NEW EVIDENCE ON THE ANATOMY, FUNCTION AND EVOLUTIONARY CONSTRAINTS IN SAUROPOD PEDES BASED ON NEW MATERIAL FROM MUPALEO SITE (CANDELEROS FORMATION), NEUQUÉN, PATAGONIA, ARGENTINA

HALUZA, ALEJANDRO; CANALE, JUAN IGNACIO

San Juan

Congreso; IV Congreso Latinoamericano de Paleontología de Vertebrados; 2011

Recent field work at the Barda Atravesada de Las Campanas (Lago Ezequiel Ramos Mexía area, Neuquén Province, Argentina), where the uppermost section of Candeleros Formation (Cenomanian) crops out, provided articulated material of both hind limbs of a small-sized sauropod. The presence of associated platycoelus anterior caudal vertebrae, with high neural spines reinforced by lateral laminae, allows the assignment of this specimen to Rebbachisauridae (Sereno et al., 1999). ␣e distal part of the left leg is preserved and the right hind limb is almost complete, lacking only the distal phalanges. The astragalus is medially tapering, showing the wedge shape that is present in other neosauropods. However, the ascending process is anteriorly oriented as in the primitive condition for sauropods (Upchurch et al., 2004; Bonnan, 2005). The pes is asymmetric like that of other neosauropods (Upchurch et al., 2004). Metatarsal I is robust and, given the extreme reduction of the medial part of the astragalus, it articulates directly with the medial surface of the tibia. Metatarsal II and III are the most developed; the latter is the largest one. Both of them articulate with the astragalus. Metatarsal IV and V articulate with the calcaneum. ␣e new material presented here allows a reinterpretation of metatarsals described for Limaysaurus tessonei (Calvo and Salgado, 1995). The fifth metatarsal shows a slightly concave distal surface bounded by medial and lateral articular condyles, suggesting the existence of an ungual phalanx. Four proximal phalanges and a fragmentary ungual were found in close association to the pes. These evidences allow the interpretation of the rebbachisaurid pes as retaining two phalanges in the first digit, an ungual phalanx in ␣fth digit and probably more phalanges for digits II, III and IV. ␣is condition is comparable to that observed in well-known diplodocoid pedes. On other hand, macronarian pedes suered consecutive reductions in the pedal phalangeal formula (González Riga et al., 2008). The observation of a “semi-plantigrade” posture in this little sauropod reinforces the proposed hypothesis of the evolutionary constraint observed in the insertion angle of the m. gastrocnemius, given the vertical reorientation of leg (Bonnan, 2005), showing its independence on the weight supported by the hind limb.