Osteology of Falcarius utahensis (Dinosauria: Theropoda): characterizing the anatomy of basal therizinosaurs

Falcarius utahensis, from the lower Yellow Cat Member of the Cedar Mountain Formation, represents the most complete and morphologically primitive therizinosaur yet discovered. Since initial publication, only the braincase, pectoral girdle, and forelimb of this phylogenetically important taxon have been subject to detailed investigation. This work completes the description of skeletal material prepared from the Crystal Geyser Quarry subsequent to the first five years of excavation - including elements of the skull, axial column, pelvis, and hind limb of this phylogenetically critical theropod - and presents an emended and significantly expanded diagnosis. Results of this study reveal a significant degree of morphological disparity between Falcarius utahensis and the evidently coeval primitive therizinosaur Beipiaosaurus inexpectus from the Yixian Formation, People's Republic of China and help characterize morphological transformations occurring in the therizinosaur lineage that are of phylogenetic significance, particularly with regard to the highly derived presacral axial column. Finally, Falcarius documents that marked heterodonty - characterized by elongate, incisiform rostral teeth - is present in basal therizinosaurs and oviraptorosaurs (i.e. Incisivosaurus gauthieri, Protarchaeopteryx robusta), and either represents a synapomorphy or symplesiomorphy for these groups or an early phase in the convergent progression toward rostral endentulism. Nonetheless, heterodonty suggests that diet was a primary factor in the early evolution of both clades. (C) 2010 The Linnean Society of London, Zoological Journal of the Linnean Society, 2010, 158, 196-230. doi: 10.1111/j.1096-3642.2009.00464.x