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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 merianae and the Virginia opossum Didelphis virginiana . 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 Massetognathus pascuali .
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Shoulder Muscle Architecture in the Echidna (Monotremata: Tachyglossus aculeatus) Indicates Conserved Functional Properties
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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- PAR ID:
- 10144102
- Date Published:
- Journal Name:
- Journal of Mammalian Evolution
- ISSN:
- 1064-7554
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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null (Ed.)Extant mammals are both taxonomically and ecologically diverse, having evolved a remarkable array of locomotor ecologies (e.g., swimming, digging, and flying). Evolution of the therian-type forelimb, with a highly reduced pectoral girdle and ball-and-socket shoulder joint, has been heralded as a key innovation that enabled mammals to co-opt their forelimbs for diverse functions. The acquisition of the mammal forelimb can be traced through their forerunners, the non-mammalian synapsids (NMS), but exactly how this musculoskeletal transformation proceeded and its impact on functional diversification have not be quantitatively tested. To explore the evolution of forelimb functional diversity in synapsids, we measured shoulder joint osteological range of motion (ROM) in a range of extant amniotes (lizards, monotremes, therian mammals), and compared their patterns of joint mobility to exemplars from each of the major grades of NMS: ‘pelycosaurs’, basal therapsids, and non-mammalian cynodonts. Three-dimensional models of the shoulder girdles and humeri were digitally aligned in an anatomical ‘neutral pose’ using a semi-automated approach based on articular surface morphology. ROM was then determined for the shoulder joint using a fully automated method, where the humerus was moved in flexion-extension, adduction-abduction, and pronation-supination until bone-to-bone contact occurred. Relative degree and directionality of mobility were then compared across taxa. We find an increase in total shoulder joint ROM through synapsid evolution, suggesting that more derived NMS could perform a wider range of limb movements. However, we also see more complex trends in directionality of shoulder mobility that may be indicators of forelimb posture. Extant lepidosaurs and monotremes had the greatest ROM in abduction-adduction, whereas therians had more ROM in flexion-extension, likely related to ‘sprawling’ vs. ‘erect’ gaits. Therapsids and cynodonts both had greatest ROM in abduction-adduction, matching previous reconstructions of these taxa as sprawling to semi-erect. However, ‘pelycosaurs’ had the greatest ROM in flexion-extension, despite having abducted forelimbs, suggesting they did not move their forelimbs in same manner as modern sprawling animals. Our results demonstrate the complex nature of forelimb evolution in synapsids and provide novel insights into the functional transformation and diversification of the mammalian forelimb. Funding Sources Funding information: NSF DEB- 1757749 (S.E.P) and NSF DEB-1754502 (K.D.A).more » « less
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1007/s10914-020-09498-6
