NEW EVIDENCE OF A TARSAL ELEMENT OF A REBBACHISAURIDAE (DINOSAURIA, SAUROPODA) FROM THE HUINCUL FORMACION OF LAS CAMPANAS CREEK, NEUQUÉN

LUCAS LERZO; PABLO A. GALLINA; ALEJANDRO OTERO; JUAN CANALE; SEBASTIÁN APESTEGUÍA; JOSE CARBALLIDO

Jornada; Reunión Anual de Comunicaciones de la APA; 2022

The presence of anPreservation of ossified calcaneum among sauropod is scarce. This small bone articulates with the fibula proximally and the with metatarsals IV and V distally. Few sauropod genera have been reported with ossified calcaneum (i.e. Vulcanodon, Barapasaurus, Shunosaurus, Diplodocus, Suuwassea, Camarasaurus, Brachiosaurus, Euhelopus, Erketu, Gobititan, Neuquensaurus) , sharing a rough surface, and a general discoidal and proximodistally compressed morphology. Only the basal early-branching sauropod Vulcanodon has a subtriangular calcaneum that resembles the morphology present in basal non-sauropod sauropodomorphs. We present a new calcaneum of a rebbachisaurid sauropod specimen (MMCh-PV70) from Huincul Formation, near Villa El Chocón, which is crescent shaped, similar to this basal morphology . The proximal surface of the calcaneum is markedly concave and has a very rugose surface, while the distal surface is flat and has a pit on the anteromedial side. Another similar calcaneum is present in the complete pes of a juvenile rebbachisaurid specimen MMCh-PV41 from Candeleros Formation at the same locality. Although the astragalus is not preserved in specimen MMCh-PV70, the great development of the calcaneum almost covering the distal surface of the fibula and seems to supply replace the space of articulation in the tarsal region left by a reduced astragalus (inferred by comparison with MMCh-PV41). The fibula of MMCh-PV70 is long, robust and has a D-shaped distal articular surface, unlike most of the sauropod fibulae which are long and slender elements with an oval-shape distal articular surface. The robustness of the fibula in relation to the tibia, and the D-shaped articular surface seems to be related to a great development of the calcaneum. The overall morphology of the rebbachisaurid calcaneum resembles that of Vulcanodon, suggesting a plesiomorphic condition within Diplodocoidea. While in most sauropods, with both tarsal elements preserved, the astragalus is the greater bone, in Rebbachisauridae the calcaneum almost reach the development of the astragalus. An ossified calcaneum has an important advantage in filling the gap between the fibula and the metatarsals IV and V, providing a more stable, relatively incompressible space, with greater resistance to high compressional forces. Future works are needed to explore the advantage of this tarsal construction, if any, on the graviportal locomotion of this clade of mid-sized sauropod dinosaurs.