Evolutionary transitions between quadrupedal and bipedal postures are pivotal to the diversification of amniotes on land, including in our own lineage (Hominini). Heterochrony is suggested as a macroevolutionary mechanism for postural transitions but understanding postural evolution in deep time is hindered by a lack of methods for inferring posture in extinct species. Dinosaurs are an excellent natural laboratory for understanding postural transitions because they demonstrate at least four instances of quadrupedality evolving from bipedality, and heterochronic processes have been put forward as an explanatory model for these transitions. We extend a quantitative method for reliably inferring posture in tetrapods to the study of ontogenetic postural transitions using measurements of proportional limb robusticity. We apply this to ontogenetic series of living and extinct amniotes, focusing on dinosaurs. Our method correctly predicts the general pattern of ontogenetic conservation of quadrupedal and bipedal postures in many living amniote species and infers the same pattern in some dinosaurs. Furthermore, it correctly predicts the ontogenetic postural shift from quadrupedal crawling to bipedal walking in humans. We also infer a transition from early ontogenetic quadrupedality to late‐ontogenetic bipedality in the transitional sauropodomorph dinosaur Mussaurus patagonicus and possibly in the early branching ceratopsian Psittacosaurus lujiatunensis but not in the sauropodomorph Massospondylus carinatus. The phylogenetic positions of these ontogenetic shifts suggest that heterochrony may play a role in the macroevolution of posture, at least in dinosaurs. Our method has substantial potential for testing evolutionary transitions between locomotor modes, especially in elucidating the role of evolutionary mechanisms like heterochrony.

中文翻译：

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一种推断个体发育过程中羊膜运动变化的定量方法及其在恐龙中的应用及其对姿势演变的影响

四足动物姿势和两足动物姿势之间的进化过渡对于陆地羊膜动物的多样化至关重要，包括我们自己的血统（Hominini）。异时被认为是姿势转变的宏观进化机制，但是由于缺乏推断灭绝物种姿势的方法，阻碍了人们在深层时期对姿势进化的理解。恐龙是了解姿势转变的出色自然实验室，因为它们证明了至少有四个由两足动物进化而来的四足动物，并且提出了异时过程作为这些转变的解释模型。我们将定量推断可靠地推断四足动物的姿势的方法用于使用成比例肢体健壮性的测量来研究自体姿势转变。我们将其应用于生活和灭绝的羊膜动物的遗传学系列，重点研究恐龙。我们的方法正确地预测了许多活着的羊膜物种中四足动物和两足动物姿势的个体发育保守的一般模式，并推断出某些恐龙的相同模式。此外，它可以正确预测人类从四足动物爬行到两足动物行走的本体姿势变化。我们还推断出过渡的蜥脚类恐龙从早期个体发育四足动物到晚期个体发育两足动物的过渡 它正确地预测了人类从四足爬行到两足行走的体态姿势转变。我们还可以推断出过渡的蜥脚类恐龙从早期个体发育的四足动物到晚期个体发育的两足动物的过渡。它正确地预测了人类从四足爬行到双足行走的本体姿势转变。我们还可以推断出过渡的蜥脚类恐龙从早期个体发育的四足动物到晚期个体发育的两足动物的过渡。巴塔哥（Mussaurus patagonicus），可能在早期分枝的角形陆嘴陆嘴鹦鹉（Psittacosaurus lujiatunensis）中，而在鳞茎变种Massospondylus carinatus中没有。这些个体转变的系统发生位置表明，异时性至少在恐龙中可能在姿势的宏观进化中起作用。我们的方法在测试运动模式之间的进化转变方面具有巨大潜力，尤其是在阐明进化机制（如异时性）中的作用方面。