tableS1_femora list_20220610.xlsx from The morphogenetic process for the dinosaurian femoral head shape shifted in evolution

Significant evolutionary shifts in locomotor behaviour often involve comparatively subtle anatomical transitions. In particular for dinosaurian locomotor apparatus, considerable medial overhang of the proximal end of the femur has been central to discussions for their locomotion. However, there is an apparent conflict with regard to the evolutionary origin of the dinosaurian femoral head, with neontological and palaeontological data suggesting seemingly incongruent hypotheses. To reconcile this, we reconstructed the evolutionary history of morphogenesis of the proximal end of the femur from early archosaurs to crown birds. Embryological comparison of living archosaurs (crocodylians and birds) suggests the acquisition of the greater overhang of the femoral head in dinosaurs results from additional growth of the proximal end in the medial-ward direction. On the other hand, the fossil comparison suggests this overhang was acquired by torsion of the proximal end, which projected to more rostral direction in ancestors. We reconcile this apparent conflict by inferring that the medial overhang of the dinosaur femoral head was initially acquired by torsion, which was then superseded by mediad growth. Details of anatomical shifts in fossil forms support this hypothesis, and their biomechanical implications are congruent with the general consensus regarding broader morpho-functional evolution on the avian stem.