The Unique Ankle Articulation of Avimimus: Examining Its Potential Existence Through Robotic Modeling

Palaeognaths, including ostriches and emus, comprise many large, terrestrial birds and are regarded as model animals for biomechanical studies of extinct dinosaurs, particularly theropods. A distinctive biomechanical feature of palaeognaths is a specialized mechanism in the intertarsal joint (one of the ankle joints), known as the engage-disengage mechanism (EDM). This passive mechanism is defined by a locally unstable equilibrium that emerges at a certain degree of flexion. It serves as a boundary between two locally stable equilibria: full extension and full flexion. The mechanism is facilitated by cam-like articular surfaces, a type of mechanical element, and is thought to contribute to efficient locomotion in palaeognaths. Avimimus, a small theropod and a member of Oviraptorosauria, exhibits osteological features resembling the intertarsal joint structure of modern birds. These features include the fusion of the tarsal bones with the tibia to form the tibiotarsus, and the fusion of the metatarsals with the tarsal bones to form the tarsometatarsus. Consequently, Avimimus is considered one of the most suitable taxa for examining whether the passive mechanisms observed in the intertarsal joints of extant birds could have also been present in non-avian dinosaurs. In this study, we investigate the potential existence of the EDM in dinosaurian ankle joints by constructing a physical model based on Avimimus specimens.