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A primitive therizinosauroid dinosaur from the Early Cretaceous of Utah


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 beaks1,2. 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 Utah4,5. 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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Figure 1: Skeletal elements of Falcarius utahensis.
Figure 2: Jaws and teeth of Falcarius utahensis.
Figure 3: Phylogenetic relationships of Falcarius among the Coelurosauria.


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We thank X. Xu, Z. Dong, P. Sereno and M. Norell for access to reference material; G. Paul for executing the skeletal reconstruction of Falcarius; The Utah Friends of Paleontology, the Utah Museum of Natural History, and H. and P. Bollan for assistance in the field and the laboratory; M. Hayden, M. Christopher, M. Suarez and C. Suarez for overseeing the volunteers in the field; J. R. Scandizzo for introductions to the site discoverer L. Walker of Moab, Utah; R. W. Gaston for providing research casts; D. Smith for comments on theropod braincase anatomy; and J. Harris, M. Hayden, M. Lowe and M. Hylland for reviewing the manuscript. Excavations were conducted under a US Bureau of Land Management permit. Funding was provided by the Utah Geological Survey, Discovery Channel Quest Grants, Paleoscene, to J.I.K. and the NSF to J.M.C.

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Correspondence to James I. Kirkland.

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Supplementary Notes

This file includes the character list and data matrix, and also contains additional references used in the character list. (DOC 107 kb)

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Kirkland, J., Zanno, L., Sampson, S. et al. A primitive therizinosauroid dinosaur from the Early Cretaceous of Utah. Nature 435, 84–87 (2005).

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