Osteology of Falcarius utahensis (Dinosauria: Theropoda): characterizing the anatomy of basal therizinosaurs LINDSAY E. ZANNO* Utah Museum of Natural History and Department of Geology and Geophysics, University of Utah, 1390 E. President’s Circle, Salt Lake City, UT84112, USA Received 22 August 2007; accepted for publication 10 March 2008 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 synapomor- phy 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. © 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 ADDITIONAL KEYWORDS: anatomy – Coelurosauria – Early Cretaceous – evolutionary trends – Maniraptora – morphology – North America – Segnosauria – Utah. INTRODUCTION Therizinosaurs were a rare and unusual group of medium to large-bodied, feathered dinosaurs charac- terized by diminutive skulls, small lanceolate teeth, elongate necks, enlarged manual unguals, broad pelves, stocky hindlimbs, and a functionally tetradac- tyl pes. As a clade, therizinosaurs are known to have inhabited North America and Asia from the Barre- mian through to the Turonian (Perle, 1979, 1981; Xu, Tang & Wang, 1999a; Kirkland et al., 2005; Zanno et al., 2009), and to have persisted in portions of Asia until the Maastrichtian (Maleev, 1954). Although first discovered over half a century ago, the taxonomic affinity of these enigmatic dinosaurs has been a subject of wide ranging debate (Maleev, 1954; Rozhdestvensky, 1970; Paul, 1984; Gauthier, 1986; Sereno, 1989; Russell & Dong, 1993). Previous uncer- tainty about their classification resulted from two main handicaps – the bizarre juxtaposition of anatomi- cal features characterizing derived Late Cretaceous taxa and a relatively poor fossil record devoid of basal forms. Fortunately, the relationship between the mor- phologically ambiguous and fragmentary therizino- saurs of the Late Cretaceous and other maniraptoran theropods was brought to light by the discovery of two successively more primitive and more complete ther- izinosaurs, the Chinese taxa Alxasaurus elesitaiensis from the Aptian age Bayin-Gobi Formation (Russell & *E-mail: lzanno@fieldmuseum.org Zoological Journal of the Linnean Society, 2010, 158, 196–230. With 19 figures © 2010 The Linnean Society of London, Zoological Journal of the Linnean Society, 2010, 158, 196–230 196