TAPHONOMIC MODES IN A TRIASSIC-JURASSIC LOESSITE FROM PATAGONIA, ARGENTINA

MANCUSO, ADRIANA CECILIA; SMITH, ROGER; POL, DIEGO; MARSICANO, CLAUDIA

Mendoza

Congreso; 4° International Palaentontological Congress; 2014

IPA

Bones of the basal sauropodomorph Mussaurus patagonicus are found in three successive loessite horizons within the Late Triassic-Jurassic high latitude El Tranquilo Basin in southern Argentina. The Laguna Colorada Formation (El Tranquilo Group) consists of reddish to brownish siltstones and claystones, fine to medium- grained sandstones and subordinate thin conglomerates deposited by a fluvial system with moderate sinuosity channels. The faunal remains are concentrated in a 3-meter thick interval of structureless mottled light reddish- brown/olive-grey massive siltstone with scattered small mudrock pebbles which we interpret as floodplain loess. The Mussaurus specimens range from neonates to adults, which can be histologically divided in six different ontogenetic stages. An unusual aggregation of at least 11 similar-sized juvenile Mussaurus patagonicus includes partially and fully-articulated individuals. Some of these skeletons retain a life-like crouched position with flexed limbs, whereas other have articulated limbs that penetrate up to 13cm into the underlying strata. Taphonomic evidence suggests these are the result of synchronous death and burial of behaviourally aggregated individuals that were all less than one year old (based on their long bone histology). The partially articulated carcasses are mainly juvenile and subadult ontogenetic stages. The bones are generally un-weathered (weathering stages 0 to 2) with low incidence of post-mortem modification such as abrasion, cracking, or breakage. All the taphonomic features documented in these carcasses suggest short post mortem/pre burial periods. In contrast, the occurrences of disarticulated but associated skeletons, disarticulated and dispersed skeletons, or isolated bones, are of mainly subadult and adult individuals. These bones display a large range of post-mortem modifications: 0 to 4 pre-burial weathering stages, very low to high abrasion, cracking and rounding, and transverse and/or irregular breakage. The main diagenetic features are the growth of calcareous nodules and discoloration of the matrix around the bones. The taphonomic signature of the Laguna Colarada Mussaurus fossils indicates a mixture of behavioural aggregations with rapid burial events interspersed with long periods of pre-burial exposure that weathered the exposed portions of embedded carcasses. We characterize this as an attritional bone accumulation from a stable sauropodomorph population living all year-round (non-migratory) on semi-arid sub-polar floodplains prone to seasonal drying and aeolian dust storms.