Hindlimb locomotor biomechanics of the derived therizinosaur Nothronychus: Functional changes in the line to birds and convergence with large-bodied neornitheans.

Abstract

Nothronychus graffami was a large therizinosaur represented by a single well-preserved individual from the Turonian Tropic Shale of southern Utah. It is characterized by an enlarged abdomen, small tail, and an extensively pneumatized axial skeleton, and is frequently regarded as herbivorous. Given the overall tail reduction and the development of a wide fused synsacrum with widely spaced acetabulae, it is reconstructed with an anteriorly rotated femur and a displaced resting ground reaction force anterior to the center of mass. The axis of the ground reaction force would shift laterally during locomotion to maintain stability as observed in extant broad abdomened neornitheans. A waddling gait is inferred for Nothronychus. This pattern is significantly different than in Falcarius, a basal therizinosaurian, where a plesiomorphic narrow abdomen, narrowly spaced acetabulae, and altiliac ilium are observed. Falcarius was capable of a more cursorial gait than derived therizinosaurs. In contrast to Nothronychus, Falcarius was probably at least omnivorous to carnivorous, so herbivory almost certainly evolved within the therizinosaur lineage. Following previous work on Tyrannosaurus and Coelophysis, moment arms were computed for Nothronychus for major muscles spanning the hip, knee, and ankle. A ball-and-socket joint is present at the hip, so three-dimensional movement was considered possible there. The knee and ankle were represented by two-dimensional hinge joints. Some muscles altered their function as the pubis shifted from a propubic orientation to an opisthopubic one. These included flexion to extension and the addition of increased abduction/adduction in the affected muscles. The results supported convergence with an avian locomotor model, such as reduction in M. caudofemoralis longus.