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 air-substrate 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.

Keywords: XROMM; dinosaur; discrete element method; footprint; locomotion.

Fig. 1.

XROMM analysis of guineafowl limb movement through a compliant substrate (poppy seeds). (A) End view of the sediment trackway, showing the volume covered by the two X-ray beams (blue and yellow), and the two calibrated light cameras (red and green). The intersection of the X-ray beams continues below the sediment surface. (B) Perspective view of the Maya scene showing the four image planes, the reconstructed skeletal model, and the photogrammetric model of the tracks. (C) Frame of the X-ray video showing subsurface imaging and the registered bone models. (D) Comparison of steps on solid (Left) and dry, granular (Right) substrates for the same individual. (Scale bars: 20 cm in A; 5 cm in D.)

Fig. 2.

Simulation of a guineafowl track. (A and B) Sequence of three video frames with registered bone models (A) and their virtual counterparts (B). (C) Colored height map of real tracked surface, indicating the location of the foot at touchdown. (D) Photograph of the track analyzed (white box in C). (E) Height map of the discrete element model, simulated from the kinematics associated with the track shown in D (white box in C). (Scale bar: 5 cm.)

Fig. 3.

Track ontogeny. Simulated track using the motions of guineafowl traversing poppy seeds as part of a discrete element simulation. Each virtual bedding plane within the sediment volume is exposed by reducing the opacity of initially overlying grains. (Scale bar: 10 cm.)

Fig. 4.

(A) Fossil dinosaur track from the Beneski Museum of Natural History, Amherst College (specimen no. ACM-ICH 37/24; Lower Jurassic). (B) Simulated track exposed at 2 cm below the original sediment–air interface. Both tracks display rounded features associated with the withdrawal of the foot, and a sinuosity to the impression of digit III. (Scale bar: 3 cm.)

Fig. 5.

Simulated track using kinematics captured from a guineafowl walking on a solid surface. (A) Perspective view late in withdrawal. (B) Top view of the track surface. (Scale bar: 2 cm.)