1. Utah Geological Survey, PO Box 146100, Salt Lake City, Utah 84114-6100, USA
2. Utah Museum of Natural History and Department of Geology and Geophysics, University of Utah, 1390 E. Presidents Circle, Salt Lake City, Utah 84112-0050, USA
3. George Washington University, Washington DC 20052, USA

Correspondence to: James I. Kirkland¹ Correspondence and requests for materials should be addressed to J.I.K. (Email: jameskirkland@utah.gov).

Abstract

Therizinosauroids are an enigmatic group of dinosaurs known mostly from the Cretaceous period of Asia, whose derived members are characterized by elongate necks, laterally expanded pelves, small, leaf-shaped teeth, edentulous rostra and mandibular symphyses that probably bore keratinized beaks^{1, 2}. Although more than a dozen therizinosauroid taxa are known, their relationships within Dinosauria have remained controversial because of fragmentary remains and an unusual suite of characters. The recently discovered 'feathered' therizinosauroid Beipiaosaurus from the Early Cretaceous of China helped to clarify the theropod affinities of the group³. However, Beipiaosaurus is also poorly represented. Here we describe a new, primitive therizinosauroid from an extensive paucispecific bonebed at the base of the Cedar Mountain Formation (Early Cretaceous) of east-central Utah^{4, 5}. This new taxon represents the most complete and most basal therizinosauroid yet discovered. Phylogenetic analysis of coelurosaurian theropods incorporating this taxon places it at the base of the clade Therizinosauroiden, indicating that this species documents the earliest known stage in the poorly understood transition from carnivory to herbivory within Therizinosauroidea. The taxon provides the first documentation, to our knowledge, of therizinosauroids in North America during the Early Cretaceous.

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