Falkingham Peter L, Gatesy Stephen M
Structure and Motion Laboratory, Department of Comparative Biomedical Sciences, Royal Veterinary College, Hatfield AL97TA, United Kingdom; and Department of Ecology and Evolutionary Biology, Brown University, Providence, RI 02912
Department of Ecology and Evolutionary Biology, Brown University, Providence, RI 02912
Proc Natl Acad Sci U S A. 2014 Dec 23;111(51):18279-84. doi: 10.1073/pnas.1416252111. Epub 2014 Dec 8.
Locomotion over deformable substrates is a common occurrence in nature. Footprints represent sedimentary distortions that provide anatomical, functional, and behavioral insights into trackmaker biology. The interpretation of such evidence can be challenging, however, particularly for fossil tracks recovered at bedding planes below the originally exposed surface. Even in living animals, the complex dynamics that give rise to footprint morphology are obscured by both foot and sediment opacity, which conceals animal-substrate and substrate-substrate interactions. We used X-ray reconstruction of moving morphology (XROMM) to image and animate the hind limb skeleton of a chicken-like bird traversing a dry, granular material. Foot movement differed significantly from walking on solid ground; the longest toe penetrated to a depth of ∼5 cm, reaching an angle of 30° below horizontal before slipping backward on withdrawal. The 3D kinematic data were integrated into a validated substrate simulation using the discrete element method (DEM) to create a quantitative model of limb-induced substrate deformation. Simulation revealed that despite sediment collapse yielding poor quality tracks at the air-substrate interface, subsurface displacements maintain a high level of organization owing to grain-grain support. Splitting the substrate volume along "virtual bedding planes" exposed prints that more closely resembled the foot and could easily be mistaken for shallow tracks. DEM data elucidate how highly localized deformations associated with foot entry and exit generate specific features in the final tracks, a temporal sequence that we term "track ontogeny." This combination of methodologies fosters a synthesis between the surface/layer-based perspective prevalent in paleontology and the particle/volume-based perspective essential for a mechanistic understanding of sediment redistribution during track formation.
在可变形基质上移动是自然界中常见的现象。足迹代表沉积变形，能为造迹生物的解剖结构、功能和行为提供见解。然而，对这类证据的解读可能具有挑战性，特别是对于在原始暴露面以下的层面上发现的化石足迹。即使在活体动物中，导致足迹形态的复杂动力学过程也因足部和沉积物的不透明度而变得模糊，这掩盖了动物与基质以及基质与基质之间的相互作用。我们使用运动形态的X射线重建（XROMM）技术对一只类似鸡的鸟类在后肢骨骼穿过干燥颗粒物质时进行成像和动画制作。足部运动与在坚实地面上行走有显著不同；最长的脚趾穿透到约5厘米的深度，在向后缩回之前达到低于水平方向30°的角度。利用离散元方法（DEM）将三维运动学数据整合到经过验证的基质模拟中，以创建肢体引起的基质变形的定量模型。模拟结果表明，尽管在空气 - 基质界面处沉积物坍塌导致足迹质量较差，但由于颗粒间的支撑作用，地下位移仍保持高度的组织性。沿着“虚拟层面”分割基质体积会暴露出更类似于足部的印记，这些印记很容易被误认为是浅足迹。DEM数据阐明了与足部进入和退出相关的高度局部化变形如何在最终足迹中产生特定特征，我们将这个时间序列称为“足迹个体发育”。这种方法的结合促进了古生物学中普遍存在的基于表面/层面的观点与对足迹形成过程中沉积物重新分布进行机械理解所必需的基于颗粒/体积的观点之间的综合。
