THE CALCANEUM OF A REBBACHISAURIDAE (DINOSAURIA, SAUROPODA) FROM THE HUINCUL FORMATION AT LAS CAMPANAS CREEK, NEUQUÉN PROVINCE, ARGENTINA

LERZO LUCAS; GALLINA, PABLO; ALEJANDRO OTERO; JUAN CANALE

Salta

Otro; Reunión de Comunicaciones de la Asociación Paleontológica Argentina; 2022

Asociación Paleontológica Argentina

Preservation of ossified calcaneum among sauropods is scarce. This small bone articulates with the fibula proximally and 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 early-branching sauropod Vulcanodon has a subtriangular calcaneum that resembles the morphology present in non-sauropod sauropodomorphs. We present here a calcaneum collected in a site near Villa El Chocón, in rocks from the Huincul Formation (Neuquén Group, Cenomanian-Turonian), as part of the skeleton of a rebbachisaurid specimen (MMCh-PV 70). It is crescent shaped, resembling the primitive morphology above-mentioned. The proximal surface of the calcaneum is markedly concave and has a very rugose surface, while the distal surface is flat, with a pit on the anteromedial side. A similar calcaneum was recognized in the complete pes of a possible juvenile rebbachisaurid specimen MMCh-PV 41 from the Candeleros Formation at the same locality. Although the astragalus is not preserved in MMCh-PV 70, the calcaneum shows a strong development, almost covering the distal surface of the fibula thus replacing part of the astragalus articulation region, suggesting a reduced astragalus (inferred by comparison with MMCh-PV 41). The primitive and large calcaneum seems to be related to an also peculiar fibula. Whereas most sauropod fibulae are long and slender elements with an oval-shape distal articular surface, the fibula of MMCh-PV 70 is robust and has a D-shaped distal articular surface, features that seem to be related to the strong development of its plesiomorphic calcaneum which, unlike most known sauropods, almost reach the development of the astragalus. An ossified calcaneum adds a solid bridge between the fibula and metatarsals IV and V, thus providing a more stable, incompressible space, with greater resistance to high compressional forces. Future works are needed to explore additional characteristics of this peculiar tarsal construction and its relevance on the graviportal evolution for this clade of sauropod dinosaurs.