several carnivorous/insectivorous lizards and theropod dinosaurs--more than is usual for extant communities, but perhaps analogous in some ways to parts of modern Australia.Item Re-assessment of varanid evolution based on new data from Saniwa ensidens Leidy, 1870 (Squamata, Reptilia). American Museum novitates, no. 3630.(New York, NY : American Museum of Natural History, 2008) Conrad, Jack L.; Rieppel, Olivier.; Grande, Lance.Saniwa ensidens is a pivotal taxon for understanding varanid evolution. A complete specimen of Saniwa ensidens was recently described, offering important new insights into the morphology of this taxon. We apply these new data to a broader-scale study of squamate relationships in order to understand the phylogenetic position of Saniwa ensidens and of varanids more generally. Among the other fossils included in our analysis were the Eocene taxon "Saniwa" feisti, the Miocene Varanus rusingensis, and the giant Pleistocene varanid Megalania prisca. We compare the phylogenetic hypothesis from our analysis of morphology with a recent molecular-based hypothesis and find numerous differences in the phylogenetic relationships within Varanus. We constrained our morphological data set to the phylogenetic pattern presented by the molecular data to further analyze the possible phylogenetic relationships of the fossil taxa. Our analyses show that Saniwa ensidens is the sister taxon to crown-group Varanus and that "Saniwa" feisti is a basal member of the varanid lineage, not closely related to Saniwa ensidens. Both Varanus rusingensis and Megalania prisca are members of the crown radiations of Varanus.Item Tooth implantation and replacement in squamates, with special reference to mosasaur lizards and snakes. American Museum novitates ; no. 3271(New York, NY : American Museum of Natural History, 1999) Zaher, Hussam.; Rieppel, Olivier."Different modes of tooth implantation in squamates are reviewed and defined. Three basic types of tooth implantation are recognized in reptiles: acrodonty, thecodonty, and pleurodonty. Only acrodonty and pleurodonty are present in squamates. Variations of the pleurodont con-dition include full pleurodonty, labial pleurodonty, and subpleurodonty. Tooth implantation and replacement in mosasaur lizards and snakes are reviewed in order to evaluate previously proposed character definitions and hypotheses of primary homologies for three distinct characters associated with tooth replacement-the presence or absence of resorption pits, a recumbent or upright position of the replacement teeth, and the mode of tooth attachment to the bone. The first character is shown to be uninformative in respect to the controversy surrounding Mosasauroidea-snake affinities. Mosasaurs show replacement teeth in a normal upright position, and the occasional presence of recumbent teeth is the result of postmortem displacement. Finally, the mode of tooth attachment in snakes and mosasaurs is fundamentally different. The highly modified condition of tooth implantation described for the alethinophidian snakes is suggested to represent an additional synapomorphy of this taxon at the exclusion of the scolecophidians, which retain the plesiomorphic, fully pleurodont, lizard condition. The lower Middle Cretaceous fossil snake Pachyrhachis shows the alethinophidian type of tooth implantation"--P. [1].