The transition from water to land by the earliest tetrapods in the Devonian Period is seen as one of the greatest steps in evolution. However, little is understood concerning changes in brain morphology over this transition. Here, we determine the brain–braincase relationship in fishes and basal lissamphibians as a proxy to elucidate the changes that occurred over the fish–tetrapod transition. We investigate six basal extant sarcopterygians spanning coelacanths to salamanders (Latimeria chalumnae, Neoceratodus, Protopterus aethiopicus, P. dolloi, Cynops, Ambystoma mexicanum) using micro-CT and MRI and quantify the brain–braincase relationship in these extant taxa. Our results show that regions of lowest brain–endocast disparity are associated with regions of bony reinforcement directly adjacent to masticatory musculature for the mandible except in Neoceratodus and Latimeria. In Latimeria this deviation from the trend can be accounted for by the possession of an intracranial joint and basicranial muscles, whereas in Neoceratodus difference is attributed to dermal bones contributing to the overall neurocranial reinforcement. Besides Neoceratodus and Latimeria, regions of low brain–endocast disparity occur where there is less reinforcement away from high mandibular muscle mass, where the trigeminal nerve complex exits the braincase and where endolymphatic sacs occupy space between the brain and braincase wall. Despite basal tetrapods possessing reduced adductor muscle mass and a different biting mechanism to piscine sarcopterygians, regions of the neurocranium lacking osteological reinforcement in the basal tetrapods Lethiscus and Brachydectes broadly correspond to regions of high brain–endocast disparity seen in extant taxa.

泥盆纪时期最早的四足动物从水到陆地的过渡被视为进化中最伟大的步骤之一。然而，人们对于这一转变过程中大脑形态的变化知之甚少。在这里，我们确定了鱼类和基础两栖动物的大脑与脑壳关系作为代理，以阐明鱼类-四足动物转变过程中发生的变化。我们使用显微 CT 和 MRI 研究了从腔棘鱼到蝾螈的六种现存的基础肉翅目动物（ Latimeria chalumnae、Neoceratodus、Protopterus aethiopicus、P. dolloi、Cynops、Ambystoma mexicanum ），并量化了这些现存类群中的脑-脑壳关系。我们的结果表明，除了新角龙属和Latimeria外，脑内铸差异最低的区域与下颌骨咀嚼肌组织直接相邻的骨强化区域相关。在Latimeria中，这种趋势的偏差可以通过拥有颅内关节和颅底肌肉来解释，而在Neoceratodus中，这种差异归因于真皮骨对整体神经颅骨的强化。除了新角属和拉蒂莫利亚之外，低脑内铸差异的区域也发生在远离高下颌肌肉块的强化较少的地方，三叉神经复合体离开脑壳的地方以及内淋巴囊占据大脑和脑壳壁之间空间的地方。 尽管基础四足动物的内收肌质量减少，咬合机制与鱼类肉翅目不同，但基础四足动物Lethiscus和Brachydectes的神经颅骨缺乏骨强化的区域大致对应于现存类群中所见的脑内铸差异较高的区域。