The form and function of the sacrum are of great relevance to understand the evolution of locomotion in tetrapods because it is a key piece of the vertebrate skeleton. The sacrum connects the caudal and presacral regions of the vertebral column and the hindlimbs through the pelvis. Here, we investigate sacrum shape evolution in pinnipeds (Carnivora: Pinnipedia) in relation to terrestrial mammalian carnivorans (fissipeds), and we include crown and stem taxa to quantify the morphological changes they experience in relation to the aquatic environment they inhabit. We use 3D geometric morphometric methods to explore the morphological variability and disparity of the sacrum in a set of terrestrial and aquatic carnivoran species. Our results show that the morphology of the sacrum of each pinniped family is remarkably different and that these differences may be related to the aquatic mode of locomotion (pectoral or pelvic oscillation), the use of hindlimbs to support body weight on land (otariids in contrast with phocids), and the presence or absence of a functional tail. In addition, disparity-through-time analyses indicate that the sacrum of pinnipeds is less constrained than that of fissipeds, which suggests a gravitational origin of such constraints in fissipeds. In conclusion, our results give further support to the important role played by this skeletal structure in the locomotory adaptations of mammals.

骶骨的形状和功能对于理解四足动物的运动进化具有重要意义，因为它是脊椎动物骨骼的关键部分。骶骨通过骨盆连接脊柱的尾部和骶前区域以及后肢。在这里，我们研究了鳍足类动物（食肉目：鳍足类动物）与陆生哺乳动物食肉动物（fissipeds）的骶骨形状演化，并且我们包括冠和茎类群以量化它们所经历的与其栖息的水生环境相关的形态变化。我们使用 3D 几何形态测量方法来探索一组陆生和水生食肉动物中骶骨的形态变异性和差异性。我们的研究结果表明，每个鳍足类动物的骶骨形态明显不同，这些差异可能与水生运动模式（胸肌或骨盆摆动）、在陆地上使用后肢支撑体重（相比之下，otariids）有关有 phocids），以及是否存在功能性尾巴。此外，时间差异分析表明，鳍足类动物的骶骨比裂足类动物的骶骨受到的约束更少，这表明裂足类动物中这种约束的引力起源。总之，我们的结果进一步支持了这种骨骼结构在哺乳动物的运动适应中发挥的重要作用。使用后肢在陆地上支撑体重（otariids 与 phocids 对比），以及是否存在功能性尾巴。此外，时间差异分析表明，鳍足类动物的骶骨比裂足类动物的骶骨受到的约束更少，这表明裂足类动物中这种约束的引力起源。总之，我们的结果进一步支持了这种骨骼结构在哺乳动物的运动适应中发挥的重要作用。使用后肢在陆地上支撑体重（otariids 与 phocids 对比），以及是否存在功能性尾巴。此外，时间差异分析表明，鳍足类动物的骶骨比裂足类动物的骶骨受到的约束更少，这表明裂足类动物中这种约束的引力起源。总之，我们的结果进一步支持了这种骨骼结构在哺乳动物的运动适应中发挥的重要作用。