242 Geological setting
The Neuquén Basin is located in the western margin of South America, between 36º and 40º south latitude (Fig. 1.1), in west-central Argentina and central Chile. It is characterized by a 2600m thick Mesozoic and Cenozoic record and covers a surface of over 120,000km2 (Yrigoyen, 1991). The basin is limited to the east and south by wide cratonic areas, and to the west by the Pacific Ocean. Due to its location, it has been considered as a back arc-retroarc basin (Digregorio et al., 1984; Legarreta and Uliana, 1991). In most of the Neuquén Basin, the Upper Jurassic to Lower
Cretaceous record is characterized by the dark bituminous shales and marls of the Vaca Muerta Formation (Weaver, 1931). These deposits concordantly overlie the clastic and mostly continental deposits of the Tordillo Formation (Fig. 1.2); this early Tithonian basal contact is an isochronous surface that marks the beginning of a widespread marine transgression (Leanza, 1981). The top of the Vaca Muerta Formation is roughly diachronic (Leanza et al., 1977) and progradational (Mitchum and Uliana, 1985; Legarreta and Gulisano, 1989). It includes younger stages passing from the south sector (middle Tithonian) to the central and northern sectors of the basin (Valanginian). The study area is located in the deepest (western sector) of
the basin, where the Vaca Muerta Formation exceeds 500m in thickness and is dominated by offshore facies. The measured section, where the remains of Pliosaurus almanzaensis n. sp. were found, represents the late Tithonian. The specimen was found in the ammonite Substeueroceras koeneni Biozone, the uppermost part of the upper Tithonian (H. Parent, A. Garrido, personal communication, 2016; Parent et al., 2015). The unit is essentially composed of two facies associations. The lower part is entirely dominated by fine-grained deposits, ranging from purely siliciclastic mudstones to hybrid siliciclastic/carbonate marls (Fig. 1.2), accumulated under anoxic bottom conditions that favored the preservation of organic matter. Toward the upper part, the succession is characterized by dark grey and greenish calcareous laminated and bioturbated mudstones showing thin intercalations of very fine- and fine-grained sand-
stones and tuffaceous sandstones, and a very conspicuous floatstone bed essentially composed of oyster shells (Fig. 1.2). Sandstones are essentially massive, although some beds show normal grading and only a few strata are characterized by ripple lamination. This facies association clearly represents a progradational arrangement and, as shown by Massaferro et al. (2014) and Sagasti et al. (2015), should be interpreted as the record of foreset-topset ramp deposits developed upon anoxic bottomset facies.
Materials and methods
The holotype specimen shown and examined in this study is deposited in the MOZ. All measurements were taken using a digital caliper with 0.01-mm accuracy. To test the phylogenetic position of Pliosaurus almanzaensis n. sp., we used a dataset based on Benson and Druckenmiller (2014) modified by Benson et al. (2013) with the addition of P. almanzaensis n. sp. and P. patagonicus. The data set (87 taxa scored for 270 characters)
Journal of Paleontology 92(2):240–253
was compiled using Mesquite Software (Maddison and Maddison, 2011). The final data set (Supplementary data I) was analyzed using TNT 1.5 version software (Goloboff and Catalano, 2016) using a heuristic search (tree bisection reconnection, with 1000 random addition sequence replicates). To evaluate the grade of homoplasy, consistency (CI) and retention (RI) indices (Farris, 1989) were calculated. Additionally, Bremer Support (Bremer, 1994) was considered to indicate clade support. IterPCR scipt (Pol and Escapa, 2009) was used to detect wildcard taxa and remove them from the reduced consensus.
Repositories and institutional abbreviations.—BHN, Musée- sur-Mer, Boulogne, France; BRSMG, Bristol City Museumand Art Gallery, Bristol, UK;CAMSM, Sedgwick Museumof Earth Sciences, Cambridge, UK; DOKDM, Dorking & District Museum, Dorking, UK; DORCM, Dorset County Museum, Dorchester, UK; MLP, Museo de La Plata, La Plata, Argentina; MOZ, Museo Provincial de Ciencias Naturales Prof. Dr. Juan A. Olsacher, Zapala, Neuquén, Argentina; NHMUK, The Natural History Museum, London, UK; OXFUM, Oxford University Museum of Natural History, Oxford, UK; PIN, Paleontological Institute, Russian Academy of Sciences, Moscow, Russia; QM, Queensland Museum, Brisbane, Australia.
Systematic paleontology Class Sauropterygia Owen, 1860
Order Plesiosauria de Blainville, 1835 Family Pliosauridae Seeley, 1874
Clade Thalassophonea Benson and Druckenmiller, 2014
Genus Pliosaurus Owen, 1842 (sensu Knutsen, 2012; Benson et al., 2013)
Type species.—Pliosaurus brachydeirus Owen, 1841 by original designation. Lower Kimmeridgian, England.
Pliosaurus almanzaensis new species
url:lsid:
zoobank.org:pub:D2F274C4-B584-42AD-B3E9- E37C203B6EB6 Figures 2–5
Holotype.—MOZ 3728P, basicranium, complete palate, mand- ible and postcranial remains comprising a posterior cervical centrum, dorsal centra, humera and proximal elements of the forelimb and femora; upper Tithonian (Jurassic), Vaca Muerta Formation, Neuquén Province, Patagonia.
Diagnosis.—Species of Pliosaurus that differs from others by two autapomorphies: angular participating in mandibular symphysis and occipital condyle without notochordal pit or irregularly-arranged grooves. Following combination of characters differentiates P. almanzaensis from other species: (1) trihedral teeth (differs from P. kevani Benson et al., 2013), (2) nine or more symphyseal teeth (differs from P. rossicus Novozhilov, 1948 and P. patagonicus), and (3) 15–17 post- symphyseal alveoli and (4) parasphenoid without ventral keel (differs from P. kevani).
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