Functional morphology and biomechanics of the locomotor apparatus in the large Late Triassic carnivore Postosuchus kirkpatricki (Archosauria: Rauisuchidae).

Postosuchus kirkpatricki was a large pseudosuchian archosaur from the Late Triassic period in North America. It is among several pseudosuchians proposed to have had derived aspects of locomotor function such as bipedalism or digitigrady, rather than plesiomorphic quadrupedalism or plantigrady, but disputes and inconsistencies about these propositions remain. These lingering disputes need resolution in order to formulate broader inferences about the evolution of bipedalism, limb posture, athleticism, and the end-Triassic mass extinctions. Here, we use 3D musculoskeletal modelling to address the disputes via a deep critical review of available evidence via multiple methods. We conclude that it is uncertain if Postosuchus spp. was quadrupedal or bipedal, plantigrade or digitigrade, due to conflicting evidence. Our analyses also reconstruct pelvic limb musculature that was relatively three times as massive as that in a similar-sized Nile crocodile, whereas the caudofemoralis was smaller than expected due to the gracile tail of Postosuchus. Aspects of hindlimb myology and morphofunctional analyses of the hindlimb joints suggest a mix of traits that are plesiomorphic archosaurian, derived "rauisuchian" and singular for Postosuchus. Our extensive modelling procedure and synthesis of current evidence forms a foundation for future studies such as predictive simulations or ichnological evidence of locomotor function.