Sauropods have a unique digitigrade and semi-tubular manus whose shape has been used as a synapomorphy that unites most sauropod taxa. The vertical orientation and semi-tubular arrangement of the metacarpals suggest the sauropod manus improved the mechanical ability of the forelimb to support great weight. However, the evolutionary mechanism responsible for modifying the relatively flat metacarpus of basal saurischians into a semi-tubular arrangement has remained uninvestigated. Furthermore, trackway evidence shows that manus pronation was more developed in sauropods than other saurischians. However, because the radius and ulna do not cross completely in sauropods, reconciling manus print orientation with forelimb osteology has been difficult. Restudy of North American neosauropod appendicular osteology and anatomy suggests that the unique manus shape of sauropods is linked temporally with reversion to a quadrupedal posture and the necessity of manus pronation. Articulation and manipulation of neosauropod forelimbs and casts, as well as a scale model of Apatosaurus louisae, suggest that, as the sauropod forelimb resumed a weight-bearing role, the primitively anterolateral position of the radius shifted to assume a more internal (anteromedial) orientation in relation to the ulna proximally and distally. The internal shift of the radius may have subsequently pronated the manus while simultaneously altering the shape of the digital arch, transforming a flat dinosaurian manus into a digitigrade, semi-tubular structure. Morphological evidence presented here suggests a semi-tubular manus was an exaptation that ultimately functioned as a weight-distributing structure, and that this unique morphology may have been present in basal sauropods.