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.

地面鸟类的脚保留了恐龙祖先的许多特征。对活物种的实验提供了对中生代足迹形成过程中解剖学、运动学和基质之间复杂相互作用的见解。然而，赛道制作过程的一个关键方面，即地下足部运动，受到基材不透明度的阻碍。在这里，我们使用双平面 X 射线对珍珠鸡走过不同稠度的射线可透基质（固体、干燥颗粒、坚硬到半液体泥浆）进行成像。尽管运动变化很大，但脚在地面下始终以循环模式移动。当脚下沉然后缩回时，三个主脚趾的爪子在不同的位置形成进入和退出路径。在增量地平线上对这些路径进行采样可以捕获二维特征，就像化石足迹一样，允许通过数字印象的存在和相对位置来表征基于深度的区域。对侏罗纪早期的深层、穿透性痕迹的检查证实，两足动物对大约 200 Ma 的软质基质具有等效的循环反应。我们对现存和灭绝证据的整合证明了基底特性对下沉深度和地下足部运动的影响，这两者都是恐龙化石记录中轨迹变化的重要来源。