A Mega track site of Mendoza (Argentina): a case study of wide-gaude sauropod trackway from the Late Cretaceous

GONZÁLEZ RIGA, BERNARDO J.; PRÁMPARO, MERCEDES; CALVO, JORGE O.

Teruel, España

Simposio; 10th International Meeting Mesozoic Terrestrial Ecosystems and Biota; 2009

Universidad Autonoma de Madrid, Universidad Complutense de Madrid

Agua del Choique is a new track site from South America. It is one of the youngest sauropod tracks site recorded in the world and offer an excellent example of wide-gauge style of locomotion produced by Late Cretaceous derived titanosaurs. This track site is preserved on a calcareous sandstone bed located in the upper part of the Loncoche Formation, late Campanian – early Maastrichthian in age (Figure 1). Almost all tracks identified (more than 200) were produced by sauropods, except by the presence of isolated small theropod tracks. The sauropod tracks exhibit a pronounced heteropody (manus-pes area ratio of 1:3) and are assigned to a new ichnotaxon (González Riga and Calvo, in press). All footprints are well separated from the midline, and the manus and pes trackway ratios (MTR and PTR) reach 18-22 and 26-31 per cent, respectively (Figures 2 and 3). These values indicate a very wide-gauge trackways, in comparison with the most known trackways recorded in Europe and South America. No manual phalanges were recorder in the site, and this aspect is in agreement with the skeletal remains of the most derived lithostrotian titanosaurs, which have no phalangeal facets in the metacarpals (Figure 2). On the other hand, we found an isolated sauropod track with a poor evidence of pes phalanges. These features, and the bone record from the Loncoche Formation, suggest that the trackmakers were, probably, middle size saltasaurine or aeolosaurine titanosaurs (total length: 10-14 m; glenoacetabular distance: 2.3-2.5 m; hip height: 1.9-2.3 m). In the northern sector of the track site, we discovered seven trackways. Most of them are parallel and show the same direction of the gait (south and southwest) suggesting that titanosaurs traveled in social group. This discovery is the second evidence of gregarious behavior recorded in South American Cretaceous sauropod; the first was documented in Bolivia. From a paleoenvironmental point of view, the Loncoche Formation includes fluvial, lacustrine and marginal marine environments produced by a marine ingression from the Atlantic that covered central-northern Patagonia during the Maastrichtian-Paleogene. The palynological content of the interval related to the track level give us relevant paleoenvironmental information about the different habitats where dinosaurs lived. The samples studied below and above the track bed are dominated by chlorophycean fresh water algae (mainly Pediastrum and Botryoccocus) and zygnematacean in less number. The terrestrial palynomorphs are scarce, mainly represented by pteridophyte/bryophyte spores and bisaccate pollen grains. The occurrence of abundant green algae taxa in the palynological association indicates the presence of freshwater ponds or lacustrine facies with an intermittent tidal influence (oolitic limestones, foraminifera, plesiosauria bones). The ocurrence of sauropod tracks in tidal facies is not a direct evidence of a permanent habitat for these huge vertebrates. Instead, this record indicates that some titanosaurs had the capacity of walking across marginal marine environments, in this case related to the Late Cretaceous Atlantic ingression. Study supported by CONICET (PIP 5132; PIP 5222), ANPCYT (PICT 2005-33984), Universidad Nacional de Cuyo (2007-2009) and IANIGLA (Mendoza, Argentina).