Abstract
Although evolutionary transformation of the pectoral girdle and forelimb appears to have had a profound impact on mammalian locomotor and ecological diversity, both the sequence of anatomical changes and the functional implications remain unclear. Monotremes can provide insight into an important stage of this evolutionary transformation, due to their phylogenetic position as the sister-group to therian mammals and their mosaic of plesiomorphic and derived features. Here we build a musculoskeletal computer model of the echidna pectoral girdle and forelimb to estimate joint ranges of motion (ROM) and muscle moment arms (MMA)—two fundamental descriptors of biomechanical function. We find that the echidna's skeletal morphology restricts scapulocoracoid mobility and glenohumeral flexion–extension compared with therians. Estimated shoulder ROMs and MMAs for muscles crossing the shoulder indicate that morphology of the echidna pectoral girdle and forelimb is optimized for humeral adduction and internal rotation, consistent with limited in vivo data. Further, more muscles act to produce humeral long-axis rotation in the echidna compared to therians, as a consequence of differences in muscle geometry. Our musculoskeletal model allows correlation of anatomy and function, and can guide hypotheses regarding function in extinct taxa and the morphological and locomotor transformation leading to therian mammals.
Highlights
The remarkable taxonomic and ecological diversity of therian mammals is underpinned by exaptation of the forelimb to serve novel locomotory styles and behaviours, e.g. cursorial horses, volant bats, aquatic whales, fossorial moles [1]
There are too many joint angle and muscle combinations to test, but we examine the effect of changing the glenohumeral anatomical reference pose on the calculated moment arms (MMA) of an important anti-gravity muscle, m. pectoralis part 2
The halved values of the elbow joint followed the body mass scaling relationship for therian mammals, but the glenohumeral joint fell below two standard errors
Summary
The remarkable taxonomic and ecological diversity of therian mammals (marsupials and placentals) is underpinned by exaptation of the forelimb to serve novel locomotory styles and behaviours, e.g. cursorial horses, volant bats, aquatic whales, fossorial moles [1]. Such diversity of forelimb function was made possible due to a profound reorganization of the ancestral synapsid musculoskeletal system over the course of approximately 300 million years [2,3] (figure 1). The increased shoulder mobility enabled functional versatility and allowed the forelimb to be brought beneath the body, which may have conferred several benefits to therians including increased efficiency [18] and/or agility [2]
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