A biplanar X-ray method for three-dimensional analysis of track formation

Abstract

Ellis, Richard G. and Gatesy, Stephen M. 2013. A biplanar X-ray method for three-dimensional analysis of track formation, Palaeontologia Electronica Vol. 16, Issue 1; 1T, 16p; palaeo-electronica.org/content/2013/371-x-ray-track-analysis A biplanar X-ray method for three-dimensional analysis of track formation Richard G. Ellis and Stephen M. Gatesy ABSTRACTTracks arise through a complex interplay between animal and substrate. Studying this dynamic process is challenging because most foot-sediment and sediment-sediment interactions are rapid and hidden from view. Herein, we describe a new method for visualizing and quantifying three-dimensional movements of both a morphologically accurate indenter and realistic sediment during track formation. Our method uses biplanar X-ray imaging and an animation-based workflow to reconstruct the trajectories of metal beads seeded throughout the sediment volume. X-rays allow sub-surface motion normally concealed by the foot and opaque matrix to be analyzed at 30 frames per second with sub-millimeter resolution. Results from two case studies of tridactyl tracks in semi-liquid mud provide novel, animated visualizations, examples of ensemble and particle-specific data, as well as measures of precision and accuracy. This methodology has the potential to mechanistically link specific track morphologies to foot movement, clarify undertrack formation, validate computational models, and set a new standard for evidence-based reconstruction of locomotion from fossil footprints.

It's in the loop: shared sub-surface foot kinematics in birds and other dinosaurs shed light on a new dimension of fossil track diversity

“…Herein, we quantify the three-dimensional (3-D) foot movements of a chicken-like bird (guineafowl) walking through a spectrum of deformable substrates. Following earlier studies of burrowing [39][40][41][42][43][44][45] and stepping [17,[46][47][48], we use X-ray imaging to see through opaque ground.…”

Interpreting ecology and behaviour from the vertebrate fossil track record

“…This is important because apparent track morphology changes within the volume, and so interpretations based on misidentified surfaces can be flawed. Many experimental studies have focused on this difficulty in considering tracks as 3D volumes, and have presented numerous methods for ‘seeing’ beneath the foot‐sediment interface including using plaster or cement between friable layers (Manning, ; Milàn & Bromley, ), coloured plasticine (Allen, ), biplaner X‐rays (Ellis & Gatesy, ) and computer simulation (Falkingham et al ., ; Falkingham, Margetts & Manning, 2010 b ; Falkingham et al ., 2011 a,b ). Even observing or defining the foot‐sediment interface can be difficult if the sediment has sealed upon removal of the trackmaker's foot; the interface or direct track sensu Gatesy (), will then exist within the volume and is unlikely to be exposed at any natural break.…”

Mitigating the Goldilocks effect: the effects of different substrate models on track formation potential

“…Plasticine [ 5 , 6 ], plaster of paris [ 7 , 8 ] and coloured cement [ 9 ] have been used to produce tracks which can be sectioned and in which subsurface deformation can be observed. Unfortunately, maintaining strict control over the mechanical properties of the substrate can be difficult in a physical experiment, and visualizing the interior of the track is a destructive process which only shows the final morphology and therefore lacks a temporal component (with the notable exception of the X-ray study described by Ellis & Gatesy [ 10 ]).…”