Crossing Slopes: Unusual Trackways of Recent Birds and Implications for Tetrapod Footprint Preservation

In a neoichnological study, the trackway of a small recent bird (Limicola- like) crossing the slope of a river bank is documented and analyzed in detail. It was preserved along the Issene River in the western High Atlas of Morocco. Photogrammetry and 3-D techniques revealed three different phases of the trackmaker's movement that were essentially controlled by the biomechanical adaptations of the limbs and by the differences in how the foot was indented from up to down slope on the tilted, mud-cracked surface of the tilted mud-cracked surface. The slope of the surface is reflected in the variation of different track parameters, such as stride length, pace length, pace angulation, and imprint depths of different digits. A “didactyl” or fully collapsed morphology preservation pattern is observed as an intra-trackway morphological variation along the whole trackway. Tracks preserve only the up-slope digits: left tracks typically preserve only digit III and IV impressions, while right tracks preserve only digit III and II impressions. The selective absence of impressions of the other digits is interpreted to be the combined product of a water saturated/damp consistency of the sediment at the time of track formation and of the “down-facing” position of the unimpressed digits the latter reflects the differences in the bearing capacities of the “up-slope” portion of the limb versus the “down-slope” portion. The study may be helpful for understanding fossil counterparts formed on similarly uneven terrain.