Department of Ecology and Evolutionary Biology, Division of Biology and Medicine, Brown University, Providence, RI 02912, USA.
Department of Computer Science and Engineering, University of Minnesota, Minneapolis, MN 55455, USA.
J Exp Biol. 2021 Jun 1;224(11). doi: 10.1242/jeb.242240. Epub 2021 Jun 4.
Feet must mediate substrate interactions across an animal's entire range of limb poses used in life. Metatarsals, the 'bones of the sole', are the dominant pedal skeletal elements for most tetrapods. In plantigrade species that walk on the entirety of their sole, such as living crocodylians, intermetatarsal mobility offers the potential for a continuum of reconfiguration within the foot itself. Alligator hindlimbs are capable of postural extremes from a belly sprawl to a high walk to sharp turns - how does the foot morphology dynamically accommodate these diverse demands? We implemented a hybrid combination of marker-based and markerless X-ray reconstruction of moving morphology (XROMM) to measure 3D metatarsal kinematics in three juvenile American alligators (Alligator mississippiensis) across their locomotor and maneuvering repertoire on a motorized treadmill and flat-surfaced arena. We found that alligators adaptively conformed their metatarsals to the ground, maintaining plantigrade contact throughout a spectrum of limb placements with non-planar feet. Deformation of the metatarsus as a whole occurred through variable abduction (twofold range of spread) and differential metatarsal pitching (45 deg arc of skew). Internally, metatarsals also underwent up to 65 deg of long-axis rotation. Such reorientation, which correlated with skew, was constrained by the overlapping arrangement of the obliquely expanded metatarsal bases. Such a proximally overlapping metatarsal morphology is shared by fossil archosaurs and archosaur relatives. In these extinct taxa, we suggest that intermetatarsal mobility likely played a significant role in maintaining ground contact across plantigrade postural extremes.
足部必须在动物整个肢体姿势范围内介导与底物的相互作用，这些姿势用于生活。跖骨，即“足底的骨头”，是大多数四足动物的主要足部骨骼元素。在跖行类物种中，如现存的鳄目动物，整个足底着地，跖骨之间的活动性为足部本身提供了连续重新配置的潜力。短吻鳄的后肢能够从腹部伸展到高步走再到急转弯等姿势的极端变化——足部形态如何动态适应这些多样化的需求？我们采用基于标记和无标记的 X 射线运动形态重建（XROMM）的混合方法，在电动跑步机和平坦表面场地测量了三只幼年美洲短吻鳄（Alligator mississippiensis）的 3D 跖骨运动学，以覆盖其运动和机动范围内的运动模式。我们发现，短吻鳄适应性地使跖骨与地面接触，在非平面足部的一系列肢体位置中保持跖行接触。整个跖骨的变形是通过可变的外展（两倍的伸展范围）和差异跖骨俯仰（45°的偏斜弧度）来实现的。在内部，跖骨还经历了高达 65°的长轴旋转。这种与偏斜相关的重新定向受到斜向扩展跖骨基部重叠排列的限制。这种近端重叠的跖骨形态与化石恐龙和恐龙近亲共享。在这些已灭绝的分类群中，我们认为跖骨之间的活动性很可能在维持跖行姿势极端的地面接触方面发挥了重要作用。
