Net cost of locomotion calculations from Effects of body plan evolution on the hydrodynamic drag and energy requirements of swimming in ichthyosaurs.

Ichthyosaurs are an extinct group of fully marine tetrapods that were well adapted to aquatic locomotion. During their approximately 160 Myr existence, they evolved from elongate and serpentine forms into stockier, fish-like animals, convergent with sharks and dolphins. Here, we use computational fluid dynamics (CFD) to quantify the impact of this transition on the energy demands of ichthyosaur swimming, to our knowledge for the first time. We run computational simulations of water flow using three-dimensional digital models of nine ichthyosaurs and an extant functional analogue, a bottlenose dolphin, providing, to our knowledge, the first quantitative evaluation of ichthyosaur hydrodynamics across phylogeny. Our results show that morphology did not have a major effect on the drag coefficient or the energy cost of steady swimming through geological time. We show that even the early ichthyosaurs produced low levels of drag for a given volume, comparable to those of a modern dolphin, and that deep ‘torpedo-shaped’ bodies did not reduce the cost of locomotion. Our analysis also provides important insight into the choice of scaling parameters for CFD applied to swimming mechanics, and underlines the great influence of body size evolution on ichthyosaur locomotion. A combination of large bodies and efficient swimming modes lowered the cost of steady swimming as ichthyosaurs became increasingly adapted to a pelagic existence.

鱼龙（Ichthyosaur）是一类已灭绝的完全水生四足动物，高度适配水生运动模式。在其约1.6亿年的演化历程中，它们从修长蜿蜒的体型逐步演变为更为粗壮的鱼形动物，演化趋同于鲨鱼与海豚。本研究首次借助计算流体动力学（Computational Fluid Dynamics, CFD），量化了这一体型转变对鱼龙游泳能量需求的影响。我们基于9种鱼龙以及1种现生功能类比类群——宽吻海豚的三维数字化模型开展水流数值模拟，据我们所知，这是首次跨系统发育尺度对鱼龙流体力学特性进行定量评估。研究结果显示，在地质时间尺度上，体型形态对稳态游泳的阻力系数与能量消耗并无显著影响。我们发现，即便早期鱼龙在对应体积下的阻力水平也较低，可与现代海豚相媲美；而深邃的‘鱼雷形’体型也并未降低运动成本。本分析还为游泳力学研究中CFD应用的缩放参数选择提供了重要参考，并凸显了体型演化对鱼龙运动模式的显著影响。随着鱼龙愈发适应远洋生活，大体型与高效游泳模式的结合降低了其稳态游泳的能量消耗。