Hindlimb musculature of the Lower Cretaceous (Barremian) Therizinosaur Falcarius utahensis (Maniraptora, Theropoda) with implications for evolution, stance, and stride

Abstract

The hindlimb musculature of the basal therizinosaur Falcarius utahensis is reconstructed using observable muscle scars, information from extant birds and crocodylians, and current models of Tyrannosaurus rex. Its ancestral phylogenetic position within Therizinosauria based on recent analyses is accepted here. Falcarius is characterized by a propubic pelvis, whereas the more derived therizinosaur Nothronychus had an incipiently opisthopubic pelvis with a highly modified ilium. Therefore, the femur of Falcarius was modelled with a plesiomorphic vertical orientation at rest. The M. caudofemoralis longus was well-developed, indicating a hip-propelled gait, with a sinuous stride. These results strongly suggest that the opisthopubic pelvis, associated muscular changes, avian stance and mode of walking evolved more than once: once within the therizinosaur lineage and once beneath the paravian node. The center of mass would have been located roughly under the acetabulum. Falcarius would have been characterized by a narrow abdomen and a non-waddling gait, in contrast to the more derived Nothronychus, with an inferred knee-based retraction system.