Forelimb biomechanics in the derived therizinosaur Nothronychus and its relation to the origin of the avian wing.

Therizinosauria is a clade of extinct unusual maniraptoran theropods. Nothronychus is a derived representative of the clade from the upper Cretaceous Moreno Hill Formation, west-central New Mexico and Tropic Shale, southern Utah. It represents an ideal taxon to establish basal function in maniraptoran theropods. This project models the function of a maniraptoran arm and establishes a good starting point to test the hypothesis that there will be significant muscular and functional changes leading to the avian forelimb. A model of Nothronychus forelimb function is presented including thirty-five muscles. It assumes a fairly conventional maniraptoran forelimb with typical ranges of motion in comparison with Allosaurus. Therefore, abduction/adduction, protraction/retraction, and long-axis rotation are modelled for the shoulder. In keeping with other maniraptorans, the elbow is modelled as a simple hinge. Little functional difference was observed between the forelimbs of Allosaurus and Nothronychus, so much muscular modification associated with flight had yet to evolve, even with the enlargement of the forelimbs. However, M. supracoracoideus and M. pectoralis had already developed antagonistic moment arms. Abduction in M. supracoracoideus was weaker than M. pectoralis adduction in Nothronychus and Allosaurus, so this relation evolved prior to flight.