Synopsis We use a series of hydrodynamic experiments on abstracted models to explore whether primitive vertebrates may have swum under various conditions without a clearly-differentiated tail fin. Cambrian vertebrates had post-anal stubby tails, some had single dorsal and ventral fins, but none had yet evolved a clearly differentiated caudal fin typical of post-Cambrian fishes, and must have relied on their long and flexible laterally-compressed bodies for locomotion, i.e., by bending their bodies side-to-side in order to propagate waves from head to tail. We approach this problem experimentally based on an abstracted model of Metaspriggina walcotti from the 506-million-year old Burgess Shale by using oscillating thin flexible plates while varying the tail fin geometry from rectangular to uniform, and finally to a no tail–fin condition. Despite a missing tail fin, this study supports the observation that the abstracted Metaspriggina model can generate a strong propulsive force in cruise conditions, both away from, and near the sea bed (in ground effect). However, when the abstracted Metaspriggina model moves in ground effect, a weaker performance is observed, indicating that Metaspriggina may not necessarily have been optimized for swimming near the sea bed. When considering acceleration from rest, we find that the Metaspriggina model's performance is not significantly different from other morphological models (abstracted truncate tail and abstracted heterocercal tail). Statistical analysis shows that morphological parameters, swimming modes, and ground effect all play significant roles in thrust performance. While the exact relationships of Cambrian vertebrates are still debated, as agnathans, they share some general characteristics with modern cyclostomes, in particular an elongate body akin to lampreys. Lampreys, as anguilliform swimmers, are considered to be some of the most efficient swimmers using a particular type of suction thrust induced by the traveling body wave as it travels from head to tail. Our current experiments suggest that Metaspriggina’s ability in acceleration from rest, through possibly a similar type of suction thrust, which is defined as the ability to generate low pressure on upstream facing sections of the body, might have evolved early in response to increasing predator pressure during the Cambrian Explosion.

中文翻译：

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没有尾鳍的鱼——探索第一脊椎动物尾巴形态的功能

概要我们在抽象模型上使用一系列流体动力学实验来探索原始脊椎动物是否可能在没有明显区分的尾鳍的情况下在各种条件下游泳。寒武纪脊椎动物有肛门后粗短的尾巴，有些只有单一的背鳍和腹鳍，但还没有进化出明显分化的尾鳍，典型的后寒武纪鱼类，并且必须依靠它们长而灵活的横向压缩身体进行运动，即，通过左右弯曲他们的身体，以便从头到尾传播波。我们基于来自 5.06 亿年历史的 Burgess 页岩的 Metaspriggina walcotti 的抽象模型，通过使用摆动的薄柔性板，同时将尾鳍几何形状从矩形变为均匀，最后变为无尾鳍条件，通过实验方法解决了这个问题。尽管缺少尾翼，但这项研究支持这样的观察结果，即抽象的 Metaspriggina 模型可以在远离海床和靠近海床的巡航条件下（在地面效应中）产生强大的推进力。然而，当抽象的 Metaspriggina 模型在地面效应中移动时，观察到较弱的性能，这表明 Metaspriggina 可能不一定针对在海床附近游泳进行了优化。在考虑静止加速时，我们发现 Metaspriggina 模型的性能与其他形态模型（抽象截尾和抽象异尾尾）没有显着差异。统计分析表明，形态参数、游泳模式和地面效应都在推力性能中发挥着重要作用。虽然寒武纪脊椎动物的确切关系仍然存在争议，但作为无颌类，它们与现代环口动物有一些共同特征，特别是类似于七鳃鳗的细长体。作为鳗鱼形游泳者，七鳃鳗被认为是最有效率的游泳者之一，当它从头到尾传播时，它们使用由行进的体波引起的特定类型的吸力。我们目前的实验表明，Metaspriggina 的从静止加速的能力，可能通过类似类型的吸力，被定义为在身体的上游面向部分产生低压的能力，可能在早期进化以响应捕食者压力的增加。寒武纪大爆发。特别是类似于七鳃鳗的细长身体。作为鳗鱼形游泳者，七鳃鳗被认为是最有效率的游泳者之一，当它从头到尾传播时，它们使用由行进的体波引起的特定类型的吸力。我们目前的实验表明，Metaspriggina 的从静止加速的能力，可能通过类似类型的吸力，被定义为在身体的上游面向部分产生低压的能力，可能在早期进化以响应捕食者压力的增加。寒武纪大爆发。特别是类似于七鳃鳗的细长身体。作为鳗鱼形游泳者，七鳃鳗被认为是最有效率的游泳者之一，当它从头到尾传播时，它们使用由行进的体波引起的特定类型的吸力。我们目前的实验表明，Metaspriggina 的从静止加速的能力，可能通过类似类型的吸力，被定义为在身体的上游面向部分产生低压的能力，可能在早期进化以响应捕食者压力的增加。寒武纪大爆发。