Osteology of the derived Therizinosaur Nothronychus with evidence for convergence in dinosaurian evolution

Abstract

Abstract Therizinosaurs were a taxon of unusual theropods from North America and Asia. The derived therizinosaur Nothronychus graffami possessed a synsacrum and hip convergent with extant birds. The osteology is figured and described in detail supplemented with material from Nothronychus mckinleyi. Both species exhibit traits convergent with extant birds, ornithischian dinosaurs, and titanosaurs. As preserved, the ilia and sacrum are minimally distorted and apomorphically modified into a synsacrum. The pubes are modified into a mesopubic condition, wherein the pubes are partially retroverted. The femoral shaft has an oval cross-section. As these characters are absent from the basal therizinosaur Falcarius utahensis, they evolved more than once within the maniraptoran lineage. An herbivorous ecology is supported. Both specimens were skeletally mature, based on fusion of the neural spines and scapulocoracoid.