The acquisition of terrestrial, limb-based locomotion during tetrapod evolution has remained a subject of debate for more than a century 1 , 2 . Our current understanding of the locomotor transition from water to land is largely based on a few exemplar fossils such as Tiktaalik 3 , Acanthostega 4 , Ichthyostega 5 and Pederpes 6 . However, isolated bony elements may reveal hidden functional diversity, providing a more comprehensive evolutionary perspective 7 . Here we analyse 40 three-dimensionally preserved humeri from extinct tetrapodomorphs that span the fin-to-limb transition and use functionally informed ecological adaptive landscapes 8 – 10 to reconstruct the evolution of terrestrial locomotion. We show that evolutionary changes in the shape of the humerus are driven by ecology and phylogeny and are associated with functional trade-offs related to locomotor performance. Two divergent adaptive landscapes are recovered for aquatic fishes and terrestrial crown tetrapods, each of which is defined by a different combination of functional specializations. Humeri of stem tetrapods share a unique suite of functional adaptations, but do not conform to their own predicted adaptive peak. Instead, humeri of stem tetrapods fall at the base of the crown tetrapod landscape, indicating that the capacity for terrestrial locomotion occurred with the origin of limbs. Our results suggest that stem tetrapods may have used transitional gaits 5 , 11 during the initial stages of land exploration, stabilized by the opposing selective pressures of their amphibious habits. Effective limb-based locomotion did not arise until loss of the ancestral ‘L-shaped’ humerus in the crown group, setting the stage for the diversification of terrestrial tetrapods and the establishment of modern ecological niches 12 , 13 . Analysis of humeri from fossils that span the fin-to-limb transition reveal that the change in the humerus shape is driven by both ecology and phylogeny, and is associated with functional trade-offs related to locomotor performance.

中文翻译：

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功能适应性景观预测四肢起源处的陆地容量

一个多世纪以来，四足动物进化过程中获得基于肢体的陆地运动一直是一个争论的话题 1 , 2 。我们目前对从水到陆地的运动过渡的理解主要基于一些示例化石，例如 Tiktaalik 3、Acanthostega 4、Ichthyostega 5 和 Pederpes 6。然而，孤立的骨元素可能会揭示隐藏的功能多样性，提供更全面的进化视角 7。在这里，我们分析了 40 个来自已灭绝四足动物的 3 维保存的肱骨，这些肱骨跨越鳍到肢的过渡，并使用功能信息丰富的生态适应性景观 8 - 10 来重建陆地运动的进化。我们表明，肱骨形状的进化变化是由生态学和系统发育驱动的，并且与与运动性能相关的功能权衡有关。为水生鱼类和陆地冠四足动物恢复了两种不同的适应性景观，每种景观都由不同的功能专业组合定义。茎四足动物的肱骨共享一套独特的功能适应性，但不符合它们自己预测的适应性峰值。相反，茎四足动物的肱骨落在冠四足动物景观的底部，表明陆地运动能力发生在四肢的起源。我们的结果表明，在陆地探索的初始阶段，茎四足动物可能使用了过渡步态 5 、 11 ，它们的两栖习性的相反选择压力使它们稳定下来。有效的基于肢体的运动直到冠群中祖先的“L 形”肱骨丧失才出现，为陆地四足动物的多样化和现代生态位的建立奠定了基础 12 , 13 。对跨越鳍到肢过渡的化石的肱骨进行分析表明，肱骨形状的变化是由生态学和系统发育共同驱动的，并且与与运动性能相关的功能权衡有关。