The ecomorphology of facultative bipedality in Lepidosauria: implications for the evolution of reptilian bipedality

Bipedality is a distinctive locomotor characteristic of some of the most noteworthy animals of all time, including dinosaurs and humans. However, the evolution of a bipedal locomotor mode is poorly understood in reptiles. It has been repeatedly hypothesised that a facultative locomotor mode, where an animal moves both bipedally and quadrupedally under different conditions, forms an intermediate stage in the evolution of obligate bipedality. I demonstrate that the evidence supporting this hypothesis is lacking, recovering facultative bipedality as an intermediate stage only once in multiple independent evolutions of bipedality, under two different topologies. In order to better understand facultative bipedality and the associated anatomies, I performed multiple studies into the ecomorphology and evolution of this behaviour in a modern clade: Lepidosauria. Linear morphometric studies accounting for variation in body size indicate that forelimb segment lengths across locomotor modes do not differ for lepidosaurs of the same size, but that distal hindlimbs segments differ greatly, contrasting with historical tropes. Using 3D landmark-based geometric morphometrics, I demonstrate that arboreal and facultatively bipedal species share many characteristics in the bony elements of the pelvis, including a straight-to-concave iliac blade and large ischial base. These shared anatomies are functionally qualified based on anatomical studies of lepidosaur pelvic girdle myology, and indicate a similarity in mechanical demands of both arboreality and facultative bipedality. Finally, I tested for the correlated evolution of facultative bipedality with substrate preference in a derived clade of squamates: Episquamata. Findings suggest that there is no correlation between substrate and a facultative locomotor mode, instead indicating that facultative bipedality is an exaptation of anatomies associated with vertically diverse environments. This echoes the evolution facultative bipedality in hominin, macropods and rodents, and is distinct from any current hypotheses concerning the evolution of archosaurian bipedality