In cursorial mammals, reduction or loss of the clavicle is usually associated with the constitution of the m. brachiocephalicus, a continuous muscle that extends from head and neck regions to the forelimb, protracting it during locomotion. Among caviomorph rodents, the Cavioidea are characterized by many adaptations that improve running performance, such as the presence of a brachiocephalic configuration of the mm. cleidomastoideus, cleido-occipitalis, and deltoideus pars clavicularis, a feature that was interpreted as one of the main modifications for speed in running. However, a comprehensive analysis of this muscular configuration in the Cavioidea is not yet available. In this study, we analyze the morphological diversity, homologies, functions, and evolutionary changes of those shoulder muscles in cavioids that constitute the m. brachiocephalicus in cursorial mammals. We hypothesize that a brachiocephalic muscular arrangement of these muscles has evolved in this group in relation to a swift mode of locomotion. Muscular dissections on seven species of cavioids were performed while myological data on three other species were taken from the literature. Our results indicate that all cavioids have a brachiocephalic arrangement of the mm. cleidomastoideus, cleido-occipitalis, and deltoideus pars clavicularis, whereas a second brachiocephalic arrangement constituted by the mm. omotransversarius and trapezius pars cervicalis is present only in the larger cavioids. Both muscular configurations act in protracting the forelimb during the gait, but we also hypothesize that the particular disposition of the m. omotransversarius around the shoulder joint of larger cavioids could also assist in the stabilization of this joint during fast half-bounding locomotion.

中文翻译：

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Cavioid Rodents (Caviomorpha) 的头臂肌肉排列：功能、解剖学和进化研究

在游动哺乳动物中，锁骨的减少或丧失通常与 m 的构成有关。头臂肌，一种从头部和颈部区域延伸到前肢的连续肌肉，在运动过程中将其拉长。在 caviomorph 啮齿动物中，Cavioidea 的特点是有许多改进运行性能的适应性，例如 mm 的头臂构造的存在。cleidomastoideus、cleido-occipitalis 和 deltoideus pars clavicularis，这一特征被解释为跑步速度的主要变化之一。然而，目前还没有对 Cavioidea 中这种肌肉结构的综合分析。在这项研究中，我们分析了构成 m.cavioids 的肩部肌肉的形态多样性、同源性、功能和进化变化。游动哺乳动物中的头臂鲸。我们假设这些肌肉的头臂肌肉排列在该组中与快速运动模式有关。对七种 cavioids 进行了肌肉解剖，而其他三个物种的肌学数据取自文献。我们的结果表明所有 cavioids 都具有 mm 的头臂排列。cleidomastoideus、cleido-occipitalis 和 deltoideus pars clavicularis，而第二个由 mm 构成的头臂排列。omotransversarius 和斜方肌 pars cheeseis 仅存在于较大的 cavioids 中。两种肌肉结构都在步态过程中起到延长前肢的作用，但我们也假设 m 的特殊倾向。