A New Rebbachisaurid Sauropod from the Lower Cretaceous (Albian) of the Rayoso Formation (Neuquén, Argentina)

VII Jornadas Internaciones sobre Paleontología de Dinosaurios y su Entorno. Salas de los Infantes, Burgos A new rebbachisaurid sauropod from the Lower Cretaceous (Albian) of the Rayoso Formation (Neuquén, Argentina) CANUDO, J.I.1,2, CARBALLIDO, J.L.3, GARRIDO, A.4,5, SALGADO, L.6 1 Aragosaurus–IUCA, Paleontología, Facultad de Ciencias, Pedro Cerbuna 12, Universidad de Zaragoza, 50009, Zaragoza, Spain, [email protected] 2 Museo de Ciencias Naturales de la Universidad de Zaragoza, Plaza Basilio Paraíso, 50008 Zaragoza, Spain 3 Conicet–Museo ‘Egidio Feruglio,’ Fontana 140, 9100 Trelew, Argentina, [email protected] 4 Museo Provincial de Ciencias Naturales ‘Prof. Dr. Juan Olsacher,’ Dirección Provincial de Minería, Etcheluz y Ejército Argentino (8340), Zapala, Argentina, [email protected] 5 Departamento Geología y Petróleo, Facultad de Ingeniería, Universidad Nacional del Comahue, Buenos Aires 1400, Neuquén (8300), Argentina 6 Instituto de Investigación en Paleobiología y Geología, Universidad Nacional de Río Negro, Conicet, Av. Gral. J.A. Roca 1242, 8332 General Roca, Argentina, [email protected] Keywords: Early Cretaceous, Rayoso Formation, Argentina, Rebbachisauridae The rebbachisaurids are a group of medium‐sized sauropods with a fundamentally Gondwanan distribution (South America, Africa) from the middle‐upper part of the Early Cretaceous to the lower part of the Late Cretaceous. They were the last diplodocoids in becoming extinct. They are also recorded in Europe, with Demandasaurus, having been described in Salas de los Infantes, Spain (Torcida Fernández‐Baldor et al., 2011). The rebbachisaurids have played a particularly significant role in the recognition of the events of dispersal and vicariance that took place within Gondwana and between Gondwana and Laurasia in the Early Cretaceous, the time of the rupture and definitive separation of Africa and South America. The recent years have witnessed a notable increase in the record of this group, with almost a dozens of new species. Since the first phylogenetic proposals, the rebbachisaurids have been situated at the base of the diplodocoids, a position they occupy at present. According to calibrated phylogenies, their origin can be traced to the Middle Jurassic, with a ghost lineage of rebbachisaurids running through most of the Jurassic and the base of the Cretaceous. However, the phylogenetic relationships among the known taxa remain uncertain, above all on account of the scarcity of cranial material. As part of a collaborative project pursued by our Spanish‐ Argentinean team for more than 15 years on the dinosaurs of the Early Cretaceous, campaigns from 2008 to 2010 resulted in the recovery of remains from several specimens of a rebbachisaurid close to the locality of Agrio del Medio (Neuquén, Argentina). The aim of this communication is to provide a brief description of this new dinosaur and to discuss its systematic and phylogenetic importance. 43 VII Jornadas Internaciones sobre Paleontología de Dinosaurios y su Entorno. Salas de los Infantes, Burgos Geologically, the fossils under study are from the Rayoso Fm. (Neuquén Basin). This unit form is a sequence of interstratified clastic levels (sandstones and clays) and evaporitic levels (gypsums and fine layers of carbonates), sporadically presenting isolated crystals of rock salt (Salgado et al., 2012). These materials were deposited in a continental environment under arid conditions. The fossils are from the middle part of the Pichi Neuquén Member, roughly 44 meters above its base. The fossil‐bearing level comprises slightly laminated sandstones of a reddish‐ brown colour with slight bioturbation. The level presents current ripples at the top and mudcracks. The geometry of the sedimentary body and its sedimentary structures suggests non‐confined deposits formed by decantation in areas with high sedimentation and low energy. Its age varies between Aptian and Albian, and is most likely to be lower Albian. The remains of three distinct individuals have been recovered at the site: one adult and two juveniles. The bones of the adult are little dispersed and are semi‐articulated. By contrast, those of the juveniles are more dispersed, covering an area of some 8 m2. The bones do not show signs of transportation, and the sequences of cervical and caudal vertebrae are articulated and arched. The presence of two juvenile specimens