The birth of a dinosaur footprint: Subsurface 3D motion reconstruction and discrete element simulation reveal track ontogeny

Abstract

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,...

Dinosaur tracks from the Langenberg Quarry (Late Jurassic, Germany) reconstructed with historical photogrammetry: Evidence for large theropods soon after insular dwarfism

“…Undertrack formation, however, is restricted to layered sediment that differs only slightly in composition (Thulborn, 1990), and undertracks are not as common as previously thought ). The Langenberg footprints are not regarded as undertracks but true tracks (i.e., tracks left on the tracking surface) or, alternatively, underprints (i.e., deep tracks left below the tracking surface due to penetration of the foot through the sediment (e.g., Thulborn, 1990;Falkingham and Gatesy, 2014)), based on the following evidence. First, the sedimentary conditions are not favorable for undertrack formation.…”

Validation of XMALab software for marker-based XROMM

“…The result is a precise and accurate XROMM animation of 3D bone meshes moving in 3D space (Brainerd et al, 2010;Gatesy et al, 2010). Over the past 8 years, researchers have used XROMM to study in vivo skeletal motion in numerous behaviors and species including: terrestrial locomotion of alligators , dogs (Wachs et al, 2016), rats (Bonnan et al, 2016) and birds (Kambic et al, 2014(Kambic et al, , 2015; arboreal locomotion of sloths (Nyakatura and Fischer, 2010); jumping in frogs (Astley and Roberts, 2012) and humans (Miranda et al, 2013); feeding in pigs (Menegaz et al, 2015), ducks (Dawson et al, 2011), geckos (Montuelle and Williams, 2015) and fish (Camp and Brainerd, 2015;Gidmark et al, 2012); flight in bats (Konow et al, 2015) and birds Heers et al, 2016;Heers and Dial, 2012); lung ventilation in iguanas ; and trackway formation in theropod dinosaurs (Falkingham and Gatesy, 2014). Methods include marker-based XROMM, in which radio-opaque markers are surgically implanted into skeletal elements (Brainerd et al, 2010;Tashman and Anderst, 2003), and markerless XROMM, which includes manual scientific rotoscoping and semi-automated bone model registration methods (Banks and Hodge, 1996;Miranda et al, 2011;You et al, 2001).…”

First Early Jurassic Small Ornithischian Tracks From Yunnan Province, Southwestern China

“…5A), and an inverted 'v' shape at the posterior end, a morphology highly similar to theropod tracks made in soft mud described by Gatesy et al (1999, fig. 1c, d), and by Falkingham and Gatesy (2014, fig. 4) in simulated dry granular media.…”