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Reconstructing dinosaur/bird locomotor evolution: 3-D track simulation and X-ray validation

Final Report Summary - TRACKEVOL (Reconstructing dinosaur/bird locomotor evolution: 3-D track simulation and X-ray validation.)

Introduction
A fossil track represents the only direct record of limb motion of extinct organisms, being the result of an interaction between an animal’s foot and a substrate. In order to fully interpret tracks and reconstruct limb kinematics and kinetics, tracks must be considered as fully three dimensional structures extending beneath the sediment surface. Additionally, an understanding is required of time-dependant track formation in order to link specific morphological features of the track with particular motions of the interacting limb. This novel study aims to investigate track formation in real time, and in three dimensions - including beneath the sediment surface, by employing state-of-the-art methods such as real-time three dimensional X-ray reconstructions, and high performance computer modelling. The principal scientific aim of the proposed research is to develop and employ unique techniques for simulating track formation, which can elucidate real-time sub-surface deformation, and to meld the new techniques with current osteology-based locomotor reconstructions and experimental data on locomotor dynamics in extant birds.

Results
Guineafowl were imaged as they traversed compliant substrates, including poppy seeds and mud of varying consistency. Using X-ray Reconstruction of Moving Morphology (XROMM, see www.xromm.org) the motions of the distal limb were reconstructed in 3-D, enabling for the first time visualization of foot movements beneath the sediment surface (Figure 1). The captured 3-D data can be viewed in real time, from any angle, allowing an unprecedented look at how the foot reacts to a compliant substrate as it sinks. Having recorded the 3-D motions of the foot, direct correlations between motions of the foot and toes could be linked with features of the surface track, captured in 3D using photogrammetric techniques (Figure 1B).

Figure 1 - Sub-surface 3-D motions of the feet of guineafowl were recorded using X-rays, as the birds traversed a loose substrate and left footprints. A, B – the locations of cameras and track surface within the X-rays. C, rotoscoped bones over X—ray images. D, Comparison of kinematics of birds walking on a solid surface or a compliant substrate. Figure from Falkingham and Gatesy (2014).

In order to understand track formation, the movement of the sediment relative to the foot is required. The sheer number of particles in the sediment precludes it from being imaged directly. Instead, a computer simulation using the Discrete Element Method was developed to simulate grain-grain and grain-foot interactions. The simulation was validated against physical test cases to ensure the behaviour of the virtual sediment matched the behaviour of the real sediments. Once validated, the motions of the guineafowl limb were transferred into the simulation, and a virtual footprint generated.
The virtual footprint could be exposed at any level within the sediment volume, allowing for direct comparison with fossil dinosaur tracks that have been exposed along natural breaks in the rock (bedding planes and laminations).
Dinosaur tracks held at the Beneski Museum of Natural History, Amherst, MA (USA) offer a rare glimpse into the 3-D volumetric nature of fossilised tracks. A number of these specimens were digitized using photogrammetric techniques for comparison with the simulated guineafowl tracks. By identifying common features between fossil and simulated tracks, the simulation could be ‘rewound’ and the features observed as they formed through the interaction of the foot and the sediment, providing insight into how previously enigmatic and unidentified structures in the fossil dinosaur tracks were made.
The ability to view a track being produced both volumetrically and dynamically led to the concept of ‘track ontogeny’ – the development of a footprint throughout the step cycle (Figure 2).

Figure 2- Track ontogeny. A single footprint observed through time and depth, allows us to identify previously enigmatic structures seen in fossil footprints.

Conclusions
• Significant and complex movements of the foot occur beneath the sediment surface. We were able to visualize and document these movements for the first time.
• Comparing the fossil tracks with the simulated guineafowl tracks enabled identification of previously enigmatic features in fossil footprints, particularly associated with overlapping entry/exit deformation.
• The development of the ‘track ontogeny’ concept provides a new, dynamic way of understanding dinosaur footprints that is in contrast to the prevailing static view of tracks as merely a print of the foot. This concept will help in reverse engineering fossil footprints in order to elucidate limb kinematics of extinct animals such as Dinosaurs.


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