Taphonomy of the titanosaur Aeolosaurus colhuehuapensis, Upper Cretaceous, central Patagonia, Argentina: an example of preservation in overbank fluvial facies

Rests of Aeolosaurus colhuehuapensis, a titanosaur from the Colhue Huapi lake, in the southern Chubut Province, Argentina, were analyzed. The materials under study include twenty one caudal vertebrae and seven haemal arches. They were recovered articulated in overbank facies related to deposits of multi-channel fluvial systems of high sinuosity in the informally called "strata of the Colhue Huapi lake" of Campanian-Maastricthian age. These systems displayed important seasonal fluctuations in the paleodischarge and they were encompassed in a semi-arid climate. The taphonomic history was inferred from the analysis of several taphonomic characteristics present in the bones and macro- and micro-scopic sedimentologic observations. The presence of tenuous longitudinal striation, the absence of bone exfoliation, the articulate condition of the materials and the dorsal bow of the caudal series, support a short period of subaerial exposure with a rapid burial episode. The absence of abrasion marks in the materials indicates that they had scarce or null transport; therefore, they could be considered as autochthonous. The dorsal bow of the tail shows an opisthotonus posture, also exhibited by some theropod and sauropod dinosaurs, but undocumented in the Chubut Group. The recent fluctuant climatic conditions, with variations of extreme humidity and drought, strongly affected the preservation of the materials. The present work is the first detailed taphonomic study of a dinosaur preserved in proximal floodplain facies for the Chubut Group. Finally, the preservation of sauropod bones in those facies is relevant because this type of sub-environment possess a high rate of sediment input as well as scarce or null re-working of the skeletal remains, which favor the conservation of articulated skeletal elements. These findings are very important for the taphonomic and phylogenetic analysis of the titanosaur clade, in which the majority of the taxa are represented by isolated and unarticulated remains.