Dinosaurs and Cretaceous environments from Northern Neuquén Basin (Argentina): a synthesis.

GONZÁLEZ RIGA, BERNARDO J.; CALVO, JORGE O.; PIRRONE, CECILIA A.; ASTINI, RICARDO A.

Mendoza

Congreso; Gondwana 12, “Geological and Biological Heritage of Gondwana”; 2005

Gondwana 12 Conference, Organising Committe

Cretaceous dinosaurs are particularly abundant in the Neuquén Basin, northwest border of Patagonia (34°-37° S). In the central area of this basin (Neuquén and Río Negro Provinces) worldwide relevance dinosaur faunas have been recovered. In contrast, dinosaur species from the northern region of this basin (Mendoza Province) were unknown until a few years ago. The sedimentary sequences of this basin comprise a complex history; this includes a backarc to foreland sedimentation characterized by cycles of continental, littoral and marine deposits. In Mendoza Province, the Neuquén and Malargüe Groups (Riográndico cycle) represent two Late Cretaceous different paleoecological scenarios. The Neuquén Group (Cenomanian-Early Campanian) comprises a thick continental succession (maximum thickness 1300 m) that represents alluvial fans, fluvial systems, aeolian dunes, and playa-lake environments. In contrast, The Malargüe Group (Late Campanian-Paleocene) includes fluvial, lacustrine, littoral and marine facies related to a first Atlantic ingression that covered central-northern Patagonia. The Neuquén Group outcrops in several sites of Mendoza, mainly in the Cerro Guillermo, Paso de las Bardas and Cañadón Amarillo areas. Abundant titanosaur sauropods were recently discovered in the middle levels of this group (Río Neuquén Subgroup, Late Turonian-Coniacian). These titanosaurs include Mendozasaurus neguyelap (18-27 m long), a large species with slightly procoelous middle caudals (González Riga, 2003). In addition, new specimens, some of them without typical strongly procoelous caudals were found (see descriptions and references in González Riga, 2002). The presence of these huge herbivores associated with theropods, crocodyliforms, Chelidae turtles, coniferous wood and Scoyenia ichnofacies suggest an important biological activity in extensive floodplains of meandering fluvial systems. Similar vertebrates and sedimentary context, but with more derived titanosaurs, were recovered in Rincón de los Sauces, northern border of Neuquén Province. In this area, mid-sized skeletons, most of them exceptionally preserved, have been found associated with theropods and crocodyliforms in numerous paleontological sites. The first species described is Rinconsaurus caudamirus (Calvo and González Riga, 2003), a slender Eutitanosauria. This species was found associated with Pehuenchesuchus enderi (Turner and Calvo, 2005), a basal form of sebecosuchian crocodyliform. In general lines, both Cerro Guillermo and Rincón de los Sauces outcrops, corresponding to the Río Neuquén Subgroup, can be assigned to the tetrapod Neuquenian assemblage sensu Leanza et al., (2004). However, some differences are observed. In Rincón de los Sauces, titanosaurs with non-procoelous anterior or middle caudals have not been found. If we follow most titanosaur cladistic analyses, this record could indicate different ages for the fossiliferous levels, being Mendoza strata the older one. However, to confirm this hypothesis, more complete taxonomic studies are necessary. Likewise, magnetic polarity study and detailed stratigraphic analyses could improve the correlation among the formations in these problematic continental sequences characterized by lateral facies changes related to the fluvial system dynamics. Study supported by the project PICT 07-08277 (ANPCyT), Universidad Nacional del Comahue (Proyecto I- 04-I121) and IANIGLA-CRICYT-CONICET, Mendoza. References: Calvo, J.O., and González Riga, B.J., 2003. Rinconsaurus caudamirus gen. et sp. nov., a new titanosaurid (Dinosauria, Sauropoda) from the Late Cretaceous of Patagonia, Argentina: Revista Geológica de Chile, no. 30 (2), p. 333-353.  González Riga, B.J., 2002. Estratigrafía y Dinosaurios del Cretácico Tardío en el extremo sur de la provincia de Mendoza, Argentina.  [Ph.D. Thesis]: Córdoba, Universidad Nacional de Córdoba (Argentina), 280 p. González Riga, B.J., 2003. A new titanosaur (Dinosauria, Sauropoda) from the Upper Cretaceous of Mendoza Province, Argentina: Ameghiniana, no. 40, p. 155-172. Leanza, H.A., Apesteguía, S., Novas, F.E., and De la Fuente, M.S., 2004. Cretaceous terrestrial beds fro the Neuquén Basin (Argentina) and their tetrapod assemblages: Cretaceous Research, no. 25, p. 61-87. Turner, A.H., and J.O. Calvo. 2005. a new sebecosuchian crocodyliform from the late cretaceous of Patagonia: Journal of Vertebrate Paleontology, no. 25 (1), p. 87-98.