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show a reduction in the total number of alveoli in the mandible as a whole, especially those of the symphyseal sector, and particularly during the Tithonian, there are no forms with eight or nine symphyseal tooth pairs. This suggests an evolutionary trend of the cranium of Pliosaurus toward a relative decrease of symphyseal alveoli. This model agrees with the condition in P. patagonicus (six symphyseal alveoli pairs; Gasparini and O’Gorman, 2014) from the middle Tithonian levels of the Vaca Muerta Formation, Neuquén. However, the condition of the upper Tithonian P. almanzaensis n. sp. (with at last nine sym- physeal alveoli pairs) shows that during the last part of this stage, Pliosaurus species with symphysis longer (in terms of alveoli count) than in P. patagonicus and P. rossicus were present in Patagonia. The upper Tithonian represents the moment just prior to the faunal turnover that affected the plesiosaurs at the end of the Jurassic (Benson and Druckenmiller, 2014). Pliosaurus almanzaensis n. sp. indicates that the decrease in morphological disparity regarding the morphology of the mandibular symphysis is not as marked as previously supposed (Fig. 7) and could be relevant in discriminating between various models (with or with- out a previous decrease of diversity) of the plesiosaur faunal turnover at the end of the Jurassic.
Acknowledgments
The authors thank J.J. Moly (Museo de La Plata) for preparation of part of the specimen, J. González for the drawing, and C. Deschamps for reviewing the English grammar. Thanks to A. Garrido and H. Parent for comments regarding the age of the stratigraphic levels where the material was collected and M.S. Fernández for reading and commenting on an earlier version of the manuscript. Author ZG also thanks the curators of the ver- tebrate paleontological collections of the following institutions during visits: CAMSM;MOZ; NHMUK; OXFUM; Bayerische Staatssammlung für Paläontologie und Historische Geologie, München, Germany; Paläontologische Sammlung der Eberhard Karls, Universität Tübingen, Germany; Muséum National d’Histoire Naturelle, Paris, France; Peterborough City Museum and Art Gallery, Peterborough, UK; Staatlische Museum für Naturkunde, Stuttgart, Germany; and Universidad Autónoma de Nueva León, Linares, México. The authors thank the Willi
Hennig Society for access to TNT software. We also thank the financial support of research grants PICT 2008-0261, PICT 2012-0748, PIP 433, UNLP N 607, and National Geographic Society Grant 5178. Finally, the authors thank T. Sato and one anonymous reviewer for comments that improved this contribution.
Accessibility of supplemental data
Data available from the Dryad Digital Repository:
http://dx.doi. org/10.5061/dryad.jc6r4.
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