is inferred from the significant difference in size of the vertebral centra and the absence of ossification between the centra and the neural archs. The low degree of dispersal of the bones and the presence of articulated parts in low‐energy, non‐confined sedimentation suggests that the site underwent an autochthonous mode of concentration. The first dinosaur fossils that came from the vicinity of Agrio del Medio were described with the name Rayososaurus agrioensis, but their geographical localization was not clearly specified in the publications (Carballido et al., 2010). Some authors doubted whether the fossils in question stemmed from the Rayoso Fm. The material that was first recovered was scarce, yet highly interesting given that the scapulae showed a morphology previously unknown in South America but similar to that of Rebbachisaurus from the early Late Cretaceous of Morocco (Wilson & Allain, 2015). This was the clearest proof of the connection between South America and Africa at the end of the Lower Cretaceous. In a detailed revision of the outcrops, photographs, field notebooks and the matrix preserved on the Rayososaurus fossils, it was shown that they had in fact been recovered from the Candeleros Formation, of Cenomanian age. It was precisely in the course of the revision work on the type‐site of Rayososaurus that the fossil remains were found belonging to the rebbachisaurid that is the subject of the present communication. They are, as such, the only dinosaur remains known from the Rayoso Fm. The rebbachisaurid of Agrio del Medio is represented by almost all the anatomical elements, with the exception of the femur and the neural arches of the dorsal vertebrae (there is only one centrum). Particularly striking is the disarticulated cranium of the adult specimen, which retains the dentary, surangular, maxilar, premaxilar, jugal, squamosal, quadrate. As regards the cranium, it should be noted that it has a mandible with a typical rectangular outline with numerous dental positions. The teeth are fine and pencil‐like, as in all other diplodocoids. The scapula shows the typical racket shape, which is one of the diagnostic characters of Rebbachisauridae. The caudal vertebrae are platycoelous, with the exception of the first ones in the series, which display a concavity in their anterior face. The specimen from Agrio del Medio shows bones similar to most of the rebbachisaurids from Neuquén Basin, making it possible to draw comparisons. Among the taxa that are closest in geographical terms, it is notable that the scapula is reasonably similar to that of Rayososaurus (Carballido et al., 2010), although it is distinguished by the development and orientation of the acromial process. 44 VII Jornadas Internaciones sobre Paleontología de Dinosaurios y su Entorno. Salas de los Infantes, Burgos A preliminary phylogenetic study situates the sauropod from Agrio del Medio together with other rebbachisaurids of the Early Cretaceous of Argentina such as Comahuesaurus, which differs from it in the position and development of the deltopectoral crest of the humerus. It is found to be clearly separated from other rebbachisaurids from Africa, Europe and the Late Cretaceous of Patagonia, such as Limaysaurus and Cathartesaura. It is thus a new taxon belonging to Rebbachisauridae and the first of the South American taxa that preserves the rostral region of the cranium. References Carballido, J.L., Garrido, A.C., Canudo, J.I., Salgado, L. (2010): Redescription of Rayososaurus agrioensis Bonaparte (Dinosauria, Diplodocoidea), a rebbachisaurid of the early Upper Cretaceous of Neuquén. Geobios, (43, 5), 493‐ 502. Salgado, L., Canudo, J.I., Garrido, A.C., Carballido, J.L. (2012): Evidence of gregrariousness in rebbachisaurids (Dinosauria, Sauropoda, Diplodocoidea) from the Early Cretaceous of Neuquén (Rayoso Formation), Patagonia, Argentina. Journal of Vertebrate Paleontology, (32, 3), 603‐613. Torcida Fernández‐Baldor, F., Canudo, J.I., Huerta, P., Montero, D., Pereda Suberbiola, X., Salgado, L. (2011): Demandasaurus darwini, a new rebbachisaurid sauropod from the Early Cretaceous of the Iberian Peninsula. Acta Palaeontologica Polonica, (56, 3), 535‐552. Wilson, J.A., Allain, R. (2015): Osteology of Rebbachisaurus garasbae Lavocat, 1954, a diplodocoid (Dinosauria, Sauropoda) from the early Late Cretaceous–aged Kem Kem beds of southeastern Morocco. Journal of Vertebrate Paleontology, (35,4), e1000701. 45.

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