Abstract: This study analyzes two trackways of the fastest running theropods in the fossil record, offering a rare opportunity to examine dinosaur biomechanics during high-speed locomotion. We focus on the distinct three-dimensional morphologies of footprints - La Torre 6B-01 and La Torre 6A-14 (Early Cretaceous, La Rioja, Spain) - produced by similar theropod trackmakers on the same surface. The La Torre 6B-01 footprints range from digitigrade to subdigitigrade, whereas the La Torre 6A-14 tracks vary from fully digitigrade to digitigrade with an elongated metatarsophalangeal area. These differences are interpreted as reflecting distinct phases of running locomotion, likely linked to changes in trackmaker behavior. Comparable morphologies occurred in other trackways that have been interpreted as produced by running trackmakers, from the Early Jurassic to the Late Cretaceous. These examples emphasize how different phases of a run - combined with variations in force exertion and autopodium positioning - can strongly influence footprint morphology. Our findings reveal that theropod dinosaurs employed running strategies more complex and dynamic than previously recognized. Subtle footprint features, such as the presence or absence of metatarsophalangeal impressions, can be directly tied to changes in posture, weight distribution, and muscular activity. This approach provides new perspectives on biomechanical evolution, showing that locomotor diversity and limb morphology shaped dinosaur adaptation and diversification.
