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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.
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The relative contributions of multiarticular snake muscles to movement in different planes
Abstract Muscles spanning multiple joints play important functional roles in a wide range of systems across tetrapods; however, their fundamental mechanics are poorly understood, particularly the consequences of anatomical position on mechanical advantage. Snakes provide an excellent study system for advancing this topic. They rely on the axial muscles for many activities, including striking, constriction, defensive displays, and locomotion. Moreover, those muscles span from one or a few vertebrae to over 30, and anatomy varies among muscles and among species. We characterized the anatomy of major epaxial muscles in a size series of corn snakes (Pantherophis guttatus) using diceCT scans, and then took several approaches to calculating contributions of each muscle to force and motion generated during body bending, starting from a highly simplistic model and moving to increasingly complex and realistic models. Only the most realistic model yielded equations that included the consequence of muscle span on torque‐displacement trade‐offs, as well as resolving ambiguities that arose from simpler models. We also tested whether muscle cross‐sectional areas or lever arms (total magnitude or pitch/yaw/roll components) were related to snake mass, longitudinal body region (anterior, middle, posterior), and/or muscle group (semispinalis‐spinalis, multifidus, longissimus dorsi, iliocostalis, and levator costae). Muscle cross‐sectional areas generally scaled with positive allometry, and most lever arms did not depart significantly from geometric similarity (isometry). The levator costae had lower cross‐sectional area than the four epaxial muscles, which did not differ significantly from each other in cross‐sectional area. Lever arm total magnitudes and components differed among muscles. We found some evidence for regional variation, indicating that functional regionalization merits further investigation. Our results contribute to knowledge of snake muscles specifically and multiarticular muscle systems generally, providing a foundation for future comparisons across species and bioinspired multiarticular systems.
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- Award ID(s):
- 2045581
- PAR ID:
- 10413338
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Journal of Morphology
- Volume:
- 284
- Issue:
- 6
- ISSN:
- 0362-2525
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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