Computer Simulations Imply Forelimb-Dominated Underwater Flight in Plesiosaurs

Plesiosaurians are an extinct group of highly derived Mesozoic marine reptiles with a global distribution that spans 135 million years from the Early Jurassic to the Late Cretaceous. During their long evolutionary history they maintained a unique body plan with two pairs of large wing-like flippers, but their locomotion has been a topic of debate for almost 200 years. Key areas of controversy have concerned the most efficient biologically possible limb stroke, e.g. whether it consisted of rowing, underwater flight, or modified underwater flight, and how the four limbs moved in relation to each other: did they move in or out of phase? Previous studies have investigated plesiosaur swimming using a variety of methods, including skeletal analysis, human swimmers, and robotics. We adopt a novel approach using a digital, three-dimensional, articulated, free-swimming plesiosaur in a simulated fluid. We generated a large number of simulations under various joint degrees of freedom to investigate how the locomotory repertoire changes under different parameters. Within the biologically possible range of limb motion, the simulated plesiosaur swims primarily with its forelimbs using an unmodified underwater flight stroke, essentially the same as turtles and penguins. In contrast, the hindlimbs provide relatively weak thrust in all simulations. We conclude that plesiosaurs were forelimb-dominated swimmers that used their hind limbs mainly for maneuverability and stability.蛇颈龙类（Plesiosaurians）是一类已灭绝的高度特化中生代海生爬行动物类群，全球广泛分布，生存时代从早侏罗世延续至晚白垩世，跨度长达1.35亿年。在其漫长的演化历程中，它们始终保持着独特的躯体构型：拥有两对巨大的翼状鳍肢，但关于其运动方式的争论已持续近200年。争议的核心焦点主要集中在两大方面：一是生物学上最具效率的肢体摆动模式——究竟为划水、水下飞行，还是改良型水下飞行；二是四肢的相对运动时序：四肢是同相位运动还是异相位运动？过往研究已通过多种手段探讨蛇颈龙的游泳行为，包括骨骼解剖分析、人体游泳实验以及机器人模拟。本研究采用了一种全新的研究路径：在模拟流体环境中构建一个数字化、三维、铰接式的自由游动蛇颈龙模型。我们在不同关节自由度条件下开展了大量模拟实验，以探究不同参数下运动模式的变化规律。在生物学可行的肢体运动范围内，模拟的蛇颈龙主要依靠前肢以未改良的水下飞行动作完成游动，其运动模式与海龟和企鹅的游泳方式基本一致。与之相对，所有模拟实验中后肢产生的推力均相对较弱。我们据此得出结论：蛇颈龙是以前肢主导的游泳者，后肢主要用于提升操控性与运动稳定性。