Meeting Abstract
A footprint is a record of the complex and dynamic interaction between foot and ground. Both fossil dinosaurian trackways and extant avian experiments have revealed track morphology can vary dramatically from a single individual through different substrates. Substrate consistency affects foot kinematics (motion): most apparent is sinking depth, but other variables such as timing and duration of weight-bearing pedal support are also affected. To explain fossil footprint diversity, how motion varies among and within substrates needs to be understood. Even in living animals, however, documenting foot motion within the substrate is difficult because the distal limb is hidden by opaque sediment. Using marker-based XROMM, we recorded and analyzed 162 steps from 82 trials of three Helmeted Guineafowl (Numida meleagris) walking through radiolucent artificial substrates of different consistency (solid, dry granular, firm mud, sloppy mud). Our initial efforts have focused on tracking the path of the tip of digit III, a highly identifiable landmark in many dinosaur tracks, above and below the ground. When viewed only as the tip of digit III, stance on deformable substrates traces a path in the form of a single loop. The geometry of these loops is formed by sinking entry and pull-out exit motions of the whole foot, and is highly variable both across and within substrate types. Importantly, however, kinematic events (eg. first and last contact of opposing foot) contribute to the toe path loop geometry of a single step. Current work is focusing on resolving the relationship between measurable track geometry and kinematics from which the track was formed. This work suggests new methods of extracting depth context, substrate conditions, and kinematic data from dinosaurian tracks.
