An articulated titanosaur from Patagonia (Argentina): new evidences of the pedal evolution.

GONZÁLEZ RIGA, BERNARDO J.; CALVO, JORGE O.; PORFIRI, JUAN

Beijing, China

Congreso; 2ª International Palaeontological Congress: "Ancient life and modern approaches"; 2006

University of Science and Technology of China Press

The study of the pedal structure is essential for understanding the locomotion, evolutionary trend  and the behavior of the huge sauropod dinosaurs. In particular, titanosaurs are the most diverse and geographically widespread Late Cretaceous herbivores in South America and other continents. In spite of that, most taxa are represented by incomplete skeletal elements lacking articulated foot. In this context, the discovery of a new specimen with exceptionally preserved pedal structure is relevant. The finding mentioned herein is the second record of South American titanosaur for which pedal morphology is completely known; the first was described in a new specimen of Epachthosaurus sciuttoi recovered in the late Cenomanian-early Turonian strata from Chubut Province, Patagonia (Martínez et al, 2004). By contrast, this new titanosaur, probably a new genus, is younger; it have been found in the Allen Formation, late Campanian-early Maastrichtian from La Invernada site, Neuquén Province, Patagonia. In this formation, Aeolosaurus and Rocasaurus genus were found, but they come from the Rio Negro Province and they do not have pedal elements. The specimen from La Invernada includes a partial caudal series and the left portion of the skeleton lacks the skull and cervical vertebrae. The anterior and posterior limbs are complete and articulated: the manus is composed by five metacarpals without phalanges, as in other most derived titanosaurs (Salgado et al., 1997). A similar condition is present in the Mongolian genus Opisthocoelicaudia (Borsuk-Bialynicka, 1977). In contrast, Epachthosaurus presents a vestigial element fused to the distal face of the metacarpal IV. These evidences suggest a progressive reduction of the pedal phalanges from the late Cenomanian-early Turonian to the late Campanian-early Maastrichthian. Moreover, the absence of manual phalanges in derived titanosaurs (Saltasaurinae and Aeolosaurini clades) is consistent with the absence of a pollex claw impression in wide-gauge trackways (Wilson and Carrano, 2000). On the other hand, the pes has five metatarsal and the phalangeal formula is: 2-2-2-2-0, in Epachthosaurus is 2-2-3-2-0 and in Opisthocoelicaudia 2-2-2-1-0. The first three digits posses sickle-shaped claws and the articular faces of ungual phalanges suggesting mobility in horizontal and vertical planes. In particular the first phalanx of the digit I is a very reduced structure (see figure). In constrast, titanosauriforms from Aptian-Albian, like a specimen from Russia, shows a probable phalangeal formula 2-2-3-2-1 (Averianov et al., 2002). The new fossils from Patagonia, in comparison with other taxa, suggests that titanosaurs had a progressive reduction of size and number of pedal phalanges and a relative lengthening of the III and IV metacarpals toward the end of the Cretaceous. Comparative study associated with cladistic analysis will confirm these interpretations.       References   Averianov, A.O., Voronkevich, A.V., Maschenko, E.N., Leshchinskiy, S.V., and Fayngertz, A.V. 2002. A sauropod foot from the Early Cretaceous of Western Siberia, Russia. Acta Palaeontologica Polonica 47 (1): 117–124. Borsuk-Bialyncka, M. 1977. A new camarasaurid sauropod Opisthocoelicxaudia skarzynskii, gen. n. sp. s. from the Upper Cretaceous of Mongolia. Palaeontologia Polonica 37: 45-64. Martínez, R.D., Jiménez, O., Rodríguez, J. Luna, M. and Lamanna, M.C. 2004. An articulated specimen of the basal Titanosaurian (Dinosauria: Sauropoda) Epachthosaurus sciuttoi from the Early Late Cretaceous Bajo Barreal Formation of Chubut Province, Argentina. Journal of Vertebrate Paleontology 24(1): 107-120. Salgado, L., Coria, R.A. and  Calvo, J.O. 1997. Evolution of Titanosaurid Sauropods. I: Phylogenetic analysis based on the postcraneal evidence. Ameghiniana 34: 3-32. Wilson J.A. and Carrano, T. 1999. Titanosaurs and the origin of “wide-gauge” trackways: a biomechanical and systematic perspective on sauropod locomotion. Paleobiology 25 (2): 252-267.