This contribution studies some paleobiological aspects of Opisthodactylus kirchneri, a fossil rheid from the Late Miocene of northwestern Argentina. We estimate the body mass of this species using an allometric equation based on long-legged birds. Besides, we estimate the maximum running speed by applying a previously published biomechanical model to study other living and fossil running birds. We perform multivariate analyses with hindlimb measurements used as biomechanical model inputs to compare the fossil rheid with other living ratite species. We found that O. kirchneri would not have been a swift runner as living rheids of South America. We identify the presence of a long distal hindlimb segment correlated with a novel adaptation to paleoenvironmental conditions in southern South America during the Late Miocene. We propose alternative interpretations based on these results and the hindlimb bone proportions.