First evidence of aggregational behaviour by the archosauromorph Hyperodapedon sanjuanensis from the Upper Triassic Ischigualasto Formation, Argentina: Evidence for burrow habitats?

Abstract

Ethological evidence in fossil vertebrates is rarely preserved in the geological record. However, the remarkable preservation of vertebrates in the Upper Triassic Ischigualasto Formation, northwestern Argentina, provides a unique opportunity to explore behavioural patterns. This study examines four juvenile specimens of the archosauromorph Hyperodapedon sanjuanensis that provide significant taphonomic insights. The assemblage is characterized by the close spatial proximity of four well-preserved, nearly complete, articulated skeletons of similar size, arranged within a confined lateral space, oriented obliquely to bedding. No evidence of weathering or large predator scavenging was observed, although some minor bone displacements indicate necrophagous activity by small animals. Multiple taphonomic scenarios were evaluated to account for this exceptional preservation; the most plausible suggests that these individuals died while aggregated, possibly within a shared structure such as a burrow, suggesting advanced social behaviour. This behaviour may reflect an adaptation to extreme seasonal climates of the Late Triassic. It could be linked to the large number of these specimens preserved in the Ischigualasto Formation and the extinction of this taxon around 229 Ma.