A biplanar X-ray method for three-dimensional analysis of track formation

Tracks arise through a complex interplay between animal and substrate. Studying this dynamic process is challenging because most foot-sediment and sediment-sediment interactions are rapid and hidden from view. Herein, we describe a new method for visualizing and quantifying three-dimensional movements of both a morphologically accurate indenter and realistic sediment during track formation. Our method uses biplanar X-ray imaging and an animation-based workflow to reconstruct the trajectories of metal beads seeded throughout the sediment volume. X-rays allow sub-surface motion normally concealed by the foot and opaque matrix to be analyzed at 30 frames per second with sub-millimeter resolution. Results from two case studies of tridactyl tracks in semi-liquid mud provide novel, animated visualizations, examples of ensemble and particle-specific data, as well as measures of precision and accuracy. This methodology has the potential to mechanistically link specific track morphologies to foot movement, clarify undertrack formation, validate computational models, and set a new standard for evidence-based reconstruction of locomotion from fossil footprints.