The origin of amniotes 320 million years ago signalled independence from water in vertebrates and was closely followed by divergences within the mammal and reptile stem lineages (Synapsida and Reptilia). Early members of both groups had highly similar morphologies, being superficially ‘lizard-like’ forms with many plesiomorphies. However, the extent to which they might have exhibited divergent patterns of evolutionary change, with potential to explain the large biological differences between their living members, is unresolved. We use a new, comprehensive phylogenetic dataset to quantify variation in rates and constraints of morphological evolution among Carboniferous–early Permian amniotes. We find evidence for an early burst of evolutionary rates, resulting in the early origins of morphologically distinctive subgroups that mostly persisted through the Cisuralian. Rates declined substantially through time, especially in reptiles. Early reptile evolution was also more constrained compared to early synapsids, exploring a more limited character state space. Postcranial innovation in particular was important in early synapsids, potentially related to their early origins of large body size. In contrast, early reptiles predominantly varied the temporal region, suggesting disparity in skull and jaw kinematics, and foreshadowing the variability of cranial biomechanics seen in reptiles today. Our results demonstrate that synapsids and reptiles underwent an early divergence of macroevolutionary patterns. This laid the foundation for subsequent evolutionary events and may be critical in understanding the substantial differences between mammals and reptiles today. Potential explanations include an early divergence of developmental processes or of ecological factors, warranting cross-disciplinary investigation.

中文翻译：

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哺乳动物和爬行动物茎谱系形态进化不同模式的早期起源

3.2 亿年前羊膜动物的起源标志着脊椎动物独立于水，紧随其后的是哺乳动物和爬行动物茎谱系（突触纲和爬行纲）的分歧。两个群体的早期成员具有高度相似的形态，表面上是“蜥蜴样”形式，具有许多 plesiomorphies。然而，它们在多大程度上可能表现出不同的进化变化模式，并有可能解释它们的活体成员之间的巨大生物学差异，但尚未得到解决。我们使用一个新的、全面的系统发育数据集来量化石炭纪-二叠纪早期羊膜动物的形态进化速率和限制的变化。我们找到了进化速度早期爆发的证据，导致形态独特的亚群的早期起源，这些亚群主要通过 Cisuralian 持续存在。随着时间的推移，利率大幅下降，尤其是爬行动物。与早期突触相比，早期爬行动物的进化也受到更多限制，探索了更有限的角色状态空间。颅后创新在早期突触中尤其重要，这可能与它们早期的大体型起源有关。相比之下，早期爬行动物主要改变颞区，这表明颅骨和颌骨运动学存在差异，并预示着今天爬行动物颅骨生物力学的可变性。我们的研究结果表明，突触和爬行动物经历了宏观进化模式的早期分歧。这为随后的进化事件奠定了基础，并且可能对于理解当今哺乳动物和爬行动物之间的实质性差异至关重要。潜在的解释包括发育过程或生态因素的早期分歧，需要跨学科调查。