Musculoskeletal modeling of sprawling and parasagittal forelimbs provides insight into synapsid postural transition

•Shoulder mobility and muscle leverage differ in sprawling vs. parasagittal tetrapods•Elevation joint versatility are key to therian posture•Reptiles monotremes both sprawl, but aspects of shoulder function•Results give predictive power for the synapsid sprawling–parasagittal transition The postural shift was a major during evolution, underpinned by reorganization forelimb, considered mammalian ecological diversity. Determining when how this occurred fossil record is challenging owing limited comparative data on extant species. Here, we built forelimb musculoskeletal models three taxa that bracket postures—tegu lizard, echidna, opossum—and tested relationship between three-dimensional mobility, action, posture. Results demonstrate clear functional variation grades, with opossum occupying distinct region pose space characterized highly retracted depressed emphasizes humeral elevation. 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Our results show posture, function, reveal sprawlers. data, combination anatomical transformations record, hypothesize Range determined neutral pose, while were relative standard reference 1 STAR Methods). simulated simultaneously, modified version existing code plotted each graphical represents orthogonal axes: retraction-protraction (Z-axis), depression-elevation (Y-axis), supination-pronation (X-axis). defined respect pose: protraction brings elbow cranially caudally plane; dorsally depression vertical pronation internally rotates supination externally humerus. Poses deemed viable if no bony intersection scapulocoracoid, head remained articulation (see Methods Figure S3). Total ROM—measured volume alpha shape wrapped poses—was greatest followed closely (alpha hull volumes 1,873,417 degrees3 1,280,653 degrees3, respectively), orders magnitude 94,550 degrees3). echidna's envelope narrowest protraction-retraction widest elevation-depression axis, meaning least 2). pattern consistent prior work (Regnault functi