Broad similarities in shoulder muscle architecture and organization across two amniotes: Implications for reconstructing non-mammalian synapsids
- Published
- Accepted
- Subject Areas
- Paleontology, Zoology, Anatomy and Physiology
- Keywords
- shoulder, forelimb, mammals, non-mammalian synapsid, musculoskeletal function, posture, locomotion
- Copyright
- © 2019 Fahn-Lai et al.
- Licence
- This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, reproduction and adaptation in any medium and for any purpose provided that it is properly attributed. For attribution, the original author(s), title, publication source (PeerJ Preprints) and either DOI or URL of the article must be cited.
- Cite this article
- 2019. Broad similarities in shoulder muscle architecture and organization across two amniotes: Implications for reconstructing non-mammalian synapsids. PeerJ Preprints 7:e27950v1 https://doi.org/10.7287/peerj.preprints.27950v1
Abstract
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 Salvator merianaeand the Virginia opossum Didelphis virginiana. By combining traditional physical dissection of cadavers with non-destructive 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 (PCSA, physiological cross-sectional area), 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 Massetognathus pascuali.
Author Comment
This is a submission to PeerJ for review.
Supplemental Information
Specimen treatment for contrast-enhanced µCT scanning
Specimen treatment for muscle architecture dissections
Full table of architecture, including functional ratios
Abbreviations and symbols: Mm, muscle mass; Mmtu, muscle-tendon unit mass; Lm, muscle length; Lmtu, muscle-tendon unit length; Lf, fascicle length; θ, pennation angle; PCSA, physiological cross-sectional area.
Measured and predicted body masses from humeral and femoral mid-diaphyseal circumferences
PCSA:Origin Area ratios for m. supracoracoideus
Homologies of m. deltoideus acromialis and m. scapulohumeralis anterior
High-contrast version of Figure 7
Functional morphospace comparing normalized PCSA against normalized fascicle length. Muscles tend to vary along either one axis or the other, consistent with a tradeoff between force production (y-axis) and working range (x-axis). Muscle abbreviations and color-coding follow Figure 1 legend.