Exceptionally-preserved titanosaurs from Mendoza Province, Argentina, provide new data on sauropod pedal evolution and diversity

GONZÁLEZ RIGA, BERNARDO J.; LAMANNA, MATHEW

Mendoza

Congreso; 4th International Palaeontological Congress; 2014

The record of sauropod dinosaurs in South America is among the most important in the world, and includes around 52 species at present. The paleobiogeography of South American sauropods was influenced by geographical factors including the fragmentation of Gondwana and changes in sea level. Two evolutionary phenomena are particularly striking: (1) the replacement of diplodocoids by titanosauriforms during the early Late Cretaceous; and (2) the remarkable proliferation of titanosaurs throughout the Late Cretaceous. The pedal anatomy of lithostrotian titanosaurs has been poorly represented in phylogenetic analyses because complete pedes of these sauropods are extremely rare: only Opisthocoelicaudia (Mongolia), Epachthosaurus (Patagonia, Argentina), and the unidentified La Invernada titanosaur (Patagonia, Argentina) preserve this part of the skeleton in its entirety. Here we report two new titanosaurian species from the Coniacian?lower Santonian Plottier Formation, discovered by the first author in Mendoza Province, Argentina. Specimen UNCUYO-LD-301/302 (housed at Laboratorio de Dinosaurios, Universidad Nacional de Cuyo) is among the largest sauropod individuals yet found (humerus length = 1.79 m), and preserves a number of elements including a complete and articulated right pes. Its phalangeal formula is 2-2-2-2-0, and it shows a spreading metatarsus with a structure that is unique among Eusauropoda: metatarsals II?V are all short and similarly robust. Metatarsal IV is the longest, but metatarsal II to V exhibits little difference in length (less than 10 percent). Moreover, the proximal and distal ends of metatarsals I and II are wider than these metatarsals are long. The unguals of digits I?III do not show the ?sickle-shape? characteristic of other eusauropods. Specimen UNCUYO-LD-309 preserves both articulated pedes (phalangeal formula: 2-2-2-2-0) and has elongate and slender metatarsals and typically ?sickle-shaped? unguals. In the field, the articulated metatarsi of both specimens were inclined at 40 to 50 degrees, confirming a semi-plantigrade hind limb posture in these animals. The unguals were deflected laterally, as is observed in exceptionally-preserved titanosaur tracks discovered in the lower Campanian Anacleto Formation of Mendoza Province. These discoveries demonstrate that lithostrotian titanosaurs encompassed a greater pedal diversity than was previously appreciated. Furthermore, variation in the shapes and sizes of titanosaurian metatarsals and pedal phalanges and the progressive reduction of the phalanges provide data critical for informing phylogenetic analyses and evaluating morphofunctional and ichnological interpretations.