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

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

Neuquen

Congreso; 3 Congreso Latinoamericano Paleontologia Vertebrados; 2008

Titanosauriform sauropods were one of the most widely distributed groups of dinosaursduring the Cretaceous. In contrast to most derived Upper Cretaceous species (Campanian-Mastrichian) the most basal forms of the group are poorly known, although they have specialinterest to understand the early evolution of Titanosauria. Chubutisaurus insignis (del Corro 1975;Salgado 1993) is represented by postcranial remains found in the Bayo Overo Member of theCerro Barcino Formation (Chubut Group) referred to the Cenomanian by Proserpio (1987). Somephylogenetic analysis interpreted Chubutisaurus as the sister group of Titanosauria (Bonaparte et al.2006; Calvo et al. 2007), although most published studies have not included this taxon probably dueto its brief description and fragmentary remains. New materials were found in the holotypequarry. These remains, housed at the Museo Paleontológico Egidio Feruglio (MPEF-PV 1129),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 of this taxon, whichis relevant to understand the early evolution of Titanosauria. The new elements showplesiomorphic and apomorphic titanosauriform characters that strengthen the support for thebasal position proposed for this taxon. The ischial shaft emarginated distal to pubic peduncle,showing the plesiomorphic state which defines the ambiguous optimization of this character asan unambiguous synapomorphy of the clade Phuwiangosaurus + Titanosauria. The scapular bladeseems to form a 45º angle respect to coracoid articulation, suggesting that this character,previously considered as synapomorphic of Titanosauria, has a broader distribution amongTitanosauriformes. As in most camarasauromorph the posterior dorsal centra is opistocoelouswith deep posteriorly acuminate pleurocoels. Recovered chevrons are not distally bifurcatedand are proximally opened (a camarasauromorph synapomorphy).A comprehensive phylogenetic analysis (36 taxa, 279 characters) of Neosauropoda basedon 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 thesister taxon of Phuwiangosaurus plus Titanosauria. The new information of Chubutisaurushelped to increase the nodal support that justifies its basal position and its exclusion from theclade formed by Phuwiangosaurus + Titanosauria.