Biomechanics of the plesiomorphic forelimb of the basal therizinosaur Falcarius

Therizinosaurs were a group of rapidly evolving basal maniraptoran theropods from the Upper Cretaceous of Asia and North America. Basal representatives were probably omnivorous, whereas more derived taxa may well have been herbivores. The earlier taxa, such as Falcarius, possessed a plesiomorphic maniraptoran body plan, but later ones, including Nothronychus, were extensively modified. Falcarius is known from many well-preserved but disarticulated elements found in an extensive bone bed in central Utah. The taxon was characterized by a well-developed but primitive forelimb. This paper presents the results of a biomechanical analysis of moment arms of major muscles spanning the shoulder and elbow. Many results are broadly similar to those found earlier for Nothronychus. Differences are attributed to allometry and changes in forelimb use related to trophic level between the two taxa. Important muscles spanning the shoulder include mm. pectoralis, supracoracoideus, and deltoid. In Falcarius, the first two exhibit abductor moment arms, while m. deltoid is complex in the abductor/adductor plane. Mm. pectoralis and supracoracoideus are synergistic protractors. Long axis rotation varies in these muscles. Muscles spanning the elbow, including mm. biceps and triceps, exhibited the expected antagonistic moment arms.