Monotremes are a group of egg-laying mammals, possessing a mosaic of ancestral and derived anatomical features. Despite much interest in monotremes from phylogenetic, morphological, and ecological perspectives, they have been the subject of relatively few biomechanical studies. In this study, we examined shoulder and proximal forelimb muscle anatomy and architecture in the short-beaked echidna, Tachyglossus aculeatus, through contrast-enhanced computed tomography and gross dissection. Muscle architecture is a major determinant of muscle function and can indicate specialized muscle roles, such as the capacity for generating large forces (through large physiological cross-sectional area, PCSA) or working ranges (through long fascicle lengths). We hypothesized that some muscles would exhibit architectural specializations convergent with other fossorial and/or sprawling animals, and that other muscles would reflect the echidna’s unusual anatomy and locomotor style. Instead, we found the shoulder and proximal forelimb muscles in echidna to have little variation in their architecture. The muscles generally had long fascicles and small-to-intermediate PCSAs, consistent with force production over a wide working range. Further, muscles did not show overt differences in architecture that, in therian mammals, have been linked to increased forelimb mobility and the transition from sprawling to parasagittal posture. Our measures of architectural disparity placed the echidna closer to the tegu lizard than other sprawling fossorial mammals (e.g., mole). The low architectural diversity found in the echidna’s shoulder and proximal forelimb muscles is interpreted as a lack of functional specialization into distinct roles. We hope our study will contribute to greater understanding of monotreme anatomy and biomechanical function, and to the reconstruction of musculoskeletal evolution in mammals.

中文翻译：

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针鼹 (Monotremata: Tachyglossus aculeatus) 的肩部肌肉结构显示保守的功能特性

单孔目动物是一组产卵哺乳动物，具有祖先和衍生的解剖特征的马赛克。尽管从系统发育、形态学和生态学的角度对单孔目动物很感兴趣，但它们一直是相对较少的生物力学研究的主题。在这项研究中，我们通过对比增强计算机断层扫描和大体解剖检查了短喙针鼹（Tachyglossus aculeatus）的肩部和近端前肢肌肉解剖结构和结构。肌肉结构是肌肉功能的主要决定因素，可以指示特定的肌肉作用，例如产生大力量的能力（通过大生理横截面积，PCSA）或工作范围（通过长肌束长度）。我们假设一些肌肉会表现出与其他化石和/或蔓延的动物趋同的建筑特化，而其他肌肉会反映针鼹不寻常的解剖结构和运动风格。相反，我们发现针鼹的肩部和近端前肢肌肉的结构几乎没有变化。肌肉通常有长束和小到中等的 PCSA，这与在广泛的工作范围内产生的力一致。此外，肌肉在结构上没有表现出明显的差异，在哺乳动物中，这与增加的前肢活动度和从伸展到旁矢状姿势的转变有关。我们对建筑差异的测量使针鼹与其他庞大的化石哺乳动物（例如鼹鼠）更接近特古蜥蜴。在针鼹的肩部和近端前肢肌肉中发现的低结构多样性被解释为缺乏对不同角色的功能专业化。我们希望我们的研究将有助于更好地理解单孔目解剖学和生物力学功能，并有助于重建哺乳动物的肌肉骨骼进化。