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Birth of a dinosaur footprint

Ben Creisler

A new online paper:

Peter L. Falkingham and Stephen M. Gatesy (2014)
The birth of a dinosaur footprint: Subsurface 3D motion reconstruction
and discrete element simulation reveal track ontogeny.
Proceedings of the National Academy of Sciences (advance online publication)
doi: 10.1073/pnas.1416252111


We reconstructed the 3D foot movements of guineafowl traversing a
granular substrate from biplanar X-rays, and then incorporated those
kinematics into a discrete element simulation. Digital track models
permitted visualization of in vivo track formation at the surface and
at virtual bedding planes for the first time. Application of these
volumetric data to fossil dinosaur tracks uncovered the developmental
origin of previously enigmatic features. A “track ontogeny”
perspective helps integrate limb and substrate dynamics into the
interpretation of track morphology, from which foot anatomy cannot be
read directly.


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.

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