The evolution of upright posture allowed synapsids and their mammalian descendants to exapt their appendicular skeletons for diverse functions and ecologies. However, the lack of skeletal correlates for posture has made it very difficult to pinpoint exactly when the transition from “sprawling” to “upright” limb posture occurred. Reconstructed muscle moment arms (MMAs) are commonly used for inferring function in extinct animals, but in order to use this approach to interpret the synapsid fossil record, we must first understand how joint range of motion (ROM) and muscle geometry interact to produce integrated function in three dimensions (3D)—something that has not been extensively studied in non-human amniotes. Here, we harnessed the power of X-ray Reconstruction of Moving Morphology (XROMM), the DeepLabCut machine learning toolkit, and a novel Maya-Python moment arm tool to create a comprehensive map of shoulder ROM and pose-varying MMAs in a functional and phylogenetic bracket for the synapsid stem: the sprawling Argentine black and white tegu and the upright Virginia opossum. We present a comparison between the two taxa, and illustrate the potential of this hybrid approach to overcome historical hurdles in the acquisition and analysis of densely-sampled 3D limb kinematics datasets. Future work will leverage these data to constrain and validate musculoskeletal models spanning synapsid evolutionary history to flesh out the story of mammalian postural evolution.