Taphonomy and early diagenesis of a semi-articulated dinosaur tail from the late maastrichtian of southwestern Europe

Articulated dinosaur remains are extremely rare in Upper Cretaceous alluvial deposits of southwestern Europe, limiting detailed reconstructions of post-mortem processes in these settings. We present a taphonomic, sedimentological, and early diagenetic analysis of a semi-articulated mid-caudal segment of a lambeosaurine hadrosaurid, recovered as a detached sandstone block from the Planta del Mestre locality (Late Maastrichtian Tremp Basin, South-Central Pyrenees). The specimen comprises eleven middle caudal vertebrae and associated neural spines and four haemal arches preserved in close anatomical association. Semi-articulated arrangement, lack of abrasion, and mineralized intervertebral fibrocartilage indicate short residence time following minimal transport. Stage 2 subaerial weathering on centra and stepped displacement of distal elements reflect differential exposure during partial entombment and post-depositional settling. Parallel grooves on neural and haemal spines record early biological interaction by a medium-sized ziphodont theropod, likely an abelisaurid, during initial decay stages. Petrographic analysis reveals early ferruginous impregnation under oxidizing conditions promoted sediment stabilization and sealing bone pores, preserving delicate structures against decay. Associated planktonic foraminifera suggest episodic marine influence within the floodplain. These results highlight the role of localized geochemical pathways in generating high-fidelity preservation within otherwise unfavourable floodplain taphonomic settings.