New materials of Chubutisaurus (Sauropoda, Titanosauriformes) and basal titanosauriform relationships

CARBALLIDO, J. L., POL, D., APESTEGUÍA, S., BELLOSI, E. S. Y KRAUSE, M.

Neuquén

Congreso; III Congreso Latinoamericano de Paleontología de Vertebrados; 2008

CEPALB

Titanosauriform sauropds were one of the most widely distributed groups of dinosaurs during the Cretaceous. In contrast to most derived Upper Cretaceous species (Campanian-Maastrichian) the most basal forms of the group are poorly known, although they have special interest to understand she early evolution of Titanosauria. Chubutisaurus insignis (del Corro 1975; Salgado 1993) is represented by postcranial remains found in the Bayo Overo Member of the Cerro Barcino Formation (Chubut Group) referred to the Cenomanian by Proserpio (1 987). Some phylogenetic analysis interpreted Chubutiraum as the sister group of Titanosauria (Bonaparte et al. 2006; Calvo et al. 2007), although most published studies havenot includedths taxon probably due to its brief description and fragmentary remains. New materials were found in the holotype quarry. These remains, housed at the Museo Paleontolbgico Egidio Feruglio (MPEF-PY 1 129), probably belong to the same individual and include 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 ofthis taxon, which is relevant to understand the early evolution of Titanosauria. The new elements show plesiomorphic and apomorphic titanasauriform characters that strengthen the support for the basal position propod for this taxon. The ischial shaft emarginated distal to pubic peduncle, showing the plesiornorphic state which defines the ambiguous optimization of this character as an unambiguous synapomorphy of the clade Phuwiungosaum + Ti tanosauria. The scapular blade seems to form a 45" angle respect to coracoid articulation, suggesting that this character, previously considered as synapornorphic of Titanosauria, has a broader distribution among Titanosauriformes. As in most camarasauromorph the posterior dorsal centra is opistocoelous with deep posteriorly acurninate pleurocoels. Recovered chevrons are not distally bifurcated and are proximally opened (a camarasauromorph spapomorphy). A comprehensive phylogenetic analysis (36 ma, 279 characters) of Neosauropoda based on character data from various datasets (Salgado et 91. 1997; Wilson 2002; Upchurch et al. 2004) corroborates the phylogenetic placement of Chubuiisaurus proposed in recent studies: as the sister taxon of Phuwiongosaurus plus Titanosauria. The new information of Chubutisnums helped to increase the nodal support that justifies its basal position and its exclusion from the clade formed by Phawiangosaunrs + Titanosauria.