Taphonomy of the basal Ischigualasto Formation (Upper Triassic, Argentina).

COLOMBI, CARINA

Oklahoma, Estados Unidos

Congreso; Sixty second Annual Meeting - Society of Vertebrate Paleontology; 2002

Society of Vertebrate Paleontology

The Ischigualasto Formation (228 Ma) consists of a sequence of fluvial sediments that preserves a rich vertebrate paleofauna, especially in the basal portion of the unit. This paleofauna includes a diverse association of therapsids, rhynchosaurs and archosaurs, including the oldest known record of dinosaurs. Despite great significance from an evolutionary perspective, the taphonomic relation ships of the paleofauna are still poorly known. Four main taphofacies are proposed for the basal portion of the Ischigualasto Formation: L1 lenticular medium- to coarse-grained sandstone with massive and stratified structure; L2 tabular medium-grained sandstone with laminar and massive structure; L3 mudstone with vertical accretion; L4 mudstone with evidence of lateral accretion. The L1 and L4 lithofacies are interpreted to represent channel deposits with different sedimentary loads. The L2 lithofacies is interpreted to represent levee and crevasse splay deposits. The L3 lithofacies is interpreted to represent deposits of the floodplain. The majority of the fossils in the Ischigualasto Formation are in the lithofacies L2 and L3. State of preservation vary, and are here relegated to three main taphofacies: T1 articulated skeleton associated with isolated bones. Bone display excellent preservation, with no evidence of pre-burial damage. Fossils are preserved in calcic-vertisols. These characteristics are interpreted to reflect attritional deaths and mummification of the carcasses on a dry floodplain. T2 disarticulated skeleton with advanced indication of weathering , embedded in sandy carbonated concretions. This taphofacie is interpreted to represent attritional mortality with prolonged weathering on a dry floodplain. Burial presumably occurred in distal overbank deposits. T3 typically isolated bones with hematite crusts, characterized by minimal weathering. The hematite-coated fossils were likely exposed as carcasses on a dry floodplain, rapidly dispersed by necrophagous agents, and then buried in a short period of time.