The application of network methodology in anatomical structures offers new insights on the connectivity pattern of skull bones, skeletal elements and their muscles. Anatomical networks helped to improve our understanding of the water-to-land transition and how the pectoral fins were transformed into limbs via their modular disintegration. Here, we apply the same methodology to tetrapods secondarily adapted to the marine environment. We find that these animals achieved their return to the sea with four types of morphological changes, which can be grouped into two different main strategies. In all marine mammals and the majority of the reptiles, the fin is formed by the persistence of superficial and interdigital connective tissues, like a ‘baby mitten', whereas the underlying connectivity pattern of the bones does not influence the formation of the forefin. On the contrary, ichthyosaurs ‘zipped up' their fingers and transformed their digits into carpal-like elements, forming a homogeneous and better-integrated forefin. These strategies led these vertebrates into three different macroevolutionary paths exploring the possible spectrum of morphological adaptations.

网络方法在解剖结构中的应用为颅骨、骨骼元素及其肌肉的连接模式提供了新的见解。解剖网络有助于提高我们对水陆过渡以及胸鳍如何通过模块化分解转变为肢体的理解。在这里，我们将相同的方法应用于二次适应海洋环境的四足动物。我们发现这些动物通过四种形态变化实现重返大海，可以分为两种不同的主要策略。在所有海洋哺乳动物和大多数爬行动物中，鳍是由表面和叉指结缔组织的持久性形成的，就像“婴儿手套”，而骨骼的潜在连接模式不影响前鳍的形成。相反，鱼龙将它们的手指“拉上拉链”，将它们的手指转变为类似腕骨的元素，形成一个均质且整合性更好的前鳍。这些策略导致这些脊椎动物进入三种不同的宏观进化路径，探索可能的形态适应谱。