TAPHONOMY OF AN ASSEMBLAGE OF JUVENILE PROSAUROPOD DINOSAURS FROM THE LATE TRIASSIC OF PATAGONIA

MANCUSO, ADRIANA CECILIA; POL, DIEGO

Teruel, España

Simposio; 10th MESOZOIC TERRESTRIAL ECOSYSTEMS SYMPOSIUM; 2009

Most of the taphonomic analyses on prosauropod dinosaurs evidence catastrophic death associated with trapping in mud or miring. An assemblage of juvenile specimens of Mussaurus patagonicus from Laguna Colorada Formation displays several features that also suggest miring although other features do not fit this scenario. The assemblage includes at least fourteen similarly-sized young individuals. They are either partially or fully articulated and posterior skeletal elements are more abundant. Some of the skeletons retain a life-like crouched position with flexed limbs, whereas others have articulated limbs that penetrate up to 13cm into the underlying strata. In rare case several articulated skeletons are intertwined. The bonebed deepens in a NNW trend, and the long axis of the concretion is aligned NE/SW. The host sediments are interpreted as having been deposited on a flood-plain with ephemeral ponds. The orientation of the skeletons shows a general E-W trend, with twelve individuals with approximately WSW trend. The bones show evidence of weathering state between 1 and 2, with fossil surfaces showing parallel and sub-parallel cracking. In some cases, the bones located more superficially show features of more exposure than the bones located deeper in the sediments. Colour haloes in the sediment around the bones may relect bacterial action during decomposition of the carcasses. The Laguna Colorada fluvial system was developed under the temperate climate, with a pronounced wet season, that was common in Gondwana during the Triassic. Indicators of miring include: limbs crosscutting the sediment, higher weathering stage in bones located more superficially, possible scavenging of anterior and dorsal parts of carcasses. However, features such as opposite inclination of concretion and the deepening trend, the limbs with life-like crouched position, the loss of parts of bodies of individuals located in lower levels, and the intertwining disposition are difficult to explain in a miring scenario. Drought is a possible cause that generates the mud-tramped surface within the flood-plain. Moreover, this condition may be enhanced by volcanism, which was abundant in this area and time. Thus, some sedimentological and taphonomical evidence supports a miring scenario for this unusual group of juvenile prosauropod dinosaurs but more evidence is needed to be conclusive.