The earliest ornithischian dinosaurs were small bipeds that possessed the derived, retroverted pubis that is synapomorphic for the clade. Their forelimbs were modified for grasping. Subsequently, ornithischians radiated into numerous body shapes and sizes. Three lineages independently evolved large size and quadrupedality, requiring profound changes to the osteology and myology of the locomotor apparatus. Using comparisons with extinct archosaurian outgroups, as well as extant birds and crocodilians, we reconstruct basal ornithischian limb musculature in order to determine the sequence of musculoskeletal changes that occurred prior to and during the early evolution of Ornithischia. The musculoskeletal anatomy of basal ornithischian forelimbs was probably more similar to that of the crown group archosaur common ancestor and crocodilians than it was to birds. Reduction or loss of the clavicle resulted in migration of the clavicular deltoid onto the proximal scapula and development of a distinct acromial process. The coracobrachialis brevis and supracoracoideus probably protracted the humerus, whereas the scapulohumeralis caudalis and deltoideus scapularis probably retracted it. The deltoideus clavicularis was likely a humeral abductor. In the hind limb, the elongate preacetabular process indicates lateral migration of important femoral protractors to the ilium from an inferred ancestral location on the dorsal vertebrae. The puboischiofemoralis externus was greatly reduced or possibly lost due to pubic retroversion. The reconstruction of basal ornithischian myology provides a baseline for determining the musculoskeletal changes associated with different locomotor modes in ornithischians, and a foundation for biomechanical studies of dinosaur locomotion.