New materials of Chubutisaurus insignis (Sauropoda, Titanosauriformes) and basal Titanosauriform relationships.

CARBALLIDO, JOSÉ L.; POL, DIEGO; APESTEGUÍA, SEBASTIAN; BELLOSI, EDUARDO; KRAUSE, MARCELO

Neuquén

Congreso; III Congreso Latinoamericano de Paleontología de Vertebrados; 2008

Universidad Nacional del Comahue

Titanosauriform sauropods were one of the most widely distributed groups of dinosaurs during the Cretaceous. In contrast to most derived Upper Cretaceous forms (Campanian-Mastrichian) the most basal forms of the group are poorly known, although studies on these forms are of special interest to understand the early evolution of Titanosauria. Chubutisaurus insignis (del Corro, 1975) is known from postcranial remains found in the Bayo Overo Member of the Cerro Barcino Formation (Chubut Group) referred to the Cenomanian by Proserpio (1987). Phylogenetic analysis that include Chubutisaurus recovered this taxon as the sister group of Titanosauria (e.g. Bonaparte et al., 2006), although most published studies have not included this taxon probably due to its brief description and fragmentary remains. Fieldwork conducted in the quarry where the holotype was found yielded new material that probably belongs to the same individual. The new material is deposited at the Museo Paleontológico Egidio Feruglio (MPEF-PV 1129) and includes dorsal and caudal vertebrae and a metatarsal. The new materials with additional undescribed elements collected by del Corro (scapula, ischium, caudal vertebrae, chevrons) provide new information on the anatomy of this taxon, which is relevant to understand the early evolution of Titanosauria. The new elements show, plesiomorphic and apomorphic titanosauriform that strengthens the character support for the basal position proposed for this taxon. The ischial shaft emarginated distal to pubic eduncle, showing the plesiomorphic state which defines the ambiguous optimization of this character as an unambiguous synapomorphy of the clade Phuwiangosaurus + Titanosauria.. The scapular blade seems to form a 45º angle respect to coracoid articulation, suggesting that this character, previously considered as synapomorphic of Titanosauria, has a broader distribution among Titanosauriformes. As in most camarasauromorph the posterior dorsal centra is opistocoelous with deep posteriorly acuminate pleurocoels. Recovered chevrons are not distally bifurcated and are proximally opened (a camarasauromorph synapomorphy). A comprehensive phylogenetic analysis (36 taxa, 279 characters) of Neosauropoda based on character data from various datasets (Salgado et al., 1997; Wilson, 2002; Upchurch et al., 2004) corroborates the phylogenetic placement of Chubutisaurus proposed in recent studies: as the sister taxon of Phuwiangosaurus plus Titanosauria. The new information of Chubutisaurus helped to increase the nodal support that justifies its basal position and its exclusion from the clade formed by Phuwiangosaurus +Titanosauria.