Locomotion over deformable substrates is a common occurrence in nature. Footprints represent sedimentary distortions that provide anatomical, functional, and behavioral insights into trackmaker biology. The interpretation of such evidence can be challenging, however, particularly for fossil tracks recovered at bedding planes below the originally exposed surface. Even in living animals, the complex dynamics that give rise to footprint morphology are obscured by both foot and sediment opacity, which conceals animal-substrate and substrate-substrate interactions. We used X-ray reconstruction of moving morphology (XROMM) to image and animate the hind limb skeleton of a chicken-like bird traversing a dry, granular material. Foot movement differed significantly from walking on solid ground; the longest toe penetrated to a depth of ∼5 cm, reaching an angle of 30°below horizontal before slipping backward on withdrawal. The 3D kinematic data were integrated into a validated substrate simulation using the discrete element method (DEM) to create a quantitative model of limb-induced substrate deformation. Simulation revealed that despite sediment collapse yielding poor quality tracks at the airsubstrate interface, subsurface displacements maintain a high level of organization owing to grain-grain support. Splitting the substrate volume along "virtual bedding planes" exposed prints that more closely resembled the foot and could easily be mistaken for shallow tracks. DEM data elucidate how highly localized deformations associated with foot entry and exit generate specific features in the final tracks, a temporal sequence that we term "track ontogeny." This combination of methodologies fosters a synthesis between the surface/layer-based perspective prevalent in paleontology and the particle/volume-based perspective essential for a mechanistic understanding of sediment redistribution during track formation.T errestrial locomotion is vital to the survival of many vertebrate animals, and is expressed in typical behaviors such as food acquisition, predator avoidance, mate finding, and population dispersal. Generalized models of legged movement are typically derived from laboratory studies of walking and running on stiff, solid surfaces; however, locomotion over compliant, yielding substrates is also important, for two reasons. First, animals frequently encounter such terrain-unconsolidated desert sand, river banks, shorelines, snow, or simply soil after rain-in their natural environments. Movement over such deformable substrates is, accordingly, a major research area in biomechanics and robotics (1-3). Second, feet that deform malleable substrates leave tracks. Footprints can be a major source of information about an animal or group of animals (4-6), and this is particularly true for extinct taxa that cannot be observed directly (7-9). Indeed, the only movements that have been recorded in the fossil record were necessarily over/through suitably compliant substrates (10).Despite tracks being so common and holding so much potential,...