Pectoral Girdle and Forelimb Muscle Reconstruction in the Basal Therizinosaur Falcarius utahensis From Central Utah

Therizinosaurs were a rapidly evolving lineage of maniraptoran theropods from the Upper Cretaceous of North America and Asia. Falcarius utahensis was a basal therizinosaur from the Berriasian/Valanginian Yellow Cat Member of the Cedar Mountain Formation of central Utah. Previous descriptions suggested that this therizinosaur had a plesiomorphic skeleton similar to other non-avian theropods. Many disarticulated elements comprising the pectoral girdle and forelimb have been recovered. They are often quite well-preserved and show identifiable muscle attachment points. In this descriptive study, inferences about the origins and insertions of 35 muscles attaching to the scapula, humerus, ulna, and radius are made using phylogenetic bracketing (with birds and crocodylians/lepidosaurs) and comparisons with other theropod dinosaurs. Resulting inferences suggest that forelimb function in Falcarius was similar to that of relatively unmodified theropods, including Tawa and Struthiomimus. Moreover, given these similarities, it can be concluded that the musculoskeletal system of Falcarius represents a plesiomorphic condition within the therizinosaur lineage.