Penetrative track morphology and sediment parameters: subsurface layers are robust to changes in substrate properties

Track morphology is controlled by the anatomy and motion of the foot, as well as by the consistency of the sediment. Penetrative tracks preserve information regarding the motion of the foot due to increased foot-sediment interaction. The track morphology in subsurface layers of a penetrative track is important for recovering these preserved motions. Using discrete element method simulations, we examined how subsurface track morphology varies with sediment consistency. Two models were used: a vertically indenting cylinder and a dynamic, simplified theropod-like 'tridactyl foot'. For both cases, we systematically changed friction and cohesion and examined changes to subsurface track morphology. Increasing friction (lower 'flowability') resulted in higher and more extensive displacement rims throughout the track volume. Increased cohesion led to tracks remaining open, preventing infilling from above. Low cohesion led to surface tracks deviating substantially from the morphology of the foot, and in cases of very low cohesion, appearing deceptively like weathered tracks. Tracks exposed along subsurface layers had morphologies more robust to changes in sediment properties than the surface tracks. Thus, changes in sediment behaviour significantly affect surface, but not subsurface, track morphology. Our findings support the reliability of motion reconstructions from penetrative tracks, regardless of sediment conditions at the time of formation.