Highlights
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Shoulder mobility and muscle leverage differ in sprawling vs. parasagittal tetrapods•
Elevation leverage and joint versatility are key to therian parasagittal posture•
Reptiles and monotremes both sprawl, but differ in aspects of shoulder function•
Results give predictive power for the synapsid sprawling–parasagittal transition
Summary
The sprawling–parasagittal postural shift was a major transition during synapsid evolution, underpinned by reorganization of the forelimb, and considered key to mammalian ecological diversity. Determining when and how this transition occurred in the fossil record is challenging owing to limited comparative data on extant species. Here, we built forelimb musculoskeletal models of three extant taxa that bracket sprawling–parasagittal postures—tegu lizard, echidna, and opossum—and tested the relationship between three-dimensional joint mobility, muscle action, and posture. Results demonstrate clear functional variation between postural grades, with the parasagittal opossum occupying a distinct region of pose space characterized by a highly retracted and depressed shoulder joint that emphasizes versatility and humeral elevation. Applying our data to the fossil record support trends of an increasingly retracted humerus and greater elevation muscle moment arms indicative of more parasagittal postures throughout synapsid evolution.
