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

Abstract

The feet of ground-dwelling birds retain many features of their dinosaurian ancestry. Experiments with living species offer insights into the complex interplay among anatomy, kinematics and substrate during the formation of Mesozoic footprints. However, a key aspect of the track-making process, sub-surface foot movement, is hindered by substrate opacity. Here, we use biplanar X-rays to image guineafowl walking through radiolucent substrates of different consistency (solid, dry granular, firm to semi-liquid muds). Despite substantial kinematic variation, the foot consistently moves in a looping pattern below ground. As the foot sinks and then withdraws, the claws of the three main toes create entry and exit paths in different locations. Sampling these paths at incremental horizons captures two-dimensional features just as fossil tracks do, allowing depth-based zones to be characterized by the presence and relative position of digit impressions. Examination of deep, penetrative tracks from the Early Jurassic confirms that bipeds had an equivalent looping response to soft substrates approximately 200 Ma. Our integration of extant and extinct evidence demonstrates the influence of substrate properties on sinking depth and sub-surface foot motion, both of which are significant sources of track variation in the fossil record of dinosaurs.

Constructing and testing hypotheses of dinosaur foot motions from fossil tracks using digitization and simulation

“…In compliant substrates, the guineafowl exhibit a looping motion of the foot (Turner et al . 2020) where toe tips initially sink into the sediment before being pulled backward as the foot withdraws (Fig. 5).…”

“…Our previous work has focused on deep tracks made by birds traversing extremely compliant substrates (Falkingham & Gatesy 2014, in press; Gatesy & Falkingham 2017, in press; Turner et al . 2020). By employing X‐ray Reconstruction of Moving Morphology, or XROMM (Brainerd et al .…”

Alligators employ intermetatarsal reconfiguration to modulate plantigrade ground contact

“…XROMM sampling of highly variable locomotor behaviors has revealed previously unseen patterns of hindlimb function (e.g. Kambic et al, 2015 ; Turner et al, 2020 ) critical to the interpretation of avian functional morphology. A much larger range of foot placement extremes is found in plantigrade quadrupedal taxa, and thus are critical to incorporate into the study of the locomotor system.…”

“…However, these studies were limited by low frame rate, an absence of markers, an inability to reconstruct long-axis rotation (LAR), and no treatment of variability in metatarsal motion. Advances in biplanar X-ray videography have enabled high-resolution 3D skeletal movement to be seen and measured inside the foot of avian ( Falkingham and Gatesy, 2014 ; Turner et al, 2020 ) and human ( Kessler et al, 2019 ; Maharaj et al, 2020 ) bipeds. The implants necessary for marker-based analysis is challenging in animals with mobile metatarsals, as they are surrounded by many small muscles, nerves and vessels that require careful surgical planning to avoid.…”

The Influence of Snow Properties on Speed and Gait Choice in the Svalbard Rock Ptarmigan (Lagopus muta hyperborea)

“…The snowshoe foot acts as a paddle pushing against the snow for propulsion ( Li et al 2012 ), evenly distributing the pressure applied to the substrate. Such process may be reflected in the amount of foot subsurface rotation required to transverse through compliant media ( Turner et al 2020 ). Notably, in other species including the Adélie penguins ( Pygoscelis adeliae ) ( Wilson et al 1991 ) and the Nearctic river otter ( Lontra canadensis ) ( Sadie and Thomas 2005 ), a common adaptation for moving over snow is switching to a “toboggan” gait, to spread their body weight more evenly when the snow is nonsupportive and deep.…”