An approach to the phylogenetic position of Argyrosaurus superbus (Sauropoda: Titanosauria).

SEBASTIÁN APESTEGUÍA

La Plata

Jornada; XX Jornadas Argentinas de Paleontologia de Vertebrados; 2004

Museo de La Plata

Argyrosaurus superbus Lydekker is a titanosaur sauropod based on a complete right forelimb (A) from Upper Cretaceous strata at the Chico River. Further explorations by Bonaparte to the Senguerr River, southern Chubut Province, provided a new specimen (B). The new remains were assigned to Argyrosaurus based on the morphology and robustness of the humerus. However, no diagnostic feature but only a general similarity indicates the inclusion of B in Argyrosaurus, as with other purported Argyrosaurus from Entre Rios and Uruguay. A study and reinterpretation of the poorly preserved vertebrae of B was made in order to conduct a phylogenetic analysis. The run of A (holotype) in a restricted data matrix comprising 82 characters and 19 taxa produced 43 trees of 160 steps which collapsed within Titanosauria and preserved Saltasaurinae. The run of B in the same data matrix produced 151 trees of 151 steps which collapsed in the base of Titanosauria, in the base of ‘Titanosauridae’ and preserved saltasaurines. The experience to combine the information of both specimens in a single Argyrosaurus taxon solved the polytomies and provided 2 trees collapsible in a small polytomy at the base of Titanosauria. The resulting tree shows a position of Argyrosaurus between Lirainosaurus and Aeolosaurus that can be described as Patagosaurus((Jobaria, Diplodocidae)(Camarasaurus (Brachiosaurus (Chubutisaurus (Phuwiangosaurus, Andesaurus, Malawisaurus)(Isisaurus (Lirainosaurus (Argyrosaurus (Aeolosaurus (Rapetosaurus (Opisthocoelicaudia (Alamosaurus (Saltasaurinae))))))))). The assignation of B to Argyrosaurus is only supported by the humerus morphology, but its peculiar shape does not preclude its inclusion.