AbstractLagosuchus talampayensis is a small‐bodied (~0.5 m long) Late Triassic dinosauriform archosaur from Argentina. Lagosuchus long has been a pivotal taxon for reconstructing the evolution of form and function on the dinosaur lineage. This importance is because it has a mix of ancestral archosaurian traits, such as a small pelvis with a mostly closed acetabulum lacking prominences that would restrict hip mobility much, with derived “dinosaurian” traits such as bipedalism, proximally shifted thigh muscle insertions, elongate hindlimbs, “advanced mesotarsal” ankle joints and digitigrade feet. Here, to quantify key functional traits related to the locomotor biomechanics of Lagosuchus, we build a three‐dimensional musculoskeletal model, focussing on morphofunctional analysis of the pelvic limb. We survey skeletal material that we have digitised, pointing out hitherto undescribed features and elements, many of which are from taxa other than Lagosuchus. Next, we select ideal elements amongst these to construct a composite model, and articulate adjacent body segments into joints, then estimate body shape including centre of mass, and add muscle paths to create a musculoskeletal model. Finally, we use two methods to quantify the hindlimb muscle parameters (“architecture”) in the model. We find that they produce similar estimates of force‐generating capacities, and compare these data to the few available data from other archosaurs in an evolutionary context, to reconstruct fundamental patterns of changes in muscle architecture and pelvic limb morphology. Our model forms a valuable basis for future quantitative analyses of locomotor function and its evolution in early archosaurs, and an example of how to navigate decision‐making for modelling problematic specimens.