Fahn-Lai Philip, Biewener Andrew A, Pierce Stephanie E
Museum of Comparative Zoology, Concord Field Station and Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA, USA.
Concord Field Station and Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA, USA.
PeerJ. 2020 Feb 18;8:e8556. doi: 10.7717/peerj.8556. eCollection 2020.
The evolution of upright limb posture in mammals may have enabled modifications of the forelimb for diverse locomotor ecologies. A rich fossil record of non-mammalian synapsids holds the key to unraveling the transition from "sprawling" to "erect" limb function in the precursors to mammals, but a detailed understanding of muscle functional anatomy is a necessary prerequisite to reconstructing postural evolution in fossils. Here we characterize the gross morphology and internal architecture of muscles crossing the shoulder joint in two morphologically-conservative extant amniotes that form a phylogenetic and morpho-functional bracket for non-mammalian synapsids: the Argentine black and white tegu and the Virginia opossum . By combining traditional physical dissection of cadavers with nondestructive three-dimensional digital dissection, we find striking similarities in muscle organization and architectural parameters. Despite the wide phylogenetic gap between our study species, distal muscle attachments are notably similar, while differences in proximal muscle attachments are driven by modifications to the skeletal anatomy of the pectoral girdle that are well-documented in transitional synapsid fossils. Further, correlates for force production, physiological cross-sectional area (PCSA), muscle gearing (pennation), and working range (fascicle length) are statistically indistinguishable for an unexpected number of muscles. Functional tradeoffs between force production and working range reveal muscle specializations that may facilitate increased girdle mobility, weight support, and active stabilization of the shoulder in the opossum-a possible signal of postural transformation. Together, these results create a foundation for reconstructing the musculoskeletal anatomy of the non-mammalian synapsid pectoral girdle with greater confidence, as we demonstrate by inferring shoulder muscle PCSAs in the fossil non-mammalian cynodont .
哺乳动物直立肢体姿势的演化可能促使前肢发生改变，以适应多样的运动生态。丰富的非哺乳类合弓纲动物化石记录是解开哺乳动物祖先从“ sprawl ”（ sprawl ：四肢向外侧伸展的姿势）到“ erect ”（ erect ：直立的姿势）肢体功能转变的关键，但要重建化石中的姿势演化，详细了解肌肉功能解剖学是必要的前提条件。在这里，我们描述了两种形态保守的现存羊膜动物肩部肌肉的大体形态和内部结构，这两种动物为非哺乳类合弓纲动物形成了系统发育和形态功能框架：阿根廷黑白泰加蜥和弗吉尼亚负鼠。通过将传统的尸体解剖与无损三维数字解剖相结合，我们发现肌肉组织和结构参数存在显著相似性。尽管我们研究的物种在系统发育上差距很大，但远端肌肉附着点明显相似，而近端肌肉附着点的差异是由胸带骨骼解剖结构的改变驱动的，这种改变在过渡性合弓纲化石中有详细记录。此外，对于数量出乎意料的肌肉，其力量产生、生理横截面积（ PCSA ）、肌肉齿轮比（羽状角）和工作范围（肌束长度）的相关性在统计学上无法区分。力量产生和工作范围之间的功能权衡揭示了肌肉特化，这可能有助于增加负鼠肩带的灵活性、重量支撑和肩部的主动稳定——这可能是姿势转变的一个信号。总之，这些结果为更有信心地重建非哺乳类合弓纲动物胸带的肌肉骨骼解剖结构奠定了基础，正如我们通过推断化石非哺乳类犬齿兽的肩部肌肉 PCSA 所证明的那样。
