Gregariousness among non-avian theropods: a case study from a Patagonian dinosaur tracksite, Candeleros Formation (Cenomanian), Argentina

Abstract

Gregarious behaviour is an important aspect of dinosaur paleoecology and has been inferred from several evidences: monospecific bonebeds, skeletal morphology, phylogenetic inferences, comparison with modern ecosystems and parallel trackways. Nevertheless, it is relatively poorly documented and understood among non-avian theropods. In the Cenomanian Candeleros Formation (northwestern Patagonia) several parallel tridactyl trackways were recorded, including five theropod trackways of medium track size (average length of 28 cm). These trackways possess similar stride, speed estimation, preservational features, uniform depth, and the same direction as non-overlapping tracks. They are preserved as shallow under tracks in a medium-grained sandstone bed interpreted as floodplain deposits. The tracking surface would have been an immediately overlying thin layer of muddy-siltstone containing a minor percentage ofexpandable clays (smectite). The latter gave a certain degree of coherence and plasticity to the substrate, sufficient to preserve several anatomical track features. Considering the theropod taxa documented in the same unit as well asthe size and track morphology, abelisaurid or carcharodontosaurid theropods are suggested as trackmakers. Physical barriers (i.e., large fluvial channels or perennial lakes) that could control the moving direction of the theropods were not recognized in the track-bearing level, nor were revealed by the analysed interval. Taking into account that there are no evidences of physical constraints and that all parallel trackways possess similar features such as stride, speed, preservation, depth, and direction, a gregarious behaviour of their producers can be suggested.