Digging Sauropods: excavation behaviour or evidence for nest building? An example from the Late Cretaceous El Molino Formation (Maragua, Bolivia)

The Late Cretaceous deposits of Bolivia contain a diverse record of fossil vertebrate tracks. Extensive surfaces within the El Molino Formation (Maastrichtian) preserve evidence of diverse animal behaviour. Here, we report an unusual sedimentary structure from a titanosaurid track-bearing surface at the Mullurina Mayu locality (Maragua, Dep. Chuquisaca). The structure occurs within a massive claystone bed, interpreted as an overbank deposit, based on associated sedimentary structures and overlying units (i.e., desiccation cracks, caliche-bearing marls). The structure is a bowl-shaped depression, longer than wide (4.2 × 3.2 m), deeper internal basin (maximum depth of 60 cm), with a sharp coherent rim and an irregular margin. Along the basin walls, groups of parallel linear grooves (1 cm wide, interspace ~1.8 cm, length ~30 cm) occur within semi-circular depressions, showing two dominant orientations (NE-SW and ESE-WNW). Isolate broader scratches, interpreted as claw traces, occur on the basin floor. Several hypotheses are considered for the structure’s formation, including scouring, weathering, to animal excavation, and nest construction. The absence of scour marks, tool marks, or weathering textures, combined with syn-sedimentary nature and distribution of the scratch-like traces, support a biogenic origin. The bundled grooves are interpreted as possible skin-slip striations produced by repeated foot movement in the soft sediment during excavation. Given the close association with titanosaurid trackways, we tentatively consider this as evidence of their digging and possible abandoned nest scrape.